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Disturbances in coarse bedload transport in a high-mountain stream channel system (Western Tatras, Poland)
Abstract
Bedload transport measurements in the formerly glaciated Chochołowski catchment located in the Western Tatras in Poland were performed in the period 1975 to 2018. Measurement of bedload displacement allowed to determine the role of bedload transport disturbances in both system connectivity and sediment transfer. Bedload transport occurred as many as triggered up to several times a year. The longest distances were observed during rain-on-snow events: between 12 m in headwaters and over 100 m in the fluvial valley. Bedload became activated along the entire length of the channel system (10.5 km) every 2 to 5 years when the stream discharge exceeded 10 m 3 s −1 (with an average of 1.25 m 3 s −1). In such situations, bedload dynamics increased downstream , and stream power was sufficient to overcome local barriers (i.e. boulder and log steps). Downstream increases in bedload dynamics may become disturbed and inverted by sudden snowmelt and locally heavy rainfall. These types of events caused the dynamics of bedload transport to be 190% to 320% greater in the upper part of the studied catchment and to decline in the downstream direction over a distance of 7 km to yield an attenuation effect. The same pattern was observed in local tributaries where the dynamics of bedload transport were 200% higher in the upper part and attenuation was observed along a distance of 1.5 km in the downstream direction. All events of this type cause seasonal disturbances in bedload transport. However, the greatest effect on bedload dynamics was produced by natural deforestation. A 16% decrease in forest cover causes a fluvial system disequi-librium manifested in an intensification of hydro-geomorphologic processes and formation of new landforms.
... According to Krzemień (1991) snow cover reaches a thickness of up to 1.5 m near the Polana Chochołowska station. However, the most recent studies showed decreasing trend in snow cover in whole Tatra Mts. in the period 1952-2013 (Szwed et al. 2017), although it usually still exceeds 1 m in Chochołowski Stream catchment (Błażejczyk 2019, Płaczkowska et al. 2020. Snow cover remains on the ground in the lower part of the Chochołowska Valley for 137 days, on average, and can last a maximum of more than 200 days. ...
... Photomaps from the years 1955/56, 1977, 1994, 1999, 2009, 2020, and 2021(made available by TNP 2023 were used to determine spatial and temporal changes in debris flow activity within the investigated track along with an orthophotomap from 2022. The debris distribution was marked on the maps starting at the outlet of the rock chute to the terminus of successive flows. ...
... However, in the most distant accumulation zone we noted 0.5 m clasts which were much bigger than those in the case of the flow in Smutná Valley, where rather small, up to 15 cm long clasts were found at the same elevation (Dlabáčková, Engel 2022). In comparison, clasts bigger than 30 cm never moved in the 2009-2018 period in Chochołowski Stream in glacial trough in Chochołowska Valley, even when 10-28 m 3 ·s −1 discharge was noted (Płaczkowska et al. 2020). Other parameters such as the location of the initiation zone, minimum height of material deposited in the accumulation zone, and difference in height between these zones achieved average values observed for debris flows in the Western Tatra Mountains (Kotarba et al. 2013). ...
In this study we used geomorphologic mapping, precipitation analysis and archival photomaps to study debris flows in the Starorobociański Cirque, which were renewed in 2018 and 2021. Debris flow in 2021 was most likely the largest single flow recorded in the Polish Western Tatras in the twenty first century. It reached a length of about 870 m and it covered an area of 0.62 to 0.65 ha. We set July 14, 2021, as the date of debris flow initiation and 52.1 mm·d−1 as its threshold value. This is one of the lowest values provided in the literature for such flows in the Western Tatra Mts. On the contrary, debris flow which occurred July 18, 2018 was linked to threshold value of 150.5 mm·d−1. On the basis of former debris flow occurrence as well as precipitation analysis we interpret the appearance of debris flow in 2021 as an effect of short and intense rainfall.
... There is an urgent need to develop and consolidate archives of extreme floods, as highlighted by Borga et al. (2008), Gourley et al., (2013Gourley et al., ( ,2014, and others. Recent studies have shown a growing interest among researchers in extreme floods in the mountains of central Europe at both regional and local scales (Kotarba, 1999;Marchi et al., 2010;Bissolli et al., 2011;Loczy, 2013;Kidová et al., 2016;Kijowska-Strugała et al., 2017;Płaczkowska et al., 2020). Developing archive records of flash floods and debris flows is an essential element in establishing verification systems that can assess the accuracy of warning techniques for these events (Borga et al., 2014;Nayrol et al., 2017). ...
... The peak discharge Q (m 3 s − 1 ) was calculated as a product of the cross-section area (m 2 ) and flow velocity (m s − 1 ). The calculated peak discharge was contrasted/compared with peak discharges indicated by direct measurement data and with specific discharges during flash/major floods triggered by extreme rainfall in the neighbouring Tatra streams (Punzet, 1981;Krzemień, 1992;Płaczkowska et al., 2020). The purpose of this comparison was to assess whether our estimation of peak discharge was realistic. ...
... The variability of these peak discharge is very high. They range from several m 3 s − 1 to more than 100 m 3 /s as shown by direct measurement data (Punzet, 1981;Krzemień, 1992;Płaczkowska et al., 2020) and estimated data (eg. Balastreos-Canovas et al., 2016). ...
The objective of this research was to investigate the geomorphological impacts of major flood events on Sucha Woda, a gravel-bed mountain stream in the Tatra Mountains, and to determine the contribution of sediment and wood in modifying the channel's geomorphology. The study included major floods in 2018, 1997 and 1973. An integrated approach was employed that included fieldwork, GIS analysis of 1974, 1999, 2018, and 2019 orthophotomaps, meteorological and hydrological analysis and literature review of the floods in 1973 and 1997. The geomorphological effects of the 2018 flood were assessed immediately after the event. This involved recording fresh undercutting of the channel banks, sediment accumulation in the channel and on the floodplain. Cross-sections of the channel were taken at selected sites, and the 25 largest boulders displaced moved during the flood were measured. At the same sites, the number of tree trunks was counted and divided into trunks aligned with or transverse to the course of the stream.
Major floods in the Sucha Woda stream result from the inflow of moist air masses from the northeastern sector that accumulate due to orographic effects. Daily precipitation ≥200 mm is the trigger for major flooding. The most common morphological response to the flood was channel widening. Changes in the channel's morphology varied along the stream and were influenced by the local relief in the valley, channel characteristics, flow rate, and availability of rock material and large wood. The supply of large wood to the Sucha Woda channel was significantly affected by windthrows and bark beetle attacks, which have been frequent in the Tatra Mountains in recent years. The intensity of remodelling of the stream increases along its course. The effects of major floods in the Sucha Woda are most significant in its lower reaches due to highly erodible glacial deposits there.
... As a result, numerous techniques have been developed for identifying bedload sediment transport, including tracers [13][14][15][16], optical methods, ultrasonic topography, and acoustic approaches. Radioactive particle tracking [17,18], radio-tracking techniques [19][20][21], magnetic tracer particles [22][23][24], painted particles [25], and the "Smartrock" tracer [26,27] are all examples of tracer techniques. This approach, however, has certain drawbacks. ...
Bedload sediment transport is critical in the natural river environmental and ecological system. It is quite challenging to measure the bedload sediment transport rate in rivers with any degree of accuracy. In this study, the authors developed the Gravel Pressure and Voice Synchronous observation system (GPVS) to estimate the bedload sediment transport rate in rivers. The core of the GPVS includes an underwater high-fidelity audio recorder and pressure sensor. The audio recorder is intended to monitor the low bedload sediment transport rate, whereas the pressure sensor is utilized to detect relatively substantial bedload sediment transport. The GPVS is tested by running flume experiments with natural gravel to evaluate the system’s reliability and feasibility. Results reveal that the GPVS has a very high sensitivity for detecting bedload transport. When the bedload sediment transport rate is relatively low, the audio recorder can measure it quantitatively, showing that the system is not impacted by ambient noise.
... This limitation might be a challenge and an arena for future research, and thus possible unanswered questions can be addressed. Our findings also show a consistent result with the current theory of sediment transport optimization as exhibited by [8,22,36], and the results show that riverbed erosion could be prevented by protecting the bed materials from flow-generated shear stress. It is tantamount to a lined channel but with the provision of locally available and low-cost armor materials that would correct shear stress-induced damage and become material groups for the armor. ...
The continuous movement of riverbed particles due to turbulent flow determines the stability of non-cohesive riverbeds and banks during riverbed and bank erosion and sedimentation. This study emulated the stable channel design by deriving the low maintenance cost of the channel through bed protection by an armor layer. The study investigated the effects of shear stress and grain size uniformity to determine the minimum non-cohesive armor layer thickness for the stability of riverbeds under steady uniform flow conditions. Experiments were conducted with four different discharges, five armor material gradations, and five bed-slope variations in a full-scale flume. We observed and recorded the behaviors of the five gradations of armor materials for given discharges and bed slopes. Eighty data points were recorded and analyzed. The hydraulic analysis of the flow along with the soil mechanics analysis of the armor materials was done. The soil mechanic analysis was particularly focused on the uniformity coefficient of the armor layer, Cu, to derive the armor layer equation. However, for the manageability of the study, we set the limit of the Cu between 3.0 and 6.0. From the viewpoint of non-erodibility, a wider Cu value indicated a thinner armor layer. Variables that govern the armor layer thickness and the layer thickness itself were derived and proposed. The variables, namely Cu, shear stress (t0 and tc), and mean diameter of the bed load and armor materials (Db50 and Da50). Our results show that these variables governed the thickness of the armor layer, and this is expected to contribute to the design of stable natural channels, which can minimize the cost of irrigation canal maintenance and development. Doi: 10.28991/CEJ-2022-08-06-01 Full Text: PDF
Windstorms account for about half of forest disturbances in Europe and are considered as drivers of natural regeneration of plant species, accompanied by significant changes in hydrological and soil moisture conditions. Our study aims to identify the effects of deforestation caused by catastrophic windthrow on streamflow characteristics in conjunction with meteorological and soil moisture data. The research was conducted in the headwater catchment (34.4 km2), in the Western Tatra Mountains, Poland. Streamflow characteristics such as average, high, and low flows, coefficient of variability, as well as flashiness of the streamflow (Richard-Baker Flashiness Index) and drought frequency (Standardised Streamflow Index) were calculated in the pre-event (2007–2013) and post-event (2013–2020) periods. The analysis was carried out for calendar months, seasons of the year, as well as for the growing and non-growing seasons. Wavelet Analysis was applied to track the changes in streamflow time series in relation to meteorological conditions (precipitation, air temperature) and remotely sensed soil wetness in the root zone. Principal Components Analysis was applied in order to identify the major drivers of the observed variability. The results revealed increasing low (+14%), average (+8%), and maximum monthly flows (16%) in the post-event period. The most significant increase in low flows (+47%, p<0.1) occurred during snowmelt. The increase in average (+31%, p<0.1) and high flows (+90%, p<0.05) occurred during mid-winter thaws. The frequency of hydrological droughts decreased by up to 36% in the post-event period, despite similar climatic conditions. The flashiness of streamflow increased during mid-winter thaws and rainfall events. The short-scale periodicity in streamflow increased in the post-event period. The results revealed the presence of 3-month periodicity in the root zone soil wetness associated with long-term soil water storage, which decreased after the catastrophic windthrow. Streamflow in the post-event period became strongly coupled with precipitation and root zone soil wetness at short period bands (∼3–16 days) in hydrologically active season. Short-scale correlation between precipitation and streamflow was decreasing with the gradual succession of vegetation.
In this study, we analyzed the effect of forest damage caused by forest fires or shallow landslides on the rainfall thresholds needed to initiate bedload transport in mountain watersheds. Two bedload monitoring systems were used: a slot sampler and a hydrophone. These were installed in a forest fire-damaged site, a shallow landslide-damaged site, and a control site. A total of 199 effective rainfall-bedload data were collected from 2014 to 2016. Total rainfall amount (TR) and peak rainfall intensity (PRI) were found to be the most effective rainfall indices to predict bedload transport initiation. We also used linear functions to estimate percent threshold lines of the rainfall indices and used performance evaluations to select a representative threshold line for each site. As a result, the forest fire-damaged and shallow landslide-damaged sites had lower TR and PRI reference values than the control site, indicating that lower TR and PRI values can initiate bedload transport in damaged mountain forest areas. Although our study results may be limited by different characteristics between the study sites and insufficient data collection, it still indicates that forest damage caused by forest fires and shallow landslides affects bedload transport. Future studies are needed to further confirm these findings.
W ostatnich dekadach XX w. i na początku XXI w. obserwuje się w skali globalnej znaczący wzrost temperatury powietrza. W odniesieniu do pozostałych elementów klimatu informacje są rozbieżne. Na przykład, relacjonowany jest wzrost opadów lub ich zmniejszanie. Brak jest praktycznie informacji, czy i w jakim stopniu zmieniają się wilgotność powietrza i prędkość wiatru. Obszary górskie są miejscami szczególnie wrażliwymi na zmiany klimatu. Dlatego też w opracowaniu tym podjęto próbę porównania sezonowych i wieloletnich zmian niektórych elementów klimatu i wskaźników klimatycznych w polskich wysokich górach (Tatry i Karkonosze) w latach 1951–2015. Stwierdzono szereg podobieństw i różnic klimatycznych pomiędzy badanymi pasmami, zwłaszcza w odniesieniu do gradientów wysokościowych temperatury powietrza (gradient średniej rocznej temperatury powietrza wynosi -0,53°C w Tatrach i -0,55°C na 100 m wzniesienia w Karkonoszach)1 i pokrywy śnieżnej (10 cm/100 m w Tatrach i 12 cm/100 m w Karkonoszach). Opady atmosferyczne cechują się gradientem wysokościowym 38 mm/100 m w Karkonoszach i 53 mm/100 m w Tatrach. Stwierdzono także podobieństwa i różnice tempa zmian różnych elementów klimatu w analizowanym wieloleciu. W przypadku średniej rocznej temperatury istotny trend zmian wynosi w obydwu pasmach około 0,3°C na 10 lat. W większości przypadków zmiany charakterystyk opadowych oraz dotyczących pokrywy śnieżnej są nieistotne statystycznie.
The natural wood regime forms the third leg of a tripod of physical processes that supports river science and management, along with the natural flow and sediment regimes. The wood regime consists of wood recruitment, transport, and storage in river corridors. Each of these components can be characterized in terms of magnitude, frequency, rate, timing, duration, and mode. We distinguish the natural wood regime, which occurs where human activities do not significantly alter the wood regime, and a target wood regime, in which management emphasizes wood recruitment, transport, and storage that balance desired geomorphic and ecological characteristics with mitigation of wood-related hazards. Wood regimes vary across space and through time but can be inferred and quantified via direct measurements, reference sites, historical information, and numerical modeling. Classifying wood regimes with respect to wood process domains and quantifying the wood budget are valuable tools for assessing and managing rivers
Connectivity describes the efficiency of material transfer between geomorphic system components such as hillslopes and rivers or longitudinal segments within a river network. Representations of geomorphic systems as networks should recognize that the compartments, links, and nodes exhibit connectivity at differing scales. The historical underpinnings of connectivity in geomorphology involve management of geomorphic systems and observations linking surface processes to landform dynamics. Current work in geomorphic connectivity emphasizes hydrological, sediment, or landscape connectivity. Signatures of connectivity can be detected using diverse indicators that vary from contemporary processes to stratigraphic records or a spatial metric such as sediment yield that encompasses geomorphic processes operate over time and space. One approach to measuring connectivity is to determine the fundamental temporal and spatial scales for the phenomenon of interest and to make measurements at a sufficiently large multiple of the fundamental scales to capture reliably a representative sample. Another approach seeks to characterize how connectivity varies with scale, by applying the same metric over a wide range of scales or using statistical measures that characterize the frequency distributions of connectivity across scales. Identifying and measuring connectivity is useful in basic and applied geomorphic research and we explore the implications of connectivity for river management. Common themes and ideas that merit further research include; increased understanding of the importance of capturing landscape heterogeneity and connectivity patterns; the potential to use graph and network theory metrics in analyzing connectivity; the need to understand which metrics best represent the physical system and its connectivity pathways, and to apply these metrics to the validation of numerical models; and the need to recognize the importance of low levels of connectivity in some situations. We emphasize the value in evaluating boundaries between components of geomorphic systems as transition zones and examining the fluxes across them to understand landscape functioning.
The aim of this paper is to determine the detailed structure of the headwater valley segment in the Western Tatra Mountains. Seven sub-catchments were chosen in Chochołowski Stream catchment for a detailed geomorphological analysis of the headwater valley segments. Selected sub-catchments represent both the crystalline and the sedimentary environment of the study area. Geomorphological mapping was used to study the structure of headwater valleys which strictly depends on the conditions of the natural environment and can be varied even in such a small area as the catchment of Chochołowski Stream. Statistical analysis showed that headwater valleys in the crystalline part differ in terms of morphometric parameters from these in the sedimentary part and are characterized by greater gradients but smaller channel widths and roughness. However, the structure of headwater valley segment in the longitudinal profile shows a similar pattern in both parts of the study area and follows a sequence of valley types: unchanneled valley-colluvial channel (channel without steps)-cascade channel.
Headwaters, defined here as first- and secondorder streams, make up 70%‒80% of the total channel length of river networks. These small streams exert a critical influence on downstream portions of the river network by: retaining or transmitting sediment and nutrients; providing habitat and refuge for diverse aquatic and riparian organisms; creating migration corridors; and governing connectivity at the watershed-scale. The upstream-most extent of the channel network and the longitudinal continuity and lateral extent of headwaters can be difficult to delineate, however, and people are less likely to recognize the importance of headwaters relative to other portions of a river network. Consequently, headwaters commonly lack the legal protections accorded to other portions of a river network and are more likely to be significantly altered or completely obliterated by land use.
The present paper examines variability of characteristics of snow cover (snow cover depth, number of days with snow cover and dates of beginning and end of snow cover) in Poland. The study makes use of a set of 43 long time series of observation records from the stations in Poland, from 1952 to 2013. To describe temporal changes in snow cover characteristics, the intervals of 1952–1990 and of 1991–2013 are compared and trends in analysed data are sought (e.g., using the Mann–Kendall test). Observed behaviour of time series of snow-related variables is complex and not easy to interpret, for instance because of the location of the research area in the zone of transitional moderate climate, where strong variability of climate events is one of the main attributes. A statistical link between the North Atlantic Oscillation (NAO) index and the snow cover depth, as well as the number of snow cover days is found.
The aim of the research was to recognize the diversity of the textural and chemical characteristics of the sediments removed from the catchments of the Tatra Mts., which were affected by the windthrow event in 2013. The diversity was related to the morphometry of the catchments and the effects of the windthrow event. In catchments sediments deposited during low transport dynamics dominated, two of the catchments were characterized by some percentage (up to 10%) of the sediments deposited in the high-energy environment. However, this diversity is connected to the morphometry of the studied catchments and up to this time impact of the windthrows is not clearly reflected in the textural characteristics of the sediments.
A rising exposure to flood risk is a predicted consequence of increased development in vulnerable areas and an increase in the frequency of extreme weather events due to climate change. In the face of this challenge, a continued reliance on engineered at-a-point flood defences is seen as both unrealistic and undesirable. The contribution of “soft engineering” solutions (e.g. riparian forests, wood in rivers) to integrated, catchment scale flood risk management has been demonstrated at small scales but not larger ones. In this study we use reduced complexity hydrological modelling to analyse the effects of land use and channel changes resulting from river restoration upon flood flows at the catchment scale. Results show short sections of river-floodplain restoration using engineered logjams, typical of many current restoration schemes, have highly variable impacts on catchment-scale flood peak magnitude and so need to be used with caution as a flood management solution. Forested floodplains have a more general impact upon flood hydrology, with areas in the middle and upper catchment tending to show reductions in peak magnitude at the catchment outflow. The most promising restoration scenarios for flood risk management are for riparian forest restoration at the sub-catchment scale, representing 20-40% of the total catchment area, where reductions in peak magnitude of up to 19% are observed through de-synchronisation of the timings of sub-catchment flood waves. Sub-catchment floodplain forest restoration over 10-15% of total catchment area can lead to reductions in peak magnitude of 6% at 25 years post-restoration. This article is protected by copyright. All rights reserved.
The most important geomorphic responses to storms are qualitative changes in system state. Minor storms produce no state change or very rapid recovery to pre‐storm state, and extinction events wipe out the system. In other cases disturbance results in a state change, which may be transitional (change to a previously existing state), state space expansion (change to a new state), and clock‐resetting events that return the system to its initial state. Recovery pathways are much more varied than the monotonic progressions represented in classic vegetation succession and linear channel evolution models. Those linear sequential pathways are only one of several archetypal recovery pathways, which also include binary, convergent, divergent, and more complex networks. Filter‐dominated systems are more likely to follow linear sequential or convergent patterns, whereas amplifier‐dominance is characteristic of divergent and more complex mesh or fully‐connected patterns. Amplifier domination is also more likely to lead to evolutionary or state space expansion responses. Amplification and filtering in geomorphic response and recovery can be assessed using the 'Four R's' framework of response, resistance, relaxation, and recursion. High resistance and resilience, rapid relaxation times, and stable recursive feedback networks reduce or offset effects of disturbances, thus filtering their impacts. Conversely, low resistance and resilience, slow relaxation, and dynamically unstable feedbacks can exaggerate disturbances, creating disproportionately large and long‐lived impacts, thereby amplifying disturbances.
Unless new filter mechanisms evolve (either autogenically or anthropically), or the number of extinction or clock‐resetting events increases, intensified storminess will result in more geomorphic variability. These ideas are applied to a case study of a flood on the Clark Fork River, Montana, USA. Copyright © 2016 John Wiley & Sons, Ltd.
This chapter describes a role of extreme rainfall events in the development and transformation of channels in the Polish Carpathians. An analysis was based on the example of four selected events, which occurred in different parts of Polish Carpathians (Western Tatra, Bieszczady, and Beskid Niski Mountains) during the period 2003–2010. The findings underline that changes of the largest extent follow short and heavy rainfalls. Furthermore, their geomorphic impacts are the most significant in small watersheds. The research showed that the largest transformations of mountain rivers occur in the main channel, while the floodplain is only locally altered. The regularities identified in the study areas are relevant for mountain river channels in forested terrains, where a large supply of woody debris, for example, stems and branches, is ensured.
In formerly glaciated mountain settings, Pleistocene glaciations are
responsible for profound spatial reorganization of the landscape structure.
By imposing local channel slope and the degree of hillslope–channel
connectivity, glacial macro-forms can exert first-order controls on the
downstream strength and continuity of the coarse sediment cascade. To
estimate quantitatively these controls we trace bedload transport for 3
years along Strimm Creek, Eastern Italian Alps. Specifically, we monitor
the travel distance of 490 PIT-tagged particles (b axis: 23–229 mm; weight:
83–6525 g) at two contrasting sites: Upper Strimm Creek (US; 4 km2),
which flows through a fluvially dominated hanging valley, and Lower Strimm
Creek (LS; 7.5 km2), located downstream, in a relict glacial trough
where it experiences periodic colluvial sediment inputs from lateral
tributaries. Tracer positioning within the streambed is periodically tracked
in the field with a portable antenna in order to assess progressive travel
distances, as well as the extent of the channel active layer, in relation to
snowmelt and rainfall-driven peak flows. Interestingly, we show that tracer
virtual velocities for selected inter-survey periods are independent of
tracer weight at both study sites. Cumulatively, tracers in US have
travelled across distances (i.e. inner quartiles) shorter than 2 m,
which correspond to over 2 orders of magnitude less than what was observed in
LS. These figures translate, after calculations of tracer inter-survey
virtual velocities, into estimated bedload volumes equal to about 3 m3 in
US and 600 m3 in LS, with most of the transport (75 % in US, and
93 % in LS) occurring during snowmelt. A similar contrast in bedload
transport rates, even without considering the additional volumes of material
mobilized by mass-wasting processes in LS, testifies the extent to which the
glacial imprinting can still affect contemporary sediment transfer, and thus
postglacial landscape evolution, in mountain drainage basins.
W pracy podjęto próbę przedstawienia dwóch zjawisk: wiatrołomów i gradacji kornika drukarza, które w okresie minionych 100 lat kilkakrotnie dotknęły drzewostany TPN. Materiały historyczne dotyczące nasilenia i zasięgu terytorialnego obu zjawisk, dostępne w postaci danych o różnym stopniu szczegółowości, poddano integracji i przedstawiono w formie numerycznej przy zastosowaniu technologii GIS, a następnie dokonano ich analizy w ujęciu czasowo−przestrzennym. Określono obszary największych szkód od wiatru w poszczególnych okresach oraz zasięg i dynamikę rozwijających się w ich następstwie gradacyjnych wystąpień kornika drukarza. Dokonano próby określenia zależności czasowo−przestrzennych między tymi zjawiskami oraz oceny niektórych ich skutków w drzewostanach Parku.
The geomorphic complexity of streams influences the residence time of carbon by creating storage sites for organic matter. Instream wood increases complexity, particularly where logjams initiate multithread channels, as documented previously for lowland alluvial channels. Analogous processes in mountain streams can substantially increase the residence time of organic matter in streams otherwise characterized by high transport capacity and limited storage. Thresholds in wood mobility that reflect valley and channel geometry and wood dimensions determine the distribution of multithread channels and the mechanics underlying these thresholds. Valleys must have a lower gradient and less lateral confinement to develop multithread channels, but also larger volumes of riparian and instream wood. Hydrostatic forces that mobilize jams increase more slowly in low-gradient, unconfined channels, facilitating jam persistence. Treefall can thus initiate complex behavior that results in a self-enhancing feedback between wood recruitment and channel geometry. Patches of old-growth forest and multithread channels were formerly more widespread. Carbon retention has likely decreased as multithread channels have become less common.
The Tatra Mountains extend along the border between Slovakia and Poland in the Western Carpathians. It is the highest and formerly one of the most glaciated massifs in the entire Carpathian mountain chain. We present a paleoglaciological map of the Tatra Mts. and its foreland (ca. 1270 km2) for the Last Glacial Maximum (LGM) at the scale 1:50 000. We re-evaluate geomorphological evidence for all Tatra glacial systems identified in the literature based on new field mapping and remote sensing data analysis. Utilizing geographic information systems (GIS) and 10 m resolution digital elevation model (DEM) as a base topography, we have reconstructed, for the first time, the detailed extent and surface geometry of all Tatra LGM glaciers (55 glacier systems, total area ca. 280 km2) based on the distribution of glacial erosional and depositional landforms. Our research results confirm stronger glaciation on the southern slopes of the Tatras due to local topography. We also conclude that distinct morainic amphitheaters, which predominate on the southern side of the highest, eastern part of the Tatras, were formed by debris-covered glaciers.
The impact of human activities on sediment yield is poorly constrained in the northeastern United States, as in other northern hemisphere forested landscapes previously occupied by Pleistocene ice sheets. This study examines changes in sedimentation in Little Kennebago Lake (LKL), Maine, in relation to extreme hydrologic events and land-use change. Historical records indicate minimal disturbance before the onset of commercial logging after 1891. Sediment cores record >1,200 years of sedimentation and allow comparison of prehistoric and anthropogenic conditions. Variations in sediment yield are evaluated in the context of hydrologic records and reconstructions of road density and timber harvest derived from historical topographic maps and aerial photographs. Cores collected before and after a highly erosive, localized extreme rainfall event in July 2018 provide a template for interpreting clastic layers found earlier in the record. The frequency of these events increased around 1900, with five layers in the previous ∼1100 years and 12 layers from 1900-2018. This timing corresponds to an increase in suspended sediment yield from 2.0 Mg/km²/yr to 6.4 Mg/km²/yr and increased abundance of pollen taxa associated with forest disturbance. A relative lack of discrete erosion events since ∼1970, in spite of increasing timber harvest, suggests that modern best management practices may be effective in reducing erosion. The 2018 event is a reminder, however, that ongoing reevaluation of management practices is necessary in light of changing hydrology, in this region and elsewhere.
Research on factors affecting sediment regime in glacierized catchments under warming climates is still scarce despite its societal relevance. In particular, coarse bedload transport has never been quantitatively related to water runoff origin (snowmelt vs glacier melt), which provides important information on the role of different sediment sources (glaciers vs hillslopes and channel bed). Drawing on data from multiple spatial and temporal scales in a paradigmatic Alpine glacierized catchment, we show that glacier melt flows play a key role in coarse sediment transport dynamics. Bedload concentration measured during glacier melt flows is up to 6 orders 2 of magnitude larger than during snowmelt. At the catchment scale and within the channel, however, minimal aggradation and degradation was detected over almost a decade. In addition, sedimentation rates at a hydropower weir, inferred from flushing frequency during the last four decades, are tightly associated to summer air temperature and not to precipitation trends, and most of sediment export occurred in July-August. However, sediment flushing frequency has been decreasing since the late 1990s despite very warm summers in the following decades. Collectively, these findings indicate that sediment is dominantly sourced from within glacier-covered areas and that transport rates are thus dictated by seasonal and multi-annual glacial dynamics. As glacier melt flows decrease due to ice mass loss, our results suggest that, for similar basins, a progressive shift from supply-limited (driven by glacier activity) to transport-limited (during rainfall-induced events) sediment transport will occur, disrupting the current near-equilibrium channel conditions.
Connectivity has become a key issue in the study of processes acting in hydro-geomorphic systems and has strong implications on the understanding of their behaviour. Given the high complexity of hydro-geomorphic systems and the large variety of the processes controlling the efficiency of water and sediment transfer through a catchment, mapping hydrological and sediment connectivity is fundamental to understand the linkages between different parts of the system and the role played by system configuration, natural landforms and man-made structures in favouring or obstacolating the continuity of runoff and sediment pathways. Furthermore, the analysis of changes on connectivity through time can help to investigate the effect of both natural and anthropic disturbance on water and sediment fluxes and associated processes. This special issue aimed to shed light on the latest advances inmapping water and sediment connectivity by means of field measurements, modelling and geomorphometric approaches along with quantitative methods for the analysis of connectivity temporal evolution.The special issue is composed of twenty-one papers presenting a huge variety of topics dealing with hydrological and sediment connectivity and their changes through time in different geographical andclimatic regions of the world, at different spatial and temporal scales. This special issue highlights the importance of connectivity assessment to properly address sediment and water-related issues and to improve management strategies in hydro-geomorphic systems.
The need for reducing erosion and sediment transport, and protecting human settlements has urged the construction of channel control works in mountain streams. Such works include different structures, the check dams being probably the most widespread and archetypical. The performance of channel control works in mountain streams has usually been evaluated based on their effectiveness in stabilizing the channels and the adjacent hillslopes, and in retaining sediment. The widespread presence of channel control works in mountain streams of various regions, however, has also a significant impact on hillslope-channel coupling and on sediment transfer across the channel system. Taking into account the impact of channel control works on sediment connectivity at the catchment scale has remarkable importance for planning new control structures and managing the existing ones and, more in general, for sediment management in mountain catchments. In this paper, we consider the channel control works within the conceptual framework of sediment connectivity and related terminology, and we analyze the spatial and temporal scales of the interactions between channel control works and sediment coupling-decoupling. Some examples from the Italian Alps outline the impact of check dams and other hydraulic structures on sediment connectivity and show the potential of geomorphometry in assessing such impacts. The overall effect of control works on sediment connectivity consists of an alteration of the sediment cascade with a decrease in the efficiency of sediment transfer. Sediment is partly retained behind grade-control dams within the catchment or in sediment traps equipped with retention check dams near the alluvial fan apex. These works contribute to decouple the alluvial fans from the upslope catchment. On the contrary, channel bed lining on alluvial fans favors sediment transfer to the receiving river, but this effect usually involves small amounts of sediment not retained by the check dams built upstream.
The aim of the study was to examine the magnitude and frequency of bedload transport in 1st–3rd order headwater catchments (with an area <2 km²) in the Western Tatras. The study was carried out in 19 headwater catchments, divided into two groups: (1) alpine catchments, and (2) montane catchments. Bedload transport measurements were carried out at intervals over a period of 40 years using the painted stone method. During large floods observed in the main stream (Q > 5 m³ s⁻¹), usually associated with prolonged rainfall or a combination of both snowmelt and rainfall, it can be expected that bedload will also be activated in its tributaries and headwater sections. Bedload transport may also occur during snowmelts, but this type of flood is of a completely local nature. During small local floods, the role of the local channel structure is quite important. Channel topography can slow down transported bedload, which is particularly visible within alluvial fans. During high-energy events, this effect disappears, and the distance of bedload transport increases downstream. In alpine channels, bedload transport occurs on a much smaller scale than is the case in montane channels and it occurs along the entire channel length. The distance of bedload transport, its size, and the frequency of occurrence of geomorphologically-active floods are smaller in alpine catchments than in montane ones. In alpine channels, the maximum distance of bedload transport was 18.2 m, and in montane channels, distances reached 165 m.
Data from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel‐bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step‐pool and riffle and pool channels. The riffle‐pool sequence seems to be a more efficient trap for travelling gravels than the step‐pool pair. Additionally, in step‐pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained‐stone conditions), and second and subsequent observations of tracer displacements (constrained‐stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained ones also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel‐bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing.
An investigation to determine the transition zone between the hillslope system and the fluvial system in connection with the degree of differentiation of sediment located upstream and downstream of log steps (LS) and boulder steps (BS) was undertaken in the channel of a stream located in a small mountain catchment area in the Silesian Beskidy Mts. (Polish Carpathians), a stream channel into which large wood was introduced artificially several years ago. Granulometric analyses of 41 samples taken from upstream (21) and downstream (20) of steps were performed to analyze the effectiveness of log steps and boulder steps in the differentiation of mineral sediment.
It was demonstrated that log steps have a significant impact on the functioning of the channel by shaping its longitudinal profile and affecting the transport of mineral material, whereas boulder steps retain less sediment, showing the beneficial effects of artificial creation of log steps in mountain stream channels. Results of granulometric analyses suggested the location of the theoretical transition zone between the hillslope system and the fluvial system, suggesting the location and range of this zone in small mountain catchment areas can change depending on the intensity and range of slope processes in the longitudinal profile of the channel.
Previous measurements of bedload transport in gravel-bed streams revealed a large temporal and spatial variability of bedload transport rates. Using an impact plate geophone system, continuous bedload transport measurements were made during six years in two mountain streams in Austria. The two streams have a snow- and glacier-melt dominated hydrologic regime resulting in frequent transport activity during the summer half year. Periods of days to weeks were identified which are associated with approximately constant Shields values that indicate quasi-stable bed conditions. Between these stable periods, the position of the bedload transport function varied while its steepness remained approximately constant. For integration time scales of several hours to one day, the fluctuations in bedload transport decreased and the correlation between bedload transport and water discharge increased. For integration times of about 70 to 100 days bedload transport is determined by discharge or shear stress to within a factor of about two, relative to the six year mean level. Bedload texture increased with increasing mean flow strength and mean transport intensity. Weak and predominantly clockwise daily hysteresis of bedload transport was found for the first half of the summer period.
The process of tree uprooting plays an important role in sediment transport within forested slopes. In this study we determine the intensity of sediment transport by uprooting and its dependencies. We use the Principal Component Analysis (PCA) to determine the most important factors influencing root plate sizes, and we calculate sediment flux by windthrow for the area of the Tatra National Park (211.6 km²), for the year 2013, when an extreme windthrow event occurred. For this purpose we combine field data with the results of windthrow areas mapping based on satellite imagery. We also use data recorded by the Tatra National Park concerning the amount of destroyed wood in the forests of the Tatra National Park to calculate sediment flux by windthrow for the period between 1968 and 2015 (48 years). To calculate the transport distance by the uprooting needed to compute sediment flux, we modified the Gabet et al.'s (2003) Eq. (11), which in our opinion gives results which are twice the actual size. The PCA identified two the most important factors influencing root plate sizes. The first factor shows that all root plate dimensions increase with increasing tree DBH (diameter at breast height), and, to a lesser degree, with an increase in the content of coarse material. The second factor shows that the depth of a root plate increases with the downslope angle of tree fall, decreasing slope inclination, and decreasing the content of coarse material. The sediment flux by windthrow in 2013 in the area of the Tatra National Park was 3.55 × 10− 4 m³ m− 1 yr− 1. Mean annual sediment flux for the period of the last 48 years was 2.76 × 10− 5 m³ m− 1 yr− 1.
Suspended sediment transport is an important contributor to ecologic and geomorphic functions of streams. However, it is challenging to generalize predictions of sediment yield because it is influenced by many factors. In this study, we quantified the relevance of natural controls (e.g., geology, catchment physiography) on suspended sediment yield (SSY) in headwater streams managed for timber harvest. We collected and analyzed six years of data from 10 sites (five headwater sub-catchments and five watershed outlets) in the Trask River Watershed (western Oregon, United States). We used generalized least squares regression models to investigate how the parameters of the SSY rating curve varied as a function of catchment setting, and whether the setting modulated the SSY response to forest harvesting. Results indicated that the highest intercepts (α) of the power relation between unit discharge and SSY were associated with sites underlain primarily by friable rocks (e.g., sedimentary formations). The greatest increases in SSY after forest harvesting (up to an order of magnitude) also occurred at sites underlain by the more friable lithologies. In contrast, basins underlain by resistant lithologies (intrusive rocks) had lower SSY and were more resilient to management-related increases in SSY. As such, the impact of forest management activities (e.g., use of forested buffers; building of new roads) on the variability in SSY was primarily contingent on catchment lithology. Sites with higher SSY, or harvest-related increases in SSY, also generally had a) lower mean elevation and slope, b) greater landscape roughness, and c) lower sediment connectivity. We used principal component analysis (PCA) to further explore the relationship between SSY and several basin physiographic variables. The PCA clearly separated sites underlain by friable geologic units from those underlain by resistant lithologies. Results are consistent with greater rates of weathering and supply of sediment to headwater streams in catchments with more friable lithologies, and limited sediment supply in catchments underlain by resistant lithologies. We hypothesize that a similar framework may aid in predicting the overall SSY of a catchment as well as its susceptibility to increases in SSY following forest harvesting.
Bedload monitoring techniques have been developed and applied for many years in rivers ranging from steep mountain torrents to the large gravel-bed Danube River in Austria. Most monitoring stations use a combination of direct (mobile bag samplers, slot samplers) and indirect (geophones, hydrophones) measurement methods. Each individual technique is adequate, yet features particular boundary conditions and limitations related to hydraulic and sampling efficiency, functionality during floods, sampling duration or grain size. We show the capabilities and limitations of the different monitoring devices with respect to technical, operational and economic criteria, evaluating their suitability for determining bedload transport parameters. Bedload monitoring results of a measuring site at the Drau River in Carinthia/Austria are used to illustrate the specific range of the device application. We present an integrated automatic and continuous bedload monitoring system. It complements the specific limitations of single monitoring methods by additional measurement devices, enabling comprehensive monitoring of the bedload transport process. We then derive the Bedload Discharge Integrated Calculation Approach and the Bedload Rating Curve Approach and discuss their application for determining bedload discharge Qb and total bedload mass Vb. Whereas the integrated approach combines data from direct monitoring methods with indirect techniques, the rating curve approach uses only data from direct bedload monitoring devices. We demonstrate that applying an integrated automatic and continuous bedload monitoring system and combining the Bedload Discharge Integrated Calculation Approach and Bedload Rating Curve Approach yields accurate bedload discharge results.
The article presents the results of a 34-day series of direct semi-continuous measurements of the bedload transport, performed during the melt season 2013 in the Scott River catchment (NW part of the Wedel-Jarlsberg Land, Spitsbergen). The daily variability of the bedload transport rates and its relation to the dissolved and suspended load was analysed in two cross-sections located in the lower course of a proglacial gravel-bed river. The bedload flux was measured by means of two 4-module sets of River Bedload Traps (RBT). In the research period, a total of 34 day-long samples of bedload and 34 water samples for the determination of the dissolved and suspended load were collected in each of the two cross-sections, 200 m apart. The measurements showed longitudinal and temporal variability of bedload flux and the remaining components (solutions, suspensions) corresponding with changes in water discharge. The mean daily bedload flux QB in consecutive measurement cross-sections XS amounted to 124 kg d− 1 XS I and 59 kg d− 1 XS II, respectively, and showed high variability and an evident relationship with temperature. The unequal bedload flux the two measuring stations XS I and XS II suggests aggradation within the alluvial fan that separates the two reaches. The volume and rate of bedload flux were determined by the frequency by the occurrence of flood flows during which approximately from 59% XS I to 77% XS II of total bedload was discharged. The highest recorded daily values constituted 12% of total bedload flux. Through the cross-section below the alluvial fan, the river discharged approximately 6 million cubic meters (M m³) of water, 3 t of bedload Lb, 2238 t of suspended load Ls, and 613 t of dissolved load Ld. The relationships between the three components of the load suggest a 500-fold excess of suspended load, and a 100-fold excess of dissolved load in relation to bedload flux. High temporal and latitudinal variability of the bedload transport rate resulted in changes in the channel morphology, and during floods – also the floodplain.
By way of introduction, sets the Tatra mountains in the high mountain systems of the temperate zone, describes climatic controls, and the postglacial evolution of the relief. There is a section on geoecological belts and description of present-day morphogenetic processes: sediment transfer in the high mountains, the morphodynamics of slopes; the morphodynamics of channels, and the Alpine rockwall-debris slope process-response systems. -K.Clayton
Clifford et al. (1992) argue that differences in hydraulic conditions at the moment of sediment entrainment in Turkey Brook, flood by flood, are attributable to variations in the form drag that arises from cluster microforms. While not denying the possibility of variable cluster concentration, we show that the time-dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold. Clifford et al. further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event. This merits further examination. However, we point out that clusters are not only observed at low flow and, therefore, already present and able to contribute significant form drag as the next bedload generating flood waxes, but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events. The revelation by Clifford et al. that flow structure in the River Quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements (perhaps clusters) is not surprising. There is, however, some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing. Nevertheless, the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport.
Climate change is expected to modify the hydrological and geomorphological dynamics of mountain watersheds significantly, so impacting on downstream water yield and sediment supply. However, such watersheds are often poorly instrumented, making it difficult to link recent and rapid climate change to landscape response. Here we combine unique records of river flow and sediment export, with historical archival imagery to test the hypothesis that climate warming has substantially increased both water yield and sediment export from small Alpine watersheds (<3 km²) characterized by small (<0.5 km² surface) glaciers. To examine ice and landform response to climate change, we apply archival digital photogrammetry to historical aerial imagery available from 1967 to present. We use the resulting data on ice loss, in combination with reliable records of stream flow from hydroelectric power intakes and climate data to approximate a water budget and to determine the evolution of different contributions to river flow. We use the stream flow records to estimate volumetric sediment transport capacity and compare this with the volumes of sand and gravel exported from the watersheds, quantified from records of intake flushing. The data show clearly that climate forcing since the early 1980s has been accompanied by a net increase in both water yield and sediment transport capacity, and we attribute these as signals of reduced snow accumulation and glacier recession. However, sediment export has not responded in the same way and we attribute this to limits on sediment delivery to streams because of poor rockwall-hillslope-channel connectivity. However, we do find that extreme climate conditions can be seen in sediment export data suggesting that these, rather than mean climate warming, may dominate watershed response.
This study uses a unique 10-year tracer dataset from a small gravel-bed stream to examine bed mobility and sediment dispersion over long timescales and at a range of spatial scales. Seasonal tracer data that captured multiple mobilizing events was examined, while the effects of morphology on bed mobility and sediment dispersion were captured at three spatial scales: within morphological units (unit scale), between morphological units (reach scale) and between reaches with different channel morphologies (channel scale). This was achieved by analyzing both reach-average mobility and travel distance data, as well as the development of 'mobility maps' that capture the spatial variability in tracer mobility within the channel. The tracer data suggest that sediment transport in East Creek remains near critical the majority of the time, with only rare large events resulting in high mobility rates and grain travel distances large enough to move sediment past dominant bedforms. While a variable capturing both the magnitude and frequency of flow events within a season yielded a better predictor to sediment mobility and dispersion than peak discharge alone, the distribution of events of different magnitude within the season played a large role in determining tracer mobility rates and travel distances. The effects of morphology differed depending on the analysis scale, demonstrating the importance of scale, and therefore study design, when examining the effect of morphology on sediment transport.
This chapter reviews sediment tracing techniques in gravel and sand bed rivers. The objective is to assess critically the use of tracers to obtain quantitative information of bedload sediment transport. The chapter presents an evaluation of the coarse particle tracing techniques for studying sediment transport in a fluvial environment. The use of tracers depends on the selection of methods and work procedures. Both study objectives and channel characteristics determine the type of tracers to be used, the duration of experiments and data analyses. Data analyses and interpretations depend largely on the study objectives, experimental design and tracing method, all of which control the quality of the data. Radioactive tracers allow in situ, continuous detection of exposed and buried particles and make possible immediate data processing and hence evaluation of the tracking strategy. The chapter also reviews several techniques that have been used in tracing bed material load in fluvial systems.
Flow conditions that produce incipient motion of bedload at threshold conditions in coarse-bed rivers are illustrated using data from the Oak Creek, Oregon, bedload research facilities. A dynamically varying transport conditions occur between threshold and full transport. The threshold for initiation of motion is not sharp. A second threshold, usually overlooked, occurs on the recession limb of runoff events and differs not only in “critical” discharge value but also in composition of the bedload and dynamics of the reforming stable bed. Bed reformation and subsequent disturbance relate to hydrograph dynamics and changing turbulence in ways that are not yet well described.
In the face of rapid climate warming, rapid glacier recession should lead to a marked increase in the spatial extent of the paraglacial zone in glaciated drainage basins. The extent of the paraglacial zone has been well established to be transient but there are very few studies of this transient response and what it means for sediment export. There is good reason to expect that glacier recession could increase basin-scale sediment connectivity as: sediment becomes less dependent on glacier surface transport; proglacial streams are more able to migrate laterally than subglacial streams and so access sediment for transport; and glacier debuttressing may aid the development of gullies that can dissect moraines and so aid hillslope to proglacial zone connectivity. By using records of the flushing of hydroelectric power installations we were able to develop a record of coarse sediment (sand and gravel) export from a basin with a rapidly retreating valley glacier, the Haut Glacier d'Arolla, from 1977 to 2014. Modelling suggested that these data could only be partially controlled by transport capacity implying an important role for sediment supply and potentially for the influence of changing sediment connectivity. Indeed, there was evidence of the effects of glacial debuttressing upon gullying processes and hence a possible increase in the ease of connection of upstream basins to the proglacial area. More recently, we were able to show possible temperature control on sediment export, which may only have become apparent because of the progressive development of better sediment connectivity. However, whilst rapid glacier recession should result in theory in a progressive increase in connectivity of sediment sources to the basin outlet, the supply to capacity ratio does not increase continually with glacier recession until maximum capacity is reached. We identified two possible examples of why. First, gullying was also accompanied by the sediment accumulation at the base of moraines that was too coarse to be transported by the proglacial stream, maintaining disconnection of the upper basins. Second, the sediment capacity ratio appeared to be elevated during periods of more rapid retreat and we attribute this to the importance of a continued supply of unworked glacial till before fluvial reworking and sorting of freshly exposed sediment increased the resistance of sediment to entrainment and hence export rates. Thus, the transient geomorphic response of glaciated basins to glacier recession may involve negative feedbacks that can reduce the extent to which increases in connectivity elsewhere in the basin lead to increased sediment export.
The aim of studies was to determine the conditions that are necessary for debris flow initiation, to recognize the morphological role of debris flows in the moulding of high-mountainous slopes. Debris flows are small, although path lengths tend to exceed 100 m. Debris flows having a recurrence-interval somewhere between 1 year and 5 years are little effective morphologically. In past times the debris flows were more effective in the moulding of the high-mountainous relief of the area examined. These events were due to the impact of man on the natural environment of the Tatra Mts during the last 200-300 years (pasture, deforestation, mining activities). -from Author
Relationships between erosion, sediment transport and deposition processes in river channels under different conditions of sediment yield formation and variable contribution of suspended (basin-derived) and bed (channel-originated) sediment are discussed. The role of the latter in total sediment yield and as a factor of morphological channel type differentiation is shown.
The term extreme rainfall refers to an event during which the thresholds of various hydrological and geomorphic processes are exceeded. The frequency of extremes varies in different climatic zones and in time. The clustering of extreme events happens when the extremes are repeated every 2-3 years, every year or even several times a year. Such clusterings disturb the equilibrium of slope and river channel systems and are separated by periods of stability and recovery. The occurences of clusters are exemplified by present - day processes, historical records and geological records. On this base a model of phases with frequent and rare clusterings during the Holocene was constructed.
Fluvial transfers through tributaries and avalanches most important sediment sources•Drainage area morphometries control supply of sediments into main stream channels•Longitudinal main valley bottom profiles control importance of in-channel storage•Low annual bedload yields of 2.4 t km- 2y- 1 (Erdalen) and 13.3 t km- 2y- 1 (Bødalen)•Bedload transport accounts for about one-third of the total fluvial load
Recession of high-mountain glaciers in response to climatic change frequently results in the development of morainedammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3km2) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an icecontact moraine-dammed lake to develop. The lake had a maximum volume of 5.5 × 105m3 and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 105m3 of material was removed from the terminal moraine during breach development. Numerical dam-breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine-dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146-2200m3 s-1. Results from two-dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9m, with maximum flow velocities exceeding 20ms-1 within 700m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300m long and 100m wide debris fan originating at the breach exit. moraine-dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine-dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact.
The timing and rate of fluvial bedload transport are of central importance within sediment budget studies and in many applications in river science and engineering. During the years 2010, 2011 and 2012 detailed field measurements with portable impact sensors as a non-invasive technique for indirectly determining fluvial bedload transport intensity were conducted in two instrumented and supply-limited drainage basin systems (Erdalen and Bødalen) in the fjord landscape in western Norway. Additional field measurements with portable impact sensors were carried out in 2010 and 2011 in selected transport-limited fluvial systems in the Coast Mountains of western Canada. The collected impact sensor field data were calibrated with laboratory flume experiments. The data from the impact sensor field measurements in western Norway and the flume experiments were combined with field data from continuous discharge monitoring, repeated surveys of channel morphometry and sediment texture, particle tracer measurements, Helley–Smith samplings, underwater video filming and biofilm analyses. The combination of methods and techniques applied provides insights into the temporal variability and intensity of fluvial bedload transport in the selected mountain streams: (i) in the transport-limited systems with generally high bedload transport rates during high discharge and with bedload material moving in clusters over the impact sensor plates, impact sensor data (based on a 1 s measuring interval) provide the opportunity to detect the start and end of bedload transport, thus to identify discharge thresholds for sediment entrainment, and to roughly estimate the intensity and relative intensity of change of bedload transport during the measuring period; (ii) in the supply-limited systems with low bedload transport rates and bedload components moving separately (as single particles) over the impact sensor plates, impact sensor data (with a 1 s measuring interval) allow the detection of the start and end of transport of bedload components > 11.3 mm, thus the identification of discharge thresholds for possible entrainment of particles, the quantification of the number of particles > 11.3 mm moving over the impact sensor plates during the measuring period, the rough estimation of grain sizes of the particles moving separately over the impact sensor plates, and the calculation of the total mass of the bedload material > 11.3 mm moving over the impact sensor plates during the measuring period; (iii) when combined with other methods and techniques (Helley–Smith sampling, particle tracer measurements, biofilm analyses, underwater video filming) which provide information on the active bedload transport channel width, on discharge thresholds for possible entrainment of particles of different grain sizes, and on transport rates of bedload material < 11.3 mm, total rates of fluvial bedload transport, covering all given grain sizes of the bedload material, can be calculated for the supply-limited mountain streams with generally low bedload transport. The higher measured annual bedload yield in Bødalen (13.6 t km− 2 yr− 1) compared to Erdalen (2.6 t km− 2 yr− 1) reflects a higher level of slope–channel coupling in Bødalen than in Erdalen.
The role of roads and footpaths in middle mountains in Europe is of key significance to present day relief. Dependencies related to the way natural road cuts, also known as hollow ways or sunken lanes, were formed and to their present day evolution are described in this paper. The relationship between land use that includes forest maintenance and the pace of transformation on and around unpaved roads (either dirt or gravel roads) is emphasized, given that the roads are found in the woodland zone. The study area selected was the middle mountain section of the Gorce Mountains. Detailed geomorphological mapping of 382.5 km of roads and paths in the area was performed. Most are dirt roads and footpaths. The mapping procedure consisted of a systematic note-taking method (logbook) and the classification of roads and paths by identifying uniformly shaped segments within them. The density of the road network in the study area was 4.14 km*km-2, which is typical of the Beskidy Range of the Carpathian Mountains in Poland. A total of 1,312 segments of roads and paths were identified using the methodology mentioned above. The segments were classified using a statistical grouping procedure into three groups of segments, where each group included segments that were shaped alike. In order to determine relationships between particular segments of roads and paths, 15 examples of roads were found that in their arrangement followed the pattern of slope catena. In order to determine the present day dynamics of shape transformations occurring within hollow ways, repetitive geodetic measurements were performed at 19 cross-sections between 2004 and 2008. A decrease in depth was observed only in 6 out of 19 cross-sections, which means that accumulation was the principal process shaping local relief and equaled 0.02- 1.98 cm*year-1. The remaining cross sections mostly showed signs of further deepening of hollow ways, progressively cutting into the ground below at an average rate of 0.08-4.13 cm*year-1 due to changes in the intensity of land use.
The study on bedload transport was conducted on the gravel-bed Scott River catchment with a glacial alimentation regime, located in the NW part of the Wedel Jarlsberg Land (Spitsbergen) with subpolar climatic conditions. In the melt season of 2010, bedload transport rate was continuously monitored at 24-hour intervals by means of four River Bedload Trap devices aligned across the width of the channel. The maximum bedload transport rate varied strongly at portions of the cross section from 16 to 152 kg m− 1 d− 1 in cross-profile I (c-p I) and 4 to 125 kg m− 1 d− 1 in cross-profile II (c-p II). The maximum channel-mean bedload transport rate (qa) amounted to 54 kg m− 1 d− 1 (c-p I) and 35 kg m− 1 d− 1 (c-p II). Mean daily bedload discharge (Qb) was estimated at a level of 97 kg day− 1 (c-p I) and 35 kg m− 1 d− 1 (c-p II), and the total bedload yield was determined at approx. 4345 kg in the measurement period (2086 kg — c-p I; 2203 kg — c-p II from 13.07 to 10.08). The analysis of the relationship between channel-mean bedload transport rate and water velocity or shear stress revealed a significant value of the correlation coefficient (R2 = 0.6). Discharge and rate of bedload transport were dependent on the weather and number of days with flood discharge. Approx. 58% of the entire discharged bedload was transported during 3 violent ablation–precipitation floods. Bedload grain size distribution was right-skewed and showed moderate sorting.
Anthropogenic modification of forests has often decoupled streams from
riparian ecosystems and altered natural wood recruitment processes.
Extensive research has shown that large wood significantly impacts
channel dynamics, especially in small and intermediate sized forested
streams where wood pieces are similar in length to channel width, and
many stream rehabilitation efforts now involve the addition of large
wood to streams. The primary objective of this research is to
investigate the relation between large wood and reach scale channel
morphology and hydraulics using a physical model, in order to better
inform stream rehabilitation programs and future modeling efforts. Four
experiments, each comprising numerous five hour runs, were conducted
using a Froude-scaled stream table with wood loads scaled to 0
m3/m2, 0.011 m3/m2, 0.016
m3/m2, and 0.022 m3/m2. The
addition of large wood significantly decreased the reach-averaged
velocity in all experiments, and was associated with decreased sediment
transport and increased sediment storage in the reach. Increases in bed
and water surface slope compensated for the loss of energy available to
transport sediment, and enabled the system to reach a new steady state
within the equivalent of 6 to 9 years. Adding wood increased pool
frequency, as well as the variability in cross-sectional depth, while
causing the reach to undergo a transition from a plane-bed to a
riffle-pool morphology. Retention of fine sediment increased the
availability of fish spawning substrate, while increased water stage
improved connectivity between the channel and the floodplain. The
changes in habitat complexity were generally related to the wood load
added to the reach, but were also dependent on the orientation and
arrangement of the pieces. These results demonstrate that wood may exert
a primary control on channel morphodynamics and the availability of
aquatic habitat in intermediate sized streams, and suggest that the
benefits from stream rehabilitation efforts are highly dependent on
project scale. The relatively long time needed to realize habitat
benefits demonstrates that long term monitoring of rehabilitation
projects is necessary.