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

The 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

Extreme flood-driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro-acoustic techniques: Direct measurement of flood-driven erosion using MLS and MBES

Copyright © 2016 John Wiley & Sons, Ltd. This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone-driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone-affected wet season. Copyright © 2016 John Wiley & Sons, Ltd

Geomorphic processes active in the Southwestern Louisiana Canal, Lafourche Parish, Louisiana

The geomorphological changes causing the destruction of the banks of the Southwestern Louisiana Canal are studied by means of field work, laboratory analyses, and infrared color imagery interpretation. Turbulence and flow patterns are mapped, and related to erosion and sediment deposition processes. The accelerated erosion rate of the last decade is discussed, with two causative factors cited: (1) development of faster boats, increasing bank and bottom erosion, and (2) a subsequently larger tidal influx, with greater erosive ability. The physical properties of the canal bank materials are also analyzed. It is concluded that channel erosion progressively increases, with no indications of stabilization, until they merge with other waterways and become indistinguishable from natural water bodies

Factors and processes influencing streambank erosion along Horseshoe Run in Tucker County, West Virginia

Factors and processes influencing streambank erosion are not fully understood and combining factors and processes into a model that predicts streambank erosion is difficult. The mechanistic Bank Stability and Toe Erosion Model (BSTEM) and the empirical Bank Assessment of Nonpoint Source Consequences of Sediment (BANCS) model were evaluated to determine their effectiveness at predicting or explaining streambank erosion along Horseshoe Run. BSTEM underpredicted erosion by between 60 and 75%, but the model was able to provide relative estimates of eroded material and was also able to predict the type of erosion present at most bank sites. Model validation revealed critical shear stress of the bed material to be locally specific and non transferable to neighboring sites on the same stream. The input parameters for the BANCS model may be used to explain the susceptibility of a streambank to erosion. However, careful consideration needs to be given when using streambank and near bank characteristics to predict relative erosion on sections of the same stream with different morphology and potentially different dominant erosional processes. When the streambank parameters were used to group sites independently of erosion, a group of streambanks with moderate rooting depths and densities, low bank angles, and surface protection emerged. This group experienced the least amount of erosion. Regression analysis showed that for noncohesive restored banks that were vulnerable to fluvial erosion, bank angle, bank height, and vegetation parameters were needed to predict susceptibility to erosion. Alternatively, for cohesive banks with non-cohesive bank toe material that were vulnerable to fluvial erosion and mass failure, bank angle, bank material, and near bank depth ratios with an emphasis on bank angle were sufficient parameters to predict susceptibility

High stage events and stream bank erosion on small grazed pasture stream reaches in the rathbun lake watershed, southern IOWA, USA

Stream bank erosion in agricultural landscapes is a major pathway for non-point source sediment and phosphorus loading of receiving waters. Previous studies have shown direct and indirect effects of land use on stream bank erosion, and identified high erosion rates within riparian pastures. One potential impact of agricultural land-use on stream bank erosion is the alteration of stream stage characteristics, including an increase in frequency of high-stage events over short periods of time (forming flash hydrographs). The objective of this study was to assess the relationship between the number of high stream stages and corresponding stream bank soil erosion. The study was conducted in six grazed pasture stream reaches within the Rathbun Lake Watershed, a reservoir on the Chariton River located within the Southern Iowa Drift Plain. The erosion pin method was utilized to measure the change in stream bank erosion in response to differences in the number of high stream-stage events, which were monitored by pressure transducers. The measured seasonal bank erosion rates were correlated with the different stream stages data to assess their impact on stream bank erosion. Based on the different model assumptions, there were generally strong linear relationships between high stage and bank erosion. Approximately 75% of the variability in stream bank erosion rates was directly linked to the number of high stages/erosive stream flow depths. Conservation practices that reduce these erosion rates will be those that increase soil-water infiltration, reduce the frequency of high stream flow events and increase bank stability through perennial vegetation cover or reducing disturbance within the riparian zone

A New Framework to Model Hydraulic Bank Erosion Considering the Effects of Roots

Floods and subsequent bank erosion are recurring hazards that pose threats to people and can cause damage to buildings and infrastructure. While numerous approaches exist on modeling bank erosion, very few consider the stabilizing effects of vegetation (i.e., roots) for hydraulic bank erosion at catchment scale. Taking root reinforcement into account enables the assessment of the efficiency of vegetation to decrease hydraulic bank erosion rates and thus improve risk management strategies along forested channels. A new framework (BankforNET) was developed to model hydraulic bank erosion that considers the mechanical effects of roots and randomness in the Shields entrainment parameter to calculate probabilistic scenario-based erosion events. The one-dimensional, probabilistic model uses the empirical excess shear stress equation where bank erodibility parameters are randomly updated from an empirical distribution based on data found in the literature. The mechanical effects of roots are implemented by considering the root area ratio (RAR) affecting the material dependent critical shear stress. The framework was validated for the Selwyn/Waikirikiri River catchment in New Zealand, the Thur River catchment and the Sulzigraben catchment, both in Switzerland. Modeled bank erosion deviates from the observed bank erosion between 7% and 19%. A sensitivity analysis based on data of vertically stable river reaches also suggests that the mechanical effects of roots can reduce hydraulic bank erosion up to 100% for channels with widths < 15.00 m, longitudinal slopes < 0.05 m m−1 and a RAR of 1% to 2%. The results show that hydraulic bank erosion can be significantly decreased by the presence of roots under certain conditions and its contribution can be quantified considering different conditions of channel geometry, forest structure and discharge scenarios

The Relationships of Streambank Angles and Shapes to Streambank Erosion Rates in the Little River Watershed, TN

Sediment is a leading cause of water quality impairment throughout the United States. In the Little River watershed in eastern Tennessee, several tributaries have been classified as impaired due primarily to sedimentation. Researchers at The University of Tennessee, in collaboration with a group of local and state organizations, began monitoring Little River tributaries to better understand their sources of pollution. To investigate the rates and processes of streambank erosion, erosion-pin monitoring sites were established on 32 banks in the watershed. This thesis complements the erosion-pin monitoring efforts by determining bank characteristics and examining the relationships of streambank angles and shapes to observed erosion rates. The specific objectives of this study were to: (1) characterize streambank angles, (2) describe the relationships between streambank angles and bank erosion rates, (3) characterize bank shape, and (4) determine if bank shapes at erosion-pin monitoring sites are representative of their immediate stream reaches. Streambank angles were measured at erosion pins. Bank angles averaged approximately 55° and varied significantly between tributaries and individual monitoring sites. Bank angle measurements were compared to erosion-pin exposure using correlation analysis. Data were then sorted into subgroups by pin position, soil texture, and bank shape, and further analyses were conducted. Results indicated streambank erosion was significantly, positively associated with bank angle for angles ≥ 30°. Significant, positive relationships were also found low on banks, where soil texture was clay, and where banks were classified as undercut. Bank profiles were documented to classify the bank shapes of erosion-pin monitoring sites and assess how well the banks at those sites represented the immediate reach. In the Little River watershed, bank profile shapes vary, but nearly three-fourths of all documented bank profiles were steeply sloping or undercut. The majority of monitoring sites (78%) were representative of the immediate stream reach with regard to bank shape. However, other factors, including surrounding land use and soil type, may differ within the immediate reach. Thus, data extrapolation from erosion pins to the reach scale should be done cautiously and take into consideration variability of individual site characteristics

Stream bank soil and phosphorus losses within grazed pasture stream reaches in the Rathbun Watershed in southern Iowa

Stream bank erosion within agricultural landscapes is a major pathway for non-point source sediment and phosphorus loading to receiving waters. The objectives of this study were to: 1) compare the effect of livestock stocking rate on sediment and phosphorus loss from stream bank erosion in the Rathbun Watershed in southern Iowa; 2) assess the relationship between stream stage and stream bank soil erosion rates, and 3) evaluate the impacts of current riparian land-uses and stream morphologic characteristics at the field and catchment scale on stream bank erosion. Stream bank erosion rates over three years were estimated using the erosion pin method, with erosion rates correlated to pasture stocking rates and the number of high stream stage events, which were monitored using pressure transducers. The effects of stream morphology and land-use on stream bank erosion were assessed using parameters such as percent of land-use, bank soil texture, stream bed slope, and sinuosity at both the field and catchment scale. While there was no significant correlation between stream bank erosion rates and stocking rates, erosion rates within sites under Conservation Reserve Program management were significantly lower than those within grazed pastures, particularly during the winter/spring season. The length of severely eroded stream banks and compaction of the riparian area were positively related to livestock stocking rates within pasture stream reaches. Approximately 75% of the variability in stream bank erosion was found to be correlated to the frequency of high stream stage events. Overall, these data and previous studies allow the speculation that, in the long term and at the catchment scale, a high percentage of agricultural land-use in riparian areas can be either directly and/or indirectly related to alteration of stream hydrologic regimes. In order to reach equilibrium state condition, where energy input to the stream channel is balanced with the minimal channel boundary resistance, such land-use changes will result in changes in stream bank erosion and channel morphology

Sub‐arctic river bank dynamics and driving processes during the open‐channel flow period

There is growing concern that rapidly changing climate in high latitudes may generate significant geomorphological changes that could mobilise floodplain sediments and carbon; however detailed investigations into the bank erosion process regimes of high latitude rivers remain lacking. Here we employ a combination of thermal and RGB colour time‐lapse photos in concert with water level, flow characteristics, bank sediment moisture and temperature, and topographical data to analyse river bank dynamics during the open‐channel flow period (the period from the rise of the spring snowmelt flood until the autumn low flow period) for a subarctic river in northern Finland (Pulmanki River). We show how variations of bank sediment temperature and moisture affect bank erosion rates and locations, how bank collapses relate to fluvial processes, and elucidate the seasonal variations and interlinkages between the different driving processes.We find that areas with high levels of groundwater content and loose sand layers were the most prone areas for bank erosion. Groundwater seeping caused continuous erosion throughout the study period, whereas erosion by flowing river water occurred during the peak of snowmelt flood. However, erosion also occurred during the falling phase of the spring flood, mainly due to mass failures. The rising phase of the spring flood therefore did not affect the river bank as much as its peak or receding phases. This is explained because the bank is resistant to erosion due to the prevalence of still frozen and drier sediments at the beginning of the spring flood. Overall, most bank erosion and deposition occurrences were observed during the low flow period after the spring flood. This highlights that spring melt, while often delivering the highest discharges, may not be the main driver of bank erosion in sub‐arctic meandering rivers

Examining Variability in Streambank Erosion Rates in the Lake Champlain Basin, Vermont

Bank erosion is a dynamic process with large variability in rates across the landscape. Although prior studies have investigated streambank and channel erosion rates on cross-section to sub-watershed scales, there is limited understanding of regional streambank erosion in the Lake Champlain Basin (LCB). Consequently, the role of erosion in watershed sediment budgets and the contribution of bank derived sediment and associated nutrients to degraded water quality in Lake Champlain is not fully understood. The goal of our work is to obtain refined rates of erosion in the LCB and to understand the source of bank erosion variability. To achieve this goal, this project compiles studies on bank erosion rates in Vermont to develop a regional dataset of erosion rates and associated stream attributes in the LCB. We quantified bank erosion rates for five field sites from LiDAR and field-surveyed cross-sections. Data from six previous bank erosion studies, that contained one or more observations were identified, creating a dataset of mass erosion rates for 190 individual stream reaches, with drainage areas that span from 0.1 km2 to 2730.9 km2 in the LCB. The reaches were associated with Vermont ANR Stream Geomorphic Assessment data, populated with stream attributes, and analyzed to identify relationships between physical characteristics of the landscape and erosion rates. From our dataset, we found that erosion rates are highly variable, but that this variability can be described in part by drainage area, slope, incision ratio, and riparian vegetation cover. A better understanding of streambank erosion rates and important driving variables provides additional context for restoration practices and sediment deposition patterns on adjacent floodplains