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Morphological Characterisation of the Severn Estuary and Solway Firth

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David Brew at Royal HaskoningDHV, Peterborough, United Kingdom
David Brew
  • Royal HaskoningDHV, Peterborough, United Kingdom
Stuart Dawks
Stuart Dawks
Stuart Dawks
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Morphological C
haracterisation of the Severn
Estuary and Solway Firth
Natural England
12 May 2015
Final Report
PB2693
A company of Royal Haskoni
ngDHV
A company of Royal Haskoni
ngDHV
Document title
Morphological Characterisation of the Severn
Estuary and Solway Firth
Document short title
Estuary Characterisation
Status
Final Report
Date
12 May 2015
Project name
Estuary Characterisation: Solway and Severn
Project number
PB2693
Client
Natural England
Reference
PB2693/R/303996/PBor
Rightwell House
Bretton
Peterborough PE3 8DW
United Kingdom
+44 1733 334455 Telephone
01733262243 Fax
info@peterborough.royalhaskoning.com E-mail
www.royalhaskoningdhv.com Internet
Drafted by
David Brew and Stuart Dawks
Checked by
Nick Cooper
Date/initials check
12/05/2015
Approved by
Nick Cooper
Date/initials approval
12/05/2015
Estuary Characterisation - i - PB2693/R/303996/PBor
Final Report 12 May 2015
SUMMARY
The Severn Estuary and Solway Firth are large estuary systems that support a variety of
habitats designated within Special Areas of Conservation (SAC). These estuarine SACs
are subject to a number of pressures including coastal squeeze and development, which
could affect the whole-estuary condition. The challenge for this study was to
characterise the landscape-scale functioning and degree of morphological equilibrium of
the estuaries to support judgements about their condition (health), in accordance with
Common Standards Monitoring (CSM) guidance of JNCC.
To define the condition of an estuary as favourable means that the special features of
the designated areas are in a healthy state and are being conserved for the future by
appropriate management. In order for this condition to be maintained over the long term,
there must be confidence that the estuary can sustain adequate habitat of the
appropriate quality, within an overall morphological equilibrium.
Morphological equilibrium in the Severn Estuary and Solway Firth was analysed using
Regime Theory, which defines empirical relationships between estuary tidal prism and
cross-sectional area. Equilibrium in these estuaries is seen as a dynamic state in which
constant adjustments take place to their overall morphology so they are able to function
effectively. The observed form of the estuary was compared to the predicted equilibrium
form to determine how far from equilibrium each estuary is. Integration of natural
(geological) and human-induced constraints then allowed an appraisal of reasons for
disequilibrium.
The critical data upon which the Regime Theory method used in this project relies are
bathymetry and tidal datum elevations. In this study, a limited number of bathymetry
datasets were received covering different parts of the estuaries, including Admiralty
Chart data, LiDAR and multibeam echosounder. These datasets were evaluated and
those that were considered to best represent the current bathymetry were integrated
and used in the analyses. The data was quality assured to check for gaps and
inconsistencies which were filled and rectified as appropriate.
The results for the Severn Estuary and Solway Firth are generally similar. Both estuaries
are under-sized compared to their predicted forms in their inner parts; the observed
channels are narrower than predicted for the present-day tidal regimes. This means that
to obtain an equilibrium form they have to widen from their current forms. They should
erode by loss of intertidal habitat because in both estuaries the high water mark is
constrained by coastal defences which do not allow it to migrate landwards. In contrast,
both outer estuaries are over-sized compared to their predicted forms; the observed
channels are wider than predicted for the present-day tidal regimes. Here, they should
accrete and develop further intertidal habitat by natural processes. The central parts of
the estuaries are tending towards equilibrium whereby their observed and predicted
widths are similar.
In addition to considering the key requirements for a baseline measure of morphological
equilibrium, a monitoring strategy was developed so that these requirements can be met
through future condition assessments. The strategy includes recommendations for
monitoring of bathymetry and changes in position of morphological boundaries,
particularly between mudflat and saltmarsh, and protocols for data management.
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Final Report 12 May 2015
CONTENTS
Page
1
INTRODUCTION 1
1.1
Objectives 1
2
METHODS 4
2.1
Extent of Study Areas and SACs 4
2.2
Bathymetry 6
2.3
Tidal Regime 6
2.4
Estuary Extent and Area of Intertidal Habitat 7
2.5
Morphological Equilibrium 7
3
RESULTS FOR THE SEVERN ESTUARY 9
3.1
Bathymetry 9
3.2
Tidal Regime 11
3.3
Extent of Estuary and Area of Intertidal Habitat 13
3.4
Morphological Equilibrium 15
3.5
Physical Constraints to Morphological Equilibrium 18
4
RESULTS FOR THE SOLWAY FIRTH 21
4.1
Bathymetry 21
4.2
Tidal Regime 25
4.3
Extent of Estuary and Area of Intertidal Habitat 27
4.4
Morphological Equilibrium 29
4.5
Physical Constraints to Morphological Equilibrium 32
5
MORPHOLOGICAL MONITORING STRATEGY 35
5.1
Bathymetry in the Severn Estuary and Solway Firth 35
5.2
Saltmarsh and Mudflat 37
5.3
Data Management 38
6
CONCLUSION 40
7
REFERENCES 41
Appendix A: Regime Theory and its Application to the Severn Estuary and Solway Firth
Appendix B: List of datasets received and used in this study
Appendix C: Observed form of the Severn Estuary at each section
Appendix D: Predicted equilibrium form of the Severn Estuary at each section
Appendix E: Observed form of the Solway Firth at each section
Appendix F: Predicted equilibrium form of the Solway Firth at each section
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1 INTRODUCTION
An assessment of the condition of the interest features and attributes of an estuary
needs to take account of the relationship between its broad-scale physical form and
function. Local measurements of physical parameters, such as signs of erosion or
accretion, aid the condition assessment of each feature attribute, but they should be
viewed within the context of the broader-scale estuary processes that are contributing to
change. This is particularly so for the Severn Estuary and Solway Firth, which are large
and dynamic systems and potentially subject to longer-term fluctuations in morphology,
reflecting estuary evolution processes as well as responses to past or present human
interventions.
Both the Severn Estuary and Solway Firth support a variety of habitats including subtidal
sand banks and intertidal sandflats, mudflats and saltmarsh, which are designated within
Special Areas of Conservation (SAC). These estuarine SACs are subject to a number of
pressures including coastal squeeze due to coastal erosion and/or sea-level rise and
development such as coast protection, ports and marinas (land claim and/or dredging).
These pressures can affect the whole-estuary condition.
1.1 Objectives
This report describes a desk study characterisation of the Severn Estuary and Solway
Firth to support judgements about estuary condition. Existing data was used to
characterise the two estuaries under five headline parameters:
• bathymetry;
• tidal regime;
• extent of estuary;
• area of intertidal habitat; and
• most importantly for the objectives of this project, (whole-estuary) morphological
equilibrium.
The key requirements for a baseline measure of morphological equilibrium were
considered using an approach aligned with that used for the Healthy Estuaries 2020
project (Natural England, 2015). Recommendations for a monitoring strategy have also
been made in the study so that these requirements can be met through future condition
assessments.
The main stages of this study in support of an assessment of morphological equilibrium
in the Severn Estuary and Solway Firth were:
• collate the essential data which are bathymetry up to the foot of flood
embankments or mean high water spring (MHWS) if no defences are present,
and tidal datums (MHWS, mean high water neap - MHWN, mean low water
spring - MLWS);
• develop a series of cross sections from the upper estuary to the estuary mouth
and measure the current form and predict the equilibrium form of the estuaries at
each section;
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• identify any natural (geological) and human-induced constraints to estuary form;
and
• provide a preliminary assessment of the condition of the morphological
equilibrium attribute, in accordance with Common Standards Monitoring (CSM)
guidance (JNCC, 2004) (Table 1.1).
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2 METHODS
Existing data have been used to characterise the key baseline geomorphological
parameters of the Severn Estuary SAC and Solway Firth SAC. These are bathymetry,
tidal regime, extent of estuary and area of intertidal and subtidal habitats. Other
parameters were then derived from these basic elements in order to determine how
close to morphological equilibrium each estuary is (see Section 2.5). These derived
components include spring tidal prism (the volume of water that enters and leaves the
estuary at a defined section during a spring tide), cross-sectional area and width.
2.1 Extent of Study Areas and SACs
2.1.1 Severn Estuary
The Severn Estuary is the largest coastal plain estuary in the United Kingdom and one
of the largest estuaries in Europe. The overall area of the European and international
conservation designations is about 740km
2
of which roughly two thirds is composed of
subtidal habitats (sandbanks and mobile gravel, sand and mud) and one third is
composed of intertidal habitats (mud and sand, saltmarsh and rocky shores). The
estuary has been a focus for human activity; a location for settlement, a source of food,
water and raw materials and a gateway for trade and exploration. The estuary and its
coastal hinterland support the cities of Cardiff, Bristol, Newport and Gloucester. Today,
major industries are sited along the estuary’s shores, including port installations,
chemical processing plants and nuclear power stations. Aggregate extraction also
occurs within the estuary.
Alongside all these competing activities, the estuary also supports a wide array of
habitats and species of international importance for nature conservation.
http://jncc.defra.gov.uk/protectedsites/sacselection/sac.asp?EUCode=UK0013030
Human activity has increasingly influenced the character of the marginal mudflats and
saltmarshes, with extensive land claim occurring since the Roman period. The
morphology of the estuary is constantly changing due to the complex hydrodynamics.
The Severn Estuary CHaMP (ABPmer, 2006) predicts losses of intertidal flats and
saltmarsh habitats over the next 100 years in response to rising sea-level against fixed
sea defences along much of the shoreline.
The defined boundary of the Severn Estuary SAC occupies the water course from Awre
downstream to a line across the estuary between Lavernock and Lilstock. However, for
the purposes of this analysis, the study area has been extended outside the bounds of
the SAC to understand if there are wider implications of disequilibrium within the SAC,
and how it could be potentially mitigated outside the SAC. Hence, the Severn Estuary is
defined from Longney, about 15km upstream of Awre, downstream to a line between
Aberthaw and Chapel Cleeve, about 15km downstream of the SAC boundary (Figure
2.1).
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Figure 2.1. Severn Estuary study area and SAC boundary
2.1.2 Solway Firth
The Solway Firth is a large shallow estuary formed by a variety of historical physical
influences including glaciation, river erosion, sea-level change and geological controls. It
is one of the least industrialised and most natural estuary systems in Europe. Located
on the west coast of the United Kingdom, it straddles the border between England and
Scotland, forming an extensive system draining into the Irish Sea. The estuary supports
extensive areas of saltmarsh, both pioneer and Atlantic salt meadow, as well as large
areas of intertidal mudflats, reefs and sandflats, and subtidal sandbanks, each of which
are of international importance in their own right.
http://jncc.defra.gov.uk/protectedsites/sacselection/sac.asp?EUCode=UK0013025
The defined boundary of the Solway Firth SAC occupies the water course from around
Gretna downstream to a line across the estuary between Sandyhills and Mawbray.
However, for the purposes of this analysis, the Solway Firth is extended about 15km
beyond the seaward boundary of the SAC to a line across the estuary between Barlocco
Bay and Siddick (Figure 2.2). The study area has been enlarged beyond the SAC
boundary for the same reasons as the Severn Estuary.
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Figure 2.2. Solway Firth study area and SAC boundary
2.2 Bathymetry
Digital bathymetries were compiled from various sources, collected using several
different methods. The best available bathymetry data for each estuary was compiled,
as far as possible (described in Sections 3.1 and 4.1 for the Severn Estuary and Solway
Firth, respectively). The bathymetry data covers all intertidal and subtidal areas up to the
seaward face of the front-line defences or up to the MHWS datum where the coastal
plain rises naturally into the hinterland, and stretches from as close to the upstream tidal
limit(s) as possible to the defined downstream boundaries (Figures 2.1 and 2.2). The
bathymetries in both the Severn Estuary and Solway Firth have been composited from
two or more surveys that cover different parts of the estuaries. The bathymetry data was
quality assured to check for gaps and inconsistencies which have been filled and
rectified, accordingly.
2.3 Tidal Regime
Although there are several methods available to determine the tidal regime in an
estuary, the simple use of the predicted tidal levels published in the 2015 UK Admiralty
Tide Tables is opted for here, in line with the Healthy Estuaries 2020 approach (Natural
England, 2015). The tidal datums were used as a characterisation tool in their own right,
but were also used along with bathymetry to calculate tidal prism and cross-sectional
area for the morphological equilibrium analysis (see Section 2.5). The critical tidal
datums for the analysis were MHWS, MHWN and MLWS.
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2.4 Estuary Extent and Area of Intertidal Habitat
MHWS was adopted as the datum to define estuary extent, rather than highest
astronomical tide. This is because MHWS is a predicted datum (2015 Admiralty Tide
Tables) at numerous points along the estuaries and so can be more readily and
accurately mapped. In addition, highest astronomical tide only occurs very infrequently
and is not the datum that truly represents the upper limit of intertidal habitat within an
estuary.
The area of intertidal habitat has been mapped between the MHWS and MLWS datums.
The original intention (in the original scope) was to screen existing data, to obtain two
specific datasets; one that has mapped intertidal areas at or close to the time of
designation (2005 for the Solway Firth and 2009 for the Severn Estuary) and one that is
as recent as possible. This has not been possible to achieve because baseline surveys
are not available (as far as we know) at the time of designation. Hence, due to the lack
of consistent available data, it is only possible to determine the extent of intertidal habitat
based on merged bathymetry data collected from different years. A time series of
intertidal habitat change has not been achieved.
An estimate of saltmarsh change along the southern shore of the Severn Estuary
Special Protection Area from 1995 (date of designation) to 2004 was carried out by
Natural England (2006). They estimated an increase of about 0.2km
2
over the nine year
period.
2.5 Morphological Equilibrium
The overall condition of an estuary can be founded on the relationship between its
physical form and function (to accommodate an energy exchange by redistributing water
and sediment), and so the estuary should be in dynamic equilibrium with natural wave,
tidal and sediment transport processes. Condition can thus be explained by overall
morphology of which the most easily measured attribute is planform (the outline of the
estuary as seen from above).
The best way to determine how far the estuary system is from the equilibrium state is
through morphological methods, which measure the long-term response of an estuary to
natural changes in forcing, and also account to a varying degree, for changes in
morphology following human interference such as land claim, engineering works or
dredging. One of the most commonly used methods is Regime Theory (adopted by
Healthy Estuaries 2020, Natural England, 2015), which uses empirical relationships
between estuary gross morphology and tidal prism, through simple power-law
equations. Indeed, the morphological equilibrium of an estuary as defined by the CSM
guidance for estuaries and coastal saltmarsh (JNCC, 2004) is the relationship between
cross-sectional area and tidal prism at the estuary mouth.
Over time, an estuary will have had its dynamic equilibrium morphology changed in
some way by human interference and different parts of its form are likely to be at
different stages of adjustment to natural process inputs. Hence, an estuary will seek to
reach a steady state over the long term by oscillating around theoretical equilibrium
morphologies over the short term to medium term. The width and depth of the estuary
will therefore change over time towards a state of dynamic equilibrium or ‘most probable
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state’. Regime Theory predicts the equilibrium width of an estuary, which when
compared with the observed width can be used to determine, at a high level, how far an
estuary is from an equilibrium form. How close an estuary is to morphological
equilibrium defines the condition of this attribute.
Regime Theory was applied to the Severn Estuary and Solway Firth through use of GIS
and Excel spreadsheet platforms, which allow step-by-step data input and calculations
developed by Healthy Estuaries 2020 (Natural England, 2015). The method relies on
bathymetry data (Section 2.2) and tidal datum data (Section 2.3) as inputs into the GIS.
Details of the principles of Regime Theory and the specifics of how the methodology is
used here are provided in Appendix A.
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3 RESULTS FOR THE SEVERN ESTUARY
3.1 Bathymetry
The best available bathymetry for the Severn Estuary was obtained in two different
formats from the Environment Agency Geomatics Group. These were LiDAR at 2m
resolution and Admiralty Chart data in vector format. These were the only two datasets
used in the analysis (a list of all bathymetry data that was received is provided in
Appendix B). Both datasets required processing and manipulation before being ‘stitched’
together to create the final bathymetry. The following procedure was followed:
• The LiDAR data, in Ordnance Datum (OD), was processed from single ASCII
files into a mosaicked dataset covering the shallower parts of the Severn
Estuary (Figure 3.1).
Figure 3.1. LiDAR data in the Severn Estuary
• The Admiralty Chart data was converted into a raster (Figure 3.2) and clipped to
the study area extent. These data were converted from Chart Datum (CD) to OD
using the conversion factors in the 2015 Admiralty Tide Tables.
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Figure 3.2. Admiralty Chart bathymetry data in the Severn Estuary
• The Admiralty Chart bathymetry and LiDAR data elevations were compared
along the length of the estuary. The LiDAR data was judged to be more accurate
in the shallower areas upstream of the M4 road bridge and was used in this part
of the estuary. Subsequently the Admiralty bathymetry was removed upstream
of the M4 road bridge.
• Admiralty Chart data was used downstream of the M4 road bridge as it provided
a wider coverage than the LiDAR data. However, where appropriate the
Admiralty data was substituted by LiDAR data to provide greater accuracy and
resolution. If the elevation difference between the two datasets was too great
(greater than 1m) it was considered that the Admiralty Chart data should be
used as the source to avoid large ‘steps’ in the overall data.
• The resulting Severn Estuary bathymetry, combining Admiralty Chart and LiDAR
data, used in our analysis is shown in Figure 3.3.
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Figure 3.3. Severn Estuary bathymetry created by combining Admiralty Chart data
with LiDAR data (in OD)
3.2 Tidal Regime
In order to calculate the spring tidal prism and cross-sectional area of the Severn
Estuary it is necessary to know the elevations of tidal datums. Table 3.1 presents the
MHWS, MHWN and MLWS tidal datum elevations at tidal stations along the Severn
Estuary.
In order to delineate the plan positions of these datums, their elevations were overlain
on to the Severn Estuary bathymetry. The elevations of the datums change with
distance upstream (Table 3.1) and to create a surface that represents them along the
estuary, the individual datum heights at each tidal station were linearly interpolated.
Figure 3.4 shows the tidal datum surfaces after they have been transposed on to the
bathymetry of the Severn Estuary. Note that upstream of Sharpness in the upper
estuary, there is no data in the Admiralty Tide Tables for datums lower than MHWS.
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Figure 3.4. Tidal datums in the Severn Estuary
3.3 Extent of Estuary and Area of Intertidal Habitat
The extent of the estuary (total area of estuary feature) was mapped using MHWS. The
intertidal area was calculated by subtracting the plan area at MLWS from the plan area
at MHWS (Figure 3.5). The area where saltmarsh could potentially form was mapped by
subtracting the plan area at MHWN from the plan area at MHWS (Figure 3.6). These
areas, which apply to the entire study area chosen (not the SAC specifically), are
presented in Table 3.2.
Table 3.2. Planform extent of the Severn Estuary and its intertidal and subtidal
areas. Note these extents are for the entire study area, not just the SAC. Also note
the upper reaches of the SAC (upstream of Sharpness) have no area data
Parameter Area (km
2
)
Estuary extent below MHWS 1,050
Intertidal area between MHWS and MLWS 250
Subtidal area below MLWS 800
Potential saltmarsh area between MHWS and MHWN 25
Due to the absence of data for MHWN and MLWS upstream of Sharpness, this reach of
the estuary is excluded from the intertidal and subtidal analysis. This reach is also
excluded from the estuary extent calculation for consistency (even though MHWS data
is available).
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Figure 3.5. Area of intertidal habitat in the Severn Estuary (area between MHWS
and MLWS datums)
Figure 3.6. Intertidal area in the Severn Estuary where saltmarsh could potentially
develop if the substrate was suitable for development of this type of habitat (area
between MHWS and MHWN datums)
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The actual area of saltmarsh in the Severn Estuary (as defined in this study) was
calculated using the saltmarsh polygons of the Land Classification Mapping (LCM)
carried out in 2007. The LCM estimated area of intertidal saltmarsh was 7.5km
2
(Figure
3.7). The NVC survey of saltmarsh habitat completed in 1998 reported a total area of
about 15km
2
within the SAC (Dargie, 1998).
Figure 3.7. Saltmarsh in the Severn Estuary (2007 Land Classification Mapping)
3.4 Morphological Equilibrium
3.4.1 Observed Estuary Form
Using the bathymetry and tidal datums in a GIS, each of the following parameters were
measured at sections spaced about 2km apart along the estuary to quantify its observed
form:
• cross-sectional area beneath MHWN;
• width at MHWN;
• mean depth beneath MHWN; and
• spring tidal prism upstream of each section.
The locations of the sections where the observed form is measured are shown in Figure
3.8 and the data at each section is presented in Appendix C. Note that observed
parameters are absent upstream of Section 1-280 (upstream of Sharpness) because no
data for MHWN or MLWS were available.
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Figure 3.8. Position of the sections in the Severn Estuary where the observed
form is measured
3.4.2 Predicted Estuary Form
The regime relationship that was used to predict estuary form is between spring tidal
prism and the cross-sectional area at MHWN tide at each of the sections defined in the
assessment of observed form (in line with Healthy Estuaries 2020, Natural England,
2015) (Appendix A, Section A.1). Two steps developed in Healthy Estuaries 2020 were
followed to determine morphological equilibrium. Details of these steps are provided in
Appendix A and they are only briefly summarised here.
The first step was to predict cross-sectional area from the re-distributed tidal prism
(Appendix A, Section A.2.3). The regime equation that encapsulates all United Kingdom
estuaries was used.
CSA = 0.024.P
0.71
(r
2
= 0.75)
where:
CSA = cross-sectional area (MHWN); and
P = upstream spring tidal prism.
The second step was to calculate planform width from cross-sectional area. Several
different methods were tested in Healthy Estuaries 2020 to develop a robust way of
estimating planform width from cross-sectional area. It was concluded that the most
reliable was the ‘constant evolution’ method (Appendix A, Section A.2.5), and this was
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adopted here. Using these two steps, the equilibrium form of the Severn Estuary was
predicted at each section, presented in Appendix D.
3.4.3 Comparison of Predicted Equilibrium Widths with Observed Widths
The results were interrogated using GIS to compare the predicted equilibrium widths
(Appendix D) with the observed widths (Appendix B) at each section. In this way,
reaches of the observed estuary which are narrower or wider than their predicted form
were mapped. The comparison for the Severn Estuary is shown in Figure 3.9.
Figure 3.9. Observed and predicted forms of the Severn Estuary using the regime
equation for all United Kingdom estuaries and the ‘constant evolution’ method
The observed widths compare with the predicted equilibrium widths in the Severn
Estuary in one of three ways. The estuary upstream of the M4 road bridge is under-
sized compared to its predicted form (i.e. the observed channel is narrower than
predicted for the present-day tidal regime) (Figure 3.9). The estuary downstream of a
line between Goldcliff and Clevedon is over-sized compared to its predicted form (i.e.
the observed channel is wider than predicted for the present-day tidal regime). The
estuary between the M4 road bridge and a line between Goldcliff and Clevedon has
observed and predicted widths which are similar, suggesting that the observed form is
close to equilibrium.
The reaches of the estuary upstream of the M4 road bridge are pressure points in the
estuary. This means that here the estuary form should be wider than it actually is and to
obtain equilibrium the estuary has to widen from its current form (i.e. it should erode
resulting in loss of intertidal habitat if the high water mark is unable to migrate
landwards). Future sea-level rise will exacerbate this trend for erosion. Where the
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channel is over-sized, downstream of a line between Goldcliff and Clevedon, it exceeds
its predicted equilibrium width and over the long term there should be a tendency for
development of intertidal habitat by natural processes.
3.5 Physical Constraints to Morphological Equilibrium
The three distinct predicted equilibrium states of the Severn Estuary suggest that
different parts are at different stages of adjustment to natural process inputs.
3.5.1 Under-sized Reaches
Upstream of the M4 road bridge, the estuary is predicted as under-sized and processes
will be attempting to widen the channel to establish an equilibrium form. However, it may
not be possible for the estuary to widen here because of constraints such as land use
(flood embankments and bridges) and geology (rock cliffs and shore platforms).
The shores of the Severn Estuary upstream of the M4 road bridge are dominated by
intertidal mudflats bordered by former floodplains, which are protected from inundation
at high water by flood embankments. The flood embankments currently artificially
constrain the natural widening of the upper estuary. Also, the locations of the two
bridges at the seaward end of the under-sized part of the estuary represent geological
constraints to widening as they are bounded by rock cliffs and shore platforms. Several
other shoreline reaches also contain higher ground in the form of cliffs.
Upstream of the M4 road bridge, the intertidal parts of the estuary have accreted over
the past 100 years (ABPmer, 2006, 2009). This is difficult to reconcile given the
significant landscape-scale morphological disequilibrium predicted, which should
correlate with a trend of erosion. It is possible that the high suspended sediment
concentrations (peaking at Sharpness, ABPmer, 2009) and long residence times in this
part of the estuary and subsequent deposition on the intertidal flats is counteracting the
effect of the disequilibrium.
3.5.2 Reaches in Near-equilibrium
Between the M4 road bridge and a line between Goldcliff and Clevedon, the estuary
appears to be a state of near-equilibrium. According to ABPmer (2006, 2009) the net
trend of erosion or accretion in the central part of the estuary (however, this does extend
further downstream than the zone of equilibrium) is not clear, although net erosion is
evident. This variability in erosion-accretion is in keeping with its near-equilibrium form,
suggesting that the estuary is in a constant state of short- to medium-term adjustment to
maintain this form.
3.5.3 Over-sized Reaches
In the predicted over-sized part of the estuary downstream of a line between Goldcliff
and Clevedon, the bed of the estuary is partially rock outcrop which constrains the
channel from deepening. The geology is sufficiently hard so that the bed is resistant to
physical processes so the estuary does not conform to the regime relationship. Here,
the width of the estuary is over-sized (wider) to compensate for the relatively shallow
depths caused by the geological constraint. The over-sized part of the estuary also
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correlates with a large volume of mudflat accretion over the past 150 years (ABPmer,
2009) suggesting gradual infilling to reduce width.
3.5.4 Overall Condition of the Morphological Equilibrium Attribute
The results of Regime Theory in the Severn Estuary SAC show that only the central part
is close to morphological equilibrium. The estuary has developed into a more
exaggerated ‘trumpet’ shape that would be expected if it was in morphological
equilibrium. The upper reaches are narrower than their predicted equilibrium form and
the lower reaches are wider than their predicted equilibrium form.
A combination of flood embankments and geological constraints control the upstream
disequilibrium. In order to allow a wider channel to develop in keeping with the
equilibrium form may necessitate realignment of the flood embankments to restore
former land-claimed intertidal areas to tidal processes. The geological constraints are
permanent and cannot be changed. Currently, the Shoreline Management Plan (Atkins,
2010) contains several managed realignment policies for the reaches of the estuary
upstream of the M4 road bridge that could act as drivers to move this part of the estuary
towards morphological equilibrium (Figure 3.10 and Table 3.3).
Figure 3.10. Location of seven potential managed realignment sites in the inner
Severn Estuary (Atkins, 2010)
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Table 3.3. Potential managed realignment sites in the inner Severn Estuary
(Atkins, 2010)
Coastal Stretch Management
Unit Epoch 1 Epoch 2 Epoch 3
Tidenham and other villages –
Guscar Rocks to Lydney Harbour TID 2 HTL HTL MR
Gloucester to Sharpness –
Frampton Pill to Royal Drift outfall SHAR 7 MR HTL HTL
Lydney to Gloucester – Brims Pill to
Northington Farm GLO 2 MR HTL HTL
Gloucester to Sharpness – Overton
Lane to upstream of Hock Cliff SHAR 4 HTL MR MR
Gloucester to Sharpness – Wicks
Green to Longley Green SHAR 2 HTL MR HTL
Gloucester to Sharpness – Severn
Farm to Wicks Green SHAR 1 HTL MR MR
Gloucester to Maisemore – West
bank at Drain from Long Brook to
west bank at railway / A40 bridge MAI 1 MR HTL HTL
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4 RESULTS FOR THE SOLWAY FIRTH
4.1 Bathymetry
The best available bathymetry for the Solway Firth was obtained in three different
formats; two from the Environment Agency Geomatics Group and one from Natural
England. These were LiDAR at 2m resolution and Admiralty Chart data in vector format
from Geomatics and multibeam data from Natural England. All three datasets required
processing and manipulation before being ‘stitched’ together to create the final
bathymetry. These were the only three datasets used in the analysis (a list of all
bathymetry data that was received is provided in Appendix B). The following procedure
was followed:
• The LiDAR data, in OD, was processed from single ASCII files into a mosaicked
dataset covering the shallow parts of the upper estuary and southern shore of
the Solway Firth (Figure 4.1)
Figure 4.1. LiDAR data in the upper Solway Firth
• The Admiralty Chart data was converted into a raster (Figure 4.2) and clipped to
the study area extent. These data were converted from CD to OD using the
conversion factors in the 2015 Admiralty Tide Tables.
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Figure 4.2. Admiralty Chart bathymetry data in the Solway Firth
• The multibeam survey (in OD) was processed and added into the GIS (Figure
4.3).
Figure 4.3. Multibeam bathymetry data in part of the Solway Firth
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• The Admiralty Chart bathymetry data was very poor along the north and
northwest sides of the estuary and was considered to be erroneous (Figure 4.2).
Unfortunately, no LiDAR or multibeam data was available to fill this gap.
Therefore, the gap was filled by creating an artificial bathymetry by interpolating
between the level of MHWS at the coast and the LiDAR data and multibeam
data combined in the offshore (Figure 4.4). This is the ‘best’ that could achieve
with the data available.
Figure 4.4. Artificially created bathymetry data in part of the Solway Firth
• The LiDAR data also contained gaps in some of the deeper channels of the
upper estuary. Linear interpolation using data either side of the channels was
carried out across these gaps.
• All the data were merged together (Figure 4.5) to create the overall bathymetry
for the Solway Firth used in our analysis (Figure 4.6) with LiDAR data taking
precedent, then the multibeam survey, the interpolated grid and finally the
Admiralty Chart bathymetry. This is a different approach to the Severn Estuary
as each site required a bespoke analysis to determine the ‘best available’
bathymetry.
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Figure 4.5. Datasets used to create the final bathymetry in the Solway Firth
Figure 4.6. Solway Firth bathymetry created by combining all datasets (in OD)
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4.2 Tidal Regime
In order to calculate the spring tidal prism and cross-sectional area of the Solway Firth it
is necessary to know the elevations of tidal datums. Table 4.1 presents the MHWS,
MHWN and MLWS tidal datum elevations at tidal stations along the Solway Firth. Figure
4.7 shows the tidal datum surfaces transposed on to the bathymetry of the Solway Firth.
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Figure 4.7. Tidal datums in the Solway Firth
4.3 Extent of Estuary and Area of Intertidal Habitat
The areas of intertidal habitat (between MHWS and MLWS) and potential saltmarsh
(between MHWS and MHWN) in the Solway Firth are shown in Figures 4.8 and 4.9,
respectively. The plan extents of the estuary parameters, which apply to the entire study
area chosen (not the SAC specifically), are presented in Table 4.2.
Table 4.2. Planform extent of the Solway Firth and its intertidal and subtidal areas.
Note these extents are for the entire study area, not just the SAC.
Parameter Area (km
2
)
Estuary extent below MHWS 860
Intertidal area between MHWS and MLWS 330
Subtidal area below MLWS 530
Potential saltmarsh area between MHWS and MHWN 90
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Figure 4.8. Area of intertidal habitat in the Solway Firth (area between MHWS and
MLWS datums)
Figure 4.9. Intertidal area in the Solway Firth where saltmarsh could potentially
develop if the substrate was suitable for development of this type of habitat (area
between MHWS and MHWN datums)
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The actual area of saltmarsh in the Solway Firth was calculated using the saltmarsh
polygons of the Land Classification Mapping (LCM) carried out in 2007. The LCM
estimated area of intertidal saltmarsh is 30km
2
(Figure 4.10). However, it should be
noted that the LCM data does not cover the Scottish coast west of the River Nith and so
the area is likely to be an underestimate.
Figure 4.10. Saltmarsh in the Solway Firth (2007 Land Classification Mapping)
4.4 Morphological Equilibrium
4.4.1 Observed Estuary Form
Using the bathymetry and tidal datums in a GIS, the observed estuary parameters at
sections spaced 1km apart were measured along the estuary in a similar way to the
Severn Estuary analysis (Section 3.4.1). The locations of the sections in the Solway
Firth where the observed form is measured are shown in Figure 4.11 and the data at
each section is presented in Appendix E. The Solway Firth is broken down into several
constituent water courses. These include the River Esk and River Eden at the head of
the estuary, the Rivers Wampool and Waver entering on the south shore in Moricambe
Bay and the River Nith entering on the north shore.
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Figure 4.11. Position of the sections in the Solway Firth where the observed form
is measured
4.4.2 Predicted Estuary Form
The same method used to predict estuary form in the Severn Estuary (Section 3.4.2) is
used in the Solway Firth and is not repeated here. Using this method, the predicted form
of the Solway Firth at each section is presented in Appendix F.
4.4.3 Comparison of Predicted Equilibrium Widths with Observed Widths
The results were interrogated using GIS to compare the predicted equilibrium widths
(Appendix F) with the observed widths (Appendix E) at each section. In this way,
reaches of the observed estuary which are narrower or wider than their predicted form
were mapped. The comparison for the Solway Firth is shown in Figure 4.12.
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Figure 4.12. Observed and predicted forms of the Solway Firth using the regime
equation for all United Kingdom estuaries and the ‘constant evolution’ method
The observed widths compare with the predicted equilibrium widths in the Solway Firth
in one of three ways. The Rivers Esk, Eden and Nith, and the upper Solway Firth
(upstream of the River Wampool on the south shore) is under-sized compared to its
predicted form (i.e. the observed channel is narrower than predicted for the present-day
tidal regime) (Figure 4.12). However, the results of the River Nith and the north shore of
the Solway Firth to the east of the River Nith may be spurious, given the artificial nature
of the created bathymetry.
The Rivers Wampool and Waver (Moricambe Bay) have observed and predicted widths
which are similar, suggesting that their observed forms are close to equilibrium.
Downstream of the Rivers Waver and Nith there is a rapid transition to an estuary that is
over-sized compared to its predicted form (i.e. the observed channel is wider than
predicted for the present-day tidal regime). However, it is difficult to judge in this
segment of the water course where the open coast might begin in a seaward direction. It
is possible that seaward of a line between Southerness and Allonby, the impact of
estuary processes is negligible and this area is subject to open coast processes only.
This would mean that Regime Theory could not be applied.
It appears that the majority of the Solway Firth is either under-sized or over-sized with
only a small proportion nearing an equilibrium form (Moricambe Bay). The upper
reaches of the Solway Firth and the confluencing Rivers Esk and Eden are pressure
points in the estuary. This means that here the estuary form should be wider than it
actually is and to obtain equilibrium the estuary has to widen from its current form (i.e. it
should erode resulting in loss of intertidal habitat if the high water mark is unable to
migrate landwards). Future sea-level rise will exacerbate this trend for erosion. The
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outer Solway Firth is over-sized, and so it exceeds its predicted equilibrium width and
over the long term there may be development of intertidal habitat by natural processes.
4.5 Physical Constraints to Morphological Equilibrium
4.5.1 Under-sized Reaches
The shores of the upper Solway Firth including the Rivers Esk and Eden are dominated
by intertidal flats and saltmarsh backed by shoreline defences, which protect adjacent
low-lying land from flooding. The estuary here is predicted as under-sized and estuarine
processes should be attempting to widen the channel to establish an equilibrium form. In
the upper estuary the fringing saltmarshes are eroding due to coastal squeeze against
the shoreline defences (Halcrow, 2010) confirming the view that the estuary is trying to
widen here. The defences are currently constraining its natural widening. Indeed the
shoreline management policy along the English side of the upper Solway Firth is
managed realignment to allow a return to a more natural shoreline, with insufficient
economic justification to maintain defences (Halcrow, 2010).
4.5.2 Reaches in Near-equilibrium
At the confluence of the Rivers Wampool and Waver (Moricambe Bay), the coast is
sheltered from higher energy conditions, which has resulted in the development of
extensive areas of saltmarsh, which is generally stable. This stability is in keeping with
the predicted near-equilibrium form of the Solway Firth in this area. However, Grune
Point spit which protects the saltmarsh in Moricambe Bay is an exception to this stability
because it is currently eroding. This erosion is likely to be related to direct wave action
and not tidal processes
4.5.3 Over-sized Reaches
It is difficult to make judgements regarding the over-sized part of the estuary
downstream of the Rivers Waver and Nith, because it is unclear where estuarine
processes end and open coast processes begin. The sea bed is sandy in this area and
there is no indication that bedrock is near the surface that would constrain the depth,
forcing the estuary to be over-wide. On the English side, between Moricambe Bay and
Silloth (north of where the water course widens significantly to the south), the coastline
is defended by sea walls and rock armour revetments, and so a constrained coastline
would be expected. Periodic beach re-nourishment has been required along this
coastline indicting erosion not accretion. It is possible that this over-sized reach is too
far seaward to be influenced by estuarine processes which would make the results of
the Regime Theory invalid.
4.5.4 Overall Condition of the Morphological Equilibrium Attribute
The overall condition of the Solway Firth SAC that is driven mainly by tidal processes is
morphological disequilibrium, whereby the estuary is narrower than its predicted
equilibrium form. The most likely cause for the disequilibrium is coastal squeeze caused
by the inability of the intertidal system to migrate landwards due to flood defences.
Currently, the Shoreline Management Plan policy for almost the entire English coastline
of the Solway Firth, east of Moricambe Bay is managed realignment (Halcrow, 2010)
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(Figure 4.13 and Table 4.3). Future actions to implement this policy could act as a driver
to move the estuary containing the SAC towards morphological equilibrium. The main
environmental rationale for managed realignment in the SMP is c
ontinued natural
shoreline evolution which will help maintain the condition of the SAC and provide
opportunities for future habitat creation to be included within the Environment Agency’s
Regional Habitat creation Programme.
Figure 4.13. Location of potential managed realignment (blue lines) along the
inner Solway Firth (English side) (Halcrow, 2010)
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Table 4.3. Potential managed realignment sites in the inner Solway Firth (Halcrow,
2010)
Coastal Stretch Policy Unit Epoch 1 Epoch 2 Epoch 3
Cardurnock to Bowness-on-Solway 8:1 MR MR MR
Bowness-on-Solway 8:2 MR MR MR
Bowness-on-Solway to Drumburgh 8:3 MR MR MR
Drumburgh to Dykesfield 8:4 MR MR MR
Dykesfield to Kingsmoor (Eden
Normal Tidal Limit) 8:5 MR MR MR
Kingsmoor (Eden Normal Tidal
Limit) to Rockcliffe 8:6 MR MR MR
Rockcliffe to Demesne Farm 8:8 MR MR MR
Demesne Farm to Metal Bridge
(Esk) 8:9 MR MR MR
Metal Bridge (Esk) to the River
Sark 8:10 MR MR HTL
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5 MORPHOLOGICAL MONITORING STRATEGY
Monitoring is continuous or repeated data collection over a period of time. The term
monitoring covers a range of activities and types of data collection usually designed to
detect changes from which a suitable coastal management strategy can be applied. The
Severn Estuary and Solway Firth are dynamic environments and their processes are
variable and in many ways unpredictable. Condition assessments must recognise this
and must be reviewed, and changed if necessary, in light of changing circumstances.
This process of review can only sensibly be carried out if the response of the estuary to
both natural and human-induced forces is monitored.
Morphological monitoring can combine a wide range of techniques of varying
sophistication including ground surveys, repeat bathymetric surveys, LiDAR and aerial
photography. The monitoring strategies for the Severn Estuary and Solway Firth should
be designed to measure attributes that contribute to an understanding of morphological
equilibrium, which can be used as a measure of estuary condition in the future. As part
of the monitoring strategy, the best way to monitor these two estuaries and some of the
advantages and disadvantages of the various data collection methods that could be
used are higlighted.
5.1 Bathymetry in the Severn Estuary and Solway Firth
The critical data upon which the Regime Theory method relies is bathymetry from which
all the resulting characterisation parameters (cross-sectional area, width and tidal prism)
can be derived. A time series of bathymetric data can provide an indication of how these
parameters are changing, which can be used to determine if the estuary (or parts of the
estuary) is moving towards or away from a more equilibrium form. There are a number
of potential techniques available for the collection of bathymetric data in the Severn
Estuary and Solway Firth including single beam echo-sounding, multibeam echo-
sounding and LiDAR (http://www.channelcoast.org/southwest/survey_techniques/).
5.1.1 Single Beam Echo Sounding
Single beam echo sounding involves using a transducer attached either to the hull of a
vessel, or to a pole mounted over the side or bow of the vessel. The echo sounder
calculates the water depth beneath the transducer, by transmitting a sound pulse that is
returned to the vessel via reflection off the sea bed. The density of soundings is
dependent on the survey line spacing, vessel speed and the echo-sounder ping rate.
Standard single beam echo sounders collect data for a narrow zone along the track of
the vessel and hence the main limitation of the system, compared to multibeam
systems, is the limited sea bed coverage. Generally the data are presented either in a
line form, or spatial interpolation is undertaken in order to provide full bathymetry
coverage. This technique has been used widely to generate marine (Admiralty) charts.
Single beam echo sounding can be combined with LiDAR and/or ground survey for a
more complete view of the whole estuary.
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5.1.2 Multibeam Echo Sounding
A multibeam echo-sounder survey provides an alternative to a single beam survey in
bathymetric data collection. The main difference between a single beam echo-sounder
and a multibeam echo-sounder is that the latter produces a number of beams forming a
‘fan’ of sound pulses or acoustic energy. A multibeam system essentially consists of a
receiver and transmitter that emit and detect multiple beams of sound energy in a
swathe (producing swathe bathymetry). These multiple soundings are taken at right
angles to vessel track, as opposed to a single sounding directly underneath a vessel
with a single beam echo sounder. This means that a multibeam system can provide a
greater density of soundings allowing faster coverage of a site.
The main advantage of multibeam systems is that they can provide 100% coverage of
the sea bed without the need to interpolate between lines. A disadvantage of multibeam
systems is the high cost compared to single beam surveys. In shallow water (less than
10m), the swathe width is also significantly reduced.
5.1.3 LiDAR
Airborne Laser Induced Direction and Range (LiDAR) is a remote sensing technique for
the collection of bathymetric and topographic data. It uses laser technology to ‘scan’ the
ground surface (or estuary bed at low tide), taking up to 10,000 observations per square
kilometre. These observations are then converted to the local co-ordinate and elevation
datum by the use of differential GPS. The system routinely achieves vertical accuracy of
+/-11-25cm and plan accuracy of +/-45cm, with a very rapid speed of data capture (up to
50km
2
per hour). It can operate on mudflats but care needs to be taken in areas of
standing water as with the normal settings the laser beam is absorbed by water rather
than reflected. The resulting data can be presented as a contour plot of bathymetry or
used to create a digital terrain model.
5.1.4 Bathymetry Monitoring Strategy for the Severn Estuary and Solway Firth
A combined echosounder and LiDAR survey is recommended for both the Severn
Estuary and Solway Firth. A survey is recommended every ten years in order to capture
estuary-scale changes in morphology as the system evolves into the future. However, if
the saltmarsh boundary measurements (see Section 5.2) indicate a deviation away from
standard limits of natural variation, then a survey should be completed to support that
observation. This is in line with the CSM guidance for estuaries and coastal saltmarsh
(JNCC, 2004) (Table 1.1).
The LiDAR survey should be undertaken at low tide along the shallower parts of each
estuary supplemented by an echosounder survey (preferably multibeam) in deeper
water where LiDAR is unable to penetrate. Care should be taken to ensure that the
LiDAR and multibeam data overlap to provide a seamless bathymetry across the
estuaries. Difficulties in overlapping the data may occur in the inner Solway Firth where
the water at low tide may be too deep for penetration of LiDAR and too shallow at high
tide for a boat to operate safely. In this case the ‘best available’ dataset should be
created by interpolation through the ensuing gaps in combination with expert
geomorphological assessment as to what the gaps should look like. The same principle
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should apply to the Severn Estuary, although the locations where gaps could occur may
be fewer.
5.2 Saltmarsh and Mudflat
There is great diversity in the morphology of saltmarshes and mudflats and the
boundary between them that relates to the changing balance of physical,
sedimentological and biological forces on the sediment. In general terms, the width of a
mudflat will be greater in areas of high tidal range (c.f. the Severn Estuary and Solway
Firth) than in areas of low tidal range, but there are considerable deviations that indicate
there are other controls. Generally mudflats undergoing erosion have low and concave-
upward profiles and those experiencing deposition have high and convex-upward
profiles.
Any changes in the fronting mudflat profile causes change in the duration of wave
attack, altering the rates of erosion and deposition, so there may be a gradual
progression to a new equilibrium. A high convex mudflat shape is desirable because
waves are progressively attenuated as they approach the shore, protecting the marsh
edge from erosion. Low concave mudflats result in steadily increasing wave height and
energy at the marsh edge, exacerbating its erosion.
The boundary between saltmarsh and mudflat in the Severn Estuary and Solway Firth
can be monitored using aerial photography supported by land-based ground-truthing
(and in combination with the LiDAR collection). It is recommended that, to understand
changes to the morphological equilibrium attribute, these types of monitoring should
take place every five years, with every other survey timed to coincide with the
bathymetric surveys. This is a deviation from the CSM guidance, as Regime Theory is
typically applied to understand the long-term changes to an estuary.
5.2.1 Aerial Photography
Vertical aerial surveys of a shoreline can provide quantitative data on large-scale
changes of the coast, such as movement of the saltmarsh edge. The process of
reviewing and assessing geomorphology from aerial photography generally requires
registration of the data into digital systems such as GIS that allow the data to be
correctly spatially located and allow accurate location and measurement to be achieved.
5.2.2 Ground-based Survey
A common form of field morphological monitoring is topographic survey. Saltmarsh and
mudflat morphology can be monitored using topographic data to assess changes in
height, width and slope. Topographic survey information can usefully be combined with
the aerial photographs and bathymetric surveys in order to gain an overall
representation of the intertidal area.
Several techniques of varying sophistication are available for collecting topographic
survey data. The least sophisticated method (although not necessarily the least
accurate) is survey using a quick set level, staff and chain. More advanced methods
include using a total station with electronic distance measurement to a survey reflector
prism and computer logging of data points. Current best practice involves the use of
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GPS (Global Positioning System). The use of Real Time Kinematic (RTK) GPS can be
very fast and efficient, entailing the establishment of a base station at a control point and
surveying the profile using a separate GPS rover unit. A survey of this kind enables
vertical accuracies of +/-30mm on hard surfaces and +/-50mm on soft surfaces, and
horizontal accuracies of +/-20mm.
5.2.3 Saltmarsh / Mudflat Monitoring Strategy for the Severn Estuary and Solway Firth
A combined aerial photograph capture along with ground survey is recommended for
both estuaries every five years. The data collection should be synchronous with the
bathymetric data capture, to obtain a specific time-slice of estuary morphology. The
ground surveys are likely to be limited in there extent given the large size of the
estuaries. However, initial analysis of the remotely sensed data should provide an
indication as to the critical areas for further investigation on the ground. The ground
survey work should tie in with the timing of SSSI condition assessments.
5.3 Data Management
Data and its collection, in all the various forms, is usually relatively costly. The return on
the investment of gathering and collating the data should be maximised through the
means by which the data is stored, maintained and accessed. In this regard, a data
governance strategy should be set up between Natural England and Scottish Natural
Heritage for the Solway Firth and Natural England and Natural Resources Wales for the
Severn Estuary to provide frameworks for the data to be stored, updated and shared.
Each of these organisations would have joint coordinating roles over the way the data is
captured, stored and used in their relevant estuary. Partners in each of these
enterprises would include the Environment Agency (both estuaries) and the Scottish
Environment Protection Agency (Solway Firth). These partners would work closely with
the lead organisations to ensure the data is managed for the good of all interested
parties. Also, the coordination of data management across organisations and
geographical boundaries would allow resources to be shared and any ongoing data
collection programmes to be efficiently streamlined into the process. For example, the
Environment Agency already monitors saltmarshes in Water Framework Directive
surveillance bodies.
5.3.1 Metadata
Data gathering should include meticulous recording of metadata. Metadata is
information about the data rather than the data itself. It should include detail regarding
the quality of the data, the parameters that have been gathered, the units/format/datums
that have been used, the scale, any geographical referencing, and the appropriateness
of the data and its intended application. In addition, information such as the spatial
extent of the data and keywords will provide means by which efficient search and
retrieval routines can interrogate the metadata. Establishing a metadata management
system to administer project datasets allows easy review of existing information and
potential cost savings through data re-use. Metadata should provide potential users with
sufficient information to make an informed decision about the quality and potential for
use of the data for any given requirement.
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It is recommended that data collected to support morphological analysis of the Severn
Estuary and Solway Firth complies with the metadata standards defined by MEDIN
(Marine Environmental Data and Information Network). Information regarding what these
standards are can be found at:
http://www.oceannet.org/submit_metadata/creating.html
5.3.2 Digital Format
Ideally, data should be collected, stored and used in digital formats. The volume of data
generally generated from survey is such that it precludes any format other than digital.
Having data in digital format can also allow the integration of datasets. As far as
possible, greatest benefit will be achieved if it can be stored in industry standard digital
formats. This will facilitate its use on a wider range of applications and increase the
longevity of the data. Digital data in standard formats also makes data transfer and
exchange more economical and less labour intensive.
5.3.3 Information Management
Information management is the capacity to efficiently store and retrieve relevant
information. A key area is the use of GIS as a management and analysis tool. A GIS is
a software package for the acquisition, storage, retrieval, manipulation and analysis of
spatially referenced data. The most sophisticated GISs are expensive and require
considerable processing power and storage capacity, whilst basic systems are also
available for use on desk-top PCs at a modest cost. All systems are based on two
components:
• a database capable of storing and retrieving information about the data, which is
mapped by geographical position
• a visualisation system capable of displaying spatially-referenced data and
interrogating the mapped data for co-ordinate information.
5.3.4 Monitoring Plan
Table 5.1 summarises a potential morphological monitoring plan for determination of
morphological equilibrium in the future.
Data Frequency Methods
Bathymetry Every 10 years Single beam echo sounder
Multibeam echosounder
LiDAR
Saltmarsh-mudflat boundary Every 5 years Aerial Photographs
Ground survey
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6 CONCLUSION
An understanding of how the Severn Estuary and Solway Firth function is essential to
ensure sustainable human uses of them into the future. This work was based on the
assumption that the ‘health’ or condition of these estuaries is founded on the relationship
between their physical forms (geometry) and the forces driving their forms
(function/process) in line with the Regime Theory concepts and approaches developed
by the Healthy Estuaries 2020 project (Natural England 2015).
To support habitat in favourable condition, the estuary morphologies need to be in
‘equilibrium’ with natural wave, tidal and sediment transport processes. Over time, these
two estuaries have had their dynamic equilibrium morphologies changed in some way
by human interference and different parts of their forms are at different stages of
adjustment to natural process inputs. Hence, into the future both estuaries will seek to
reach a steady state over the long term and their widths and depths will change over
time towards a state of dynamic equilibrium or ‘most probable state’.
Regime Theory has been used in the Severn Estuary and Solway Firth to predict their
equilibrium widths, which have been compared with their observed widths to determine,
at a high level, how far they are from equilibrium forms. How close each estuary is to
morphological equilibrium defines the condition of this attribute. The method has been
combined with known natural and human constraints on morphology, where adjustment
of the estuary form may not be possible due to hard geology or essential infrastructure.
The method also supports identification of potential locations to restore intertidal habitat
in such a way that a more sustainable estuary form is produced.
Estuary Characterisation PB2693/R/303996/PBor
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7 REFERENCES
ABPmer. 2006. Severn Estuary CHaMP Morphological Assessments. Part E: Historical
Trends Analysis. Report to Jacobs Babtie (on behalf of the Environment Agency), June
2007.
ABPmer. 2009. Severn Estuary SMP2: Baseline Understanding of Coastal Behaviour
and Dynamics. Report to Atkins, May 2009.
Atkins. 2010. Severn Estuary Shoreline Management Plan Review (SMP2). Available at:
http://www.severnestuary.net/secg/smpr.html
Dargie, T. 1998. NVC Survey of saltmarsh habitat in the Severn Estuary 1998.
Countryside Council for Wales Science Report 341.
Halcrow. 2010. North West England and North Wales Shoreline Management Plan
SMP2. Available at:
http://www.allerdale.gov.uk/downloads/nw_shoreline_management_plan_2.pdf
HR Wallingford, ABPmer and Pethick, J. 2007. Review and formalisation of
geomorphological concepts and approaches for estuaries. Defra/Environment Agency
R&D Technical Report FD2116/TR2.
JNCC. 2004. Common Standards Monitoring Guidance for Estuaries. Version August
2004.
Natural England. 2006. Coastal Squeeze, Saltmarsh Loss and Special Protection Areas.
English Nature Research Report 710.
Natural England. 2015. Healthy Estuaries 2020: Towards Addressing Coastal Squeeze
in Estuaries. Improvement Programme for England’s Natura 2000 Sites. IPENS002.
Natural England.
O’Brien, M.P. 1931. Estuary tidal prism related to entrance areas. Civil Engineering, 1,
738-739.
Spearman, J. R. 1995. Estuary regimes and methods for prediction of long-term
changes. PhD Thesis, Oxford Brookes University.
Spearman, J. R. 2001. A simulation of tidal creek response to managed retreat using a
hybrid regime model. HR Wallingford Report TR 118.
Townend, I., Wright, A. and Price, D. 2000. An investigation of the gross properties of
UK estuaries. In EMPHASYS Consortium. 2000. Modelling Estuary Morphology and
Process, 73-81.
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 42 - 12 May 2015
Appendix A: Regime Theory and its Application to the Severn Estuary and Solway
Firth
A.1. General Principles of Regime Theory
Regime Theory is based on empirical relationships between estuary properties that
reflect their size and shape. The most widely used of these regime relationships is
between channel cross-sectional area and upstream tidal prism (or discharge). This
relationship, first proposed by O’Brien (1931), is between the spring tidal prism (the
volume of water that enters and leaves the estuary during a spring tide) and the cross-
sectional area at mean sea (tide) level at the mouth. This equation takes the form:
CSA = a.P
b
where:
CSA = cross-sectional area (mean sea level);
P = upstream spring tidal prism;
a = constant coefficient; and
b = constant exponent.
In the regime equation adopted in the Severn Estuary and Solway Firth, the cross-
sectional area at MHWN tide is used instead of mean sea level. This is because MHWN
tide is deemed to be the boundary of the active estuarine channel geomorphology,
because when the water level is at this datum, maximum discharge takes place
(immediately before inundation of the saltmarsh). Areas higher than MHWN tide within
the tidal environment will have tidal current velocities that approach zero.
A.1.1. Applying Regime Theory to Inter-estuary Analysis
When the regime relationship is applied to a number of estuaries it is found to be linear
when both datasets are transformed into their log values. The best-fit regression line
that is constructed through a log-log plot represents the theoretical equilibrium
morphology for those estuaries in general. This theoretical equilibrium has been applied
successfully across a range of estuaries in the United Kingdom. Townend et al. (2000)
described an empirical regime relationship for 66 estuaries around the United Kingdom
coast (Figure A.1). The regression (regime) equation for the whole dataset is:
CSA = 0.024.P
0.71
(r
2
= 0.75)
This is the regression equation that was used in the Severn Estuary and Solway Firth.
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Figure A.1 shows that although individual estuaries may depart from the ideal
relationship between flow (tidal prism) and form (cross-section) (i.e. a linear regression
line through the data) due to, for example human intervention or natural constraints such
as geology, these departures will form a random scatter around the fundamental
relationship that can be expressed as the best-fit regression to the data. The relationship
is in this way, a useful tool to describe the overall condition of a given estuary compared
to others in a regional group (but see uncertainties below).
A.1.2. Applying Regime Theory to Intra-estuary Analysis
As well as being applicable between estuaries, the relationship can equally be applied
within a single estuary. Thus a downstream increase in tidal prism in a given estuary will
be matched by an increase in the cross-sectional area of successive channel profiles.
This provides a measure of the equilibrium morphology of an estuary along its length
and a tool to assess condition by determining how the tidal prism / channel cross-
sectional area relationship changes with distance along the estuary.
A.1.3. Uncertainties with Regime Theory
The Regime Theory only requires geometric and water level information to be used as
inputs. This is so the method is simple to apply. HR Wallingford et al. (2007) showed
that the use of only bathymetry as input to the method is an oversimplification because it
does not take into account other important mechanisms controlling estuary evolution.
These may include the effects of waves, fluvial discharge, longshore sediment transport
and geology.
The potential weakness of the method related to these parameters is acknowledged, but
it is beyond the scope of this study to include what are more complicated mathematical
formulae (which are still not fully understood and to date haven’t been applied
successfully). It is understood that the level of uncertainty in the regime equation is
important for understanding the uncertainty in the corresponding equilibrium predictions
arising from its use.
A.2. Methods used to Predict Estuary Equilibrium Form in the Severn Estuary and
Solway Firth
The two main parts to the analysis in the Severn Estuary and Solway Firth are:
1. Measure the observed forms; and
2. Predict the equilibrium forms.
These two forms are then compared to see how close the estuaries are to morphological
equilibrium.
A.2.1. Development of Sections and Observed Estuary Form
The observed (present-day) cross-sectional area and tidal prism have been calculated in
each estuary using the bathymetric datasets relative to the tidal elevations at specific
sections along each of the estuaries. The number of sections is typically determined by
the size of the estuary. Given the relatively large-scales of the two estuaries, the
spacings of the sections are approximately 2km in the Severn Estuary and 1km in the
Solway Firth. The sections stretch between MHWS tide on either side of the estuary and
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 45 - 12 May 2015
are perpendicular (as far as possible) to a line along the centre of the channel. It is then
possible to create a table in GIS with values for each estuary parameter calculated at
each section. This data is defined as the observed morphology of the estuary
(Appendices C to F).
A.2.2. Morphological Equilibrium based on the Predicted Estuary Form
In order to provide a preliminary assessment of the condition of the morphological
equilibrium attribute, the observed forms of the estuaries are compared to the
equilibrium forms predicted using a set of calculations at each of the sections originally
defined in the measurement of observed form. The prediction of the equilibrium forms
was carried out in three main stages using the methodology developed for Healthy
Estuaries 2020 (Natural England, 2015):
• distribute throughout the estuary the total observed tidal prism at the mouth to
predict the tidal prism upstream of each section;
• calculate equilibrium cross-sectional areas from the upstream tidal prisms at
each section; and
• calculate mean depths and equilibrium widths at each section.
The calculations of predicted form are automated in the Excel tool and the outputs
defined as the predicted morphology of the estuary. The results obtained are then
interrogated using GIS to compare the predicted form with the observed form at each
section to gauge how far from equilibrium the estuary is.
A.2.3. Distributing the Observed Tidal Prism at the Mouth throughout the Estuary
One result of the measurement of observed form using GIS is the spring tidal prism of
the entire estuary (i.e. the tidal prism observed at the estuary mouth). In order to predict
the equilibrium form of the estuary at each section this total tidal prism has to be
distributed throughout the estuary from its mouth to its head. The tidal prism at each
section is calculated using an equal distribution model with the following equation:
P
x
= e
[-3.(x/l)]
.P
tot
where:
P
x
= tidal prism at each section (m
3
);
x = distance to section from estuary mouth (m);
l = total estuary length from mouth to head (m); and
P
tot
= total tidal prism (observed) (m
3
).
This equation distributes the total tidal prism along the estuary according to distance
from the mouth. The calculation of tidal prism upstream of a particular section from the
mouth is based on a cubic exponent, which is multiplied by the ratio of the distance to
the section from the mouth (x) and the total length of the estuary (l). The ratio x/l is a
non-dimensional distance along the estuary axis; i.e. it varies from 0 at the mouth to 1 at
the head. The use of an exponential set at 3 has been verified by empirical calibration
using United Kingdom estuaries (unpublished).
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The calculation of P
x
is straightforward in an estuary with a single channel. However, an
estuary typically has a main channel with one or more smaller channels joining it, which
makes the designation of x and l in the equation complicated. For example, both the
Severn Estuary and Solway Firth have major channels with smaller channels joining at
points along their lengths. In this situation, the equal distribution equation is first applied
to each joining channel; the tidal prism is apportioned based on the observed tidal prism
at the channel mouths with l as the total channel length. The equation is then applied to
the main channel only, but the observed tidal prism at the mouth is reduced by the sum
of the observed tidal prisms at the mouths of the joining channels. The sum of the tidal
prisms of the joining channels is then added back on to the predicted tidal prism at each
section of the main channel. The calculation of tidal prism at each section is automated
in the Excel tool from files imported directly from GIS.
A.2.4. Calculating Equilibrium Cross-sectional Areas
The calculation of equilibrium cross-sectional area from predicted tidal prism at each
section is based on the regime equation for all United Kingdom estuaries:
CSA = 0.024.P
0.71
(r
2
= 0.75)
A.2.5. Predicting Estuary Width using the ‘Constant Evolution’ Method
Using the regime equation the equilibrium cross-sectional area at each section is
predicted. However, the crucial parameter in the assessment is regime width (planform).
In order to predict the regime width from the equilibrium cross-sectional area, it is
necessary to predict the equilibrium mean depth. In this study, the ‘constant evolution’
method is used as described in Healthy Estuaries 2020 (Natural England, 2015).
One of the main difficulties with Regime Theory is that in most cases, an estuary system
does not conform to a smooth relationship of the type:
CSA = a.P
b
Instead an estuary presents considerable scatter around a best fit relationship of that
form. Adopting the best fit relationship and implementing the regime equation to derive
the equilibrium cross-sectional area of an estuary may provide results that are driven
mainly by the scatter in the data and the uncertainty inherent in the method (Spearman,
1995, 2001; HR Wallingford et al., 2007).
To overcome this problem, Spearman (2001) suggested that the discrepancies between
the observed estuary cross-sectional area and the equilibrium cross-sectional area
given by the regime equation at each section are held to be constant throughout the
evolution. In this way the observed cross-sectional area at each section is assumed to
be in regime (for reasons that are not fully understand) and is adjusted in proportion to
the relative change between its form and the equilibrium form (HR Wallingford et al.,
2007).
Using this methodology it is possible to predict mean depths and equilibrium widths
based on the relationship between the observed and predicted cross-sectional areas at
each section. Equilibrium width is predicted using the observed mean depth to width
ratio at each section and applying the same ratio to the predicted cross-sectional area:
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 47 - 12 May 2015
W
E
= (CSA
E
.W
O
/D
O
)
0.5
where:
W
E
= equilibrium width (m);
CSA
E
= equilibrium cross-sectional area (m
2
);
W
O
= observed width (m); and
D
O
= observed mean depth (m).
The same principle can be applied to calculate equilibrium mean depth:
D
E
= (CSA
E
/[W
O
/D
O
])
0.5
where:
D
E
= equilibrium mean depth (m).
Estuary Characterisation PB2693/R/303996/PBor
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Appendix B: List of datasets received and used in this study
Estuary Name File Name Type Description Source Used
Severn and Solway Healthy Estuary 2020 toolbox Healthy Estuaries 2020 Toolbox.tbx ESRI Toolbox Python toolbox used to produce the Healthy Estuaries 2020 output Royal HaskoningDHV Yes
Severn and Solway Admiralty Charts Charts_Astrium.gdb Raster Geodatabase of all UK admiralty charts Environment Agency - Geostore No
Severn and Solway Admiralty Charts UKHO_Vector_Defra.gdb Vector Geodatabase containing vector admiralty datasets Environment Agency - Geostore Yes
Severn and Solway DiGMapGB-50 - Artificial dgm_50_artificial Vector 1:50,000 map representing artificial geology (Study extent) Environment Agency - Geostore Yes
Severn and Solway DiGMapGB-50 - Bedrock dgm_50_bedrock Vector 1:50,000 map representing bedrock geology Environment Agency - Geostore Yes
Severn and Solway DiGMapGB-50 - Superficial dgm_50_superficial Vector 1:50,000 map representing superficial geology Environment Agency - Geostore Yes
Severn and Solway Landcover Map 2007 lcm2007_25m_gb Raster CEH Land classification Mapping Environment Agency - Geostore No
Severn and Solway Landcover Map 2007 lmap_2007_ceh Vector CEH Land classification Mapping Environment Agency - Geostore Yes
Severn and Solway 2m Lidar Grids multiple types Text ASCI LIDAR tiles Natural England (EA Geomatics) Yes
Severn and Solway Port locations and tidal datum's Tidal Datum Text Location of Tidal datum's from Admiralty Tide Tables (2015) UKHO Yes
Severn 2012_EACCWNE Grab Sample data multiple types Vector Severn Sabellaria Samples_i Natural England No
Severn CCW data multiple types Vector Subtidal reef GIS layers Natural England No
Severn Channel Coast observatory Single Beam multiple types Vector Single beam Natural England No
Severn Seastar Survey Severn Estuary SAC
Sandbanks Bathymetry
Severn Estuary SAC Final SBES_RTK and
Manual Tides 2D Fill All Lines 24_09_2013 2m
Sort 225 min leg 1 step Vector (dxf) Bathymetry Survey Natural England No
Severn Sidescan Mosaics UTM multiple types Raster Bathymetry Survey Natural England No
Solway Allonby Bay Survey CS0286_SolwayFirth_XYZ_2011 Text Solway Firth Bathymetric Survey Natural England No
Solway Multibeam Survey Solway_MBES_Coverage_061110 Text Solway Firth Bathymetric Survey Natural England Yes
Solway Single Beam Survey SBES Summary 061110-V2 Text Solway Firth Bathymetric Survey Natural England No
Solway Single Beam Survey SBES_Bathy_upto_4_October_All_vessels Text Solway Firth Bathymetric Survey Natural England No
Solway Single Beam Survey Summary SBES Summary 061110-V2a Text Solway Firth Bathymetric Survey Natural England No
Estuary Characterisation PB2693/R/303996/PBor
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Appendix C: Observed form of the Severn Estuary at each section
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
1-0
24
#N/A
#N/A
#N/A
1-20
514,509
#N/A
#N/A
#N/A
1-40
2,128,563
#N/A
#N/A
#N/A
1-60
3,437,534
#N/A
#N/A
#N/A
1-80
5,661,526
#N/A
#N/A
#N/A
1-100
8,648,420
#N/A
#N/A
#N/A
1-120
12,551,041
#N/A
#N/A
#N/A
1-140
16,125,550
#N/A
#N/A
#N/A
1-160
20,673,766
#N/A
#N/A
#N/A
1-180
26,185,575
#N/A
#N/A
#N/A
1-200
41,022,826
#N/A
#N/A
#N/A
1-220
61,377,463
#N/A
#N/A
#N/A
1-240
82,477,399
#N/A
#N/A
#N/A
1-260
100,034,068
#N/A
#N/A
#N/A
1-280
114,360,065
3,715
1,087
3.42
1-300
135,090,011
6,375
1,601
3.98
1-320
165,517,582
8,623
2,338
3.69
1-340
202,145,941
11,864
2,626
4.52
1-360
245,153,384
13,651
2,603
5.24
1-380
296,992,047
15,646
3,166
4.94
1-400
362,477,783
17,409
3,672
4.74
1-420
427,693,696
21,056
3,598
5.85
1-440
492,189,289
16,572
2,487
6.67
1-460
537,184,802
21,506
2,828
7.60
1-480
611,002,813
27,019
3,566
7.57
1-500
684,307,775
23,865
3,835
6.22
1-520
791,173,724
61,768
6,029
10.25
1-540
932,978,751
68,064
7,644
8.90
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Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
1-560
1,102,314,394
69,797
8,383
8.33
1-580
1,277,145,257
79,573
8,362
9.52
1-600
1,437,647,740
79,229
7,623
10.39
1-620
1,598,117,050
84,079
7,765
10.83
1-640
1,782,203,269
97,944
8,838
11.08
1-660
1,985,397,190
117,827
10,295
11.45
1-680
2,227,141,284
138,337
12,328
11.22
2-0
2,826
96
42
2.23
2-20
3,982,063
1,375
226
6.08
2-40
9,638,829
2,457
334
7.36
2-60
22,139,618
5,662
1,213
4.66
3-0
2,539,670,055
162,648
13,968
11.64
3-20
2,850,424,644
181,508
15,033
12.08
3-40
3,174,234,615
192,819
15,815
12.19
3-60
3,484,324,230
210,382
16,319
12.89
3-80
3,830,102,749
223,768
15,864
14.10
3-100
4,173,990,465
225,205
14,732
15.29
3-120
4,503,723,010
244,598
16,374
14.94
3-140
4,839,280,618
253,991
14,621
17.37
3-160
5,157,305,369
257,806
15,025
17.16
3-180
5,478,310,850
274,198
14,578
18.81
3-200
5,795,814,600
352,370
18,560
18.98
3-220
6,105,707,924
362,128
20,518
17.65
4-0
2,969
44
31
1.47
4-20
355,186
97
55
1.76
4-40
885,454
341
158
2.16
4-60
3,100,397
341
119
2.89
4-80
4,929,500
611
170
3.60
4-100
7,134,166
958
230
4.18
4-120
10,484,613
924
216
4.26
Estuary Characterisation PB2693/R/303996/PBor
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Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
4-140
15,926,627
1,618
364
4.44
4-160
23,250,990
3,294
803
4.11
4-180
36,177,218
6,134
1,296
4.73
5-0
6,910,239,269
400,175
24,433
16.38
5-20
7,474,225,354
415,746
22,803
18.23
5-40
7,993,902,903
427,495
22,357
19.12
5-60
8,500,979,236
431,380
21,957
19.65
5-80
9,002,299,746
440,544
22,222
19.83
5-100
9,507,896,229
461,863
23,271
19.85
5-120
9,977,818,279
459,835
22,188
20.72
5-140
10,434,857,355
484,283
22,383
21.64
5-160
10,852,156,317
509,093
21,903
23.24
5-180
11,269,484,005
536,617
22,143
24.24
5-200
11,650,165,858
536,861
20,102
26.71
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Appendix D: Predicted equilibrium form of the Severn Estuary at each section
Section
Tidal Prism (m³)
Cross
-
Sectional Area (m²)
Width (m)
Mean Depth (m)
1-0
110,882,835
12,361
#N/A
#N/A
1-20
121,111,233
13,160
#N/A
#N/A
1-40
132,283,149
14,011
#N/A
#N/A
1-60
144,485,620
14,917
#N/A
#N/A
1-80
157,813,709
15,882
#N/A
#N/A
1-100
172,371,249
16,908
#N/A
#N/A
1-120
188,271,651
18,001
#N/A
#N/A
1-140
205,638,786
19,165
#N/A
#N/A
1-160
224,607,955
20,404
#N/A
#N/A
1-180
245,326,936
21,724
#N/A
#N/A
1-200
267,957,141
23,128
#N/A
#N/A
1-220
292,674,872
24,623
#N/A
#N/A
1-240
319,672,692
26,215
#N/A
#N/A
1-260
349,160,929
27,910
#N/A
#N/A
1-280
381,369,310
29,714
3,074
9.67
1-300
416,548,757
31,635
3,566
8.87
1-320
454,973,335
33,681
4,622
7.29
1-340
496,942,391
35,858
4,565
7.85
1-360
542,782,887
38,176
4,353
8.77
1-380
592,851,944
40,645
5,103
7.97
1-400
647,539,625
43,272
5,790
7.47
1-420
707,271,977
46,070
5,321
8.66
1-440
772,514,344
49,048
4,278
11.47
1-460
843,775,000
52,219
4,408
11.85
1-480
921,609,101
55,595
5,116
10.87
1-500
1,006,623,016
59,190
6,039
9.80
1-520
1,099,479,048
63,016
6,089
10.35
1-540
1,200,900,592
67,091
7,589
8.84
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Section
Tidal Prism (m³)
Cross
-
Sectional Area (m²)
Width (m)
Mean Depth (m)
1-560
1,311,677,776
71,428
8,481
8.42
1-580
1,432,673,611
76,046
8,174
9.30
1-600
1,564,830,719
80,962
7,706
10.51
1-620
1,709,178,670
86,197
7,862
10.96
1-640
1,866,842,011
91,769
8,555
10.73
1-660
2,039,049,022
97,702
9,374
10.42
1-680
2,227,141,284
104,019
10,689
9.73
2-0
1,102,267
468
94
4.96
2-20
2,996,271
952
188
5.07
2-40
8,144,710
1,936
296
6.53
2-60
22,139,618
3,938
1,012
3.89
3-0
2,441,281,097
111,025
11,541
9.62
3-20
2,501,481,214
112,962
11,857
9.53
3-40
2,580,556,594
115,486
12,240
9.44
3-60
2,684,425,423
118,767
12,261
9.69
3-80
2,820,861,488
123,022
11,763
10.46
3-100
3,000,075,984
128,521
11,129
11.55
3-120
3,235,481,744
135,603
12,192
11.12
3-140
3,544,697,087
144,681
11,035
13.11
3-160
3,950,864,413
156,265
11,698
13.36
3-180
4,484,382,238
170,970
11,511
14.85
3-200
5,185,180,285
189,537
13,613
13.92
3-220
6,105,707,924
212,855
15,731
13.53
4-0
1,801,158
663
118
5.61
4-20
2,513,718
840
162
5.17
4-40
3,508,176
1,065
279
3.81
4-60
4,896,054
1,349
235
5.73
4-80
6,832,994
1,709
284
6.01
4-100
9,536,211
2,166
345
6.28
4-120
13,308,855
2,744
373
7.35
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 54 - 12 May 2015
Section
Tidal Prism (m³)
Cross
-
Sectional Area (m²)
Width (m)
Mean Depth (m)
4-140
18,574,003
3,476
534
6.52
4-160
25,922,110
4,405
928
4.75
4-180
36,177,218
5,581
1,236
4.51
5-0
6,416,126,291
220,483
18,135
12.16
5-20
6,512,071,972
222,819
16,693
13.35
5-40
6,641,585,095
225,956
16,254
13.90
5-60
6,816,409,525
230,163
16,038
14.35
5-80
7,052,397,821
235,793
16,257
14.50
5-100
7,370,948,701
243,306
16,890
14.41
5-120
7,800,947,412
253,300
16,468
15.38
5-140
8,381,384,959
266,542
16,606
16.05
5-160
9,164,893,694
284,002
16,359
17.36
5-180
10,222,519,861
306,900
16,745
18.33
5-200
11,650,165,858
336,750
15,921
21.15
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 55 - 12 May 2015
Appendix E: Observed form of the Solway Firth at each section
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
1-0
63,063
N/A N/A N/A
1-10
266,685
N/A N/A N/A
1-20
485,522
N/A N/A N/A
1-30
1,067,330
N/A N/A N/A
1-40
2,026,106
N/A N/A N/A
1-50
3,851,452
43
261
0.17
1-60
6,777,733
237
489
0.48
1-70
11,033,457
322
560
0.58
1-80
16,423,915
767
1,226
0.63
1-90
24,136,788
1,881
1,614
1.17
1-100
33,219,289
2,259
1,952
1.16
1-110
41,620,242
2,943
1,772
1.66
2-0
151,090
N/A N/A N/A
2-10
423,202
N/A N/A N/A
2-20
751,639
14 101 0.14
2-30
1,241,623
42 108 0.38
2-40
2,543,218
130
336
0.39
2-50
4,797,631
307
456
0.68
2-60
6,499,155
400
415
0.97
2-70
9,391,532
422
307
1.37
2-80
12,897,515
707
628
1.13
2-90
16,781,705
590
310
1.90
2-100
19,011,324
745
453
1.64
2-110
22,529,570
1,067
692
1.54
2-120
26,722,310
1,403
938
1.50
2-130
76,244,371
4,578
2,631
1.74
3-0
89,767,688
4,833
2,348
2.06
3-10
100,410,968
5,437
2,391
2.27
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 56 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
3-20
112,410,464
5,771
2,086
2.77
3-30
123,001,782
5,644
2,000
2.82
3-40
133,386,869
5,387
1,754
3.07
3-50
146,011,719
6,696
2,327
2.88
3-60
158,521,301
6,784
2,228
3.05
3-70
173,201,685
8,976
2,918
3.08
3-80
191,649,797
11,163
3,739
2.99
3-90
215,123,211
12,540
4,327
2.90
3-100
237,000,243
13,288
4,792
2.77
3-110
10,984
N/A N/A N/A
4-0
76,031
N/A N/A N/A
4-10
215,619
N/A N/A N/A
4-20
473,130
N/A N/A N/A
4-30
850,924
36 76 0.48
4-40
1,805,162
38
215
0.18
4-50
4,332,898
275
536
0.51
4-60
7,262,659
935
758
1.23
4-70
10,488,338
997
800
1.24
4-80
19,463
N/A N/A N/A
5-0
68,623
N/A N/A N/A
5-10
212,527
N/A N/A N/A
5-20
569,889
50 140 0.36
5-30
1,411,858
101
121
0.83
5-40
4,209,194
472
494
0.96
5-50
23,321,897
4,084
2,585
1.58
6-0
32,761,428
3,946
1,927
2.05
6-10
431,276,886
29,308
7,754
3.78
7-0
529,088,915
38,433
9,395
4.09
7-20
599,087,474
50,495
10,111
4.99
7-30
655,154,255
50,499
10,364
4.87
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 57 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
7-40
2,327
N/A N/A N/A
8-0
76,391
N/A N/A N/A
8-10
169,399
N/A N/A N/A
8-20
353,305
N/A N/A N/A
8-30
630,283
33 66 0.49
8-40
967,018
48
81
0.59
8-50
1,366,900
69
118
0.59
8-70
2,792,424
311
421
0.74
8-80
4,559,781
358
424
0.85
8-90
6,412,587
439
462
0.95
8-100
9,314,849
970
945
1.03
8-110
14,094,387
1,425
1,272
1.12
8-120
20,592,683
2,328
1,928
1.21
8-130
30,381,840
3,495
3,065
1.14
8-140
41,461,121
4,877
3,600
1.35
9-0
927,149,475
67,029
11,037
6.07
9-10
1,009,604,877
78,823
10,884
7.24
9-20
1,084,020,595
80,923
10,744
7.53
9-30
1,167,480,801
83,660
11,031
7.60
9-40
1,248,293,251
89,257
11,792
7.94
9-50
1,334,019,963
96,098
12,192
8.42
9-60
1,416,161,492
99,855
11,979
8.88
9-70
1,502,400,975
116,410
13,237
8.79
9-80
1,612,682,340
131,423
14,967
8.78
9-90
1,766,625,739
146,520
16,546
8.86
9-100
1,889,689,818
155,312
17,305
8.97
9-110
2,025,265,448
164,095
18,877
8.69
9-120
2,174,879,126
193,954
19,811
9.79
9-130
2,304,759,172
197,374
20,139
9.80
9-140
2,458,273,677
206,124
20,974
9.83
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 58 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
9-150
2,599,041,306
212,218
20,965
10.12
9-160
2,762,558,069
232,452
21,913
10.61
9-170
2,956,034,451
256,631
22,953
11.18
9-180
3,111,588,665
271,023
23,461
11.55
9-190
3,315,116,368
291,857
24,609
11.86
9-200
3,470,719,956
306,250
25,413
12.05
9-220
3,848,737,666
347,267
24,923
13.93
9-230
4,012,429,385
372,723
25,335
14.71
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 59 - 12 May 2015
Appendix F: Predicted equilibrium form of the Solway Firth at each section
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
1-0
2,072,150
733
N/A N/A
1-10
2,721,856
889
N/A N/A
1-20
3,575,272
1,079
N/A N/A
1-30
4,696,269
1,310
N/A N/A
1-40
6,168,747
1,589
N/A N/A
1-50
8,102,908
1,929
1,739
1.11
1-60
10,643,510
2,341
1,537
1.52
1-70
13,980,696
2,841
1,663
1.71
1-80
18,364,231
3,449
2,598
1.33
1-90
24,122,188
4,185
2,399
1.74
1-100
31,685,506
5,080
2,926
1.74
1-110
41,620,242
6,165
2,563
2.40
2-0
1,675,762
630
N/A N/A
2-10
2,110,737
742
N/A N/A
2-20
2,658,618
874
809
1.08
2-30
3,348,712
1,030
539
1.91
2-40
4,217,932
1,213
1,028
1.18
2-50
5,312,774
1,430
982
1.46
2-60
6,691,803
1,684
851
1.98
2-70
8,428,785
1,984
666
2.98
2-80
10,616,633
2,337
1,142
2.05
2-90
13,372,376
2,753
670
4.11
2-100
16,843,424
3,243
945
3.43
2-110
21,215,448
3,821
1,309
2.92
2-120
26,722,310
4,501
1,676
2.69
2-130
76,739,524
9,519
3,792
2.51
3-0
79,372,327
9,749
3,333
2.93
3-10
82,830,624
10,049
3,251
3.09
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 60 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
3-20
87,373,243
10,437
2,805
3.72
3-30
93,340,163
10,939
2,784
3.93
3-40
101,177,963
11,583
2,572
4.50
3-50
111,473,242
12,408
3,166
3.92
3-60
124,996,522
13,459
3,135
4.29
3-70
142,759,919
14,790
3,745
3.95
3-80
166,092,886
16,469
4,541
3.63
3-90
196,741,717
18,573
5,265
3.53
3-100
237,000,243
21,197
6,051
3.50
3-110
522,184
275
N/A N/A
4-0
759,773
359
N/A N/A
4-10
1,105,463
469
N/A N/A
4-20
1,608,439
612
N/A N/A
4-30
2,340,264
799
357
2.24
4-40
3,405,065
1,042
1,124
0.93
4-50
4,954,340
1,360
1,192
1.14
4-60
7,208,522
1,775
1,045
1.70
4-70
10,488,338
2,317
1,220
1.90
4-80
209,563
144
N/A N/A
5-0
381,850
220
N/A N/A
5-10
695,775
338
N/A N/A
5-20
1,267,785
517
450
1.15
5-30
2,310,055
791
340
2.33
5-40
4,209,194
1,212
792
1.53
5-50
15,596,881
3,071
2,242
1.37
6-0
32,761,428
5,201
2,212
2.35
6-10
288,949,237
24,400
7,074
3.45
7-0
355,754,380
28,283
8,060
3.51
7-20
451,808,237
33,514
8,237
4.07
7-30
655,154,255
43,633
9,634
4.53
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 61 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
7-40
2,064,228
731
N/A N/A
8-0
2,557,530
851
N/A N/A
8-10
3,168,720
990
N/A N/A
8-20
3,925,969
1,153
N/A N/A
8-30
4,864,184
1,343
424
3.17
8-40
6,026,610
1,563
465
3.36
8-50
7,466,829
1,820
602
3.02
8-70
9,251,227
2,119
1,098
1.93
8-80
11,462,053
2,468
1,112
2.22
8-90
14,201,216
2,873
1,181
2.43
8-100
17,594,974
3,345
1,755
1.91
8-110
21,799,761
3,895
2,102
1.85
8-120
27,009,394
4,535
2,690
1.69
8-130
33,464,007
5,280
3,767
1.40
8-140
41,461,121
6,148
4,042
1.52
9-0
861,700,035
53,005
9,814
5.40
9-10
884,700,230
54,005
9,009
5.99
9-20
910,904,895
55,136
8,868
6.22
9-30
940,760,489
56,413
9,046
6.24
9-40
974,775,674
57,854
9,269
6.24
9-50
1,013,529,977
59,478
9,280
6.41
9-60
1,057,683,671
61,306
9,093
6.74
9-70
1,107,989,018
63,362
9,766
6.49
9-80
1,165,303,088
65,672
10,580
6.21
9-90
1,230,602,363
68,264
11,294
6.04
9-100
1,304,999,371
71,170
11,714
6.08
9-110
1,389,761,639
74,422
12,713
5.85
9-120
1,486,333,295
78,057
12,568
6.21
9-130
1,596,359,666
82,117
12,990
6.32
9-140
1,721,715,309
86,645
13,598
6.37
Estuary Characterisation PB2693/R/303996/PBor
Final Report - 62 - 12 May 2015
Section
Tidal Prism (m³) Cross-Sectional Area (m²) Width (m) Mean Depth (m)
9-150
1,864,535,957
91,689
13,780
6.65
9-160
2,027,254,896
97,301
14,177
6.86
9-170
2,212,644,429
103,538
14,579
7.10
9-180
2,423,863,106
110,462
14,978
7.38
9-190
2,664,509,536
118,141
15,657
7.55
9-200
2,938,683,701
126,648
16,342
7.75
9-220
3,606,950,869
146,481
16,187
9.05
9-230
4,012,429,385
157,990
16,495
9.58
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