Bank Erosion Rates Research Articles

Spatial and Temporal Variability of Bank Erosion during the Period 1930–2016: Case Study—Kolubara River Basin (Serbia)

The fluvial process is characterized by an intense meandering riverbed. The aim of this study was to perform a reconstruction of the lateral migration of a 15 km length of an active meandering river during the period 1930–2016. River morphological changes were analyzed and quantified from cadastral maps and aerial photographs as well as by geodetic survey and GIS. Hydrological characteristics and extreme hydrological events were evaluated in relation to bank erosion rate. The rate of bank erosion was markedly different from the long-term studied meanders, just like in the short-term period. During the 87 years of observation (from 1930 to 2016), the length of the Kolubara River was enlarged by 3.44 km. The average migration rate of the Kolubara River for monitored meanders in the period 1930–2010 was 1.9 m·year−1, while in the period 2010–2016, the average migration rate was 3.3 m·year−1. The rate of bank erosion was more intensive across the entire short-term period than during the longer period, and the maximum annual rate of bank erosion during the period 2010–2016 varied between 0.3 and 11.5 m. It is very likely that in the period from 2010, frequent discharge variations and rapid change of its extreme values caused more intensive bank erosion. These research results will be valuable for river channel management, engineering (soft and hard engineering), and planning purposes (predicting changes in river channel form) in the Kolubara River Basin.

Introducing nested spatial scales in multi-stress models: towards better assessment of human impacts on river ecosystems

We investigated the relationships between the ecological status of wadeable rivers and the intensity of various stressors related with hydromorphological alterations, nutrient, and organic matter contaminations. The French invertebrate-based multimetric index (I2M2), which efficiently responds to the effects of both physical and chemical and hydromorphological stressors, was used as descriptor of river reach ecological status. We developed a model focusing on the effects of hydromorphological and physical and chemical stressor gradients on the I2M2 in different physiographic contexts. The potential confounding effects of natural geographic conditions and watershed scale pressure gradients were taken into account and neutralized by gathering watersheds into homogeneous clusters integrated as an interaction factor in the model. Whatever effects were considered (general or within-spatial clusters), the I2M2 was impaired similarly by the same stressor types, being negatively influenced by an increase in BOD5, ammonium, nitrite, nitrate, and total phosphorus concentrations. The I2M2 was also negatively influenced by variables describing hydromorphology at the reach scale, especially by the ‘loss of sinuosity,’ ‘increasing rates of bank erosion,’ ‘flow reductions,’ and ‘alteration of pool/riffle succession.’ The I2M2 was generally more strongly impaired by physical and chemical pressures than by hydromorphological alterations.

Volumetric measurement of river bank erosion from sequential historical aerial photography

Understanding of the relative contribution of bank erosion to sediment budgets in New Zealand is limited. Few measurements of bank erosion rates exist, and this is a major limitation to the development of a locally calibrated model of bank erosion. The New Zealand sediment budget model, SedNetNZ, predicts bank erosion based on preliminary data, and this study aims to underpin the development of an improved model for bank erosion. Photogrammetric techniques and LiDAR were used to collect data on bank erosion rates for five different river reaches, ranging from 3 to 14km in length, in the Kaipara Catchment, Northland, New Zealand. Changing river channel planform between the 1950s and 2015 was assessed using four to five well-spaced dates of historical aerial photographs. Changes in planform were combined with bank height, to calculate erosion and accretion volumes which were compared with SedNetNZ modelled estimates. Erosion and accretion is relatively evenly balanced in the study sites. The largest difference in terms of relative proportions of erosion and accretion are found along the Tangowahine River (13.4km reach length), where 492,000m3 of sediment eroded between 1956 and 2015 compared to 364,000m3 of accretion. Lateral migration rates (erosion) for the five river reaches range between 0.14myr−1 and 0.21m yr−1 and are comparable with those measured by previous assessments in New Zealand. The migration rates in channel widths per year for the three larger rivers (stream order 5–6) range between 0.4% and 0.8% of channel width per year. In contrast, the smaller streams (stream order 3–4) are retreating more rapidly, with width-averaged rates of 1.7% and 3.0%. Current SedNetNZ modelling tends to underestimate the bank height and greatly overestimates the migration rate.

The geomorphic impacts of culverts at paved forest roads: Examples from Carpathian headwater channels, Czech Republic

Road-stream crossings constructed with culverts have the potential to alter geomorphic processes in adjacent stream channels. We examined changes in channel geometry and bed sediments in longitudinal profiles related to culverts at five paved forest road crossings with perennial headwater streams and two gullies initiated by point releases of water through ditches. A dendrogeomorphic approach was used for dating of exposed tree roots and estimation of bank erosion rates in incised cross-sections downstream of road crossings. The studied forest road crossings concentrate surface runoff from the road net, act as stabilizing elements in stream longitudinal profiles and decelerate bedload transport in the case of culvert clogging, which caused notable upstream and downstream changes in fluvial processes. Notable fining of the coarsest fraction in upstream cross-sections was observed only for two culverts with the highest depositional tendencies. Much lower width-depth ratios (<4) and typically higher channel slopes were measured in channel reaches downstream of the culverts, compared with upstream cross-sections. Two road-induced gullies typically had very low width/depth ratios close to 2. At five of a total of seven studied culverts, the highest documented erosion rates occurred immediately downstream of these structures. The mean annual erosion rates reconstructed from cross-sections at the positions of individual tree root samples ranged from 0.2 to 6.5cm·year−1, and the erosion occurred over 2–5year intervals, depending on the studied location. The highest number of observed changes in cross-sectional geometry caused by channel erosion were connected with relatively high-magnitude flow events (with recurrence intervals >10years; specific discharge >1500l·s·km−2), whereas more frequent flows (with recurrence intervals of 2–10years) were not always recorded by exposed tree roots.

Improving bank erosion modelling at catchment scale by incorporating temporal and spatial variability

AbstractBank erosion can contribute a significant portion of the sediment budget within temperate catchments, yet few catchment scale models include an explicit representation of bank erosion processes. Furthermore, representation is often simplistic resulting in an inability to capture realistic spatial and temporal variability in simulated bank erosion. In this study, the sediment component of the catchment scale model SHETRAN is developed to incorporate key factors influencing the spatio‐temporal rate of bank erosion, due to the effects of channel sinuosity and channel bank vegetation. The model is applied to the Eden catchment, north‐west England, and validated using data derived from a GIS methodology. The developed model simulates magnitudes of total catchment annual bank erosion (617–4063 t y‐1) within the range of observed values (211–4426 t yr‐1). In addition, the model provides both greater inter‐annual and spatial variability of bank eroded sediment generation when compared with the basic model, and indicates a potential 61% increase of bank eroded sediment as a result of temporal flood clustering. The approach developed within this study can be used within a number of distributed hydrologic models and has general applicability to temperate catchments, yet further development of model representation of bank erosion processes is required. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Analysis of fundamental physical factors influencing channel bank erosion: results for contrasting catchments in England and Wales

Channel bank erosion processes are controlled by numerous factors and as such are both temporally and spatially variable. The significance of channel bank erosion to the sediment budget is difficult to quantify without extensive fieldwork/data analysis. In this study, the importance of key physical factors controlling channel bank erosion, including channel slope, upstream catchment area, channel confinement, and sinuosity, was explored using regression analysis. The resulting analysis can be used in practical studies to provide a first approximation of bank erosion rates (in catchments similar to those investigated). A data set of channel bank erosion rates covering eight contrasting river catchments across England and Wales, over a time period of up to 150 years, was created using a modified GIS methodology. The best predictors were found to be upstream area, channel confinement, and sinuosity with respect to dimensionless width-averaged retreat rates (m m−1 yr−1). Notwithstanding these relationships, the results highlight the variability of the magnitude of sediment production by channel bank erosion both within and between catchments.

URBAN STREAM RESTORATION AND APPLIED PRACTICES IN NORTHEAST ILLINOIS

INTRODUCTION: In-stream and watershed dynamics in urban and urbanizing areas have significant impacts on local property and infrastructure, as well as the quality of the stream itself including: water quality, habitat, physical characteristics, and biodiversity. As land development occurs, natural vegetation and exposed soils are converted to buildings, pavement and other impervious surfaces. This leads to increased runoff during storm events as well as decreasing the time that it takes that stormwater to reach streams, wetlands, and other stormwater storage and conveyance systems. These hydrologic changes in a watershed often occur at a rapid pace which results in rapid destabilization and degradation of streams and rivers. Rivers and streams are naturally dynamic systems. They naturally erode and reshape themselves based on changes to the watershed or the stream itself. Erosion and deposition are natural processes that have always been important components of stream systems and in and of themselves are not undesirable. When natural stream dynamics are rapidly accelerated, however, an entire series of negative impacts to the stream and the biological systems that are depended on the stream occur. Rapid destabilization of streams often leads to significant bank and bed erosion that negatively impact stream health and frequently leads to negative impact to property, buildings and structures, as well as public infrastructure. Past approaches to stream bank and bed stabilization often involved channelization, armoring, and other gray infrastructure techniques to protect public and private property in the effected reaches of streams and rivers without taking into account the overall stream system dynamics. Early stabilization efforts frequently led to other unintended consequences by accelerating the rate of bank and bed erosion in untreated reaches, inadvertent flooding, and other infrastructure impacts. The complex nature of stream dynamics and fluvial geomorphology when applied to urban stream systems and significantly modified watersheds require the need for detailed analysis of the morphology of the stream. Consideration of the complex factors and processes that make up fluvial morphology are critical when selecting practices or methods of stream restoration. Many agencies and cooperative partners work to accumulate and analyze case studies and detailed research in order to develop a method of evaluating and prescribing different stream restoration techniques based on the morphologic conditions in the stream reach (Lyn D.A., and Newton J.F., 2015). An accumulation of case studies, research, and scholarly work on stream restoration techniques and practices helps shape and inform designers across multiple agencies in order to effectively select and design restoration practices. Ultimately, in urban streams, the designer is working to establish a condition of dynamic equilibrium in the treated stream reach. Dynamic equilibrium is defined as a stream reach that is in balance with sediment transport, aggradation, degradation, and bank and bed erosion. When those characteristics are in balance based on the inputs of sediment within the watershed, the bed load and sediments the stream transports, and discharge rate and volume, then the stream is considered to be in a relatively stable state (FISRWG, 1998). The selection then of stream restoration and stabilization practices in urban areas is dependent on not only the reach being treated, but also on the overall watershed dynamics. In addition to the physics of the actual practices implemented, including resistance to shear stresses and velocity of the water flow within the stream channel being treated, the practices must also take into account the larger picture of stream dynamics including sediment delivery and transport, within the watershed and not just within the treated reach. Successful urban stream restoration and stabilization techniques mimic the structures found in more undisturbed systems through the utilization of similar materials in an engineered configuration. In many streams the use of a combination of hard and soft armorment and stabilization solutions including stone, woody debris materials, modern geosynthetic reinforcement devices and native vegetation to stabilize and naturalize stream channels, thereby provided enhanced habitat, better water quality, and protecting property and infrastructure.

Development and Testing of a Physically Based Model of Streambank Erosion for Coupling with a Basin‐Scale Hydrologic Model SWAT

AbstractA comprehensive streambank erosion model based on excess shear stress has been developed and incorporated in the hydrological model Soil and Water Assessment Tool (SWAT). It takes into account processes such as weathering, vegetative cover, and channel meanders to adjust critical and effective stresses while estimating bank erosion. The streambank erosion model was tested for performance in the Cedar Creek watershed in north‐central Texas where streambank erosion rates are high. A Rapid Geomorphic field assessment (RAP‐M) of the Cedar Creek watershed was done adopting techniques developed by the Natural Resources Conservation Service (NRCS), and the stream segments were categorized into various severity classes. Based on the RAP‐M field assessment, erosion pin sites were established at seven locations within the severely eroding streambanks of the watershed. A Monte Carlo simulation was carried out to assess the sensitivity of different parameters that control streambank erosion such as critical shear stress, erodibility, weathering depth, and weathering duration. The sensitive parameters were adjusted and the model was calibrated based on the bank erosion severity category identified by the RAP‐M field assessment. The average observed erosion rates were in the range 25‐367 mm year−1. The SWAT model was able to reasonably predict the bank erosion rates within the range of variability observed in the field (R2 = 0.90; E = 0.78). Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.

Bank erosion and accretion dynamics explored by GIS techniques in lower Ramganga river, Western Uttar Pradesh, India

The rivers of Indo-Ganga–Brahmaputra (IGB) plain are widely known for their dynamicity and hazard proneness. All of the large rivers that originated from Himalaya and flowing into the IGB plain (like Ganga, Ghagra, Ramganga, Rapti, Bagmati, Koshi, Teesta, Brahmaputra, etc.), shift their position vigorously and create massive kind of bank erosion. In the present study, we have selected 35.30 km alluvial reach of lower Ramganga river which is one of the major tributaries of the Ganga River. The study aims to quantify the amount and rate of bank erosion and accretion phenomena, also to find out the nature of bank erosion and accretion. In the present paper, we assessed the bank erosion and accretion of 91 years. We have taken 1923 toposheets, and Landsat images of 1972, 1981, 1993, 2003, and 2014 for the study. Demarcation of bank lines and estimation of bank erosion and accretion areas (for left and right bank separately) were done by using ArcGIS. Detailed field works were also done for investigating the nature of bank erosion and accretion as well as for bank line demarcation. The study reveals that in all of the evaluated periods, the river has experienced differential rates and amounts of bank erosion and accretion. The widening of channel, extension of meander bends along with lateral movements of the channel and chute cutoff are mainly responsible for bank erosion and accretion, while liquefaction and flowage of bank materials and shear failure were the principle mechanisms for that.

Assessing channel changes of the Ganges-Padma River system in Bangladesh using Landsat and hydrological data

The Ganga/Ganges11Nomenclature is always a problem with international rivers. The river known as the Ganges in Bangladesh is known as the Ganga in India. In this paper the system will be called the Ganga but that section with at least one bank in Bangladesh will be called the Ganges. Similarly, the Brahmaputra is sometimes referred to in Bangladesh as the Jamuna. is an important river system in South Asia which supports the life and livelihoods of millions of people both in India and Bangladesh. The system has a number of names throughout its length. Below its confluence with the Brahmaputra at Aricha it is known as the Padma, which in turn merges with the Upper Meghna at Chandpur below which the channel is known as the Lower Meghna. There is a growing concern about this large river system because its channels are subject to frequent migration, threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability, land use change, and agricultural intensification as the basin experiences rapid population growth. Concerns have been expressed that the construction of a barrage just upstream of the Indo–Bangladesh border has adversely affected the Ganges reach in Bangladesh. Partly in order to investigate this, the planform changes of the Ganges and the Padma within Bangladesh was analysed over the period 1973 to 2011 using multitemporal Landsat images and long–term flow data in eight epochs with an average duration of 4.5years. The Padma reach is less affected by the barrage and provides a useful control study. Areas of erosion and deposition were determined from sequential changes in the bankline positions. Mean channel width, sinuosity and braiding index were analysed using a Geographic Information System (GIS). Flood frequency, duration and magnitudes were studied using long-term discharge records. Generally, channel planform evaluation indicated that both the Ganges and the Padma experienced contraction, expansion and readjustment in configuration over the last 38years. Erosion and deposition statistics of the Ganges indicate that 57km2 of land was lost along the right bank whereas around 59km2 has been gained along the left bank during the assessment period, suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank. The width of the Ganges varied from a maximum of 5.36 to a minimum of 3.23km during the observation period. Changes to sandbar area are, in general, much more radical than changes to the overall width and area of the channel. Measurement of areas of erosion and accretion showed that both banks of the Padma experienced considerable loss of land. The total net loss for left bank and right bank was 155 and 28km2, respectively. The Padma is approximately twice the width of the Ganges and the changes to its channel area are not as temporally dynamic as the Ganges. The relationship between bank curvature and erosion/accretion of the river banks for both rivers was analysed and the results contradict established meander theory. Regression analysis between bank erosion rates, annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the Padma (r2=0.6283) and that the Ganges bank erosion rate is influenced by mean flood flow (r2=0.6738). The flood frequency shows generally good stability across the first eight of the nine epochs for the Ganges but for the Padma the frequency showed even greater stability. We were unable to support the widely held belief that the upstream barrage has a deleterious effect on the Ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices.

Effects of forested floodplain soil properties on phosphorous concentrations in two Chesapeake Bay sub-watersheds, Virginia, USA.

Aquatic ecosystems are known to undergo fluctuations in nutrient levels as a result of both natural and anthropogenic processes. Changes in both extrinsic and intrinsic fluvial dynamics necessitate constant monitoring as anthropogenic alterations exert new pressures to previously stable river basins. In this study, we analyzed stream water and riparian zone soil phosphorous (P) dynamics in two third-order sub-watersheds of the lower Chesapeake Bay in Virginia, USA. The Ni River is predominantly forested (70% forested), and Sugarland Run is a more human impacted (>45% impervious surfaces) sub-watershed located in the suburbs of Washington D.C. Total stream P concentrations were measured during both high and low flows and Mehlich-3 methods were used to evaluate potential P fluxes in riparian soils. The results show total stream P concentrations in Sugarland Run ranged from 0.002 to 0.20ppm, with an average of 0.054ppm. In contrast, the forested Ni River had typical stream P concentrations <0.01ppm. Total soil P was significantly higher in the more urbanized Sugarland Run basin (23.8 ± 2.1ppm) compared to the Ni River basin (16 ± 3.7ppm). Average stream bank erosion rates and corresponding cut-bank P flux rates were estimated to be 7.98cmyear(-1) and 361kg P year(-1) for Ni River and 9.84cmyear(-1) and 11,600kg P year(-1) for Sugarland Run, respectively. The significantly higher values of total P in the stream water and floodplain cut-banks of Sugarland Run suggests erosion and resuspension of previously deposited legacy sediments is an important processes in this human-impacted basin.

Bank vulnerability and avulsion modeling of the Bhagirathi-Hugli river between Ajay and Jalangi confluences in lower Ganga Plain, India

The emergence of the study of fluvial dynamics of the alluvial monsoon rivers especially in India is being enriched through the application of remote sensing and GIS techniques. To study fluvial dynamics over time, channel avulsion no doubt is one of the most considerable phenomena. The study section (~75 km) in between the two confluences of the Bhagirathi River i.e., Ajay (Katwa) and Jalangi River (Nabadwip) confluence in Lower Ganga Plain of West Bengal is highly oscillatory in nature. Several satellite images and toposheets of different time periods (1955, 1973, 1990 and 2011) have been used in order to analyse the avulsive nature in this section of the river. These fluvial features (cut offs, ox-bow lakes, point bars etc.) form as a result of channel avulsion and have been identified with the help of Landsat satellite images. The decadal differences in sinuosity ratio in different parts of the studied section, channel migration rate and avulsion model explain the dynamic nature and crucial portions of high bank erosion rate of the channel in the mentioned time period. For both the reaches (R1 and R2), meander loop cut offs are dominant rather than lateral avulsion in the context of aggregate channel migration that also reflects the instability and erosional vulnerability at different meander bends.

The alluvial architecture of a suspended sediment dominated meandering river: the Río Bermejo, Argentina

AbstractThe alluvial architecture of fine‐grained (silt‐bed) meandering rivers remains poorly understood in comparison to the extensive study given to sand‐bed and gravel‐bed channels. This paucity of knowledge stems, in part, from the difficulty of studying such modern rivers and deriving analogue information from which to inform facies models for ancient sediments. This paper employs a new technique, the parametric echosounder, to quantify the subsurface structure of the Río Bermejo, Argentina, which is a predominantly silt‐bed river with a large suspended sediment load. These results show that the parametric echosounder can provide high‐resolution (decimetre) subsurface imaging from fine‐grained rivers that is equivalent to the more commonly used ground‐penetrating radar that has been shown to work well in coarser‐grained rivers. Analysis of the data reveals that the alluvial architecture of the Río Bermejo is characterized by large‐scale inclined heterolithic stratification generated by point‐bar evolution, and associated large‐scale scour surfaces that result from channel migration. The small‐scale and medium‐scale structure of the sedimentary architecture is generated by vertical accretion deposits, bed sets associated with small bars, dunes and climbing ripples and the cut and fill from small cross‐bar channels. This style of alluvial architecture is very different from other modern fine‐grained rivers reported in the literature that emphasize the presence of oblique accretion. The Río Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Modern examples of this type of highly active fine‐grained river have been reported rarely in the literature, although ancient examples are more prevalent and show similarities with the alluvial architecture of the Río Bermejo, which thus represents a useful analogue for their identification and interpretation. Although the full spectrum of the sedimentology of fine‐grained rivers has yet to be revealed, meandering rivers dominated by lateral or oblique accretion probably represent end members of such channels, with the specific style of sedimentation being controlled by grain size and sediment load characteristics.

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

AbstractCompound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three‐dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one‐third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations. Copyright © 2016 John Wiley &amp; Sons, Ltd.

River bank erosion and channel evolution in sand-bed braided reach of River Chenab: role of floods during different flow regimes

Braided reaches of large rivers in alluvial plains show major morphological changes, particularly the external bank erosion, due to the flood events. This paper highlights the bank erosion and channel evolution induced by eleven different flood events in a 7-km long reach of the River Chenab, Pakistan. The impact of floods on river bank erosion and channel evolution is analyzed under low and high flow conditions. Flood-induced changes, for river’s external banks and channel evolution, were assessed by processing Landsat ETM+ images in ArcGIS tool, and their inter-relationship is evaluated through regression analysis. The results revealed that the major morphological changes were triggered by the flood events occurred during the high flow or Monsoon season (July–September), whereas the flood events of similar magnitude occurring during low flow season (October–March) did not induce such changes. Mostly, the erosion remained limited to the middle part of the reach, where the branch channel flows along the bank. The average annual bank erosion rates are much higher as compared with a global scale. Data analysis showed a strong correlation between the mean high flows and total bank erosion indicating that Monsoon seasonal flows and floods are responsible for bank erosion. The present study further identifies the river bank locations highly susceptible to erosion by developing the correlation between bank erosion and branch channel progression. Strong correlation for bank erosion could be established with the shift of branch channels position flowing along the banks in braided reaches of sand bed rivers. However, the presence of sand bars along the river banks resulted in reduced erosion that weakens this relationship. The findings of the present study can help develop better understanding about the bank erosion process and constitute a key element to inform and improve river bank management.

Modulation of outer bank erosion by slump blocks: Disentangling the protective and destructive role of failed material on the three‐dimensional flow structure

AbstractThe three‐dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion. Although submerged slump blocks and associated large‐scale bank roughness elements have both previously been proposed to divert flow away from the bank, direct observations of the interaction between eroded bank material and the 3‐D flow field are lacking. Here we use observations from multibeam echo sounding, terrestrial laser scanning, and acoustic Doppler current profiling to quantify, for the first time, the influence of submerged slump blocks on the near‐bank flow field. In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may also deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion. We use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

DEVELOPMENT AND DYNAMICS OF THE SUNGACHA RIVERBED

The Sungacha river valley was laid in clay-loamy sediments of the Lake Khanka. Its meanders are dominantly steep with a complex configuration. The development of the Sungacha riverbed (activation or deceleration of the riverbed deformations) in the recent past was affected by fluctuations in the water level of the Lake Khanka. The Sungacha channel runoff, rate of bank erosion and riverbank deformations were rosen with increase of water level in the lake. By reducing of the water level the channel runoff was decreasing and deformations were slowing. Today, however, banks erosion and riverbed deformations ceased almost throughout the river except for the river mouth. Reducing the rate of riverbed deformations had begun in the second half of the XX century as a result of reducing the channel runoff due to increased water intake for agriculture and the growth of the sand bar in the Sungacha river mouth. However the possibility of the channel deformations growth is maintained due to both natural reasons (increase in water content of the rivers on the Khanka plain), and the weakening of anthropogenic impact on the lake and the rivers.

EROSION PROCESSES IN COMPOSITE RIVERBANKS: EXPERIMENTS AND MODELLING

Erosion processes in a composite bank is a very complex phenomenon due to many factors involved such as: seepage, variability of the bank stratifications, river stage variations, fluvial processes, soil stabilities. This study was focused to understand the primary processes responsible for bank erosion, and methodology to predict the bank erosion. Finally, a stochastic bank erosion model is proposed to estimate the seasonal bank erosion in a composite river bank. In the first stage, the developed model was simulated with known stage hydrograph, bank soil erodibility, and the bank stratification of a river bend in a reach of the Brahmaputra River in India. The critical shear stresses of the bank soils were determined through submerged-jet tests and the seepage erosion parameters were determined through laboratory lysimeter experiments. The predicted seasonal bank erosion rates for four consecutive years were compared with the observed rates. The results showed a good agreement between the observed data and modelling results. In the second stage, the uncertainty in the stage hydrograph, soil erodibility and the bank stratifications were considered. The stochastic bank erosion predictions were compared with the observed distribution of the bank erosion in 45 bends of the Brahmaputra River over 700 km long reach. Since the model has predicted satisfactorily therefore, the developed model can be used to obtain morphological dynamics of a sand-bed river bend with composite banks.

Spatial variability in bank resistance to erosion on a large meandering, mixed bedrock-alluvial river

Spatial heterogeneity in the erosion-resistance properties of the channel banks and floodplains associated with sediment characteristics, vegetation, or bedrock can have a substantial influence on the morphodynamics of meandering rivers, resulting in highly variable rates of bank erosion and complex patterns of planform evolution. Although past studies have examined the spatial variability in bank erodibility within small rivers, this aspect of the erosion-resistance properties for large rivers remains poorly understood. Furthermore, with the exception of recent numerical modeling that incorporates stochastic variability of floodplain erosional resistance, most models of meandering river dynamics have assumed uniform erodibility of the bank and floodplain materials. The present paper investigates the lateral and vertical heterogeneity in bank material properties and riparian vegetation within two elongate meander loops on a large mixed bedrock-alluvial river using several geotechnical field and laboratory methods. Additionally, the bank stability and toe-erosion numerical model (BSTEM) and repeat terrestrial LiDAR surveys are used to evaluate the capacity of the bank material properties to modify the rates and mechanisms of bank retreat. Results show that the textural properties of the bank materials, soil cohesion, and critical shear stress necessary for sediment entrainment differ substantially between the two bends and are also highly variable within each bend — laterally and vertically. Trees growing along the banks increase the resistance to erosion by contributing to the shear strength of the bank materials and are capable of increasing bank stability along a large river. Locally outcropping bedrock also influences bank erodibility in both bends. The results of this study demonstrate that spatial variability in the erosion-resistance properties of the channel banks is an important factor contributing to spatial variability in the rates and mechanisms of bank erosion determined from short-term studies of bank retreat and long-term analysis of changes in channel planform.

Characteristics of Point-Bar Development under the Influence of a Dam: Case Study on the Dráva River at Sigetec, Croatia

Abstract Before the extensive engineering works the Dráva River had braided pattern. However in the 19-20th centuries river regulation works became widespread, thus meanders were cut off, side-channels were blocked and hydroelectric power plants were completed. These human impacts significantly changed the hydro-morphology of the river. The aim of the present research is to analyse meander development and the formation of a point-bar from the point of view of indirect human impact. Series of maps and ortho-photos representing the period of 1870-2011 were used to quantify the longterm meander development, rate of bank erosion and point-bar aggradation. Besides, at-a-site erosion measurements and grain-size analysis were also carried out. As the result of reservoir constructions during the last 145 years floods almost totally disappeared, as their return period increased to 5-15 years and their duration decreased to 1-2 days. The channel pattern had changed from braided to sinuous and to meandering, thus the rate of bank erosion increased from 3.7 m/y to 32 m/y. On the upstream part of the point-bar the maximum grain size is 49.7-83.4 mm and the mean particle size is 7.6 mm, whilst on the downstream part the maximum grain size was only 39.7-39.9 mm and mean sediment size decreased to 6.1 mm. Due to the coarse sediment supply and the decreasing stream energy the point-bars develop quickly upstream and laterally too.