Magnitude and processes of bank erosion at a small stream in Denmark

River banks are important sources of sediment and phosphorus to fluvial systems, and the erosion processes operating on the banks are complex and change over time. This study explores the magnitude of bank erosion on a cohesive streambank within a small channelized stream and studies the various types of erosion processes taking place. Repeat field surveys of erosion pin plots were carried out during a 4‐year period and observations were supplemented by continuous monitoring of volumetric soil water content, soil temperature, ground water level and exposure of a PEEP sensor. Bank erosion rates (17·6–30·1 mm year^?1) and total P content on the banks were relatively high, which makes the bank an important source of sediment and phosphorus to the stream, and it was estimated that 0·27 kg P_tot year^?1 ha^?1 may potentially be supplied to the stream from the banks. Yearly pin erosion rates exceeding 5 cm year^?1 were mainly found at the lower parts of the bank and were associated with fluvial erosion. Negative erosion pin readings were widespread with a net advance of the bank during the monitoring period mainly attributed to subaerial processes and bank failure. It was found that dry periods characterized by low soil water content and freeze–thaw cycles during winter triggered bank failures. The great spatial variability, in combination with the temporal interaction of processes operating at different scales, requires new tools such as 3‐D topographical surveying to better capture bank erosion rates. An understanding of the processes governing bank erosion is required for riparian management using vegetational measures as root size and structure play different roles when it comes to controlling bank erosion processes. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of riparian vegetation on stream bank subaerial processes in southwestern Virginia,USA

Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on subaerial processes have not been quantified. Subaerial processes, such as soil desiccation and freeze–thaw cycling, are climate‐related phenomena that deliver soil directly to the stream and make the banks more vulnerable to fluvial erosion by reducing soil strength. This study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks. Soil temperature and water tension were measured at six paired field sites in southwestern Virginia, USA, for one year. Results showed that stream banks with herbaceous vegetation had higher soil temperatures and a greater diurnal temperature range during the summer compared to forested stream banks. Daily average summer soil water tension was 13 to 57 per cent higher under herbaceous vegetation than under woody vegetation, probably due to evapotranspiration from the shallow herbaceous root system on the bank. In contrast to summer conditions, the deciduous forest buffers provided little protection for stream banks during the winter: the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze–thaw cycles. During the winter, the stream banks under the deciduous forests were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze–thaw cycling. Study results also indicated that freeze–thaw cycling and soil desiccation were greater on the upper stream bank due to thermal and moisture regulation of the lower bank by the stream. Therefore, subaerial erosion and soil weakening may be greater on the upper stream banks. Additional research is needed on the influence of subaerial processes on both subaerial and fluvial erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Magnitude and variation in the contribution of bank erosion to the suspended sediment load of the River Severn,UK

The wide range of studies describing the role of bank erosion in fluvial sediment supply have mostly lumped amounts of bank erosion into coarse temporal units, such as years. This paper investigates sediment yields from individual bank erosion events within the upper River Severn, UK (basin area 380 km²). Manual erosion pins and photo-electronic erosion pins were used to estimate bank erosion, and turbidity meters were used to determine suspended sediment transport. At the annual time-scale, the silt-clay fraction of bank-derived sediment accounted for an equivalent of 17 per cent of the suspended load, increasing to an average of 38 per cent at the monthly timescale, and then to an average of 64 per cent at the event timescale. This research highlighted that for an upland catchment, bank erosion was an important supply of suspended sediment, and that for some flood events bank erosion can supply more sediment than is transported. © 1997 John Wiley & Sons, Ltd.     

Insights into river bank erosion processes derived from analysis of negative erosion‐pin recordings: observations from three recent UK studies

Recent studies of river bank erosion in three catchments in the UK have been characterized by the persistent occurrence of negative erosion‐pin results. The cause of these negative recordings is considered with reference to field data from the Afon Trannon, Nant Tanllwyth and River Arrow, and to a laboratory study of freeze–thaw and desiccation processes. It seems that there is potential for, and in some cases evidence of, a number of different circumstances that generate negative results, but none of these alone is sufficient to explain all incidents. Factors considered include: deposition of sediment during high flows; soil fall from the upper parts of the bank on to lower erosion pins; loosening of the soil surface and expansion/contraction of the soil mass with fluctuations in temperature and moisture content; movement of the erosion‐pin within the bank and human interference. Each has its own implications for the use of erosion pins. Further issues arise when including negative data in subsequent data analysis, and it is demonstrated that attempts to correlate erosion rates with hydro‐meteorological data in order to ascertain causes of erosion will be influenced by the way in which negative data are handled. It is thus suggested that any study of river bank erosion using erosion pins should state whether or not negative data were obtained, and if so, how they were included in data analysis. Failure to include this information could lead to comparison of mean erosion rates that reflect bank processes very differently. The studies presented here offer a clear example of the value of ‘anomalous’ field data: results which do not appear to fit expected patterns can reveal as much about the processes in operation as those that do. Copyright © 2002 John Wiley & Sons, Ltd.     

Defining the moment of erosion: the principle of thermal consonance timing

Geomorphological process research demands quantitative information on erosion and deposition event timing and magnitude, in relation to fluctuations in the suspected driving forces. This paper establishes a new measurement principle – thermal consonance timing (TCT) – which delivers clearer, more continuous and quantitative information on erosion and deposition event magnitude, timing and frequency, to assist understanding of the controlling mechanisms. TCT is based on monitoring the switch from characteristically strong temperature gradients in sediment, to weaker gradients in air or water, which reveals the moment of erosion. The paper (1) derives the TCT principle from soil micrometeorological theory; (2) illustrates initial concept operationalization for field and laboratory use; (3) presents experimental data for simple soil erosion simulations; and (4) discusses initial application of TCT and perifluvial micrometeorology principles in the delivery of timing solutions for two bank erosion events on the River Wharfe, UK, in relation to the hydrograph. River bank thermal regimes respond, as soil temperature and energy balance theory predicts, with strong horizontal thermal gradients (often >1 K cm^?1 over 6·8 cm). TCT fixed the timing of two erosion events, the first during inundation, the second 19 h after the discharge peak and 13 h after re‐emergence from the flow. This provides rare confirmation of delayed bank retreat, quantifies the time‐lag involved, and suggests mass failure processes rather than fluid entrainment. Erosion events can be virtually instantaneous, implying ‘catastrophic retreat’ rather than ‘progressive entrainment’. Considerable potential exists to employ TCT approaches for: validating process models in several geomorphological contexts; assisting process identification and improving discrimination of competing hypotheses of process dominance through high‐resolution, simultaneous analysis of erosion and deposition events and driving forces; defining shifting erodibility and erosion thresholds; refining dynamic linkages in event‐based sediment budget investigations; and deriving closer approximations to ‘true’ erosion and deposition rates, especially in self‐concealing scour‐and‐fill systems. Copyright © 2005 John Wiley & Sons, Ltd.     

Monitoring and numerical modelling of riverbank erosion processes: a case study along the Cecina River (central Italy)

Riverbank retreat along a bend of the Cecina River, Tuscany (central Italy) was monitored across a near annual cycle (autumn 2003 to summer 2004) with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale. Seven flow events occurred during the period of investigation, with the largest having an estimated return period of about 1·5 years. Bank simulations were performed by linking hydrodynamic, fluvial erosion, groundwater flow and bank stability models, for the seven flow events, which are representative of the typical range of hydrographs that normally occur during an annual cycle. The simulations allowed identification of (i) the time of onset and cessation of mass failure and fluvial erosion episodes, (ii) the contributions to total bank retreat made by specific fluvial erosion and mass‐wasting processes, and (iii) the causes of retreat. The results show that the occurrence of bank erosion processes (fluvial erosion, slide failure, cantilever failure) and their relative dominance differ significantly for each event, depending on seasonal hydrological conditions and initial bank geometry. Due to the specific planimetric configuration of the study bend, which steers the core of high velocity fluid away from the bank at higher flow discharges, fluvial erosion tends to occur during particular phases of the hydrograph. As a result fluvial erosion is ineffective at higher peak discharges, and depends more on the duration of more moderate discharges. Slide failures appear to be closely related to the magnitude of peak river stages, typically occurring in close proximity to the peak phase (preferentially during the falling limb, but in some cases even before the peak), while cantilever failures more typically occur in the late phase of the flow hydrograph, when they may be induced by the cumulative effects of any fluvial erosion. Copyright © 2008 John Wiley & Sons, Ltd.     

Bank erosion of an incised upland channel by subaerial processes: Tasmania,Australia

The headwaters of many rivers are characterized by gullies and incised streams that generate significant volumes of sediment and degrade downstream water quality. These systems are characterized by harsh climates, ephemeral flows that do not reach bank top, and bare cohesive banks of clay and weathered bedrock. We investigated the rates and processes of bank erosion in an incised canal that has such characteristics. Detailed measurements of bank position were made over two years with a purpose‐built groundprofiler and photo‐electronic erosion pins (PEEPs). Stage height and turbidity were also monitored. The bare banks eroded at 13 ± 2 mm a⁻¹. Erosion is controlled by subaerial processes that loosen bank material. Observations show that needle‐ice growth is important in winter and desiccation of clays predominates in summer. Flows are unable to erode firm cohesive clays from the banks, and erosion is generally limited by the availability of loosened material. This produces strong hysteresis in turbidity during events. Peak turbidity is related to the number of days with low flow between events, and not peak stage. Rehabilitation with a moderate cover of grass is able to prevent bank erosion by limiting the subaerial erosion processes. Projections of current erosion suggest that without vegetation cover the banks are unlikely to stabilize for many years. Copyright © 2000 John Wiley & Sons, Ltd.     

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

Bank 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 & Sons Ltd.     

Quantifying different riverbank erosion processes during an extreme flood event

Riverbank erosion is a major contributor to catchment sediment budgets. At large spatial scales data is often restricted to planform channel change, with little information on process distributions and their sediment contribution. This study demonstrates how multi‐temporal LiDAR and high resolution aerial imagery can be used to determine processes and volumes of riverbank erosion at a catchment scale. Remotely sensed data captured before and after an extreme flood event, enabled a digital elevation model of difference (DoD) to be constructed for the channel and floodplain. This meant that: the spatial area that could be assessed was extensive; three‐dimensional forms of bank failures could be mapped at a resolution that enabled process inference; and the volume and rates of different bank erosion processes over time could be assessed. A classification of riverbank mass failures, integrating form and process, identified a total of 437 mass failure polygons throughout the study area. These were interpreted as wet flow mass failures based on the presence of a well defined scarp wall and the absence of failed blocks on the failure floor. The failures appeared to be the result of: bank exfiltration, antecedent moisture conditions preceding the event, and the historic development of the channel. Using one‐dimensional hydraulic modelling to delineate geomorphic features within the main boundary of the macrochannel, an estimated 1 466 322 m² of erosion was interpreted as fluvial entrainment, occurring across catchment areas from 30 to 1668 km². Only 8% of the whole riverbank planform area was occupied by mass failures, whilst fluvial entrainment covered 33%. A third of the volume of material eroded came from mass failures, even though they occupied 19% of the eroded bank area. The availability of repeat LiDAR surveys, combined with high‐resolution aerial photography, was very effective in erosion process determination and quantification at a large spatial scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Stochastic erosion of composite banks in alluvial river bends

The erosion of composite river banks is a complex process involving a number of factors including fluvial erosion, seepage erosion, and cantilever mass failure. To predict the rate of bank erosion with these complexities, a stochastic bank erosion model is suitable to define the probability distribution of the controlling variables. In this study, a bank erosion model in a river bend is developed by coupling several bank erosion processes with an existing hydrodynamic and morphological model. The soil erodibility of cohesive bank layers was measured using a submerged jet test apparatus. Seasonal bank erosion rates for four consecutive years at a bend in the Brahmaputra River, India, were measured by repeated bankline surveys. The ability of the model to predict erosion was evaluated in the river bend that displayed active bank erosion. In this study, different monsoon conditions and the distribution functions of two variables were considered in estimating the stochastic bank erosion rate: the probability of the soil erodibility and stochastic stage hydrographs for the nth return period river stage. Additionally, the influences of the deflection angle of the streamflow, longitudinal slope of river channel, and bed material size on bank erosion rate were also investigated. The obtained stochastic erosion predictions were compared with the observed distribution of the annual‐average bank erosion rate of 45 river bends in the Brahmaputra River. The developed model appropriately predicted the short‐term morphological dynamics of sand‐bed river bends with composite banks. Copyright © 2014 John Wiley & Sons, Ltd.     

Braiding process and bank erosion in the Brahmaputra River

The present work explores relations between stream power,braiding intensities and bank erosion in certain stretches of the Brahmaputra River.In this paper,an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure.The braiding index is compared for discrete years to understand the morphological behaviour.Subsequently,a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion.The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years.The procedure addresses the selection of input parameters from digital satellite images,comprising scenes for the years 1990,1997 and 2007 with specific dates,from Dhubri near Indo-Bangladesh Border to Upper Assam.Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach.Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes.The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power,there is an occurrence of wide spread braiding.This in turn incurs substantial yearly land loss due to bank erosion,caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River.     

Field instrumentation for high‐resolution parallel monitoring of bedrock erosion and bedload transport

River incision is fundamental in shaping the Earth's surface. In mountainous regions with steep river beds, fluvial bedrock erosion by bedload transport is an important mechanism forming channels. However, there are only a few complete field datasets that can be used to improve process understanding and evaluate erosion models, especially at the process scale. To provide a simultaneous dataset of hydraulics, bedload transport and bedrock erosion at high temporal and spatial resolution, a new measuring device has been installed in the Erlenbach, a gauged stream in the Swiss Pre‐Alps. In this stream, bedload transport rates can be calculated from surveying deposits and from geophone plate sensors and bedload transport samples can be taken directly by an automated moving basket system. To measure bedrock erosion rates simultaneously, two natural stone slabs were mounted flush with the channel bed in a steel frame hosting various measurement devices. Force sensors below the slabs record normal stress and shear stress. At‐a‐point erosion rates on the slab surfaces are continuously measured at sub‐millimetre precision at three locations on each slab. In addition, the slab topography is monitored following erosive flood events. In this article (i) the ‘erosion scale’ device is described, (ii) data resolution and data quality is assessed by means of tests and event data, and (iii) the first transport event is discussed. The erosion scales are confirmed to provide data at high spatio‐temporal resolution for process analysis. The preliminary data show evidence for the tools effect in bedrock erosion. The bedrock slabs can be exchanged to obtain measurements for catchments with different lithologies for comparison. Copyright © 2014 John Wiley & Sons, Ltd.     

Distinct patterns of bank erosion in a navigable regulated river

Distinct bankline patterns appeared after the removal of protection works along a navigable reach of the Meuse River. A series of oblique embayments now dominate the riverine landscape after ten years of bank erosion, but their location and asymmetry cannot be explained yet. This work analyses and integrates field measurements of flow, ship waves, bank composition, bed topography and historical maps to explain the observed patterns along two reaches of the river. An extraordinary low-water-level event generated by a ship accident provided the unique opportunity to also analyse the subaqueous bank topography. The results indicate that the formation of oblique embayments arises from the combination of floodplain heterogeneity, structured by scroll-bar deposits, and the regulation of water levels, resulting in ship-wave attack at a narrow range of bank elevation for 70% of the time. Substrate erodibility acts on the effectiveness of trees to slow down local bank erosion rates, which is possibly enhanced by a positive feedback between woody roots and cohesive soil. The strong regulation of water levels and the waves generated by the intense ship traffic produce an increasingly long mildly-sloping terrace at the bank toe and progressively dominate the bank erosion process. This study demonstrates the important role of floodplain and scroll bar formation in shaping later bank erosion, which has implications for predictive numerical models, restoration strategies, and understanding the role of vegetation in bank erosion processes. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Understanding mass fluvial erosion along a bank profile: using PEEP technology for quantifying retreat lengths and identifying event timing

This study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses. We explore the relationship between the two regimes in a representative, US Midwestern stream with semi‐cohesive bank soils, namely Clear Creek, IA. Photo‐Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion. We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt‐sized particles at low shear stresses over sand‐sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several‐fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows. The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright © 2017 John Wiley & Sons, Ltd.     

Critical caving erosion width for cantilever failures of river bank

The cantilever failure is one of the typical bank failures, in which the lateral caving erosion at the bottom of the bank plays an important role. When the caving erosion width is larger than a certain value, the cantilever failures such as shear, toppling and stress failures may occur. In order to understand the condition of the cantilever failure, the collapse mechanisms of the cantilever failures are studied based on the bank stability theory and flume experiment. According to the bank stability equation with the lateral erosion, the critical caving erosion width (CCEW) formulas for the shear and toppling failures of simple slope bank were derived in this paper. The formulas show that the CCEW increases as the overhanging soil thickness and soil cohesion increase, and decreases as the crack depth on the bank surface and the slope angle of the bank increase. And these formulas were tested with experimental data, which shows the predicted values are good agreement with experimental data. The paper reveals a quantitative expression on the process of the river cantilever failure.     

Bank erosion and channel width change in a tropical catchment

Catchment sediment budget models are used to predict the location and rates of bank erosion in tropical catchments draining to the Great Barrier Reef lagoon, yet the reliability of these predictions has not been tested due to a lack of measured bank erosion data. This paper presents the results of a 3 year field study examining bank erosion and channel change on the Daintree River, Australia. Three different methods were employed: (1) erosion pins were used to assess the influence of riparian vegetation on bank erosion, (2) bench‐marked cross‐sections were used to evaluate annual changes in channel width and (3) historical aerial photos were used to place the short term data into a longer temporal perspective of channel change (1972–2000). The erosion pin data suggest that the mean erosion rate of banks with riparian vegetation is 6·5 times (or 85%) lower than that of banks without riparian vegetation. The changes measured from cross‐section surveys suggest that channel width has increased by an average of 0·74 (±0·47) m a^?1 over the study period (or ～0·8% yr^?1). The aerial photo results suggest that over the last 30 years the Daintree River has undergone channel contraction of the order of 0·25 m a^?1. The cross‐section data were compared against modelled SedNet bank erosion rates, and it was found that the model underestimated bank erosion and was unable to represent the variable erosion and accretion processes that were observed in the field data. The reach averaged bank erosion rates were improved by the inclusion of locally derived bed slope and discharge estimates; however, the results suggest that it will be difficult for catchment scale sediment budget models to ever accurately predict the location and rate of bank erosion due to the variation in bank erosion rates in both space and time. Copyright © 2008 John Wiley & Sons, Ltd.     

Three-dimensional modeling of bank erosion and morphological changes in the Shishou bend of the middle Yangtze River

This paper presents a three-dimensional (3-D) numerical model to simulate morphological changes in alluvial channels due to bank erosion. A method for the simulation of bank erosion is established. This is incorporated into a 3-D mathematical model for turbulent flow and non-uniform, non-equilibrium sediment transport. This model is applied to simulate morphological changes in the Shishou bend of the middle Yangtze River in China, where serious bank erosions occurred during the last two decades. The double-layer sediment structure of the riverbank on the middle Yangtze River is taken into account in the bank erosion module. Both cohesive and non-cohesive bank material in the different layers are considered. The bank erosion module also includes other factors affecting the rate of bank erosion, such as the longitudinal length of failed bank, the thickness of each layer in the double-layer structure, and the erosion-resisting effect of cohesive material from the top layer of failed bank. A locally-adaptive grid system is proposed to efficiently simulate the lateral migration of alluvial channel due to bank erosion. The predictive capability of the 3-D model is examined by laboratory data. Simulated processes of bank erosion agree with field observations in the Shishou bend during the period of October 1996–October 1998, and the bank erosion module plays a significant role in simulating morphological changes of the study reach. In addition, the equivalent channel-forming discharge, which is defined as a constant discharge that can create the same amount of bank erosion in an alluvial channel as that created by natural runoff processes during the same period of time, is proposed to improve calculation efficiency for feasibility studies.     

Channel erosion along the Carmel river,Monterey county,California

Historic maps, photographs, and channel cross-sections show that the channel of the Carmel River underwent massive bank erosion, channel migration, and aggradation in a major flood in 1911, then narrowed and incised by 1939. The channel was stable until 1978 and 1980, when bank erosion affected some reaches but not others. The narrowing and incision were in response to a lack of major floods after 1914 and construction in 1921 of a dam that cut off sediment supply from the most actively eroding half of the basin. Localized erosion in 1978 and 1980 occurred during low magnitude events along reaches whose bank strength had been reduced by devegetation. These events illustrate that the stability of a fluvial system can be disrupted either by application of a large erosive force in a high magnitude event (the 1911 flood) or in a low magnitude event, by reducing the resistance to erosion (bank devegetation). The Carmel River is a potentially unstable system. Its discharge and slope characteristics place it near the threshold between meandering and braided. On the Lower Carmel, the presence of bank vegetation can make the difference between a narrow, stable meandering channel and a wide shifting channel with braided reaches.     

Distribution of erosion across bedrock channels

Lateral erosion in bedrock rivers is an important control on the shape of channel cross‐sections, and the coupling of channels and hillslopes. Recent observations link lateral erosion to the variability of flow. We propose two mechanisms to explain this. One is based on changing shear stress distributions within the channel with varying flood level, the other on the competition between cover and tool effects in fluvial bedrock erosion. We assess these processes for the Liwu River, Taiwan, and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case. Copyright © 2007 John Wiley & Sons, Ltd.     

An empirical model of event scale cohesive bank profile evolution

The highly stochastic nature of riverbank erosion has driven the need for spatially explicit empirical models. Detailed bank profile surveys along a meander bend of the Brandywine Creek in Pennsylvania, USA, before and after 28 high flow events over a 2·5 year period are used to develop an empirical model of cohesive bank profile erosion. Two hundred and thirty‐six bank erosion observations are classified as hydraulic erosion or subaerial erosion. Threshold conditions required to initiate bank erosion cannot be defined based on field measurements. Using the near‐bank velocity and the number of freeze–thaw cycles as predictors, regression equations are derived for hydraulic erosion that specify the length, thickness, and location on the bank face of eroded blocks. An empirical discriminant function defines the critical geometry of overhang failures, and the volumes removed by overhang failures are computed using another regression equation. All the regression equations are significant, but have low correlation coefficients, suggesting that cohesive bank erosion has a strong stochastic component. Individual events typically remove small masses of soil (average volume 0·084 m³/m) a few centimeters thick (median = 0·057 m) and a few decimeters in length (median = 0·50 m) from the lower third of the bank. Hydraulic erosion is responsible for 87% of all erosion. When applied to three survey sites not used in its development, the profile model predicts the total volume of erosion with errors of 23%, 5% and 1%. Twenty‐four percent of computed erosion volumes for single events are within 50% of observed volumes at these three sites. Extending the approach to decadal timescales and to entire bends will require three‐dimensional observations of bank failure, and spatially and temporally explicit methods to account for the influence of individual large trees on bank failures and near‐bank hydraulic processes. Copyright © 2009 John Wiley & Sons, Ltd.