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THEME: Environment (including climate change)
TOPIC: ENV.2011.2.1.2-1 Hydromorphology and ecological objectives of WFD
Collaborative project (large-scale integrating project)
Grant Agreement 282656
Duration: November 1, 2011 – October 31, 2015
REstoring rivers FOR effective catchment Management
Deliverable
D6.2 Part 4
Title
The Geomorphic Units survey and classification System (GUS)
Authors
(authors of D6.2 Part 4*) M. Rinaldi1, B. Belletti1, F. Comiti2, L. Nardi1, M.
Bussettini3, L. Mao4, A.M. Gurnell5
1UNIFI, 2Free University of Bozen-Bolzano, 3ISPRA, 4Pontificia Universidad
Católica de Chile (voluntary contribution), 5QMUL
Due date to deliverable: 31 July 2015
Actual submission date: 30 October 2015
Project funded by the European Commission within the 7th Framework Programme (2007 – 2013)
Dissemination Level
PU
Public
X
PP
Restricted to other programme participants (including the Commission Services)
RE
Restricted to a group specified by the consortium (including the Commission Services)
CO
Confidential, only for members of the consortium (including the Commission Services)
!
!
!
D6.2 Methods for HyMo Assessment.
Part 4. Geomorphic Units Survey
ii
* Please cite the whole of Deliverable 6.2 as follows:
M. Rinaldi, B. Belletti, M.I. Berga Cano, S. Bizzi, B. Blamauer, K. Brabec, G. Braca, M.
Bussettini, F. Comiti, L. Demarchi, D. García de Jalón, M. Giełczewski, B. Golfieri, M.
González del Tánago, R. Grabowski, A.M. Gurnell, H. Habersack, S. Hellsten, S.
Kaufman, M. Klösch, B. Lastoria, F. Magdaleno Mas, L. Mao, E. Marchese, P.
Marcinkowski, V. Martínez-Fernández, E. Mosselman, S. Muhar, L. Nardi, T. Okruszko, A.
Paillex, C. Percopo, M. Poppe, J. Rääpysjärvi, M. Schirmer, M. Stelmaszczyk, N. Surian,
M. Toro Velasco, W. Van de Bund, P. Vezza, C. Weissteiner (2015) Final report on
methods, models, tools to assess the hydromorphology of rivers. Deliverable 6.2, a
report in five parts of REFORM (REstoring rivers FOR effective catchment Management),
a Collaborative project (large-scale integrating project) funded by the European
Commission within the 7th Framework Programme under Grant Agreement 282656.
Please cite Part 4 of Deliverable 6.2 as follows:
M. Rinaldi, B. Belletti, F. Comiti, L. Nardi, M. Bussettini, L. Mao, A.M. Gurnell (2015) The
Geomorphic Units survey and classification System (GUS), Deliverable 6.2, Part 4, of
REFORM (REstoring rivers FOR effective catchment Management), a Collaborative project
(large-scale integrating project) funded by the European Commission within the 7th
Framework Programme under Grant Agreement 282656.
D6.2 Methods for HyMo Assessment.
Part 4. Geomorphic Units Survey
iii
Summary
Background and Introduction to Deliverable 6.2
Work Package 6 of REFORM focuses on monitoring protocols, survey methods,
assessment procedures, gudelines and other tools for characterising the consequences of
physical degradation and restoration, and for planning and designing successful river
restoration and mitigation measures and programmes.
Deliverable 6.2 of Work Package 6 is the final report on methods, models and tools to
assess the hydromorphology of rivers. This report summarises the outputs of Tasks 6.1
(Selection of indicators for cost-effective monitoring and development of monitoring
protocols to assess river degradation and restoration), 6.2 (Improve existing methods to
survey and assess the hydromorphology of river ecosystems), and 6.3 (Identification
and selection of existing hydromorphological and ecological models and tools suitable to
plan and evaluate river restoration).
The deliverable is structured in five parts. Part 1 provides an overall framework for
hydromorphological assessment. Part 2 includes thematic annexes on protocols for
monitoring indicators and models. Part 3 is a detailed guidebook for the application of
the Morphological Quality Index (MQI). Part 4 (this volume) describes the Geomorphic
Units survey and classification System. Part 5 includes a series of applications to some
case studies of some of the tools and methods reported in the previous parts.
Summary of Deliverable 6.2 Part 4
This part provides a detailed description of the Geomorphic Units survey and
classification System (GUS). This method is used to identify, characterise and analyse
the assemblage of geomorphic units within a given reach. The system is suitable for
integrating the MQI and is also aimed at allowing the establishment of links between
hydromorphological conditions at reach scale, characteristic geomorphic units, and
related biological conditions.
The document is organised in two parts. Part A provides the general background and
describes characteristics, analysis, testing, and typical applications of the method. Part B
is an Illustrated Guidebook to the identification and classification of geomorphic units. A
series of Forms for the application of the GUS are reported in Appendix 1. The list of
gemorphic units included in the GUS is reported in Appendix 2. A brief glossary of
significant terms is reported in Appendix 3.
Acknowledgements
REFORM receives funding from the European Union’s Seventh Programme for research,
technological development and demonstration under Grant Agreement No. 282656.
D6.2 Methods for HyMo Assessment.
Part 4. Geomorphic Units Survey
iv
Table of Contents
Summary ............................................................................................................................................ iii
A. The Geomorphic Units survey and classification system (GUS) ...................... 6
A.1.Introduction ................................................................................................. 6
A.2. General background .................................................................................... 7
2.1 Existing methods for characterising physical habitats .................................................................. 7
2.2 The geomorphic units ................................................................................................................... 8
A.3. Description of the GUS .............................................................................. 13
3.1 Overall characteristics of the method ........................................................................................ 13
3.2 The Geomorphic Units and the spatial hierarchical framework ................................................. 13
3.3 Spatial settings ............................................................................................................................ 13
3.4 Methods and levels of characterisation...................................................................................... 14
3.4.1 Methods ............................................................................................................................... 14
3.4.2 Types of data and information............................................................................................. 14
3.4.3 Levels of characterisation .................................................................................................... 14
3.4.4 Survey and compilation of the GUS forms ........................................................................... 16
A.4. Analysis of geomorphic units and GUS indices .......................................... 18
4.1 GUS Indices ................................................................................................................................. 18
4.2 Interactions between GUS and MQI ........................................................................................... 19
A.5. Testing phase ............................................................................................ 21
A.6. Applications of GUS ................................................................................... 23
B. Guide to the application of the GUS .............................................................. 24
B.1 Guide to the compilation of the survey forms ............................................. 24
1.1 Sheet 1: Survey plan .................................................................................................................... 24
1.2 Sheet 2: General information and field sketch ........................................................................... 25
1.3 Sheet 3: Broad level .................................................................................................................... 27
1.4 Sheets 4 to 6: Basic level ............................................................................................................. 28
1.5 Sheets 7 to 15: Detailed level (macro-units and units) ............................................................... 29
1.6 Sheet 16: Detailed level (sub-units) ............................................................................................ 31
B.2. Illustrated guidebook for the identification and delineation of spatial units
......................................................................................................................... 34
2.1 Bankfull channel units ................................................................................................................. 34
2.1.1 Macro-unit: baseflow or 'submerged' channels .................................................................. 34
2.1.2 Macro-unit: emergent sediment units ................................................................................. 50
2.1.3 Macro-unit: in-channel vegetation ...................................................................................... 64
D6.2 Methods for HyMo Assessment.
Part 4. Geomorphic Units Survey
v
2.2 Floodplain units ........................................................................................................................... 81
2.2.1 Macro-unit: riparian zone .................................................................................................... 81
2.2.2 Macro-unit: floodplain aquatic zones .................................................................................. 90
2.2.3 Macro-unit: human-dominated areas (land use included) .................................................. 93
2.3 Artificial features ......................................................................................................................... 95
2.4 Sub-units ................................................................................................................................... 100
2.4.1 Bankfull channel sub-units ................................................................................................. 100
2.4.2 Floodplain sub-units ........................................................................................................... 102
References ..................................................................................................... 103
Appendix 1: Survey and classification form .................................................... 112
Appendix 2: Geomorphic units and macro-units list ....................................... 129
Appendix 3: Glossary ..................................................................................... 131
D6.2 Methods for HyMo Assessment
Part 4. Geomorphic Unit Survey
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A. The Geomorphic Units survey
and classification system (GUS)
A.1.Introduction
The assessment of stream hydromorphological conditions is required for the classification
and monitoring of water bodies by the Water Framework Directive 2000/60, and is useful
for establishing links between their physical and biological conditions. The spatial scales
of geomorphic units and smaller (hydraulic, river element) units are the most
appropriate to assess these links, since geomorphic units represent the physical
template for habitats.
Geomorphic units (e.g., riffles, pools, etc.) constitute distinct habitats for aquatic fauna
and flora, and may also provide temporary habitats for organisms (refugia from
disturbance or predation, spawning, etc.). Procedures to assess physical habitats need to
be ecologically and geomorphologically meaningful, enabling ecologically relevant scales
and physical variables to be placed into a geomorphological characterisation template.
Because geomorphic units constitute the physical structures that underpin habitat units,
an assessment of the assemblage of geomorphic units can provide information about the
existing range of habitats occurring in a given a reach.
Several methods for the survey or assessment of physical habitats have been developed
worldwide since the 1980s. However, physical habitat methods are affected by a series
of important limitations (Rinaldi et al., 2013a; Belletti et al., 2015) (see section 2.1).
First, in most methods the spatial scale of investigation is not well placed within a multi-
scale approach, encompassing rather small areas (‘site’ scale) that are often of a fixed
length. Second, these procedures tend to associate high status conditions with maximum
morphological diversity for all types of rivers, failing to recognise that in some cases the
natural geomorphic structure of a particular stream type may be very simple whereas in
other cases it may be complex (Fryirs, 2003). Lastly, a notable gap exists in the
terminology used to describe geomorphic units in most habitat surveys when compared
to the present state of the art in fluvial geomorphology.
To address some of these limitations, a new system for the survey and classification of
geomorphic units (GUS, Geomorphic Units survey and classification System) in streams
and rivers has been developed. The system fits within the multi-scale, hierarchical
framework developed in REFORM Deliverable 2.1, is suitable for integration with the
MQI, and also allows links to be established between hydromorphological conditions at
reach scale, characteristic geomorphic units, and related biological conditions.
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A.2. General background
2.1 Existing methods for characterising physical habitats
Several methods and protocols have been developed for the survey, characterisation,
and classification of physical habitat elements which can be described as ‘river habitat
surveys’ or ‘physical habitat assessments’ (e.g., Platts et al. 1983; Plafkin et al. 1989;
Raven et al. 1997; Ladson et al. 1999; National Environmental Research Institute 1999;
LAWA 2000, 2002a, b). They provide a framework within which habitat units can be
efficiently inventoried and sampled, and so they are useful for characterising the range
of physical habitats that are present, their heterogeneity and the contemporary physical
structure of ecosystems. Additionally, these methods often inventory some features of
ecological relevance, which are not addressed within truly morphological assessment
methods, such as the presence of refuge areas, organic matter, shading, etc. Therefore,
they are potentially helpful in establishing links between morphology and ecological
conditions and communities (e.g., supporting explanation of the distribution patterns of
organisms, the composition and structure of biological communities or aspects of
ecosystem functioning).
Nevertheless, existing physical habitat assessment methods have a series of limitations
(Belletti et al., 2015). Among these, is the way that physical habitat methods
characterize channel forms and geomorphic units. There is a notable gap in the
terminology used to describe geomorphic units in most habitat surveys when compared
to the present state of the art in fluvial geomorphology. For example, most refer only to
riffles and pools when describing the configuration of the river bed, probably because
most habitat survey methods have been developed to address small, single-thread,
sand- or gravel-bed rivers. As a result, the wide variety of bed morphologies found in
steep, mountain, cobble- or boulder- bed streams, where other geomorphic units may
occur (cascades, rapids, glides, step-pools, etc.) is not considered. Although
considerable progress has been made recently in naming and describing geomorphic
units found in mountain streams (e.g., Halwas and Church 2002; Wohl, 2010; Comiti
and Mao 2012), these have not being incorporated in most physical habitat assessment
methods. Furthermore, the variety of geomorphic units found in rivers with complex,
transitional or multi-thread patterns (i.e., wandering or braided) is poorly incorporated in
these methods, although some effort has been made recently to represent some of these
morphologies (including ephemeral or temporary streams typical of some Mediterranean
regions in Southern Europe). In the case of large rivers with complex morphologies
(e.g., many piedmont Alpine rivers), field surveys alone are insufficient to characterize
channel forms and geomorphic units, thus the incorporation of remote sensing
techniques is essential. This implies that existing procedures fail to identify correctly the
variability and complexity of geomorphic units that exist in nature. At the same time,
they tend to associate high status conditions with maximum morphological diversity for
all types of rivers, failing to recognize that in some cases the natural geomorphic
structure of a particular stream type may be very simple whereas in other cases it may
be more complex (Fryirs, 2003; Barquín et al., 2011). Furthermore, considerable
progress has been achieved recently in developing new procedures to identify and
analyse geomorphic units within the context of a more appropriate spatio-temporal
framework (e.g., Fryirs and Brierley, 2013; Brierley et al., 2013), but existing physical
habitat assessment methods do not appear to adopt this type of approach and so are not
placed within an appropriate spatio-temporal framework that takes account of recent
progress in the field of fluvial geomorphology.
Besides the methods discussed above, other procedures, developed since the early
1990s to map, characterise and/or classifying physical habitats (e.g. Thomson et al.,
2001; Hill et al., 2013; Zavadil & Stewardson, 2013), but which do not include a quality
assessment based on one or more synthetic indices, are worthy of some consideration.
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These include the methods described by Hawkins et al. (1993), Jowett (1993), Wadeson
(1995), Maddock & Bird (1996), Padmore et al. (1996), and more recently, by Thomson
et al. (2001), Clifford et al. (2006), Harvey & Clifford (2009), and Zavadil et al. (2012).
Most of these methods focus on aquatic habitats, in response to the interest of scientists
and river managers in aquatic organisms. Many of these approaches are based on the
identification and classification of flow types (e.g., free fall, broken standing waves, etc.;
Padmore, 1998; Newson & Newson, 2000; Zavadil & Stewardson, 2013), which are used
to indicate the template of physical habitats at the microhabitat scale. However, it is
important to note that such flow types are highly temporally variable, depending
strongly on discharge conditions at the moment of observation (Zavadil & Stewardson,
2013).
Another methodological advance has been the development of habitat modelling tools.
Habitat simulation models quantify the spatial variability of hydraulic parameters (e.g.,
flow velocity, water depth, etc.) for different flow discharges. Modelling methods include:
(i) 1D models applied at the micro-habitat scale and based on preference curves for
different species and their life stages (e.g., PHABSIM, Bovee et al., 1998); (ii) models
based on fuzzy logic (e.g., CASIMIR, Jorde et al., 2000); and (iii) 2D models applicable
at the meso-habitat scale (e.g., MesoHabsim, Parasiewicz, 2001, 2007; Vezza et al.,
2015; RHASIM, Liefeld & Schulze, 2005; MEM, Hauer et al., 2007; MesoCASiMiR,
Schneider et al., 2006). Various hydromorphological and habitat indices have also been
developed, providing a quantitative assessment of spatio-temporal habitat variability
(e.g., HDMI, Gostner et al., 2013; IHQ and IHSD, Vezza et al., 2015).
Due to the shortcomings and limitations these existing methods, a systematic procedure
for collecting and interpreting data and information on physical habitats at appropriate
spatial scales and based on the present state of the art in fluvial geomorphology remains
to be developed. Procedures to assess physical habitat need to be ecologically and
geomorphologically meaningful, incorporating ecologically relevant scales and physical
variables (Frissell et al., 1986) into a geomorphological characterisation. An assessment
of the assemblage of geomorphic units provides information about the range of habitats
occurring in a given a reach, because geomorphic units constitute the physical
foundation of habitat units.
2.2 The geomorphic units
A geomorphic unit is defined as an area containing a landform created by erosion and/or
deposition inside (in-channel or bankfull geomorphic unit) or outside (floodplain
geomorphic unit) the river channel. Geomorphic units can be sedimentary units, or can
include living or dead (e.g. large wood) vegetation (‘biogeomorphic units’).
Referring to the multi-scale, hierarchical framework developed in REFORM Deliverable
2.1, a geomorphic unit may include one to several hydraulic units (i.e. spatially distinct
patches of relatively homogeneous surface flow and substrate character), each of which
can include a series of river elements (i.e. individuals and patches of sediment particles,
plants, wood pieces, etc).
The spatial scales of geomorphic and smaller (hydraulic, river element) units are the
most appropriate to assess the presence and diversity of physical habitats. Geomorphic
and hydraulic units are generally associated with the mesohabitat scale (about 10-1 - 103
m; Bain & Knight, 1996; Kemp et al., 1999; Hauer et al., 2011; Parasiewicz et al., 2013;
Zavadil & Stewardson, 2013), whereas river elements coincide with the microhabitat
scale (approximately 1 - 50 cm). Geomorphic units (e.g., riffles, pools, bars, islands,
etc.) constitute distinct habitats for fluvial (aquatic and riparian) fauna and flora, and
may also provide temporary habitats (refugia from disturbance or predation, spawning,
etc.).
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Geomorphic units are linked to the reach scale, because the processes of water flow and
sediment transport that control the geomorphic units are influenced by factors acting at
the reach (e.g., slope, substrate, and valley configuration) and larger scales. Reaches of
the same morphological type usually exhibit similar assemblages of geomorphic units. As
a consequence, physical habitat characteristics and associated biotic conditions are
strongly influenced by reach scale physical factors, which in turn are constrained by
regional-, catchment-, and segment scale considerations.
Figure A2.1 Sketch of a typical succession of geomorphic units occurring from upstream
to downstream.
Moving downstream along the fluvial system, different geomorphic units may occur as a
result of changing boundary conditions, such as valley and bed slope, flow discharge,
sediment size, etc. (Fig. A2.1). Erosional units sculpted into bedrock (e.g. plunge pools,
rock steps) and/or composed of coarse sediment (e.g. cascades, rapids) prevail along
confined, high-gradient reaches. Along unconfined, alluvial reaches, riffles, pools, glides,
depositional bars and islands become dominant, along with floodplain features (e.g.
D6.2 Methods for HyMo Assessment
Part 4. Geomorphic Unit Survey
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secondary channels, oxbows, backswamps, etc.). Downstream transitions in the
assemblage of geomorphic units may occur as a function of overall boundary conditions
(slope, discharge, etc.) and local factors (e.g. local change in bed slope, presence of a
tributary, as well as the presence of a dead tree or a local bedrock outcrop).
The typical assemblage of geomorphic units occurring along a river reach is one of the
factors determining the overall channel pattern (or type). In the REFORM Extended River
Typology (ERT, Fig. 2.2 and 2.3), characterisation of geomorphic units supports the
assessment of the functioning of each type. The set of distinguishing morphological
attributes determining each river type may vary between biogeographical regions and
may be degraded or reduced by human interventions, but a check-list of the geomorphic
units that may be present within the channel and its floodplain is provided in Table A2.1.
Figure A2.2 River types 0 to 6 of the Extended River Typology.
D6.2 Methods for HyMo Assessment
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Figure A2.3 River types 7 to 22 of the Extended River Typology.
Table A2.1 Description of the 22 morphological types of the ERT. Geomorphic units: AB:
Alternate bar; AC: Abandoned channel; B: Bar; Be: Bench; BL: Boulder levées; Bs:
Backswamp; C: Cascade; CC: Crevasse channel; Ch: Chutes; Co: Cut-off channel; CS:
Crevasse splay; F: Forced; G: Glide; I: Island; L: Levées; LB: Lateral bar; MB: Marginal
bar; MCB: Mid-channel bar; P: Pool; PB: Point bar; PBe: Point bench; Po: Pond; R: Riffle;
Ra: Rapids; RD: Ripples (and Dunes); RS: Rock step; RSw: Ridge and Swale; SB: Scroll
bar; Sc: Scroll; SP: Step-Pool; SS: Sand splay; VI: Vegetation induced.
ERT
Geomorphic
Units
Stability
Description
0
Possible
occasional B
Very Stable
Highly modified reaches
1
RS, C, Ra
Usually strongly confined and
highly stable
Sediment supply-limited channels
with no continuous alluvial bed
2
BL, C, SS, AC
Can be highly unstable
Small, steep channels at the
extremities of the stream network
3
Poorly defined,
featureless
channels.
Very stable, shallow (often
ephemeral) channels
Small, relatively low gradient
channels at the extremities of the
stream network
4
C, P
Stable for long periods but
occasional catastrophic
destabilisation
Very steep with coarse bed material
consisting mainly of boulders and
local exposures of bedrock
5
SP
Stable for long periods but
occasional catastrophic
destabilisation
Sequence of channel spanning
accumulations of boulders and
cobbles (steps) separated by pools
6
G, Ra, FB, FP
Relatively stable for long
periods, but floods can induce
lateral instability and
avulsions
Predominantly single thread but
secondary channels are sometimes
present
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Table A2.1 Description of the 22 morphological types of the ERT (continued).
ERT
Geomorphic
Units
Stability
Description
7
R, P, G, LB
Subject to frequent shifting of
bars
Coarse cobble-gravel sediments
sorted to reflect the flow pattern and
bed morphology
8
MCB, R, P
Usually highly unstable both
laterally and vertically
Multiple channels separated by active
bars (bar-braided)
9
I, MCB, R, P
Usually unstable both laterally
and vertically
Distinguished from type 11 by > 20%
channel area covered by islands of
established vegetation
10
I, R, P
Lateral instability usually
present
Islands covered by mature vegetation
extend between channels
11
I, MCB, MB, R, P
Usually highly unstable both
laterally and vertically
Exhibit switching from single to multi-
thread
12
Large, continuous
AB, R, P
Usually unstable both laterally
and vertically
Differs from type 11 in its lower
sinuosity and very pronounced
alternating lateral bar development
13
Large alternate
(continuous) PB,
R, P
Subject to frequent shifting of
bars
Sinuous pattern with discontinuous
bars of coarse sediment
14
R, P, PB, Ch, Co,
SB, Pbe
Laterally unstable channels
subject to lateral migration
Meandering pattern with frequent
point bars of coarse sediment
15
B, RD
Unstable both laterally and
vertically
Same morphology of 8 but with
predominant sand material
16
Continuous, large
AB, P, RD
Vertically unstable due to bar
movement and sometimes
migrate laterally
Highly sinuous baseflow and
alternating bars within a straight to
sinuous channel
17
R, P, PB, RD,
occasional Be,
SB, L, Bs
Laterally unstable channels
subject to lateral migration
Same morphology of 13 but with
predominant sand material
18
P, PB, RD, S, L,
RSw, Bs, AC
Unstable channels subject to
meander loop progression and
extension with cut-offs
Same morphology of 14 but with
predominant sand material
19
I, RD, L, VIB,
VIBe, RD, AC
Stable
Vegetation stabilising bars between
channel threads, forming islands that
develop by vertical accretion of fine
sediment
20
L, Bs
Very stable
Silt to silt-clay banks often with high
organic content are highly cohesive
21
L, Bs, Pbe
Very stable
Similar to 20 but with higher
sinuosity
22
I, L, CC, CS, Po,
VIB, VIBe, AC, Bs
Very stable
Silt to silt-clay banks often with high
organic content are highly cohesive;
extensive islands covered by wetland
vegetation
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A.3. Description of the GUS
3.1 Overall characteristics of the method
The overall characteristics of the GUS can be summarized as follows:
- The method is designed to provide a general framework for the survey and
classification of geomorphic units. However, it does not aim to assess the deviation
from any given reference conditions and/or to assess the status or quality of the
stream by the use of synthetic indices.
- It is an open-ended, flexible framework, where the operator can set up the level of
characterisation and the specific focus of the survey, depending on the objectives and
on available resources.
- The system is embedded into an appropriate spatially-nested hierarchical framework.
- The analysis of geomorphic units could be inserted within a wider spatial-temporal
framework of analysis of morphological conditions (e.g. Brierley et al., 2013). The
collected information can be used to better understand the morphology of a given
reach and to support the analysis of river reach behavior and evolution.
- The collected information may allow a link to be established between river
hydromorphology at the reach scale and the biota.
3.2 The Geomorphic Units and the spatial hierarchical framework
Geomorphic units are organized within a nested hierarchical framework as follows:
(1) Macro-unit: assemblage of units of the same type, e.g. water, sediment, vegetation.
The spatial scale of the Macro-units is the reach or the sub-reach. The minimum size of a
macro-unit coincides with the size of the corresponding unit (e.g. a bar, an island).
(2) Unit: basic spatial unit, and corresponds to a feature with distinctive morphological
characteristics and significant size located within a macro-unit, e.g. riffle, bar, island.
(3) Sub-unit: corresponds to patches of relatively homogeneous characteristics in terms
of vegetation, sediment and/or flow conditions located within a unit.
Units and Sub-units correspond to the mesohabitat scale. Small Sub-units can also
correspond to the microhabitat scale (i.e. river elements).
These spatial units are analyzed at the reach or sub-reach scale, where sub-reaches
contain characteristic assemblages of geomorphic units that characterize the morphology
at the reach scale.
3.3 Spatial settings
The overall spatial domain of application of the GUS is potentially the entire genetic
floodplain, i.e. the part of the valley floor delimited by hillslopes or ancient terraces
which can be directly affected or potentially influenced by fluvial processes. The main
focus of the survey is the portion of the fluvial corridor that is most directly or frequently
connected with contemporary fluvial processes, that is the relatively natural corridor of
spontaneous riparian vegetation. However, depending on the aims of the study, the
survey can be extended to human-dominated portions of the floodplain (agricultural
lands, urbanized areas).
Two spatial settings are distinguished: (1) the bankfull channel; (2) the floodplain.
Accordingly, the geomorphic units can be first classified in the following groups:
(1) Bankfull channel units: this group includes all the geomorphic units located within
the bankfull channel, comprising ‘submerged’ units (bed configuration, submerged
vegetation) and ‘emergent’ units (bars, islands, large wood jams), and features located
within the bankfull channel margins at the interface with the floodplain (e.g., banks,
berms, benches).
D6.2 Methods for HyMo Assessment
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(2) Floodplain units: they comprise all the units occupying the floodplain (e.g., modern
floodplain, recent terraces, wetlands, oxbow lakes, natural levées etc.). The size of the
floodplain units is generally larger than the bankfull channel.
3.4 Methods and levels of characterisation
3.4.1 Methods
Methods for the survey and characterisation include: (1) remote sensing - GIS analysis;
(2) field survey. It is preferable to combine remote sensing and field survey methods,
but in some cases it may only be possible to use one of these methods, depending on
the selected level of characterisation (see later), the size of the river, and the resolution
of the available remotely sensed data and imagery.
For remote sensing, aerial photos of sufficient resolution are needed. Satellite images
can also be used for preliminary reconnaissance of morphological characteristics and
range of possible units, but the delineation of macro-units within a GIS requires the
higher spatial resolution of aerial photos and LiDAR data, which is especially useful for
defining floodplain units (e.g. different levels of recent terraces) and emergent units
within the bankfull channel (e.g. bars, benches and high bars). The increasing
development of remote sensing platforms and techniques (e.g. ultra-light systems,
bathimetric LiDAR, structure from motion photogrammetry, hyper spectral image
systems; Carbonneau & Piégay, 2012) will very likely lead to their increasing use for
characterising geomorphic units, although a field check of their geomorphological
interpretation is strongly recommended.
3.4.2 Types of data and information
The following data and information can be obtained through the application of the GUS,
to provide an increasing level of detail.
(1) list of existing geomorphic units (i.e. presence/absence) in a given reach (or sub-
reach);
(2) number (frequency) of each unit;
(3) size (length and/or area) of each unit.
(4) detailed characterisation of geomorphic unit sub-types (i.e. presence/absence);
(5) description of sediment characteristics (size, substrate alteration), hydraulic
conditions, vegetation characteristics;
(6) identification of formative processes;
(7) additional size measures (e.g. width, height);
(8) other physical characteristics (e.g. D50, water temperature, etc.).
3.4.3 Levels of characterisation
The survey of geomorphic units can be carried out at different levels of detail (Fig.3.1
and 3.2, Tab.3.1), as follows:
(1) Broad level: a general characterisation of macro-units, i.e. presence/absence, areal
extent and/or percentage cover in relation to the two spatial settings (i.e. bankfull
channel, floodplain). The broad level characterisation is entirely based on remotely
sensed data sources, analysed within a GIS analysis. Therefore, it can only be applied to
rivers of sufficient size in relation to image resolution.
(2) Basic level: a complete delineation and first level of characterisation of all
geomorphic units, i.e. presence/absence, number, area/length. Some macro-unit types
can also be described at this level. It is mainly carried out by field survey, but remote
sensing and GIS analysis can also be used for large rivers or where very high spatial
resolution imagery is available.
(3) Detailed level: (i) provides more detailed information and data for units (and some
macro-units) on genetic processes, morphological, hydrological, vegetation and sediment
properties; (ii) describes macro-units and unit sub-types (when applicable); (iii)
characterises sub-units.
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In Table A3.1, ‘large rivers’ generally indicate channels of relatively large size, i.e. with a
channel width >30 m, whereas ‘small rivers’ indicate channels with a size ranging from
intermediate to small (channel width ≤30 m).
The survey methods for each level of characterisation, spatial scale and setting are
summarised in Table A3.2.
Figure A3.1 Levels of characterisation and spatial units associated with a bankfull
channel setting; examples of geomorphic units (types and sub-types) for different
spatial contexts are also reported.
Table A3.1 Levels of characterisation, methods, and types of collected information.
Broad
Basic
Detailed (optional)
Spatial unit
Macro-units
Macro-units
(some)
Macro-units
(some)
Units
Units
Sub-units
Method
Remote sensing
Field survey
Remote sensing
(when possible)
Field survey
Type of
collected
information
Presence/absence
(minimum level)
Presence/absence
(minimum level)
Presence/absence
(Sub-types / Sub-units)
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Area (optional)
(necessary for application
of GUS sub-indices)
Frequency (%)
(optional)
Number
(minimum information
for application of GUS
indices)
Linear or areal
extension (%)
(optional)
Number
Formative processes,
morphological
characteristics, hydraulic
conditions, vegetation
type, sediment
Specific measures
Applications
Required for large rivers
(all morphologies)
Required for single-
thread and small rivers
Always optional
Required for unconfined /
partly confined large rivers
(floodplain units)
Optional for multi-
thread and transitional
channels
(always required for
application of GUS
indices)
Table A3.2 Survey methods for each level of characterisation, spatial scale and setting:
RS = remote sensing; FS = field survey.
Bankfull channel
Floodplain
Submerged
Emergent
Broad
Macro-units
RS
RS
Basic
Macro-units (types)
RS**/FS
Units
RS*/FS
RS*/FS
RS*/FS
Detailed
Macro-units
RS**+FS
Units
FS
FS
FS
Sub-units
FS
FS
FS
(*large rivers and VHR images; **large rivers and HR/VHR images)
3.4.4 Survey and compilation of the GUS forms
In this section, some general information related to the survey and compilation of the
GUS forms is provided. A detailed Illustrated Guide to the classification and the Forms
for the survey and classification of geomorphic units are reported in Part B and Appendix
1, respectively.
As previously described, the GUS is applied through a combination of remote sensing -
GIS analysis and field survey. A collection of existing material (images, previous surveys,
other available data and information) precedes the remote sensing - GIS analysis
phase (e.g. delineation of macro-units, measurement of their sizes, etc.). Field survey
is essential to characterise the units. Field survey is applied to the entire investigated
reach or to a sub-reach that is selected to include the range of geomorphic units
observed along and so characteristic of the investigated reach.
For safety reasons, the timing of the field survey, should avoid periods of high flows,
and seasons of typically high to intermediate flows should be avoided as these conditions
make identification of submerged units difficult. Low-flow periods are the most suitable
for field survey not only because they are safest and also allow better visibility of
submerged units, but also because macro-units (i.e. Broad level) are most consistently
identified during such flow conditions. Partially or totally dry conditions should be
excluded because they would impede the classification of submerged units (except in the
case of intermittent or temporary streams, see below). However, depending on the
survey objectives, surveys under a range of different flow conditions may be informative.
For example multiple, stage-dependent surveys may help to quantify spatio-temporal
variations in habitat availability. In the case of intermittent or temporary streams, the
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field survey is carried out during periods that represent the dominant hydrological
regime conditions, and in any case in the same conditions observed during the remote
analysis (i.e. Broad level).
The GUS should be carried out by surveyors trained in fluvial geomorphology.
Similar to other fields of the river sciences (e.g., freshwater biology), application of
geomorphological methods without the necessary background and skills could seriously
affect the quality of the survey data that are obtained.
Time required for the application of the GUS
The time required for an application of the GUS depends on many factors, including: (1)
the expertise and experience of the operator; and (ii) the availability of necessary
materials (particularly aerial photographs at good resolution). The time required for
surveying a single reach (or sub-reach) also depends on the number of investigated
reaches within the same river segment or in the same area, since this affects the
number and diversity of data sources incorporated in the remote sensing – GIS analysis
and the time taken travelling between field survey sites. Approximately one day is
required to survey one or more sub-reaches within a single reach but this time may be
significantly reduced when surveying streams with a simple and relatively uniform
channel morphology, and may increase when surveying large rivers and/or those with a
complex channel morphology (e.g. braided or wandering reaches).
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A.4. Analysis of geomorphic units and GUS indices
The analysis carried out through the GUS can be used to address several aims and at
different Stages of the Deliverable 6.2 framework, including:
(i) as a characterisation tool to support the extended classification of the river type
(ERT) (Stage I);
(ii) to support the analysis of morphological quality of the reach by integrating a
morphological assessment method (e.g. the MQI) (Stage II);
(iii) as a monitoring tool, in order to detect small scale morphological changes (Stage
II);
(iv) to evaluate the effects of management actions on hydromorphology (Stage IV).
To support these applications, two synthetic indices have been developed to describe the
spatial heterogeneity of a given reach in terms of its geomorphic units.
4.1 GUS Indices
Two synthetic GUS indices (GUSI) are defined using information from the survey of
geomorphic units. They can be used (i) to better characterise the assemblage of
geomorphic units, and (ii) to monitor the trend of changes in geomorphic units in a given
reach (decrease or increase in richness and density) as a consequence of possible
pressures or interventions. The results of the GUS (including the indices) at the site-
scale must be combined with a morphological assessment at reach-scale to properly
interpret the significance and relevance of the diversity (richness and density) of
geomorphic units.
(1) Geomorphic Units Richness Index (GUSI-R)
The Geomorphic Units Richness Index (GUSI-R) evaluates how many types of
geomorphic units and macro-units (e.g. bar, island, riffle, secondary channel: see Part B,
Appendix 2) are observed within a given reach in comparison with the maximum number
of possible units:
GUSI-R = Σ NTGU / n
where NTGU is the total number of types of units and macro-units within the investigated
reach (or sub-reach) (e.g., in the case of presence of riffles, pools and side bars, NTGU
=3), whereas n is the total number of possible types of units and macro-units, i.e. 35.
For the calculation of this index, the presence/absence of each type of unit is required
(this is carried out at the Basic level).
(2) Geomorphic Units Density Index (GUSI-D)
The Geomorphic Units Diversity Index (GUSI-D) calculates the total number of
geomorphic units (independently of the type) within the investigated reach per unit
length:
GUSI-D = Σ NGU / L
where NGU is the total number of geomorphic units observed along the investigated reach
(or sub-reach) (e.g., in the case of 7 riffles, 6 pools and 3 bars, NGU = 16), whereas L is
the length (in km) of the investigated reach (or sub-reach).
The calculation of this index requires the number of units and macro-units of each type
to be measured (this is carried out at the Basic level).
(3) Sub-indices
It is also possible to calculate a series of sub-indices expressing the abundance and
density of geomorphic units for each spatial setting, i.e. bankfull channel and floodplain.
The following richness and density sub-indices are defined:
GUSI-RBC = Σ NTBCGU / n
GUSI-RFP = Σ NTFPGU / n
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GUSI-DBC = Σ NBCGU / n
GUSI-DFP = Σ NFPGU / n
where GUSI-RBC is the richness sub-index of bankfull channel geomorphic units, NTBCGU is
the total number of types of bankfull channel geomorphic units, GUSI-RFP is the richness
sub-index of floodplain geomorphic units, NTFPGU is the total number of types of
floodplain geomorphic units, GUSI-DBC is the density sub-index of bankfull channel
geomorphic units, NBCGU is the total number of bankfull channel geomorphic units
(independent of the type), GUSI-DFP is the density sub-index of floodplain geomorphic
units, NFPGU is the total number of floodplain geomorphic units (independent of the type).
Lastly, it is possible to calculate a series of sub-indices expressing the density of
geomorphic units for each macro-unit (see Part B for the definition of macro-units). The
calculation requires measurement of the area of each macro-unit (this is carried out at
Broad level). The sub-indices are defined as follows:
GUSI-DC = Σ NCGU / AC
GUSI-DE = Σ NEGU / AE
GUSI-DV = Σ NVGU / AV
GUSI-DF = Σ NFGU / AF
GUSI–DW = Σ NWGU / AW
where, for bankfull channel macro-units, GUSI-DC is the density sub-index of baseflow
channel geomorphic units, NCGU is the number of baseflow channel geomorphic units, AC
is the area (in km2) of the baseflow channel geomorphic units, GUSI-DE is the density
sub-index of emergent sediment geomorphic units, NEGU is the number of emergent
sediment geomorphic units, AE is the area (in km2) of the sediment emergent
geomorphic units, GUSI-DV is the density sub-index of in-channel vegetation geomorphic
units, NVGU is the number of in-channel vegetation geomorphic units, AV is the area (in
km2) of the in-channel vegetation geomorphic units; for floodplain macro-units, GUSI-DF
is the density sub-index of riparian zone geomorphic units, NFGU is the number of riparian
zone geomorphic units, AF is the area (in km2) of the riparian zone geomorphic units,
GUSI-DW is the density sub-index of floodplain aquatic zones geomorphic units, NWGU is
the number of floodplain aquatic zones geomorphic units, AW is the area (in km2) of
floodplain aquatic zones geomorphic units.
4.2 Interactions between GUS and MQI
Use of GUS in combination with MQI can provide an overall assessment of stream
reaches that is useful for understanding their functioning, and, therefore, for supporting
the identification of appropriate management actions.
It is important that the outputs of the GUS are interpreted in combination with the
results of the MQI and MQIm. For example, an increase in the abundance and diversity
of geomorphic units in a given reach is not necessarily related to an improvement of
morphological conditions but may be associated with the presence of artificial structures
(e.g., weirs). On the contrary, a low diversity of geomorphic units can be the result of
the ‘natural’ simple geomorphic structure of a particular stream type. Therefore, the
survey of geomorphic units at the site-scale must be combined with a MQI assessment
at reach-scale to better interpret the significance and relevance of the diversity of
geomorphic units. Some examples include:
(1) Reach-scale morphological assessment (MQI) results in very good status. This means
that geomorphic processes are unaltered or scarcely altered, and the geomorphic units
at site-scale represent the typical assemblage that could be expected for this river type
under current conditions.
(2) Reach-scale morphological assessment results in a very poor status. This implies that
geomorphic processes are intensely altered, and the geomorphic units at site-scale do
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not represent the typical assemblage that could be expected for such a river in
undisturbed conditions.
(3) A repeated application of GUS reveals an increase in abundance and/or diversity of
geomorphic units. If the MQIm tends to increase as well, the increase of geomorphic
units is likely due to enhanced morphological processes. On the contrary, an increase of
geomorphic units associated with a decrease in MQIm may be the result of additional
artificial elements within the reach.
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A.5. Testing phase
The main case study is the Cecina River (Tuscany, Central Italy), which has also served
as a Case Study for the REFORM hierarchical framework (REFORM Deliverable 2.1 Part 3,
Case study 4). The main test sites (Fig. 5.1) are located along an unconfined reach
flowing within a relatively narrow plain in a hilly physiographic unit (reach 3.7 in REFORM
D2.1 Part 3). The reach length is 6500 m, and it has a watershed area of about 635
km2. The channel type is ‘pseudo-meandering’ (ERT type 12), with a gravel bed, a mean
slope of about 0.003, and mean width of about 50 m. The main artificial elements within
the reach are some sills and a bridge. The MQI of the reach is 0.78 (i.e. ‘good’
morphological quality).
Figure A5.1 Cecina River, reach 3.7 (near Casino di Terra, Pisa), and location of the two
sub-reaches used for testing the GUS.
The GUS was applied at the Broad level to the entire reach, and at the Basic level to two
sub-reaches (sub-reach 1: 1500 m; sub-reach 2: 1100 m), which were selected as
representative of the full range of geomorphic units observed along the entire reach. For
the remote sensing analysis, high-resolution (15 cm) aerial photographs were used. The
remote sensing – GIS analysis was integrated with a detailed field survey. Figure 5.2
shows an example of GIS mapping of the geomorphic units.
Lastly, the GUS indices and sub-indices for the two sub-reaches were calculated and are
summarised in Table A5.1.
Table A5.1 Results of the application of the GUS indices and sub-indices.
Sub-reach 1 (upstream)
Sub-reach 2 (downstream)
GUSI-R
0.43
0.37
GUSI-RBC
0.34
0.31
GUSI-RFP
0.09
0.06
GUSI-D
62
73.64
GUSI-DBC
57.33
67.27
GUSI-DFP
4.67
6.36
GUSI-DC
29.88
16
GUSI-DE
5.75
9.17
GUSI-DV
12.56
19.21
GUSI-DF
0.46
0.77
GUSI-DW
/
/
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Figure A5.2 Example of the application of the GUS to the Cecina River (Base level): map
of the types of geomorphic units within sub-reach 2 (downstream).
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A.6. Applications of GUS
The data and information collected through the GUS can be used for a series of potential
applications, including the following:
(i) Spatial and temporal analyses of geomorphic units at different spatial scales
- Survey and characterisation of physical habitats at the meso (Units, Sub-units) and
micro (substrates, flow types, etc.) scale, as well as analysis of the fluvial
landscape at the Macro-unit scale; these can be carried out by calculating diversity
indices (e.g. Shannon, richness, dominance, etc.) and landscape description
metrics (e.g. patch form, connectivity, ecotones length, etc.).
- More detailed characterisation of the morphology at the reach scale and its
evolution through time.
- Monitoring of the geomorphic units across time, in order to assess the effect of
interventions (e.g. restoration) or of different hydrological conditions.
(ii) Analyses of the relationships between geomorphic units (i.e. physical habitats) and
biota
- As a physical basis for biological surveys at a scale that is geomorphologically
meaningful.
- As a key tool to link the morphological status at the reach scale with the biological
status at the site scale.
- As a tool for the survey and mapping of mesohabitats in order to: (i) apply habitat
simulation models for the fauna (e.g. MesoHABSIM, Parasiewicz et al., 2013); (ii)
calculate the spatio-temporal variation of habitats for the fauna (e.g. Vezza et al.,
2014, 2015).
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B. Guide to the application of the
GUS
B.1 Guide to the compilation of the survey forms
In this section a detailed description of the compilation of the survey forms is provided.
The survey form is composed of several sheets:
- The method requires the compilation of a first sheet (Survey plan), which aims to
organize the survey in the context of its objectives, by recording (i.e. using a
tick) the kind of information that is planned to be collected (which spatial setting,
which level, and which spatial scale).
- The second sheet records general information on the river, the surveyed reach
and sub-reach; a part of the sheet is reserved for a field sketch.
- The following sheets are the core of the survey (Broad, Basic and Detailed levels).
1.1 Sheet 1: Survey plan
The first part of the sheet requires the following basic information (tick when the case;
Fig. B1.1):
- The type of river (small, ≤ 30 m; large, > 30 m) and of valley setting (confined
vs. partly confined/unconfined);
- The kind of remotely-sensed data available: satellite images, high resolution (HR,
i.e. 20-50 cm) or very high resolution (VHR, < 15 cm) photographs, Lidar;
- Information on the kind of survey and data that will be recorded (i.e. which
sheets will be completed).
Figure B1.1 Detail of the first part of Sheet 1 (Survey plan).
The second part summarises the macro-unit and unit information recorded on
subsequent survey sheets (sheets 3 to 15) for all the levels of characterisation
(Broad, Basic and Detailed) and all spatial settings (Bankfull channel, Floodplain; Fig.
B1.2). The operator must tick the box when he/she plans to survey a specific feature:
- Broad level: presence/absence (P/A), area (Are) or percentage (%) (for macro-
units);
- Basic level: presence/absence (P/A), number (Num), length or area (L/A) (for
types of macro-units and units);
- Detailed level: characteristic features for macro-units and units, i.e. sub-types of
units and macro-units (S-T), sediment characteristics (Sed), hydraulic conditions
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(Hyd), vegetation characteristics (Veg), bank morphology (Mor) and composition
(Com), type of processes (Pro), specific measures (Mea), other (Oth).
At the Basic level, the survey method (remote sensing or field survey) is indicated. The
Broad and Detailed levels are applied using remote sensing and field survey,
respectively. Finally, for the Basic level it is possible to indicate whether the GUS indices
have been calculated.
Figure B1.2 Detail of the second part of Sheet 1 (Survey plan). C, S, E, V, F/H, W, A
represent the identifcation codes of the macro-units; see Section 2 for more details.
The third part summarises the sub-unit information recorded on the sub-unit
survey forms, carried out at the Detailed level, for all spatial settings.
At the end of the survey, the operator should return to this page and check if all required
information has been correctly acquired (last two parts). When a specific feature could
not be analyzed, it must be marked N.S. (i.e. not surveyed) in the appropriate box.
1.2 Sheet 2: General information and field sketch
The first part of this page concerns an initial desk-study phase, during which the
operator should collect all the available information on the studied river (Fig. B1.3). The
sub-reach is selected during this phase, as representative of the variety of geomorphic
units observed at the reach scale (see Part A, section 2.2). Some of the information
required can be obtained from other morphological surveys (i.e. the overall bed
configuration and channel pattern), such as the delineation and characterisation phases
of the REFORM D2.1 framework (Gurnell et al., 2014a, 2015a, 2015b; see also D6.2
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Main report) or the segmentation phase of the MQI (Rinaldi et al., 2013b, 2014; see also
D6.2 part 3).
The information concerns:
(i) General information on the river and the survey:
- Name of the surveyor;
- Date of the survey;
- Name of the river;
- A place name for location;
- Reach mean altitude (m a.s.l.);
- Reach length (m);
- Sub-reach length (m);
- Sub-reach X and Y coordinates (upstream point).
(ii) General morphological characteristics at the reach scale:
- Mean floodplain width (m);
- Mean width of the river corridor (m), that includes the bankfull channel and the
entire ‘functioning’ riparian zone (in some cases the river corridor may
encompass the entire floodplain);
- Mean bankfull width (m);
- Degree of confinement (i.e. the % of bank in contact with hillslopes; Rinaldi et
al., 2013b);
- Mean reach slope (%);
- Dominant surrounding land use (3 classes: NA, natural; AG, agricultural areas;
UR: urban and industrial areas including transport infrastructure);
- Mean baseflow channel width (m); it can be measured from remote sensing if
high-resolution images are available;
- Mean baseflow channel depth (m or classes: < 20 cm, between 20 cm and 1 m,
more than 1 m); it is defined in the field (or from previous surveys);
- River is wadeable/not wadeable (W/NW);
- Bed morphology: colluvial, bedrock, alluvial, semi-alluvial, artificial;
- Channel pattern: straight, sinuous (including pseudo-meandering), meandering,
wandering, braided, anabranching;
- Hydrological regime: permanent, intermittent, temporary stream or other;
- Other available information: it is possible to indicate the discharge during the
survey (field or remote images), as well as further information (i.e. date, flood
magnitude or return period when available) of recent flood events (e.g. greater
than the 1.5 years return period discharge) which may have modified the
presence and extent of geomorphic units. It is also possible to indicate the
number of events (if known), which have occurred between two consecutive
observations or during the last hydrologic year, having a discharge greater than
the bankfull or 1.5 years return period discharge.
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Figure B1.3 Detail of the first part of Sheet 2 (General information).
The second part of the sheet is reserved for a field sketch. This should represent, in a
schematic way, all the geomorphic units present in the surveyed reach or sub-reach. See
the section titled “Sheets 4 to 6: Basic level” for a detailed description of the survey of
geomorphic units (and some macro-units).
1.3 Sheet 3: Broad level
The first part of the sheet concerns the following information on the remotely sensed
data used for the survey (Fig. B1.4):
- Date of the photo;
- Source/ownership of the photo;
- Scale and/or the resolution (m) of the photo;
- Scale at which macro-units (or units) are mapped;
- Spatial scale of application (reach or sub-reach).
A photo of the reach or sub-reach can be included in the sheet (this is also useful for the
field survey).
Figure B1.4 Detail of the first part of Sheet 3 (Broad level).
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The second part of the sheet contains the macro-unit survey form (Fig. B1.5).
According to their spatial setting (bankfull channel, floodplain), macro-units are
organized as follows (see section 2 for the definition of single macro-units):
(i) Bankfull channel macro-units: baseflow or ‘submerged’ channels (C/S),
‘emergent’ sediment units (E), in-channel vegetation units (V);
(ii) Floodplain macro-units: riparian zone (F), floodplain aquatic zones (W); human-
dominated areas (H; land use included);
(iii) All spatial settings: artificial features (A).
The following information can be recorded:
- Presence/absence of a macro-unit (minimum level) (P/A);
- Area of each macro-unit (m2);
- Percentage of each macro-unit relative to the spatial setting (Bankfull channel,
Floodplain) (%).
Figure B1.5 Detail of the second part of Sheet 3 (Broad level, macro-units survey c).
The survey of macro-units is always required for large rivers (all spatial settings), and
for small unconfined or partly-confined rivers (but only for the Floodplain). The scale of
analysis (reach or sub-reach) must be defined, based on the objective of the analysis, as
follows:
- if it is planned to survey only the macro-units at the Broad level, the entire reach
length must be surveyed;
- if it is planned to survey also units (Basic level), the choice of the survey scale for
macro-units is optional (reach or sub-reach).
In the case of small confined rivers, the survey of macro-units is always optional (it
depends on available data), as well as the choice of the scale of survey (reach or sub-
reach). Finally, the survey of features such as human-dominated areas (H, land use
included) and artificial features (A) is always optional.
It should be noted that the survey and calculation of the areal extent of macro-units is
needed for the calculation of some GUS sub-indices.
1.4 Sheets 4 to 6: Basic level
The survey of units (and some macro-units) at the Basic level is carried out in the field
(Fig. B1.6). On the basis of available remotely-sensed data (high or very high resolution
images) the Basic level can be carried out from remote sensing, but a field check is
always recommended.
The first column of the sheet corresponds to the identification code of the corresponding
macro-unit. The recorded information is:
- Presence/absence of macro-units (baseflow or ‘submerged’ channels) and unit
types (minimum level) (P/A);
- Their number (N or code): indicate the unit identification code and progressive
number, in order to match with the field sketch of sheet 2 (e.g.: 3 rapids = CR1,
CR2, CR3 both in the sketch and in the sheet);
- Length (by field survey) or area (by remote sensing) for each identified unit
(and/or macro-unit) (L/A);
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- Number or reference of the photograph taken during the field survey, for each
unit (optional) (Picture number).
Figure B1.6 Detail of the Sheet 4 (Basic level).
It is possible to indicate information for a limited number of units. If more units are
present, use an additional sheet (the same for the Detailed level).
At the end of sheet 5, the total number of unit types and units can be indicated in order
to calculate the GUS indices (GUSI-R and GUSI-D) and sub-indices (see part A, section
4.1).
Below, there are some recommendation on how to conduct the field survey at the Basic
level:
(i) Proceed from upstream to downstream and for spatial settings (i.e. bankfull channel,
floodplain; mainly in case of large rivers).
(ii) According to the survey objectives:
- 1 surveyor, if only presence/absence and number of units is required;
- At least 2 surveyors, if the unit size must be measured (length and/or area).
The survey of units at the Basic level is required for single-thread and small rivers, but is
optional for multi-thread and transitional rivers. However it is necessary if the GUS
indices and sub-indices (presence/absence and unit number) are to be calculated (see
Part A, section 4.1).
1.5 Sheets 7 to 15: Detailed level (macro-units and units)
The Detailed level is an optional in-depth characterisation of units and some macro-
units (for sub-units, see below) conducted mainly using field survey, although some
characteristics of units and macro-units can be obtained from remote sensing if
sufficiently high resolution data are available.
At this level, the method allows collection of data on morphological, hydrological, and
sedimentary features, as well as on genetic mechanisms (Fig. B1.7).
The detailed characterisation of units is accomplished by recording:
- Presence/absence of a specific unit ‘sub-type’, which reflects:
i. the genetic mechanisms, such as a ‘forced pool’ for bankfull channel
submerged units;
ii. the development stage, such as a 'mature island' for islands.
- Dominant substrate (sediment size) and any substrate alteration: bedrock (Bed),
boulder (Bou), cobble (Cob), gravel (Gra), sand (San), silt (Sil), clay (Cla),
clogging (Clo), armouring (Arm), artificial (Art).
- Dominant hydraulic conditions, which convey information on hydrological
connectivity between units and the water channel:
i. Three classes of flow velocity are assigned to bankfull channel ‘submerged’
units (i.e., bed configuration): high (Hig), intermediate (Int) and low
(Low);
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ii. Three classes of frequency of submersion in relation to the Bankfull
channel, for ‘emergent’ units (i.e., bars, islands and LW jams) (excluding
bank-related features, see below): below bankfull (<BC), bankfull (BC)
and above bankfull level (>BC);
iii. Two classes of flow depth, for in-channel aquatic vegetation: < 1m, > 1m;
iv. Two classes of topographic height with respect to the Bankfull level, for
Floodplain units: modern floodplain (Flo), recent terrace (Ter);
v. For banks, the information on hydraulic conditions is provided in terms of
connection with other surfaces:
Modern floodplain bank (Flo), i.e. bank connecting a Bankfull
channel unit with the modern floodplain;
Recent terrace bank (Ter), i.e. bank connecting a Bankfull channel
unit with a recent terrace.
- Presence of vegetation features:
i. Spatial structure: absent (Abs), sparse (Spa), patches (Pat), dense (Den);
ii. Height structure: trees (Tre), shrubs (Shr), herbs (Her); for submerged
units: presence of algae or floating vegetation (Alg), rooted vegetation
that has floating leaves (Flo), submerged leaves (Sub), emergent leaves
(Eme);
iii. Dominant species or vegetation type (Species);
iv. Presence of large wood (not jam) (Woo);
v. Presence of roots (only for banks and benches) (Roo).
- In terms of processes:
i. The formative process for islands: floodplain dissection island (Dis) and
mid-channel island (Mid) (sensu Gurnell et al. 2001);
ii. The acting (or dominant) process for benches: channel incision (Inc),
channel narrowing (Nar).
iii. Bank stability/instability ‘status’: retreating (Ret), stable (Sta), advancing
(Adv) bank.
- Some relevant measures (Mea):
i. Mean width (Wid) (m), for almost all units;
ii. Mean size (Siz) (m) and number (Num) (three classes: < 5, > 5, > 10) of
logs, for large wood jams;
iii. Mean height (Hei) (m), for aquatic vegetation and benches;
iv. Bank length (Len) (m), height (Hei) (m) and slope (Slo) (%).
- Number or reference of the picture taken during the field survey (Pic).
- Other additional available information, as for example the determination of the
D50, the dominant flow type during the survey, the presence of moss or
peryphyton on the bed substrate, the water temperature (Other).
Figure B1.7 Detail of the Sheet 11 (Detailed level).
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For banks, further information is provided in terms of (see section 2 for detailed
description):
- Bank morphology (geometry): near vertical (Ver), vertical undercut (Und), planar
(Pla), with toe (Toe), convex upwards (Con), concave upwards (Coc), complex
(Com).
- Bank composition (material): non-cohesive (NoC), cohesive (Coe), composite
(Com), multi-layered (Lay).
It is worth noticing that, for the banks, the characterisation is averaged for all the bank
profile and length. If the operator is interested in the detailed characterisation of banks,
it is suggested to divide the bank length into smaller portions and to characterise them
separately.
In some cases, also selected macro-units (i.e. baseflow channels) can be optionally
further characterised at the Detailed level, by means of remote sensing and/or coupled
with field survey, as follows:
- The definition of ‘sub-types’ of secondary channels on the basis of their size and
their connection to the main channel (from remote sensing and validation on the
field).
- The characterisation of macro-units types and sub-types, in terms of:
i. Sediment characteristics (same as units);
ii. Three classes of hydraulic conditions, in terms of frequency of the
connection to the main channel/network: below bankfull (<BC), bankfull
(BC) and above bankfull (>BC) level;
iii. Vegetation characteristics (same as units);
iv. Relevant measures, i.e. width (Wid);
v. Number or reference of the picture taken during the field survey (Pic);
vi. Other additional information as, for example, water temperature or
conductivity, to determine the connection with the groundwater (Other).
The characterisation of units and macro-units at the Detailed level is always optional for
all types of rivers.
1.6 Sheet 16: Detailed level (sub-units)
The Detailed level also allows, if needed, the definition (mapping) and
characterisation of sub-units (Fig. B1.8). Once identified (see section 2.5 for a list of
some example of sub-units for each spatial setting) the operator can characterise sub-
units in a similar manner to units:
- Indicate the macro-unit and unit (code and progressive number) at which the
sub-unit belongs.
- Name of the sub-unit.
- Number of the sub-unit (Num): this can be “total”, i.e. the sum of all the sub-
units of a same type, or “progressive”, if the aim is to map and characterise each
sub-unit of a same type.
- Sediment characteristics (same as units).
- Presence of vegetation features:
i. Spatial structure: absent (Abs), dense (Den), sparse (Spa);
ii. Presence of other (secondary) types of vegetation structures: trees (Tre),
shrubs (Shr), herbs (Her);
iii. Presence of other (secondary) types of aquatic vegetation: algae (Alg),
floating (Flo), rooted with floating leaves (Rfl), submerged leaves (Sub),
emergent leaves (Eme);
iv. Dominant species or vegetation type (Species);
v. Presence of woody debris (not jam) (Woo);
vi. Presence of roots (only for banks) (Roo).
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- Some relevant measures in terms of length (Len), width (Wid) and height (Hei)
(m), or surface area (Are) (m2).
- Number or reference of the picture taken during the field survey (Pic);
- Other additional information which can be relevant for the characterisation of the
sub-unit.
The characterisation of sub-units at the Detailed level is always optional for all types of
rivers.
Figure B1.8 Detail of the Sheet 16 (Detailed level for sub-units).
Table B1.1 summarises the characteristics surveyed at the Detailed level for each spatial
unit and spatial setting.
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Table B1.1. Summary of the characteristics surveyed at the Detailed level for each
spatial unit and spatial setting.
Bankfull channel
Floodplain
Submerged
Emergent
(Land use excluded)
Macro-
units
Sub-type
Sediment characteristics
Hydraulic conditions
Vegetation characteristics
Measures
Picture number
Other (e.g. temp.)
Units
Sub-type
Sub-type
Sub-type
Sediment characteristics
Sediment characteristics
Sediment characteristics
Hydraulic conditions
Hydraulic conditions
Hydraulic conditions
Vegetation characteristics
Vegetation characteristics
Vegetation characteristics
Processes
Bank morphology
Bank composition
Measures
Measures
Measures
Picture number
Picture number
Picture number
Other (e.g. D50)
Other (e.g. D50)
Other
Sub-
Macro-unit (code)
Macro-unit (code)
Macro-unit (code)
units
Unit (code)
Unit (code)
Unit (code)
Type/Name
Type/Name
Type/Name
Number
Number
Number
Sediment characteristics
Sediment characteristics
Sediment characteristics
Vegetation characteristics
Vegetation characteristics
Vegetation characteristics
Measures
Measures
Measures
Picture number
Picture number
Picture number
Other
Other
Other
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B.2. Illustrated guidebook for the identification
and delineation of spatial units
In this section geomorphic units (and related macro-units and sub-types, as well as
examples of sub-units) that can be identified in fluvial systems are listed and described,
organised following the spatial setting to which they belong.
Appendix 2 reports the list of all geomorphic units (and relative macro-units and sub-
types).
2.1 Bankfull channel units
The bankfull channel corresponds to the area occupied by baseflow channels, bars,
islands, and other possible vegetation units. Bankfull channel units include all
geomorphic units located within the bankfull channel. They comprise three macro-units:
(1) baseflow or ‘submerged’ channel units; (2) 'emergent' sediment units, i.e. ‘emergent’
depositional and erosive sediment features; (3) in-channel vegetation units.
2.1.1 Macro-unit: baseflow or 'submerged' channels
This macro-unit includes all the geomorphic units which are part of the baseflow
channels (i.e. submerged at baseflow).
All the portions within the bankfull channel can be flooded with different frequency of
inundation. For practical reasons, at the Broad level of classification, baseflow channels
are considered the portions of the bankfull channel that are submerged at the time of
observation (provided that the survey is never carried out during high flow conditions).
This also includes submerged bars that are considered as part of the submerged channel
and are not further classified and characterised within the GUS. Intermittent or
temporary channels which do not have flow during the survey are included within the
emergent sediment units (see the definition of dry channel).
It should be noted that although baseflow is usually maintained in perennial rivers and
streams during extreme low-flow conditions by groundwater seepage, in some hydrologic
(prolonged dry periods without precipitation) or hydrogeologic conditions, even
‘perennial’ rivers may dry up for short periods of time.
At the Basic level, two types of baseflow channels can be distinguished: (1) baseflow
channel or main channel; (2) secondary channel/s. At the Detailed level, further sub-
types of secondary channels can be identified (see below).
Identification code of the macro-unit: C or S (see macro-unit types and sub-types
below).
Macro-unit types
Baseflow channel or main channel
Identification code: C
Definition
In single-thread perennial streams, this corresponds to the single channel containing
flowing water, and is termed the baseflow channel. Where depositional bars are absent
or scarce the baseflow channel can occupy most of the bankfull channel.
In multi-thread patterns, this is identified as the main channel, i.e. the one carrying the
larger proportion of water. If a main channel is not clearly recognizable, more than one
baseflow channel can be recorded.
Equivalent terms: low-flow channel, low-water flow, main thread or main branch or
main anabranch (in multi-thread systems)
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Distinctive characteristics: In transitional and multi-thread systems, the main channel
(C) can be easily distinguished from remotely sensed imagery (high resolution aerial
and satellite photos) as it is wider and deeper (i.e. dark blue color) than the other
branches.
(a)
(b)
(c)
Macro-unit type baseflow channel or main channel in (a) single-thread river, (b) multi-
thread braided river and (c) multi-thread anabranching (anastomosing) rivers (which
may possess several baseflow channels).
Secondary channel (within the bankfull channel)
Identification code: S
References: Arscott et al. (2000, 2002); Tockner & Malard (2003); Van der Nat et al.
(2003); Ashmore (2013); Belletti et al. (2013)
Definition
Baseflow channels are classified as ‘secondary’ when their size (and corresponding flow)
is significantly smaller than the main channel. They may exist in single-thread,
transitional or multi-thread systems.
Equivalent terms: secondary thread, branch or anabranch, side channel
Distinctive characteristics: secondary channels are smaller, narrower and shallower than
the main channel.
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(a)
(b)
Macro-unit type secondary channel in (a) single-thread river, (b) multi-thread braided
river.
Macro-unit sub-types
References: Arscott et al. (2000, 2002); Tockner & Malard (2003); Van der Nat et al.
(2003); Bertoldi et al. (2009); Ashmore (2012); Belletti et al. (2013); Welber et al.
(2012)
(a)
(b)
Sub-types of macro-unit secondary-channel (S) in (a) single-thread and (b) transitional
and multi-thread (braided) rivers (modified from Belletti et al. (2013) and Belletti
(2012).
Chute cut-off channel
Definition
In single-thread or transitional systems, a chute cut-off channel is a secondary channel
located on the inner portion of a side or point bar and generated by a chute cute-off,
that is a shortcut across an emergent bar, typically a bank-attached bar.
Equivalent terms: side channel, side arm
Two-way connected branch
Definition
In multi-thread or transitional systems, a two-way connected branch is a secondary
channel connected upstream and downstream to the main channel. Two-way connected
branches are narrower and shallower than the main channel. Some authors also identify
primary, secondary and tertiary order of two-way connected branches, depending on
the degree of connection to the main channel or to other branches.
Equivalent terms: upstream-to-downstream channel, surface-connected channel, side
braid/channel
One-way connected branch
Definition
In multi-thread or transitional systems, a one-way connected branch is a secondary
channel connected usually only at the downstream end to the main channel although
sometimes the connection may be at the upstream end (mixed channels: Tockner &
Malard, 2003; Belletti et al., 2013). The downstream connected channels are fed by
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groundwater seepage from the alluvial aquifer, whereas those with an upstream
connection disappear as their flow seeps into the alluvial aquifer. Upstream connected
channels are most frequent in alpine and glacial systems with a highly variable flow
regime (i.e. snow and glacier melt) and coarse bed material. Some authors also identify
primary, secondary and tertiary order of one-way connected branches (e.g. Arscott et
al., 2000).
Equivalent terms: upstream-connected channel, groundwater-fed channel, backwater,
mixed channel
Pond
Definition
A pond is a geomorphic unit, more common in multi-thread systems, consisting of a
portion of secondary channel that becomes completely disconnected from the channel
network at baseflow.
Equivalent terms: isolated pool or isolated standing water
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Bed configuration units
Within each baseflow channel macro-unit, a series of geomorphic units can be identified
that are associated with the configuration of the river bed.
In confined single-thread streams, the spatial scale (longitudinal extent) of these units is
similar to the bankfull channel width, whereas in relatively large alluvial rivers featuring
wandering or braided patterns, their spatial scale is similar to the baseflow channel
width.
Most units are found in alluvial or semi-alluvial channels (Fig. B2.1), but some are
exclusively found in bedrock channels. Geomorphic units of alluvial and semi-alluvial
channels can be erosional (e.g. pool), depositional (e.g. step) or mixed (e.g. cascade),
whereas bedrock channels are characterised by specific erosional units (e.g. pothole).
In ecohydraulics, bed configuration geomorphic units are commonly named
'hydromorphological units' (HMU; e.g. Vezza et al., 2014).
Figure B2.1 Main geomorphic units of mountain alluvial bed streams (modified from
Halwas & Church, 2002).
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Pothole
Identification code: CH
References: Brierley & Fryirs (2005); Fryirs & Brierley (2013)
Definition
Erosional geomorphic unit, typical of bedrock channels. It is a deep, circular scour
feature that occurs in areas where flow energy is highly concentrated. These features
are sculpted from bedrock due to the abrasion induced by transported particles trapped
in the hole. They are commonly associated to weak lithological layers or to the presence
of structural discontinuities.
Distinctive characteristics: in contrast to pools (plunge pool), potholes are not located
downstream of a step unit.
(a)
(b)
Bed configuration unit type: pothole (a, b). (a) Modified from Brierley & Fryirs (2005).
Cascade
Identification code: CC
References: Halwas & Church (2002); Montgomery & Buffington (1997); Buffington &
Montgomery (2013)
Definition
Alluvial or semi-alluvial geomorphic unit mainly formed by boulders and/or large
cobbles. Sediments are not organized either in lateral ribs or longitudinal stone lines,
and are transported only by infrequent large floods. Small pools between boulders are
shallow, characterised by very turbulent flow, and are usually smaller than the channel
width (named pocket pools; see the sub-units). Tumbling flow dominates at all flow
stages, and so energy dissipation is controlled by spill resistance with the additional
contribution of wake turbulence around large clasts. These units are typical of very
steep (S>7%), confined reaches, that are well connected to a supply of coarse sediment
(hillslopes, debris-flow channels, etc.).
Distinctive characteristics: in comparison to sequences of step and pool units, which
also feature tumbling flow, the organization of the large clasts in cascade units is more
chaotic and channel-spanning pools are lacking. In comparison with rapids, cascades
maintain a dominant tumbling flow characteristic at flood stage, and clasts show less
organization.
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(a)
(b)
Bed configuration unit type: cascade (a, b). (a) Modified from Halwas & Church (2002).
Rapid
Identification code: CR
References: Grant et al. (1990); Halwas & Church (2002)
Definition
Rapids are an alluvial or semi-alluvial geomorphic unit, mainly formed by boulders and
large cobbles. Boulders are very stable and partially organized into irregular ribs or
stone lines oriented more or less perpendicular to the channel and partly or totally
spanning the channel width. These transverse ribs (see sub-units), if present, are visible
only at low flow, being fully submerged during bankfull flows. Pools are shallow and
poorly developed, and do not form distinct, separate geomorphic units.
Distinctive characteristics: compared to cascade and step units, the larger clasts within
rapids become submerged at bankfull flows, such that tumbling flow only occurs at low
to medium flows. In contrast to riffles, rapids are characterised by coarser grains, some
of which are organized in lines or transverse ribs which protrude from the flow at low to
medium stages, and flow is more turbulent with higher air concentrations (white-water),
producing broken standing waves during low flows.
(a)
(b)
Bed configuration unit type: rapid (a, b). (a) Modified from Halwas & Church (2002).
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Riffle
Identification code: CF
References: Grant et al. (1990); Church (1992); Wood-Smith & Buffington (1996);
Knighton (1998)
Definition
Riffles are characterised by relatively shallow and fast flow (near to super-critical)
compared to adjacent units, and by relatively uniform sediment (gravel to small
cobbles) which rarely protrude out of the flow. Differences in water depth and velocity
between riffles and nearby units (typically pools and glides) decrease as stage
increases. Riffles tend to occur at the inflection point between bends in sinuous alluvial
channels, where the channel is dominated by a sequence of alternating bars at the
bends.
Distinctive characteristics: compared to rapids, riffles are characterised by less turbulent
flows, frequently showing unbroken standing waves at intermediate to high stage,
before they get completely drowned out. Compared to glides, riffles are characterised
by a locally higher bed slope inducing accelerating flow velocity, and presenting an
undulating but unbroken flow surface (waves may become broken on steeper riffles
composed of relatively coarser sediment).
(a)
(b)
Bed configuration unit type: riffle (a, b). (a) Modified from Halwas & Church (2002) and
Brierley & Fryirs (2005).
Sub-types
References: Brierley & Fryirs (2005)
Forced riffle
Definition
Longitudinally undulating alluvial sediment accumulation that acts as a locally high area
on the river’s longitudinal profile. These features are formed by bedrock outcrops,
accumulation of coarse sediments or large wood elements. They tend to occur at wider
sections and in bedrock-confined systems (Brierley & Fryirs, 2005).
Equivalent terms: constriction riffle
(a)
(b)
Bed configuration unit sub-type: forced riffle (a, b). (a) Modified from Brierley & Fryirs
(2005).
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Step
Identification code: CT
References: Chin (2003); Halwas & Church (2002); Church (1992); Comiti & Mao
(2012)
Definition
A relatively short unit typical of alluvial, semi-alluvial and bedrock steep channels. Steps
are characterised by near-vertical drops in the channel bed which span the entire width,
and are higher than the bankfull flow depth just upstream of the crest of the step, such
that the relative jet is not submerged during bankfull flows (Comiti & Mao, 2012). Steps
feature accelerating and convergent flow conditions as a consequence of the
downstream overfall of water, thus turbulence fluctuations are limited (Wilcox et al.,
2011) and the water surface is fairly smooth. Besides steps composed of sediment
(alluvial or semi-alluvial), these features can be totally or partially created by wood (log
steps) or they can be scoured into the bedrock (rock steps) (see the sub-types).
Distinctive characteristics: step units span the entire channel cross section with the
same natural drop structure, and so they differ from cascade units which present drops
that only extend across a part of the channel cross section (partial steps). In contrast to
transverse ribs and stone lines that are found in rapids or glides, the drops created by
steps are not submerged by bankfull flows, and so tumbling flow occurs up to the
annual flood stage and flow is dominated by spill resistance (Comiti & Mao, 2012).
(a)
(b)
Bed configuration unit type: step (a, b). (a) Modified from Brierley & Fryirs (2005) and
from Halwas & Church (2002).
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Sub-types
References: Knighton (1998); Brierley & Fryirs (2005); Zimmermann et al. (2010);
Waters & Curran (2012); Comiti & Mao (2012); Wohl (2010)
Rock step
Definition
This is an erosional feature formed by turbulent flow plunging over a locally resistant
area of bedrock, forming a channel-wide drop. Transverse rock steps >1m high may
separate a backwater pool upstream from a plunge pool downstream (Brierley & Fryirs,
2005).
Equivalent terms: bedrock step, rockstep
(a)
(b)
Bed configuration unit sub-type: rock step (a, b). (a) Modified from Brierley & Fryirs
(2005).
Waterfall
Definition
A waterfall is a near-vertical step of significant height formed by high turbulent flow
plunging over a locally resistant area of bedrock that forms a channel-wide drop.
Waterfalls are higher than rock steps, typically exceeding 3m in height, and are
observed as single units rather than being part of a regularly spaced sequence of steps.
Equivalent terms: knickpoint
(a)
(b)
Bed configuration unit sub-type: waterfall (a, b). (a) Modified from Brierley & Fryirs
(2005); (b) from www.gocime.com.
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Boulder step
Definition
A boulder step is a step unit composed of large clasts (mainly boulders, but also
cobbles). Typically these steps are not entirely alluvial but contain large boulders
supplied from local hillslopes which, because of their size, are very stable. The stability
of boulder steps depends on: the size of clasts composing the step, the channel width
(greater the width lower the stability), the dowstream distance from other steps
(greater the distance lower the stability), and the magnitude of peak flows. Boulder
steps can occasionally be disrupted by over-bankfull floods, and when they collapse the
instability may migrate headward or downstream to other steps.
(a)
(b)
Bed configuration unit sub-type: boulder step (a, b). (a) Modified from Brierley & Fryirs
(2005).
Log step
Definition
A step unit totally or partially imposed by a large wood element (log) fallen from the
bank and spanning completely or partially the channel. Log steps units are very
common in temperate old-growth forested basins. Log steps can be oriented normal or
oblique to flow.
(a)
(b)
Bed configuration unit sub-type: log step (a, b). (a) Modified from Halwas & Church
(2002) and Abbe & Montgomery (2003).
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Glide
Identification code: CG
References: Bisson et al. (1982); Church (1992); Grant (1990); Sullivan (1986);
Halwas & Church (2002)
Definition
Glides feature a regular longitudinal bed profile, with a smooth or rippled water surface
that is approximately parallel to the bed, with low turbulence.
In relatively steep gravel-bed rivers, glides are often armoured, and in the steepest
cases may incorporate some coarse grains (cobbles and boulders) but these rarely
protrude from the flow. Glides are also common in low gradient gravel-bed and sand-
bed rivers, where they are typically located downstream of pools and upstream of riffles.
Distinctive characteristics: compared to riffles or rapids, glides are characterised by a
lower local slope and rippled or smooth water surface. Standing waves are not present,
except where isolated boulders emerge through the water surface. Compared to pools,
glides are characterised at low stages by a more disturbed water surface and a channel
bed that is approximately parallel to the water surface.
Equivalent terms: run (generally used to indicate a glide of limited length located
between a pool and a step or a riffle unit and/or in low slope reaches)
(a)
(b)
Bed configuration unit type: glide (a, b). (a) Modified from Brierley & Fryirs (2005).
Sub-types
References: Wohl (1998)
Rock glide
Definition
This unit is found in bedrock channels and features the hydrodynamic characteristics
described above for glides (smooth or rippled flow and a water surface that is near
parallel to the channel bed).
Equivalent terms: bedrock glide
(a)
(b)
Bed configuration unit sub-type: rock glide (a, b). (a) Modified from Wohl (1998).
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Pool
Identification code: CP
References: Church (1992); Grant et al. (1990); Wood-Smith & Buffington (1996);
Halwas and Church (2002)
Definition
A pool unit is a channel-spanning topographic depression in the channel bed, with a
reversed bed slope at the downstream end. Pools are characterised by deep, relatively
slow velocity flows but with complex hydrodynamic patterns. Bed sediments often
appear to be finer than the adjacent units if deposition has occurred, but the substrate
can also be coarse. Pools reflect the interactions between flowing water and sediment
transport, often alternating with steps or riffles, along boulder- and gravel-bed rivers.
Pools are also found in sand-bed rivers in association with channel bends. Indeed,
different flow processes are responsible for pool formation, and thus several sub-types
can be identified.
Distinctive characteristics: All types of pools are characterised by a topographic
depression with a reverse bed slope in their downstream part which makes them quite
different from all the other units characterised by low flow velocity (e.g. low gradient
glides).
(a)
(b)
Bed configuration unit type: pool (a, b). (a) Modified from Knighton (1998).
Sub-types
References: Bisson et al. (1982); Montgomery et al. (1995); Brierley and Fryirs (2005)
Forced pool
Definition
Pool unit originated by constriction scouring associated with irregularly spaced bedrock
outcrops, large wood elements, forced riffles, and from the deposition of coarse material
of several origins (e.g. from glacial deposits in north-European rivers formed on glacial
valleys). Often they form from erosional processes due to local reduction of the flow
section or due to the formation of vortex with vertical axis.
Equivalent terms: constriction pool
(a)
(b)
Bed configuration unit sub-type: constriction pool (a, b). (a) Modified from Montgomery
et al. (1995).
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Scour pool
Definition
Pool unit derived from local scouring of thebed sediment downstream of rock, clast or
wood step units.
(a)
(b)
Bed configuration unit sub-type: scour pool (a, b). (a) Modified from Brierley & Fryirs
(2005) and from Halwas & Church (2002).
Plunge pool
Definition
Pool unit, typically quite deep and circular, formed in bedrock channels by corrosion and
cavitation processes below rock steps or waterfalls by the action of a plunging jet.
(a)
(b)
Bed configuration unit sub-type: plunge pool (a, b). (a) Modified from Brierley & Fryirs
(2005).
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Dammed pool
Definition
A dammed pool may form immediately upstream of a rock step, boulders, a log step,or
a wood accumulation and it persists until it is completely filled by sediment or the
obstruction (boulders, wood) is removed.
Equivalent terms: backwater pool
(a)
(b)
Bed configuration unit sub-type: dammed pool (a, b). (a) Modified from Bisson et al.
(1982), Brierley & Fryirs (2005) and from Halwas & Church (2002).
Meander pool
Definition
Deep pool unit formed by erosion by secondary flows close to the concave bank of a
meander bend.
(a)
(b)
Bed configuration unit sub-type: meander pool (a, b). Modified from Knighton (1998).
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Dune system
Identification code: CD
References: Simons & Richardson (1966); Knighton (1998)
Definition
This unit is typical of low-gradient, alluvial sand-bed rivers. The surface flow is
influenced by the presence of the dunes, showing 'bulges' not in phase with the dunes.
A single dune or a few occasional dunes should be classified as sub-units (same for a
single boulder or a single tree). A set of dunes is classified as a geomorphic unit (dune
system) if dunes extend to the length of a channel width. A dune system is often
associated with ripples (see the sub-units) generating a dune-ripple morphology
(Montgomery & Buffington, 1997).
Dune systems are difficult to observe, particularly in deep channels when the bed is
frequently not visible (except in case of availability of a detailed bathymetric survey). In
most cases the presence of this bed morphology can be assumed on the basis of the
knowledge of the bed material (mainly sand) and from the undulating water surface,
showing well-structured and recurring turbulent fluctuations.
(a)
(b)
Bed configuration unit type: dune system (a, b). (a) Modified from Montgomery &
Buffington (1997).
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2.1.2 Macro-unit: emergent sediment units
In alluvial and semi-alluvial streams, in-channel ‘emergent’ (exposed at baseflow)
sediment units are mainly depositional bars and unvegetated banks, but some erosional
units can be identified such as channels which are dry at the time of observation and
thus not baseflow channels, and bedrock outcrops may also be present.
At the Broad level, depositional bars and erosional channels are included in the same
macro-unit (Fig. B2.2). At the Basic level and Detailed level, the following geomorphic
units and related sub-types are classified.
Identification code: E
Figure B2.2 Example of macro-unit ‘emergent sediment units’ (E).
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Bank-attached bar
Identification code: EA
References: Kellerhals et al. (1976); Brierley & Fryirs (2005)
Definition
Bars are macro-scale bed features consisting of a depositional surface composed of
channel bed sediment. They are elevated above the water surface for most of the year,
but are submerged as flow increases towards bankfull. Vegetation may be completely
absent from bar surfaces, but in some cases a partial, discontinuous cover of grasses
and herbaceous vegetation, shrubs or isolated trees may exist.
Bank-attached bars are located along one side of the bankfull channel and are attached
to the channel bank or to other units located at the bankfull margins (i.e. benches) or
are separated from the bankfull channel edge by an emergent (dry) channel.
Equivalent terms: more specific terms are used as sub-types
Sub-types
Sub-types of bank-attached bars. Modified from Church & Jones (1982) and Brierley &
Fryirs (2005).
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Side bar
References: Kellerhals et al. (1976); Church & Jones (1982); Hooke (1995)
Definition
Lateral bar, generally elongated and located on one side of a channel. Side bars often
alternate from one side of the channel to the other and are normally attached to the
bank, although they may occasionally be separated by a dry chute cut-off channel. Side
bars are typical of straight to sinuous sand- or gravel-bed channels with alternate bars
(or ‘pseudomeandering’). They may occur as an early phase of meander development.
Equivalent terms: lateral bar, alternate bar (Thorne, 1998), bank-attached or attached
bar (Hooke, 1995)
Point bar
References: Kellerhals et al. (1976); Church & Jones (1982); Hooke (1995); Thorne
(1998)
Definition
Arc-shaped bar developed along the convex side of meander bends. Bank-attached or in
some cases separated from the bank by a dry chute cut-off channel. Point bars are
characteristic of meandering rivers, but can also occur locally along sinuous channels.
Counterpoint bar
References: Thorne and Lewin (1979); Page and Nanson (1982); Lewin (1983); Hickin
(1984)
Definition
Bar type that develops in the flow separation zone along the concave bank of tight river
bends. Sediments are usually finer than nearby point bars because of differences in the
local hydrodynamic conditions associated with each bar type.
Equivalent terms: concave bar (Hooke, 1995)
Junction bar
References: Kellerhals et al. (1976); Thorne (1998)
Definition
Bar that develops immediately downstream of a tributary confluence. These delta-like
features have an avalanche face and are generally comprised of poorly sorted gravel,
sand and mud with complex and variable internal sedimentary structures.
Equivalent terms: tributary confluence bar (Brierley & Fryirs, 2005)
Forced bank-attached bar
References: Brierley & Fryirs (2005)
Definition
Bar formation induced by a flow obstruction (e.g. bedrock outcrop, boulder, large wood
jam, vegetation).
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Mid-channel bar
Identification code: EC
References: Hooke (1995); Thorne (1998)
Definition
Mid-channel bars are macro-scale depositional features located within the bankfull
channel and clearly separated by flowing water (i.e. baseflow channels) from the banks
or other units on both sides. The separation from floodplain, due to flowing channels on
both sides, makes their distinction from lateral bars ecologically relevant (habitat
disconnection). Under extreme low flow conditions as well as in the case of temporary or
ephemeral streams, a mid-channel bar may be surrounded by dry channels (see the
definition of dry channel and chute cut-off).
Equivalent terms: more specific terms are used as sub-types
Sub-types
Sub-types of mid-channel bars. Modified from Church & Jones (1982), Kellerhas et al.
(1976) and Brierley & Fryirs (2005).
Longitudinal bar
References: Kellerhals et al. (1976); Church & Jones (1982)
Definition
Mid-channel, elongate bar, with variable shape (lozenge, diamond, teardrop or lobate
shaped). Multiple longitudinal bars are common in braided rivers, but may also occur in
wandering or single-thread channels with local flow bifurcation and braiding.
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Transverse bar
References: Church & Jones (1982); Brierley & Fryirs (2005)
Definition
Mid-channel bar, oriented perpendicular to flow. They are generally found at points of
abrupt channel and flow expansion. They have a lobate or sinuous front with avalanche
face. The upstream ramp may be concave creating an arc shape.
Equivalent terms: linguoid bar (Church & Jones, 1982)
Diagonal bar
References: Kellerhals et al. (1976); Thorne (1998)
Definition
Mid-channel bar that runs obliquely across the channel (bank-attached bars included in
a sequence of bars forming an overall diagonal bar are classified as side bars).
Equivalent terms: diamond bar (Brierley & Fryirs, 2005)
Medial bar
References: Church & Jones (1982)
Definition
Large, complex mid-channel bar made up of a mosaic of erosional and depositional
forms comprising an array of smaller-scale geomorphic units. Variable morphology
depends on material texture, flow energy and flood history responsible of its formation
and subsequent re-working. Medial bars may include chute cut-off channels and a series
of sub-units such as ramps, dissection features, lobes, ridges, vegetation patches.
Bedrock core bar
References: Brierley & Fryirs (2005)
Definition
Elongated bedrock ridge over which sediments have been draped (after a large flood
event) and in some cases colonized by vegetation.
Forced mid-channel bar
References: Brierley & Fryirs (2005)
Definition
Mid-channel bar induced by a flow obstruction (e.g. bedrock outcrop, boulder, large
wood jam, vegetation).
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Bank-attached high bar
Identification code: EAh
References: Hupp & Rinaldi (2007); Surian et al. (2009)
Definition
High bars are not usually distinguished in morphological classifications, but represent
significant and distinctive habitat units in terms of morphological and sedimentary
characteristics.
High bars are depositional features which differ from the previous types of bars as a
result of their: (1) higher topographic elevation; (2) higher sediment heterogeneity
(mainly gravel, cobble and sand), with coarse sediment associated with a significant
proportion of fine material; (3) sparse grass, herbaceous and/or shrub vegetation cover
(highest areas with fine sediment may be colonized by scattered trees).
In many cases they represent transitional features between bars and modern floodplain
or islands. For these specific characteristics, the distinction between high bars and other
types of ‘lower’ bars is ecologically relevant.
While bars are commonly deposited and reshaped during formative (e.g. 1 to 2 year and
even lower discharges), high bars are generally deposited during more intense flood
events (typically a return period >10 years, often of the order of 30÷50 years) and are
generated by intense bedload and coarse-grained bedload sheets (Whiting et al., 1988).
Indeed, high magnitude events are able to erode and deposit large amounts of coarse
material (even coarser than those present in the channel bed or on bars). Although high
bars are commonly submerged during formative flow events that produce overbank
deposition of fine material, higher discharges are generally required for a full
remobilization of coarse sediments and high bar reshaping (Surian et al., 2009).
Because of these characteristics, high bars are commonly observed along cobble- or
gravel-bed streams with relatively high energy (e.g. partly confined single-thread,
wandering or braided).
Bank-attached high bars are located along one side of the bankfull channel and are
attached to the bank or to other units located at the bankfull margins (i.e. benches) or
are separated from the bank by an emergent or dry channel (e.g. a dry cut-off).
Distinctive characteristics: Bank-attached high bars differ from bank-attached bars
because of (1) higher sediment heterogeneity; (2) higher vegetation cover (herbaceous
and shrubs); (3) higher topographic position. Bank-attached high bars differ from the
modern floodplain because coarse-grained sediment still prevails and the vegetation
cover is less dense. Bank-attached high bars differ from bank-attached boulder berms
because the latter are characterised by coarser sediment (prevailing cobble and
boulders) and by a more pronounced topography.
(a)
(b)
Emergent sediment unit type: bank-attached high bar.(a, b). Photo in (b) is taken from
Surian et al. (2009).
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Mid-channel high bar
Identification code: ECh
References: Hupp & Rinaldi (2007); Surian et al. (2009)
Definition
Mid-channel high bars have the same characteristics of bank-attached high bars, except
that they are separated from the banks or other units by a baseflow channel on both
sides.
Distinctive characteristics: mid-channel high bars differ from bank-attached high bars
only because of their relative position within the channel. Differences with mid-channel
bars, mid-channel boulder berms or islands are the same as for bank-attached high
bars.
Emergent sediment unit type: mid-channel high bar.
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Bank-attached boulder berm
Identification code: EB
References: Stewart & La Marche (1967); Carling (1987, 1989)
Definition
This is an elongated, bank-attached, stepped feature commonly occurring along
mountain confined or partly-confined, high energy streams. It is composed of coarse
materials (mainly boulder, with some cobble or gravel) with a very limited finer grained
matrix and may have a characteristic convex cross-section.
Boulder berms are characteristic overbank coarse deposits associated with large, high
energy, mainly flash or catastrophic floods, during which sediment transport may occur
as a ‘debris flood’ (i.e. a very rapid, surging flow of water, heavily charged with debris,
that typically occurs in steep channels (Hungr, 2005)). They are normally deposited in a
single flood event under very high velocity conditions during the flood peak, and they are
formed in the zone of expansion and large velocity gradients (Carling, 1987; 1989).
Similar to other emergent depositional units (bars and high bars), bank-attached and
mid-channel boulder berms are distinguished according to their position within the
bankfull channel. Bank-attached boulder berms are located along one side of the bankfull
channel and are attached to the bank or to other units located at bankfull margins (i.e.
benches).
Equivalent terms: (bank-attached) boulder bar, cobble berm, boulder bench (Brierley &
Fryirs, 2005)
Distinctive characteristics: They differ from bank-attached high bars or bars because of
their coarser material and higher topographic position. Bank-attached boulder berms
differ from mid-channel boulder berms because of their relative position within the
channel.
(a)
(b)
Emergent sediment unit type: bank-attached boulder berm.
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Mid-channel boulder berm
Identification code: EM
References: Stewart & La Marche (1967); Carling (1987, 1989)
Definition
Mid-channel boulder berms have the same characteristics as bank-attached boulder
berms, with a linguoid shape that is separated from the banks or other units located at
the bankfull margins (i.e. benches) by a baseflow channel on both sides. Mid-channel
boulder berms are deposited under high velocity conditions and are characterised by a
cluster of boulders without any significant fine matrix, and they fine distinctly in a
downstream direction.
Equivalent terms: (mid-channel) boulder bar, boulder mound (Brierley & Fryirs, 2005)
Distinctive characteristics: They differ from mid-channel high bars or bars because of
the coarser material and higher topographic position. Mid-channel boulder berms differ
from bank-attached boulder berms because of their relative position within the channel.
(a)
(b)
Emergent sediment unit type: mid-channel boulder berm.
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Dry channel
Identification code: ED
Definition
A dry channel is an erosional feature that occupies a portion of the bankfull channel bed
where water flow is absent at the time of observation (i.e. baseflow). It forms a
preferential flow path during flows in excess of baseflow. Rivers and streams with a
temporary or ephemeral hydrological regime have an entirely dry channel (or channels)
since they support no surface flow under baseflow conditions.
Equivalent terms: emergent channel, dry cut-off
Distinctive characteristics: a dry channel differs from a baseflow channel because of the
absence of flowing water at the time of observation (i.e. under baseflow conditions).
Emergent sediment unit type: dry channel.
Bedrock outcrop
Identification code: EO
Definition
Emergent bedrock outcrops within the bankfull channel can be observed not only along
confined or semi-alluvial channels, but also along alluvial streams, especially where bed
incision has occurred.
Emergent sediment unit type: bedrock outcrop.
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Unvegetated bank
Identification code: EK
References: Thorne (1982, 1999)
Definition
A bank is a sloping surface that usually separates the bankfull channel from the
floodplain. Therefore, banks included in the margins of a bankfull channel are only those
delimiting the edge of the bankfull channel (other sloping surfaces that do not delimit
the edge of the bankfull channel but are found within the floodplain are termed scarps).
Only banks composed of alluvial sediments are characterised (hillslopes or old terraces
delimiting the bankfull channel of confined streams are not classified as banks because
their upper surface does not correspond to the level of the floodplain).
Unvegetated banks are distinguished from vegetated banks (see 'in-channel vegetation'
units) because are characterised by the absence or scarce presence of vegetation.
Equivalent terms: streambank, riverbank
Distinctive characteristics: sloping surface composed of alluvial sediments. Compared to
vegetated banks the vegetation is absent or negligible.
Emergent sediment unit type: Eroding, cohesive, terrace bank (on the left) connecting
the channel with a low terrace.
Even if sub-types are not considered, at the Detailed level banks (both unvegetated and
vegetated) can be further characterised depending on: (1) bank morphology; (2) bank
material; (3) stability/instability ‘status’.
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(1) Characterisation of bank morphology
Bank morphologies: (a) Near-vertical; (b) Vertical undercut; (c) Planar; (d) With toe
sediment deposition; (e) Convex upwards; (f) Concave upwards; (g) Complex.
(a) Near vertical: bank having a steep slope up to 90 degree. Frequently this shape is
the result of slab or cantilever failures occurring more frequently on cohesive bank.
(b) Vertical undercut: near-vertical bank having scour at the toe, which is related to
the erosion processes exerted by the flow (hydraulic action). This shape is frequently
observed in composite banks.
(c) Planar: bank presenting a nearly flat and graded surface with a variety of slope-
height ratios depending on the degree of cohesion of the sediments, sediment packing
or cementation. This shape is the result of mass failure which occurs by shearing along
shallow, planar or slightly curved surfaces and it is often observed on non-cohesive
banks.
(d) With toe sediment deposition: bank with generally planar morphology but
exhibiting a typical basal wedge of sediment with slope near to the angle of repose,
which is generated from failures that have occurred in the upper part of the bank. The
toe has no direct effects on the stability of the upper cohesive layer, but it acts as a
protection from toe erosion, until it is entrained by the flow.
(e) Convex upwards: bank presenting a curved (convex upwards) surface. Along
convex upwards banks gradual mass movements mechanisms may be inferred (Brierley
& Fryirs, 2005).
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(f) Concave upwards: bank composed of a curved (concave upwards) surface which is
frequently the result of rotational slip failures. This geometry is often observed in
cohesive banks.
(g) Complex: a bank is complex when its surface is irregular and cannot be described
by one of the aforementioned types.
(2) Characterisation of bank material
Bank material: (A) Non-cohesive; (B) Cohesive; (C) Composite; (D) Multi-layered
(pictures from the Cecina River, Italy).
(A) Non-cohesive: the bank is entirely composed of non-cohesive sediments (gravel,
cobble, coarse sand). The maximum slope angle of loose, non-cohesive banks is equal
to the angle of repose of the material. However, higher slope angles can be observed in
case of packing or partially cemented banks (Nardi et al., 2012) or in the presence of
apparent cohesion which develops within a finer matrix.
(B) Cohesive: the bank is entirely composed of cohesive soils (in general sandy-silt,
silt, clay). Thanks to the apparent cohesion, cohesive banks are stable for very steep
angles, up to vertical, and can reach a considerable height (of the order of meters).
(C) Composite: the bank is composed of two different layers of sediments. Typically, in
composite banks non-cohesive deposits formed from relic bars are overlain by cohesive
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materials deposited by overbank flow on emergent bars. Composite banks are often
cantilevered, as a result of erosion of the underlying gravels by the action of the flow
which produces an overhanging of the cohesive layer. Piping and seepage processes are
also frequently observed at the boundary of different bank sediment layers.
(D) Multi-layered: also known as stratified, a multi-layered bank is composed of more
than two different layers of sediments. Similarly to the composite banks, multi-layered
banks often exhibit cantilevered blocks. Piping and seepage processes are also
frequently observed at the boundary of different bank sediment layers.
(3) Bank stability/instability ‘status’
Retreating: a retreating bank is an unstable bank on which one or more erosion
processes are active. Processes that act on a retreating bank can be both erosion
processes and mass failure. Piping, seepage, and particle-by-particle detachment
exerted by the near-bank flows are the main erosion processes, whereas all the
mechanisms related to the failure of blocks under the action of gravity (e.g. planar,
rotational, slab and cantilever failures) are defined as mass failure (Thorne, 1982).
Referring to the 'basal point control' concept (Thorne, 1982), a bank can display: (1) a
condition of equilibrium, when the processes of sediment supply and removal balance
each other (unimpeded removal), determining a retreat parallel to the bank and through
which the bank is in a condition of dynamic equilibrium; (2) the erosion causes a
complete removal of material at the bank toe and in some case is able to entail a bed
scour, causing further instability (excess basal capacity); (3) a condition of
accumulation (impeded removal), where the rate of material deposition by mass
movements at the bank toe is greater than the rate of removal by fluvial erosion. In the
latter case the bank is evolving into a stable or advancing bank.
Stable: a stable bank is a bank on which no eroding or deposition processes are active.
Frequently stable banks are vegetated.
Advancing: an advancing bank is a bank where depositional processes prevail,
determining a progressive shifting towards the opposite bank.
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2.1.3 Macro-unit: in-channel vegetation
In-channel vegetation macro-unit include geomorphic features of a significant size (see
below) that include: (i) well-developed vegetation cover on emergent sediment surfaces
(islands), (ii) large wood jams, (iii) rooted aquatic vegetation often associated with
submerged sediment units, (iv) vegetated features located at the margins of the bankfull
channel (benches), and (v) vegetated banks. At the Broad level, all in-channel
vegetation units are included in the same macro-unit (Fig. B2.3Figure B2.). At the Basic
level, several geomorphic units are classified.
Identification code: V
Figure B2.3 Example of 'in-channel vegetation' macro-unit (V).
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Island
Identification code: VI
Definition
Islands are units within the bankfull channel characterised by a cover of perennial
vegetation and by other features common to the floodplain (e.g. a significant layer of
fine sediment superimposed on the top of gravel layers in gravel-bed rivers, and a
higher elevation than unvegetated or sparsely vegetated bars), but they differ from the
floodplain in that they are entirely surrounded by baseflow channels or emergent
sediment units (e.g. bars). In the past, some authors have used the term vegetated
islands, but the adjective 'vegetated' is not needed as islands, by definition, have a
cover of established vegetation. A bar covered with entirely annual or biennial plants, no
matter how dense, cannot be considered as an island.
Vegetation (perennial grasses, herbs, shrubs, young or mature trees) does not need to
cover the entire island unit surface, as patches of bare sediment can occupy a minor
part (<1/3) of the island.
The unit surface is typically but not always higher than bar surfaces and approaches or
is equal to that of the floodplain.
A patch of in-channel vegetation should only be classified as an island unit if the
following conditions are satisfied: (1) the vegetation cover is comprised of at least 3
individual plants; and (2) the patch has an area larger than approximately 5 m2.
These are meant to provide a broad indication of the scale of island units. They do not
need to be strictly applied, but a single large tree, for example, cannot be classified as
an island in the same way as a single large boulder does not represent a channel units,
unless it forms a step.
Island characterisation in terms of vegetation type (e.g. herbaceous vs. woody, woody
pioneer vs. woody mature) is carried out at the unit sub-type level (Detailed level).
Islands form under different processes, that involve sediment retention around
vegetation, surface aggradation to approach floodplain level accompanied by continuing
vegetation colonisation and growth: (i) colonization of bar surfaces by germination of
deposited seeds; (ii) large wood (jam or not) deposition on bars, which induces fine
sediment deposition and colonization by seedlings; (iii) regeneration (vegetative
sprouting) of uprooted trees or tree fragments deposited on bars or within the baseflow
channel; (iv) fine sediment retention byrooted aquatic vegetation in low energy
streams, promoting bed aggradation and stabilization to form emergent bars that are
further colonised by plants. In all of these cases the resulting island is classified as a
'building island' (Gurnell et al., 2001). Islands can also form by avulsion of main or
secondary channels across the floodplain. In this case the resulting islands are classified
as 'dissection islands' (Gurnell et al., 2001).
Equivalent terms: vegetated island; see sub-types
Distinctive characteristics: in order to be classified as a geomorphic unit and to be
distinguished from mid-channel bars, mid-channel high bars and mid-channel boulder
berms or part of them, the vegetated patch of an island should: (1) be composed of at
least 3 perennial plants; (2) occupy an area greater than 5 m2; (3) have a distinct
surface layer of finer sediment when found in a gravel or coarser bed river; (4) have a
surface elevation typically bu not always higher than bar surfaces and close to that of
the floodplain; (5) the perennial vegetation cover does not have to be continuous but
the area of bare sediment or annual/biennial vegetation cover should not occupy more
then 1/3 of the island surface).
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Sub-types (all having >10 m2 area)
Grassy island
Island with a predominantly perennial grass and herb cover. These islands are
frequently found in low-gradient sandy rivers.
Young woody island
Island with a cover of woody vegetation (shrubs or trees) typically < 10 m tall. Canopy
height is a surrogate for island age, although tree height-age relationships vary widely
with species and environmental conditions. However, for the poplar and willow species,
typical of European rivers, this type of island is likely to be <10 yr old.
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Established or adult woody island
Island formed by woody vegetation (shrubs or trees) whose typical height is 10÷20 m.
Canopy height is a surrogate for island age, although tree height-age relationships vary
widely with species and environmental conditions. However, for the poplar and willow
species typical of European rivers, this type of island is likely to be 10-20 yr old.
Mature woody island
Island formed by woody vegetation (shrubs or trees) whose dominant height is > 20 m.
Canopy height is a surrogate for island age, although tree height-age relationships vary
widely with species and environmental conditions. However, for the poplar and willow
species typical of European rivers, this type of island is likely to be > 20 yr old. In most
river systems, trees growing on islands rarely reach an age exceeding 50 yr because the
rate of turnover of islands (growth, establishment, erosion) is usually < 50 yrs.
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Complex woody island
Island formed by woody vegetation (shrubs or trees), with or without grassy patches,
often show a mosaic-like pattern of patches of different age. They typically result from
the dissection and coalescence of mature, adult or young islands, by processes of
erosion, deposition and vegetation establishment and growth.
Large wood jam
Identification code: VJ
References: Wallerstein et al. (1997); Gurnell et al. (2002); Abbe & Montgomery (2003)
Definition
Elements of large wood or LW (i.e. wood pieces or entire uprooted trees >10 cm in
diameter and > 1 m in length) - lying within the bankfull channel either dead or still
able to sprout – may form a unit if both of the following conditions are satisfied: (1) LW
elements are organized in a jam or accumulation (≥ 3 logs); (2) the LW jam has an
area (an envelope that encloses the wood pieces and intervening air/sediment gaps)
larger than approximately 5 m2.
As for islands, these conditions should be considered as general indications.
Nonetheless, a single log forming a step will be characterised as a bed configuration unit
(log step), and not as a vegetation unit. A large LW jam determining a step unit will be
characterised both as LW jam unit and step unit.
LW jams are further characterised at the Detailed level (sub-types).
Equivalent terms: LW accumulation; see sub-types
Distinctive characteristics: in order to be classified as geomorphic unit, the LW
accumulation: (1) should be composed by ≥ 3 logs (being >10 cm in diameter and > 1
m in length); (2) should have an area larger than approximately 5 m2.
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Sub-types (all having >10 m2 area)
References: the terminology for the classification of sub-types is mainly taken from
Abbe & Montgomery (2003). Other relevant references: Wallerstein et al. (1997);
Gurnell et al. (2002, 2014b, 2015c)
Meander jam
Accumulation of transported LW on the outer banks of river bends (which do not have to
be meanders in strict terms) determined by the flow curvature and leading floating
material becoming trapped against that bank or on the bank top. These jams are typical
of meandering rivers, but can be found on all river types if relatively sharp bends exist
including in bedrock channels.
Equivalent terms: counterpoint jam
Bench jam
Accumulation of transported or locally fallen LW retained by oblique key wood pieces
which are wedged into irregularities in the channel margins (banks). The key pieces
create a physical barrier to water flow behind which fine sediment and additional wood
accumulates to form a bench (see the definition of bench unit). Picture is taken from
Gurnell et al. (2014b).
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Bar apex jam
Accumulation of transported LW that forms on the upstream face of (mid-channel)
medial bars which intercept floating wood due to the decrease in water depth. These
jams are typical of braided rivers but can be observed anywhere on medial bars. Pioneer
islands (see the definition in the sub-unit section) often develop from bar apex jams, as
a result of seed germination in the lee of the wood and/or sprouting of new shrubs from
the deposited wood. The jam should be ascribed to such a sub-type if there is evidence
that the island has been initiated by the jam, otherwise, where the jam appears to have
been trapped by a pre-existing (and usually larger) island it should be classified as a
vegetation-trapped jam (see later).
Bar top jam
Accumulation of transported LW found on the surface of bars of any kind, where floating
material becomes stranded due to the decrease in water depth. These jams are typical
of braided and transitional rivers but can be observed anywhere on medial bars and
lateral bars. Pioneer islands (see the definition in the sub-unit section) often develop
from bar top jams, as a result of seed germination in the lee of the wood and/or
sprouting of new shrubs from the deposited wood. The jam should be ascribed to such a
type if there is evidences that the island has been initiated by the jam, otherwise where
the jam appears to have been trapped by a pre-existing and usually larger island it
should be classified as a vegetation-trapped jam (see later).
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Dam jam
Accumulation of mainly transported LW that spans the entire channel width (baseflow or
bankfull channel), and so forms a channel dam whose porosity varies widely. In some
cases (particularly where wood supply is high and where the channel is fairly confined)
these jams can occupy the entire valley width (valley jam; Abbe & Montgomery, 2003).
A log step formed by a single wood element cannot be classified as a jam. Often dam
jams form where LW elements are retained in channel constrictions, by riparian trees
and shrubs on the channel margins, or by islands within the bankfull channel.
Equivalent terms: debris dams, channel-spanning jam, valley jam
Bank input jam
Accumulation of in situ LW produced when trees or other large wood pieces fall from the
banks as a result of wind throw or bank erosion (Abbe & Montgomery, 2003). These
jams are typical, but not confined to, confined or partly-confined mountain rivers.
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Flow deflection jam
Accumulation of mainly transported LW which extends from the bank in a direction that
is oblique with respect to the flow. These are typically but not always triggered by a
large tree that has fallen from the bank. Compared to bank input jams, flow deflection
jams are largely comprised of transported rather than in situ wood and are typical of,
but not confined to, larger rivers, where are large wood piece or entire tree can lie
oblique to the flow while only occupying a part of the channel width, causing flow
deflection and trapping other floating material and fine sediment.
Landslide jam
Accumulation of LW generated by mass wasting processes or by a debris flow which has
transported and deposited the wood in the river channel. These jams are typical of
confined mountain rivers, where debris flows may occur down hillslopes or along
confined valleys depositing very large and chaotic accumulations of wood.
Equivalent terms: the sub-type also includes debris flow jams (confined channels) and
debris torrent jams (steep confined channels)
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Vegetation-trapped jam
Accumulation of large wood which does not fall into any of the preceding types but is
retained by standing trees/shrubs lying within the bankfull channel (islands, pioneer
islands or isolated woody plants). These jams are formed at flood flows and can achieve
very large dimensions. If the LW accumulation occupies the entire channel width then is
classified as a dam jam (or debris dam).
Equivalent terms: flood jam, wood ridge, wood pile
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Aquatic vegetation
Identification code: VA
Definition
Perennial aquatic vegetation rooted in the channel bed can form a geomorphic unit
when the patch has an area larger than approximately 5 m2 and can induce sediment
retention to support the development of sediment units on the channel bed such as
shelves, particularly in low energy, sand and finer bed rivers.
At Detailed level different aquatic vegetation sub-types can be classified according to
whether the leaves are emergent, floating or submerged (see the sub-types).
Distinctive characteristics: area at least 5 m2 composed of perennial aquatic plants that
are rooted into the channel bed.
Sub-types
Floating leaves
Perennial aquatic vegetation rooted in the channel bed with floating leaves; the latter
are usually large and leathery.
Submerged leaves
Perennial aquatic vegetation rooted in the channel bed with submerged leaves; the
latter are usually thin and narrow.
Emergent leaves
Perennial aquatic vegetation rooted in the channel bed with emergent leaves whose
shape is usually similar to terrestrial plants in the surrounding.
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Bench (berm or shelf)
Identification code: VB
References: Hupp & Osterkamp (1996); Brierley & Fryirs (2005); Rinaldi (2008); Surian
et al. (2009); Gurnell et al. (2012, 2014b); Gurnell (2014)
Definition
Bench, berm, or shelf, are generic terms used to indicate features with a flat or slightly
convex upper surface and steeper edge that are deposited at the margins within the
bankfull channel. They are vegetated features that form at an intermediate position
between submerged or emergent bars within the baseflow channel and the floodplain (if
any) and are distinguished from bars by their relatively flat, vegetated upper surfaces,
their steeper inner (towards the baseflow channel) surfaces and the presence of
vegetation that has usually retained sediment to produce the relatively falt upper
surface.
The terms bench, berm, or shelf have been used with slightly different meanings by
various authors, but for the scope of this classification, they have been included in a
generic type termed bench, but various sub-types can be distinguished.
A clear distinction between bench, berm and shelf is provided by Gurnell (2014) and
Gurnell et al. (2012, 2014b), on the basis of the degree of surface development and of
vegetation submersion. According to these authors, these features are quite common
along one or both banks in low energy single-thread sinuous and meandering systems,
and are formed by fine sediment trapping and stabilization by aquatic vegetation
(macrophytes) which is then replaced by riparian species as the surface of the feature
emerges above the baseflow water surface (see sub-types). These features are
indicative of channel adjustments (migration, narrowing), which in low energy, finer
sediment systems, requires vegetation to stabilise the deposited fine sediments. Initially,
submerged shelves are formed, which evolve into berms when the surface reaches that
of the baseflow water surface, and then benches as the surface becomes elevated above
the baseflow water surface level.
Tree roots can also contribute to the development of bench features, by trapping fine
sediment (tree-induced shelf, berm or bench; Gurnell et al., 2014b; see sub-types).
As well, wood material can contribute to the development of benches along banks
(Camporeale et al., 2013; bench jams, according to Abbe & Montgomery, 2003). In that
case these surfaces are classified as LW jams (see sub-types of LW jams), and can
develop until reaching the floodplain level. In all of these cases, the wood, roots or
aquatic plants provide structures that retain and stabilise mobile sediment.
As already noted, these features are indicative of channel adjustments (e.g. channel
migration or narrowing) and may be very significant features when major channel
changes are occurring (Brierley & Fryirs, 2005; Hupp & Rinaldi, 2007). According to
Brierley & Fryirs (2005), benches are a major mechanism of channel contraction
(narrowing) in over-widened channels, whereas the term ledge is preferred to indicate
depositional features associated to channel expansion (widening) (see sub-types). In
other cases, benches (or berms) may represent relic bar surfaces abandoned by incision
and small amounts of narrowing in meandering rivers. In such cases they may occur at
intermediate positions between bars and modern floodplain or between modern
floodplain and terrace (Hupp & Rinaldi, 2007).
Additionally, on the basis of their position within the channel, Brierley & Fryirs (2015)
define two sub-types of benches (point bench; concave bank bench; see sub-types).
Hupp & Osterkamp (1996) describe a channel shelf along relatively steep-gradient
reaches (see sub-types).
Finally, bench features are also attributed to bank processes (mass movement) and to
the scouring and abrasive effect of ice on banks or on the channel bed, as observed in
low energy north-European rivers (Kling, pers. comm.; see sub-types).
Equivalent terms: see sub-types
Distinctive characteristics: these features are generally narrow and discontinuous. They
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form within the margins of the bankfull channel at the bank toe and in some cases may
progressively aggrade to floodplain level as their vegetated surface retains sediment.
Sub-types
Submerged shelf
References: Gurnell (2014); Gurnell et al. (2012, 2014b)
It is a vegetation-induced feature located between the baseflow channel and the
floodplain and is completely submerged under baseflow condition. It is usually associated
with aquatic vegetation, but can also form in association with submerged tree roots
(tree-induced shelf) that trap fine sediment. It should be noted that in some cases this
tree-induced feature may in part represent erosional processes in that the tree roots
may become exposed by sediment removal, but it is difficult to identify the precise
balance of deposition, retention and erosion without considering the geomorphological
context of the feature.
A submerged shelf is indicative of channel migration or narrowing (i.e. they occur along
one or both banks) in low energy single-thread sinuous and meandering systems.
(a)
(b)
Sub-type of bench: submerged shelf. (a, b) From Gurnell et al. (2014b).
Berm
References: Gurnell (2014); Gurnell et al. (2012, 2014b)
It is a vegetation-induced feature, whose upper surface approximates the baseflow water
surface within the bankfull channel. It is quite common along one or both banks in low
energy single-thread sinuous and meandering systems, usually colonised by wetland
species following aggradation of a submerged shelf to baseflow level.
Like for submerged shelf, this feature can also form in association with tree roots (tree-
induced berm) that trap fine sediment. It should be noted that in some cases this tree-
induced feature may in part represent erosional processes in that the tree roots may
become exposed by sediment removal, but it is difficult to identify the precise balance of
deposition, retention and erosion without considering the geomorphological context of
the feature.
Equivalent terms: emergent shelf
(a)
(b)
Sub-type of bench: berm. (a, b) From Gurnell et al. (2014b).
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Bench (sensu strictu)
References: Gurnell (2014); Gurnell et al. (2012, 2014b)
It is a vegetation-induced feature located between the baseflow channel and the
floodplain, is completely emerged at baseflow condition and is colonised by riparian
species following the surface aggradation of an emergent shelf or berm. It is quite
common along one or both banks in low energy single-thread sinuous and meandering
systems.
Like for submerged shelf and berm, this feature can also form in association with tree
roots (tree-induced bench) that trap fine sediment. It should be noted that in some
cases this tree-induced feature may in part represent erosional processes in that the tree
roots may become exposed by sediment removal, but it is difficult to identify the precise
balance of deposition, retention and erosion without considering the geomorphological
context of the feature.
(a)
(b)
Sub-type of bench: bench (s.s.). (a, b) From Gurnell et al. (2014b).
Ledge
References: Brierley & Fryirs (2005)
According to Brierley & Fryirs (2005), ledges reflect channel widening and/or incision,
whereas benches are a major mechanism of channel contraction (narrowing) in over-
widened channels. Ledges are composed of the same material as the basal floodplain,
whereas the sedimentary structure of a bench is quite different from the floodplain.
Sub-type of bench: ledge. Modified from Brierley & Fryirs (2005).
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Point bench
References: Brierley & Fryirs (2005)
Bench that develops at the convex bank in meandering rivers, slightly above the level of
the point bar. It displays a convex planform with planar surface. The sediment deposit
shows a vertical or oblique gradient (layers of sand and mud) indicating slow lateral
migration or lateral accretion within an overwidened bend. However, in some cases,
point benches may simply be aggraded, vegetated point bars, where vegetation has
interacted with flows to trap finer sediment and the inner edge of the point bench has
become trimmed to a steeper slope by fluvial processes, creating the classic bench
profile described above as an aggradational extension of the point bar within a
migrating meander bend.
Concave bank bench
References: Brierley & Fryirs (2005); Gurnell et al. (2014b)
It forms along the concave bank of relatively tight bends that usually abut bedrock
valley margins or a flow obstruction (e.g. wood accumulation), because of the formation
of secondary flows during high flood-stage. However, neither are essential for the
formation of this feature, which is usually characterized by finer, more organic-rich
sediments than a point bench within the same system. It is often characterised by the
presence of a ridge (for definition see sub-units) on the top of the feature, parallel to
the baseflow channel. This feature is often inset against the floodplain. The sediment is
mainly fine-grained (layers of sand, silt and clay) and organic material.
In meandering rivers, these features may form from LW jams that accumulate in the
low velocity zone upstream the meander bend at the concave bank (counterpoint or
meander jam; Gurnell et al., 2014b; see sub-types of LW jam). The LW jam entails the
deposition of fine sediment as well as of organic material (small wood pieces).
Modified from Brierley & Fryirs (2005).
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Shelf
References: Hupp & Osterkamp (1996)
It is a horizontal to gently sloping surface that normally extends the short distance
between the break in the relatively steep bank slope and the lower limit of persistent
woody vegetation that marks the channel bed edge. The shelf is best developed along
relatively steep-gradient reaches where its presence is related to the presence of
colonising vegetation, but it is more patchy and irregular than the finer sediment shelf-
berm-bench features that depend upon vegetation for their presence in lower energy
systems, and an extensive development of both floodplain and channel shelf along the
same reach is rare, although they are not mutually exclusive.
Sub-type of bench: shelf in the Passage torrent (USA). Picture from Hupp & Osterkamp
(1996).
Slump bench
References: Kling (pers. comm.), Gurnell (1997)
Its formation can be attributed to bank processes (mass movement). It is commonly
observed along low energy north-European rivers, mainly in meandering rivers with silty
soils. In these systems, a large part of the meandering morphology can be related to
slides or slumps along the river, and the meander migration occurs jerkily. Most of the
slides occur along the outer bank, but may also be found in between two meanders.
Once vegetation is established and sediment is trapped and smooths feature
morphology, it can be quite difficult to distinguish this feature from an erosional or
sedimentary bench.
Equivalent terms: subsidence bench, slide bench
Sub-type of bench: slump bench in northern Sweden (picture: J. Kling).
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Ice abrasion and ice ploughing bench
References: Kling (pers. comm.)
Bench features that are formed under the scouring and abrasive effect of ice on banks
or on the channel bed when moving downstream. The term ice abrasion bench can be
employed to indicate the scouring effect of ice on banks; ice ploughing bench, can be
employed to indicate the scouring effect of ice on the channel bed. These features are
commonly observed along low energy north-European rivers that are subject to
significant ice build-up during the winter. In the spring, when the ice may break up
quickly, large volumes of ice move rapidly downstream during meltwater floods, and
severe ice scouring of the channel may occur.
Equivalent terms: ice scouring bench
Sub-type of bench: ice abrasion bench in northern Sweden (picture: Åsa Widén).
Vegetated bank
Identification code: VK
References: Thorne (1982, 1999)
Definition
Vegetated banks have the same characteristics as unvegetated banks but are
characterised by the significant presence of vegetation.
As for vegetated banks, at the Detailed level unvegetated banks can be further
characterised depending on: (1) bank morphology; (2) bank material; (3)
stability/instability ‘status’ (see unvegetated banks).
Equivalent terms: streambank, riverbank
Distinctive characteristics: compared to unvegetated banks the presence of vegetation is
significant. The criterion for considering the presence of vegetation as significant can be
assumed as the same for the identification of islands and aquatic vegetation, i.e. at least
5 m2.
In-channel vegetation unit: stable, vegetated, modern floodplain banks (both sides).
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2.2 Floodplain units
Floodplain units consist of surfaces and morphological features included in the overall
floodplain delimited by hillslopes or ancient alluvial deposits (i.e. old terraces).
2.2.1 Macro-unit: riparian zone
This includes the portion of the floodplain affected by various fluvial processes (e.g.
channel mobility, flooding) and characterised by spontaneous vegetation or relatively
natural conditions, where there is a natural absence of vegetation. Agricultural and
urbanised lands are not included. This macro-unit includes Floodplain units which
normally cannot be discriminated by remote sensing at the Broad level because they
need some field information on their elevation (e.g. modern floodplain or terrace) (Fig
B2.4).
Identification code: F
Figure B2.4 Example of macro.unit 'riparian zone' (F).
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Modern floodplain
Identification code: FF
References: Hupp & Osterkamp (1996); Simon & Castro (2003)
Definition
It is an alluvial, flat surface adjacent to the river, created by lateral and vertical
accretion under the present river flow and sediment regime. A river in dynamic
equilibrium builds a floodplain that is generally inundated for discharges just exceeding
channel-forming flows (return interval of 1÷3 years).
In many cases, such as where there has been recent channel incision, areas that fit this
definition represent a minor part of the whole floodplain. In other cases, it corresponds
to the entire floodplain (e.g., when no significant bed level adjustments have occurred
in historical time). Where several, longitudinally extensive, alluvial surfaces are present
at different levels, the lowest surface is considered as the modern floodplain, while the
higher surfaces are classified as recent terraces.
The modern floodplain is identified in the field as follows: (1) morphological-topographic
continuity with depositional features within the bankfull channel (bank-attached bars);
(2) presence of finer sediment material (from overbank deposition) compared to
bankfull units; (3) extensive cover by vegetation (perennial grasses herbaceous plants,
shrubs and trees, both young and adult), with significant presence of woody vegetation
where the vegetation has not been removed or modified by humans; (4) evidence of
reasonably frequent flooding such as large wood deposition. In addition, some limited
areas of bare sediment may be present, particularly following a recent flood. Note that
the types of field evidence described in (1) to (4) are not always observed together. For
example, bare fields close to the bankfull channel can still form part of the modern
floodplain if there is no channel incision. Furthermore, the extensive vegetation cover
described in (3) would also characterise terraces that have not been cleared or modified
by human activities.
Equivalent terms: active floodplain, genetic floodplain
Distinctive characteristics: compared to bank attached bars, high bars or boulder berms,
the modern floodplain is characterised by finer sediment and extensive vegetation cover
(the same characteristics that distinguish an island from a mid-channel bar); compared
to benches, a modern floodplain is commonly wider and continuous; compared to a
recent terrace, the modern floodplain is topographically lower and inundated with
smaller return periods (commonly 1÷3 years).
(a)
(b)
(a) Vegetated modern floodplain, on the left. On the right it is possible to see the
transition between a bank-attached bar and the modern floodplain (the latter is
topographically higher). (b) A modern floodplain on both sides. It is characterised by
grasses and herbaceous vegetation because of the agricultural activity.
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Recent terrace
Identification code: FT
References: Hupp & Osterkamp (1996); Simon & Castro (2003)
Definition
A recent terrace is a former floodplain that has become a terrace because of recent
channel incision (i.e. the last 100-200 years), which in most cases is driven by human
alterations. The relative level of a recent terrace above the modern floodplain can vary
widely, but the likelihood of inundation is always lower than that for the modern
floodplain (i.e. an average frequency greater than once in 3 years, Hupp & Osterkamp,
1996).
Frequently, more than one terrace level can be found. In such a case, an order (by
roman numerals) is assigned to each different terrace level.
Equivalent terms: modern terrace, terrace, low terrace
Distinctive characteristics: compared to a modern floodplain, a recent terrace is
topographically higher and inundated with a larger return periods (i.e. >3 years).
(a)
(b)
(a) Recent terrace due to incision delimited by a cohesive bank. (b) Cultivated recent
terrace delimited by a steep and high, cohesive bank.
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Scarp
Identification code: FS
Definition
This is a generic term to indicate various types of slopes included in the floodplain,
which are not at the interface with the bankfull channel (in this latter case the scarp is
included in the bankfull channel units and indicated as bank). The interface between a
recent terrace and a modern floodplain is represented by a scarp. Another example is
represented by meander scars, i.e. steep scarp slopes left in the floodplain by meander
progression.
Distinctive characteristics: compared to the banks, the scarps are located within the
floodplain and not at the boundary of the bankfull channel.
Floodplain unit type: scarp (Cecina River, picture: G. Consoli).
Levée
Identification code: FL
Definition
A natural levée is a raised, elongated ridge above the floodplain surface adjacent to the
channel, usually containing relatively coarser overbank sediments (although finer than
the bed sediments and usually finer than the bank sediments) deposited as flood flows
spread out from the channel across the floodplain. These are most frequently found at
the concave banks. Levée crests may be up to some meters higher than the floodplain
or may be absent or nearly imperceptible.
Distinctive characteristics: compared to the modern floodplain is (slightly)
topographically higher; it displays an inverse slope from the channel.
Floodplain unit type: natural levée. From Brierley & Fryirs (2005).
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Overbank deposits
Identification code: FD
References: Brierley & Fryirs (2005); Fryirs & Brierley (2013)
Definition
This terms indicates several sedimentary features close to the channel generated by
overbank deposition.
Equivalent terms: see sub-types
Floodplain unit type: overbank deposits (Cecina River; picture: G. Consoli).
Sub-types
References: Brierley & Fryirs (2005); Fryirs & Brierley (2013)
Crevasse splay
Definition
It is a lobate or fan-shaped feature composed of relatively coarser sediment (gravel,
sand), fining from the channel, generated by breaching of levée. The surface may have
multiple distributary channels.
Compared to other overbank deposits, crevasse splays are only found in the context of
levées.
The term floodout is employed when a similar deposit forms not in relation to levée
breaching but where the channel bed becomes raised to the level of the floodplain
during a flood.
Equivalent terms: crevasse channel-fill, floodout
Sub-type of overbank deposit: crevasse splay
(http://www.geodz.com/deu/d/crevasse_splay)
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Sand wedge
Sandy deposit with wedge-shaped cross-section at channel margins in non-levee
settings. They form on the proximal floodplain surface in moderate to high energy
systems.
Sand sheet
Flat, tabular, laterally extensive sheet in non-levee setting with massive, often poorly
sorted facies. It shows little lateral variation in thickness, mean grain size or internal
structure. They are associated with rapid sediment charged bedload deposition on the
floodplain surface during extreme flood events.
A sand sheet is differentiated from other floodplain deposits by its shape, extensive
area, and lack of distal thinning.
Ridges and swales
Identification code: FR
Definition
Ridges and swales are arcuate, alternating features, where the ridge is a rising,
elongated deposit and the swale is a depression, and which have been incorporated in
the floodplain. They have been produced by point bar (or scroll bar, see sub-units for
definition) migration on the advancing, convex bank during meander growth, and
through aggradation of their surfaces, they are elevated sufficiently to have become
incorporated into the floodplain.
Distinctive characteristics: compared to overbank deposits, they are characterised by a
typical undulating surface.
(a)
(b)
Floodplain unit type: ridges and swales. (a) Modified from Brierley & Fryirs (2005).
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Floodplain island
Identification code: FI
Definition
In anabranching systems, sufficiently large islands separating the anabranches can be
classified as floodplain islands because their surface elevation and surface sediment
texture corresponds to that of the floodplain. They may be formed by a combination of
within bankfull channel, coalescence and aggradation of pioneer islands and vegetated
mid-channel bars (termed building islands by Gurnell et al., 2001) or they may be
dissected from the floodplain as a result of avulsions (termed floodplain dissection
islands by Gurnell et al., 2001) but in either case, their surface elevation at floodplain
level leads them to be described as floodplain islands (or established islands; Gurnell
et al., 2001).
Equivalent terms: established island (Gurnell et al., 2001)
Distinctive characteristics: compared to most of the islands of the bankfull channel
units, these islands are larger in size and, because their surface elevation, correspond
to the level of the floodplain. Therefore, they cannot be considered to be located
‘within’ the bankfull channel but form part of the floodplain.
Floodplain unit type: floodplain island.
Terrace island
Identification code: FN
Definition
Terrace islands may (locally) occur in anabranching systems, where bed incision has
generated island surfaces which are significantly higher than the level of the modern
floodplain (see the characteristics of recent terraces).
Distinctive characteristics: compared to the floodplain islands, terrace islands are
topographically higher and thus inundated by higher return period floods (>3 years).
!
Floodplain
islands
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Secondary channel (within the floodplain)
Identification code: FC
Definition
Floodplain secondary channels indicate erosive features periodically conveying water
during high flow events, or even containing continuous flow but having a distinctly
smaller size than the baseflow channel within the bankfull channel, and being located
outside of the bankfull channel. These channels may occur in single-thread, transitional
or braided systems.
Equivalent terms: see sub-types
Distinctive characteristics: compared to the secondary channels included in the bankfull
channel units, these channels are within the floodplain, at a significant distance from the
bankfull channel.
Floodplain unit type: secondary channel. Modified from Nanson and Croke (1992) and
Brierley & Fryirs (2005).
Sub-types
Flood channel (back channel)
References: Brierley & Fryirs (2005)
Definition
A flood channel is a subsidiary channel, occupied by flow starting at approximately
bankfull stage. It can be located behind natural levées (if any; see the definition of
levées). Where the channel is weakly defined (smaller depression) and conveys
floodwater with a relatively higher return period, it is termed a flood runner.
Equivalent terms: flood runner, back channel
Abandoned channel
Definition
It indicates an old, inactive channel on the floodplain left by a previous channel cut-off
or an avulsion. It may be partially or entirely filled by sediment, and it may be
occasionally inundated during high flow events.
Equivalent terms: paleochannel, prior channel, ancestral channel, side arm, inactive
secondary channel
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Abandoned meander
Definition
It is a specific case of an abandoned channel incorporating only one meander
wavelength and thus generated by a meander cut-off. It is formed by neck cut-off
(abrupt) or chute cut-off (gradual). The terms paleochannel is employed to indicate an
abandoned channel including more than one meander wavelength.
An abandoned channel created meander cut-off and occupied by water is called an
oxbow lake (see later).
Sub-type of secondary channel: abandoned meander formed by a recent meander cut-
off.
Abandoned
meander
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2.2.2 Macro-unit: floodplain aquatic zones
This macro-unit represents the presence of water within the floodplain (e.g. lakes,
ponds, wetlands). However, it may incoprorate emergent sediment or vegetation
(submerged and emergent) within its units.
Identification code: W
Floodplain lake
Identification code: WO
Definition
Floodplain lakes are relatively deep features that are also larger in area than ponds. The
limnetic zone is significantly developed and these features display lake-like temperature
stratification. Aquatic vegetation may be present but not in the deepest areas.
Floodplain unit type: floodplain lake (picture from:
http://commons.wikimedia.org/wiki/File:Lake_Chicheri._Volga-Akhtuba_floodplain.JPG).
Sub-types
Oxbow lake
Definition
It is a water body which was once part of a meander bend, but which has been
abandoned because of a meander cut-off and continues to contain water (unlike
floodplain secondary channels and, particularly, abandoned meanders.
Sub-type of floodplain lake: oxbow lakes generated by meander cut-off (picture from:
http://clasfaculty.ucdenver.edu/callen/1202/Landscapes/Fluvial/Fluvial.html).
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Wetland
Identification code: WW
References: Brierley & Fryirs (2005)
Definition
Wetland is a general term to indicate shallow areas including minor depressions that are
occupied by water, including floodplain ponds and swamps (see the sub-types). These
features commonly form where lower order tributaries drain directly onto the floodplain,
or where surface water (from precipitation, flooding, or groundwater seepage) persists
on the floodplain surface. Wetlands are naturally colonised by dense aquatic/wetland
vegetation that can trap fine grained suspended sediments to form cohesive, mud- and
organic-rich floodplain surface deposits.
Equivalent terms: see sub-types
Distinctive characteristics: compared to floodplain lakes, wetlands are smaller and
shallower.
Sub-types
Swamp
Definition
Swamps form on relatively flat surfaces covered by water and usually contain vertically
accreted mud that is usually organic-rich and forms around the swamp vegetation.
Swamps form as a consequence of insufficient drainage of surface water or are fed by
near-surface groundwater. They may include ponds and discontinuous channels or
drainage lines.
Equivalent terms: backswamp or swampy meadow (Brierley & Fryirs, 2005)
Sub-type of wetland: swamp (picture from:
http://www.tulane.edu/~bfleury/envirobio/swamp.html)
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Floodplain pond
Ponds located in floodplain deposits are relatively small, often elongated and scoured
features formed along preferential drainage lines. They are often fed by small
tributaries. They displays a significant, aquatic vegetation cover, even in the deepest
areas (up to 5 m). Compared to floodplain lakes, floodplain ponds are smaller, shallower
and the shore areas is more developed than the limnetic area (which is usually absent).
Sub-type of wetland: floodplain pond (picture from:
http://tangalor.blogspot.it/2010/03/lassenza-di-verita-crea-una-palude.html).
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2.2.3 Macro-unit: human-dominated areas (land use included)
This macro-unit includes the portion of overall floodplain external to the fluvial corridor,
that is dominated by human elements or activities (urbanised areas, infrastructures,
agriculture), i.e. not occupied by relatively natural areas and infrequently interested by
fluvial processes (Fig. B2.5).
The units are basically the same as for the macro-unit 'riparian zone', but they are
classified in this macro-unit when they are dominated by human elements (urban and
industrial areas, infrastructures) or activities (agriculture).
This macro-unit can be absent (e.g. confined streams) or very wide (e.g. in very large
lowland river systems). Its definition is not mandatory for the classification system and
the outer limit can be defined by the operator on the basis of the objectives.
This macro-unit allows, if needed, to include the land use adjacent to the fluvial corridor
in the analysis, as well as to contextualise typical fluvial forms plunged in a human-
dominated matrix, as typically occurs in lowland rivers. For this aim, the following
generic types of land use are distinguished and can be added to the denomination of
each geomorphic unit. For further detailed analysis of land use, see the Corine Land
Cover (CLC) project.
Identification code: H
Figure B2.5 Example of macro-unit 'human-dominated areas' (H).
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Land use types
Agriculture
Definition
It includes all areas dominated by agriculture activities (plantation excluded), where
tree vegetation does not dominate: arable lands, pastures and heterogeneous areas
(i.e. with presence of natural vegetation).
Identification code: Hag
Plantation
Definition
It includes all permanent agriculture areas, i.e. orchards, vineyards, olive tree groves.
This are distinguished from previous category in terms of vertical structure.
Heterogeneous areas are also included (i.e. with presence of natural vegetation), where
tree vegetation dominates.
Identification code: HPi
Urban
Definition
It includes all those areas occupied by human settlements: urban and factory areas,
highways, railways, including artificial vegetated areas (urban parks, resorts, etc.).
Identification code: HPi
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2.3 Artificial features
Artificial features cannot be defined as geomorphic units, but are important elements of
the fluvial landscape, given that they can significantly modify the fluvial processes and
the morphology and assemblage of units. Therefore, a range of artificial features that
should be mapped during the survey of geomorphic units have been defined, and which
may allow to better understand the mosaic of geomorphic units at a given reach. The list
of artificial features reported below is extracted from the indicators of artificiality of the
Morphological Quality Index (MQI, Rinaldi et al. 2013b, 2014).
Identification code: A
Artificial features
Dam
Definition
Structure that creates a reservoir and
induces a significant alteration of flow and
sediment discharges with complete (and
permanent) interception of bedload.
Identification code: AA
Check dam
Definition
In mountain areas are distinguished: (a) retention check dams (on the left) aiming at
intercepting the bedload; in case of great size (> 5-6 m height) it can be considered as
a dam; (b) consolidation check dams (on the right), aiming at stabilizing the channel
bed by reducing the channel slope
Identification code: AB
Retention check dam
Consolidation check dam
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Weir
Definition
In lowland areas, the following types of weirs can be identified: (a) weirs or
consolidation check-dams (on the left), aiming at stabilizing the channel bed and/or at
intercepting the bedload; (b) abstraction weirs (on the right), for water diversion
purposes (e.g. for agriculture), but having significant effect on the bedload. Run-of-the-
river structures used for hydropower generation where little or no water storage is
provided are also included in this category.
Identification code: AC
Weri or consolidation check-dam
Abstraction weir
Retention basin
Definition
Two types are distinguished: (1) lateral
retention basin (located outside of the
channel, delimited by artificial levées and
periodically flooded (picture); (2) instream
retention basin (transversal structure within
the bankfull channel that causes a partial
storage of peak discharges).
Identification code: AD
Diversion or spillway
Definition
Diversion is an in and out-flow channel
which conveys water flow from other water
courses at all flow discharges.
Spillways are specific diversion channels for
flood protection purposes.
Identification code: AE
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Culvert
Definition
It is a structure aiming at crossing the water
channel and located below other structures
(e.g. a road, a town).
Identification code: AF
Ford
Definition
It is a structure aiming at crossing the water
channel that can be submerged at high flow
conditions. It can be associated with culverts
to allow the water flow at low-flow condition.
Identification code: AG
Bridge
Definition
It is a above-ground structure aiming at
crossing the river channel (road, railway,
crosswalk). It can have piles within the
channel.
Identification code: AH
Bed revetment
Definition
It concerns revetements of the channel bed
(and in case of the banks). They can be
formed by large wood, concrete and
unconsolidated coarse material.
Identification code: AI
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Bed sill
Definition
Transverse structure with low height (< 1÷2
m), aiming at stabilizing the channel bed
and at reducing bed erosion.
Identification code: AJ
Ramp
Definition
Transverse structure with low height (< 1÷2
m), aiming at stabilizing the channel bed
and at reducing bed erosion. In general it is
made with boulders arranged longitudinally
along the water channel.
Identification code: AK
Bank protection
Definition
Structure aiming at preventing bank erosion and/or bank mass movement (on the left).
Different techniques and materials can be employed, such as bio-engineering techniques
based on the use of vegetation and geotextile, or rigid structures such as windrows and
trenches, sacks and blocks or gabions and mattresses. In some case the bank can be
completely covered by artificial material (artificial bank; on the right).
Identification code: AL
Bank protection.
Artificial bank.
Artificial levée or embankment
Definition
Longitudinal structure located above-
ground, aiming at protecting against
floods for discharges higher than bankfull
discharge.
Identification code: AM
!
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Mining site / sediment removal
Definition
It includes sites for alluvial sediment mining (commercial purposes; on the left), as well
as sediment removal for channel maintenance or prevention of the flood risk (on the
right).
Identification code: AN
Mining site
Sediment removal
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2.4 Sub-units
Here below a list (not complete) of sub-units that can be identified on the field during
the survey.
2.4.1 Bankfull channel sub-units
Baseflow channel sub-units
Backwater area. Small pocket located at the margins of the baseflow channel, along
the channel shoreline as consequence of local erosion (e.g. between two consecutive
trees) or of the presence of single element of large wood which determines slow flow
condition and backwater effect. It may form also usptream large wood jams (e.g. dam
jams). These areas often represent refugia from flow for several aquatic organisms and
promote the development of aquatic vegetation even in reaches where this is commonly
absent (i.e. because of high flow velocity conditions).
Boulder patch. Small accumulation of boulders (>256 mm in diameter) deposited in
the baseflow channel; the diameter of sediment is significantly different from that of the
unit where the accumulation is. Around the deposit specific local conditions (i.e.
hydraulic and sediment conditions), different form the surrounding areas and relevant in
terms of habitat, may occur.
Cobble patch. Small accumulation of cobbles (64÷256 mm in diameter) deposited in
the baseflow channel; the diameter of sediment is significantly different from that of the
unit where the accumulation is. Around the deposit specific local conditions (i.e.
hydraulic and sediment conditions), different form the surrounding areas and relevant in
terms of habitat, may occur.
Dune. Bed form typical of low-gradient, alluvial sand-bed (>0.1 mm in diameter) rivers.
Dunes can be distinguished from ripples (see later) by their larger height (10-1÷101 m)
and wavelength (proportional to the water depth). Dunes migrate downstream and water
surface is only in part influenced by the presence of dunes. Dunes and ripples are often
associated and superimposed, generating dune-ripple morphology.
Gravel patch. Small accumulation of gravels (2÷64 mm in diameter) deposited in the
baseflow channel; the diameter of sediment is significantly different from that of the unit
where the accumulation is. Around the deposit specific local conditions (i.e. hydraulic
and sediment conditions), different form the surrounding areas and relevant in terms of
habitat, may occur.
Isolated emergent boulder. An individual boulder (> 256 mm in diameter) or a group
of few boulders partially emerged within the baseflow channel, form a sub-unit since
they are significant in terms of physical habitats for aquatic flora and fauna. Around
them specific local conditions (i.e. hydraulic and sediment conditions), different form the
surrounding areas and relevant in terms of habitat, may occur.
Pocket pool. Small pool areas which form between boulders in cascade units, shallow
and with turbulent flow, having smaller size compared to the channel width.
Ripple (Simons & Richardson, 1966; Knighton, 1998). Sub-unit typical of alluvial fine-
grained (i.e. sand) and unconfined, low-gradient channels. Ripples are usually less than
0.04 h high and 0.6 m long, and tend not to interact with the water surface, which is
usually quite even. With active sand transport, ripples migrate downstream. Ripples and
dunes are often associated and superimposed, generating dune-ripple morphology.
Sand patch. Small accumulation of sand (0.06÷2 mm in diameter) deposited in the
baseflow channel; the diameter of sediment is significantly different from that of the unit
where the accumulation is. Around the deposit specific local conditions (i.e. hydraulic
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and sediment conditions), different form the surrounding areas and relevant in terms of
habitat, may occur.
Silt-clay pacth. Small accumulation of silt-clay (<0.06 mm in diameter) deposited in
the baseflow channel; the diameter of sediment is significantly different from that of the
unit where the accumulation is. Around the deposit specific local conditions (i.e.
hydraulic and sediment conditions), different form the surrounding areas and relevant in
terms of habitat, may occur.
Transverse rib (Lenzi et al., 2000). Group of cobbles or boulders organized in lines
across the baseflow channel width, protruding from the flow at low to medium stages.
Transverse ribs generally constitute a portion of rapid.
Emergent sub-units
Boulder patch. Small accumulation of boulders (>256 mm in diameter) deposited on a
bar or other emergent unit (both in the bankfull or on the floodplain); the diameter of
sediment is significantly different from that of the unit where the accumulation is.
Around the deposit specific local conditions (i.e. sediment and moisture conditions),
different form the surrounding areas and relevant in terms of habitat, may occur.
Cobble pacth. Small accumulation of cobbles (64÷256 mm in diameter) deposited on a
bar or other emergent unit (both in the bankfull or on the floodplain); the diameter of
sediment is significantly different from that of the unit where the accumulation is.
Around the deposit specific local conditions (i.e. sediment and moisture conditions),
different form the surrounding areas and relevant in terms of habitat, may occur.
Gravel patch. Small accumulation of gravels (2÷64 mm in diameter) deposited on a bar
or other emergent unit (both in the bankfull or on the floodplain); the diameter of
sediment is significantly different from that of the unit where the accumulation is.
Around the deposit specific local conditions (i.e. sediment and moisture conditions),
different form the surrounding areas and relevant in terms of habitat, may occur.
Ramp (Brierley & Fryirs, 2005). Coarse sediment deposit which forms at the upstream
end of a bend, raising from the baseflow channel and deposited on the bar surface like a
ramp. In some case it represents the sediment filling of a chute cut-off channel.
Ridge (Brierley & Fryirs, 2005). Rising, elongated and arcuate or near-straight deposit
located at the bar top (bank-attached or mid-channel bars). The sediment tends to be
finer downstream. Ridges may form as consequence of the presence of vegetation or
other blocking structures on the bar surface.
Sand patch. Small accumulation of sand (0.0625÷2 mm in diameter) deposited on a
bar or other emergent unit (both in the bankfull or on the floodplain); the diameter of
sediment is significantly different from that of the unit where the accumulation is.
Around the deposit specific local conditions (i.e. sediment and moisture conditions),
different form the surrounding areas and relevant in terms of habitat, may occur.
Scour hole. Local sediment erosion on the bar surface (bank-attached or mid-channel
bar) as consequence of a flood event.
Scroll bar (Brierley & Fryirs, 2005; Nanson, 1980, 1981). Elongated ridge-like bar
formed along convex banks of meander bends, commonly on point bars. They are
caused by deposition in the shear zone between the helical flow cell in the thalweg zone
and flow in a separation zone adjacent to the convex bank of a bend, often cored by
trees deposited on point bars during floods.
Silt-clay patch. Small accumulation of silt-clay (<0.0625 mm in diameter) deposited on
a bar or other emergent unit (both in the bankfull or on the floodplain); the diameter of
sediment is significantly different from that of the unit where the accumulation is.
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Around the deposit specific local conditions (i.e. sediment and moisture conditions),
different form the surrounding areas and relevant in terms of habitat, may occur.
Instream vegetation sub-units
Isolated woody plants. Group of 1 to 3 trees or shrubs located within the bankfull
channel, that cannot be classificed as unit (< 3 individuals).
Pioneer island. Sub-unit typical of large gravel-bed rivers, formed by shrubs or trees
(of whatever any height), covering a small area (approximately < 102), featuring little to
no fine sediment (sand) deposition. Three of more woody plants should be present to
classify the sub-unit as pioneer island (see the definition of island unit), otherwise it is
classified as 'isolated woody plant' (see the definition below). Pioneer islands often
originate by resprouting of wood elements, but this is not a condition for their
identification. However, large wood jams are quite commonly associated with pioneer
islands.
Seedling-induced levee (Gurnell et al., 2014b). It is a characteristic type of vegetated
ridge, which forms as consequence of the presence of seedlings at an elevation that is
sufficiently low on the bar for the seedlings to have a sufficient moisture supply but high
enough to avoid uprooting of the seedlings by flow pulses. The sediment is trapped as
the seedlings grow to form a ridge-like feature which may evolve into an island. It is
typical of large alluvial rivers and has small size (< 102). Compared to a pioneer island,
this sub-unit is characterised by fine sediment (sand).
Small aquatic vegetation patch. Patch of aquatic vegetation smaller than
approximately 10 m2 in area.
Small herbaceous vegetation patch. Patch of terrestrial herbaceous vegetation
smaller than approximately 10 m2 in area.
Small large wood accumulation. It concerns the large wood accumulations or
individual pieces of large wood that cannot be classified as unit because of the small size
(< 10 m2).
Vegetated ridge. Compared to a ridge it is characterised by the presence of vegetation
(herbs, shrubs or trees). It can't be classified as unit because of the small size.
2.4.2 Floodplain sub-units
Vegetated patch. Patch of vegetation (herbs, isolated woody plants, wood
accumulations, aquatic vegetation) of whatever size, located on the floodplain.
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Appendix 1: Survey and
classification form
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Appendix 2: Geomorphic units
and macro-units list
Spatial setting
Macro-unit
Unit (type)
Unit sub-type
Bankfull
channel
(‘submerged’
units)
Baseflow or
submerged
channels
(C/S)
Pothole (CH)
Cascade (CC)
Rapid (CR)
Riffle (CF)
Forced riffle
Step (CT)
Rock step
Waterfall
Boulder step
Log step
Glide (CG)
Rock glide
Pool (CP)
Forced pool
Scour pool
Plunge pool
Dammed pool
Meander pool
Dune system (CD)
Bankfull
channel
(‘emergent’
units)
Emergent
sediment
units (E)
Bank-attached bar (EA)
Side bar
Point bar
Counterpoint bar
Junction bar
Forced bank-attached bar
Mid-channel bar (EC)
Longitudinal bar
Transverse bar
Diagonal bar
Medial bar
Bedrock core bar
Forced mid-channel bar
Bank attached high-bar (EAh)
Mid-channel high-bar (ECh)
Bank-attached boulder berm (EB)
Mid-channel boulder berm (EM)
Dry channel (ED)
Bedrock outcrop (EO)
Unvegetated bank (EK)
In-channel
vegetation
(V)
Island (VI)
Grassy island
Young woody island
Established/Adult woody
island
Mature woody island
Complex woody island
Spatial setting
Macro-unit
Macro-unit type
Macro-unit sub-type
Bankfull
channel
(‘submerged’
units)
Baseflow or
submerged
channels (C/S)
Baseflow channel or main
channel (C)
Secondary channel (within
bankfull) (S)
Chute cut-off
Two-way connected branch
One-way connected branch
Pond
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In-channel
vegetation
(V)
Large wood jam (VJ)
Meander jam
Bench jam
Bar apex jam
Bar top jam
Dam jam
Bank input jam
Flow deflection jam
Landslide jam
Vegetation-trapped jam
Aquatic vegetation (VA)
Floating leaves
Submerged leaves
Emergent leaves
Bench (VB)
Submerged shelf
Berm
Bench (sensu stricto)
Ledge
Point bench
Concave bank bench
Shelf
Slump bench
Ice abrasion and ice
ploughing bench
Vegetated bank (VK)
Floodplain
Riparian
zone (F)/
Human
dominated
areas (H)
Modern floodplain (FF/HF)
Recent terrace (FT/HT)
Scarp (FS/HS)
Levee (FL/HL)
Overbank deposits (FD/HD)
Crevasse splay
Sand wedge
Sand sheet
Ridges and swales (FR/HR)
Floodplain island (FI/HI)
Terrace island (FN/HN)
Secondary channel (FC/HC)
Flood channel
Abandoned channel
Abandoned meander
Floodplain
aquatic
zones (W/H)
Floodplain lake (WO/HO)
Oxbow lake
Wetland (WW/HW)
Swamp
Floodplain ponds
Spatial setting
“Macro-units”
Feature types
Floodplain
Human dominated areas (H)
Agriculture (HAg)
Plantation (HPl)
Urban (HUr)
All
Artificial features (A)
Dam (AA)
Check-dam (AB)
Weir A(C)
Retention basin (AD)
Diversion or spillway (AE)
Culvert (AF)
Ford (AG)
Bridge (AH)
Bed revetment (AI)
Bed sill (AJ)
Ramp (AK)
Bank protection (AL)
Artificial levee or embankment (AM)
Mining sites / Sediment removal (AN)
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Appendix 3: Glossary
Alluvial channel
It is a channel which is modelled within its alluvial sediment, previously transported and
deposited. The layer of alluvial sediment is continuous and thick. In case of significant
presence of bedrock or coarser sediment (e.g. large boulders) the channel is classified as
semi-alluvial. Alluvial channels are typical of lowland reaches but are also common in
mountain-hilly areas. In the latter case banks can be formed by bedrock. In case of
single-thread channel, several morphologies of bed configuration can be observed,
depending on the bed slope and the bed sediment size (see bed configuration units).
Armouring
Where the river bed surface is comprised of coarser particles than the underlying river
bed layers as a result of removal (mobilisation and transport) of the finer particles from
the bed surface layer. In gravel- and cobble-bed rivers a certain degree of armouring is
common. In case of strong degree of bed armouring, it can be related to local channel
alterations that cause a water flow transport capacity higher than the sediment supply.
Bankfull channel
It includes the water channel network, the bars and islands. Its limits coincide with
banks, but often are difficult to be identified, as in case the transition between the
bankfull channel and the floodplain is vague. The bankfull limits are thus identified with
the bankfull stage (or level) (see the definition below).
Bankfull discharge
It is the discharge or river flow that fills the river channel up to the bankfull level. The
frequency of bankfull discharge is usually 1 to 3 years. For rivers in dynamic equilibrium
the bankfull discharge corresponds to the formative or dominant discharge, i.e. at which
changes in channel forms and dimensions occur.
Bankfull stage (or level)
The bankfull stage determines the limit of the bankfull channel, and corresponds to the
flow stage at which water starts to spill out of the channel (on one or both banks) onto
the surrounding floodplain. It corresponds to the bankfull discharge (see the definition
above). The identification on the field of the bankfull level is rather difficult (e.g. in case
of incised rivers).
Baseflow channel
In its broad meaning, it corresponds to the part of the bankfull channel which conveys
mean annual flow (and lower).
Bedrock channel
It is characterised by the absence of alluvial sediment, because the high flow energy is
able to carry downstream all the material coming from the hillslopes. However some
alluvial material can be stored within pools or downstream blocking structures. It is
typical of mountain-hilly areas.
Boulders
Sediment particles having a diameter >256 mm.
Clay
Sediment particles having a diameter <0.002 mm.
Cobbles
Sediment particles having a diameter of 64÷256 mm.
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Clogging (or embeddedness)
The infiltration of fine sediment particles (mainly silt and clay) into the gaps between the
larger sediment particles of a river bed.
Colluvial channel
Colluvial reaches are incised within colluvial material. They are common in low order
reaches (first order), are small in size with steep slopes, and the bedload transport is
intermittent and impulsive (debris flow). They can be related to gullies. The channel is
poorly organised in geomorphic units.
Confined channel
A river without floodplain, where more than 90% of the river banks are directly in
contact with hillslopes, ancient terraces, landslides, tributaries' alluvial fans or glacial
deposits. The floodplain is limited to some isolated pockets (< 10% bank length). It is
typical of mountain and hilly areas, or locally in lowplain areas (e.g. in presence of
separation zones between catchments).
Flow type
Above-water spatial unit formed by the interaction between local hydraulic and sediment
conditions which produces a series of distinc flow patterns at the flow surface. Different
flow types are distinguished: free fall, chute, broken standing waves, unbroken standing
waves, rippled, upwelling, smooth, no perceptible flow.
Fluvial or river corridor
Near-natural area of land including the fluvial geomorphic units that are directly (or
more frequently) concerned by fluvial processes. It is usually delimited by near-natural
vegetation (i.e. it includes the bankfull channel and floodplain units). In some case it
corresponds to the entire floodplain.
Geomorphic unit
Area containing a landform (e.g. bar, riffle, floodplain) created by erosion and/or
deposition inside (bankfull channel geomorphic unit) or outside (floodplain geomorphic
unit) the river channel. Some geomorphic features are formed in association with living
and dead (e.g. large wood) vegetation (also named biogeomorphic units).
GIS (Geographic Information System)
It is a computerized informatic system (software) that allows the collection, entry,
analysis, visualisation and return of information coming from georeferenced geographic
data.
Gravel
Sediment particles having a diameter 2÷64 mm.
Hydraulic unit
Spatially distinct patch of relatively homogeneous surface flow and substrate character.
It can include several single river elements or small groups of sediment, plants, wood
elements, etc. A single geomorphic unit can include from one to several hydraulic units.
Large river
A river whose width is significantly greater than the bed sediment size and that is
completely laterally unconstrained. In general it concerns lowland unconfined rivers,
larger than 30 m and with bankfull discharge at least 20÷50 m3/s.
Macrohabitat
A generic zone where a given species lives, and that is defined on the basis of
geomorphic, hydrologic and climatic conditions observed at the reach or sub-catchment
scale, about 10 m in size (Heggenes & Wollebaek, 2013).
Mesohabitat
Eco-hydraulic characteristics at the reach scale in terms of habitat types, about some
meter to 10 m in size (Heggenes & Wollebaek, 2013).
D6.2 Methods for HyMo Assessment
Part 4. Geomorphic Unit Survey
Page 133 of 133
Microhabitat
Small areas within a mesohabitat, about 10 cm in size (Heggenes & Wollebaek, 2013).
Partly-confined channel
A river with a discontinuous floodplain, i.e. river banks are in contact with the floodplain
for between 10 and 90% of their total length. It is common in pedmont areas, alpine
valleys, or in separation areas between river catchments.
Sand
Sediment particles having a diameter 0.0625÷2 mm.
Silt
Sediment particles having a diameter 0.002÷0.0625 mm.
Small river
A river with coarse bed sediment and width 1 to 10 times the bed sediment particles
(usually ≤ 30 m). In general it concerns mountain confined rivers.
Unconfined channel
A river with continuous floodplain, where less than 10% of the river bank length is in
contact with hillslopes or ancient terraces, and the river has no lateral constraints to its
mobility. It is typical of lowplain areas, but it can be observed also in mountain and hilly
areas (e.g. the case of glacial valleys or recent alluvial fans).
