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.     

Distributed evaluation of the contribution of soil erosion to the sediment yield from a watershed

The correct determination of the sediment yield from a basin is of paramount importance in several hydraulic and environmental applications, such as the evaluation of the storage reduction of artificial reservoirs. However, due to the highly episodic nature of sediment supply and transport in many environments and to the extreme complexity of the processes involved, the evaluation of the sediment load in a river is still highly uncertain. When the time scale of interest is sufficiently long, and when the primary sediment source comes from distributed erosion in the watershed, the problem can be tackled in an indirect fashion, by computing the contribution to the annual suspended yield from soil erosion. In order to accomplish this task, we propose a distributed application of the widely used USLE formula. The formula is automatically applied along drainage networks derived from a digital elevation model and properly modified to take into account the presence of deposition zones in the watershed. © 1997 John Wiley & Sons, Ltd.     

Comparative application of two erosion models to a basin

Abstract

Two mathematical models were used to estimate the annual sediment yield resulting from rainfall and runoff at the outlet of the Nestos River basin (Toxotes, Thrace, Greece). The models were applied to that part of the Nestos River basin (838 km²) which lies downstream of three dams. Both models consist of three submodels: a simplified rainfall-runoff submodel, a physically-based surface erosion submodel and a sediment transport submodel for streams. The two models differ only in the surface erosion submodel: that of the first model is based on the relationships of Poesen (1985) for splash detachment and splash transport, while the corresponding submodel of the second model is based on the relationships of Schmidt (1992) for the momentum flux exerted by the droplets and the momentum flux exerted by the overland flow. The degree of conformity between the annual values of sediment yield at the basin outlet according to both models is satisfactory.     

Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure,precipitation and stream stage

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K_s) with depth, but differences between lateral K_s of a concretion layer and the vertical K_s of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low‐flow (LF) seeps (flow rates typically less than 0·05 L min^?1), persistent high‐flow (HF) seeps (average flow rate of 0·39 L min^?1) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than ?5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min^?1 and sediment concentrations commonly approached 100 g L^?1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min^?1) and sediment concentrations (989 g L^?1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant. Copyright © 2007 John Wiley & Sons, Ltd.     

Hydrological and sediment yield response to summer rainfall in a small high Arctic watershed

In cold regions, the response and related antecedent mechanisms that produce flood flows from rainfall events have received limited study. In 2007, a small watershed at Cape Bounty, Melville Island, Nunavut, was studied in detail during the melt season. Two rainfall events on June 30 and July 22, totalling 9·2 and 10·8 mm, respectively, represented significant contributions to seasonal discharge and sediment transport in a year with a low winter snowpack. The precipitation events elevated discharge and suspended sediment concentrations to twice the magnitude of the nival melt, and generated the only measurable downstream lacustrine turbidity current of the season. In two days, rainfall runoff transported 35% of the seasonal suspended sediment load, in contrast to 29% transported over the nival freshet. The magnitude and intensity of the rain events were not unusual in this setting, but the rainfall response was substantial in comparison with equivalent past events. Exceptional temperatures of July 2007 generated early, deep permafrost thaw, and ground ice melt. The resultant increase in soil moisture amplified the subsequent rainfall runoff and sediment transport response. These results demonstrate the importance of antecedent moisture conditions and the role of permafrost active layer development as an important factor in the rainfall runoff and sediment transport response to precipitation events. Copyright © 2009 John Wiley & Sons, Ltd.     

Shift from transport limited to supply limited sediment concentrations with the progression of monsoon rains in the Upper Blue Nile Basin

Long‐term erosion monitoring data in the Ethiopian highlands are only available from the Soil Conservation Research Program (SCRP) watersheds including the Anjeni watershed. The 113 ha Anjeni watershed was established in 1984 and fanya juu terraces were installed in 1986. Runoff and erosion data are available from three different plot sizes and at the watershed outlet. The objective of this study was to investigate how erosion processes and sediment rating parameters vary with plot size and the progression of the rainy monsoon phase. We analyzed runoff and sediment loss data from 40 plots and the watershed outlet. The dataset included erosion data from 24 newly constructed plots (3 m length) during the rainy monsoon phase of 2012 and 2013, and 16 long‐term plots (with 12, 16, 22, and 24% slopes and 3, 15 and 30 m lengths) and the watershed outlet during the period between 1986 to 1990. Sediment concentration (C) was fitted to runoff (Q) using a power regression equation (C = aQ^b). Sediment concentration and yield increased with increasing plot length from 3 m to 15 m, but sediment yield decreased as plot length increased to 30 m.The coefficients (a and b) were affected by plot size and the progression of the rainy monsoon phase. As plot size increases, the a value increased, while the b value decreased. Greater a values were observed during the beginning of the monsoon phase, while values of b were greater towards the end of the monsoon phase. Overall findings suggest that erosion from cultivated fields is primarily controlled by transport limitations at the beginning of the monsoon phase, while towards the end of the monsoon phase, as surface covers emerge, sediment availability will be reduced, and thus sediment source would be a limitation. Copyright © 2016 John Wiley & Sons, Ltd.     

Bank erosion of legacy sediment at the transition from vertical to lateral stream incision

Streambank erosion is a primary source of suspended sediments in many waterways of the US Atlantic Piedmont. This problem is exacerbated where banks are comprised of fine sediment produced by the intensive land use practices of early European settlers. A stream in this region, Richland Creek incises into banks comprised of three stratigraphic layers associated with historic land use: pre‐European settlement, early European agriculture and development, and water‐powered milldam operation. This study aims to identify the bank processes along a reach of Richland Creek that is eroding towards its pre‐disturbance elevation. The volume of material that has eroded along this stream since the milldam breached was calculated by differencing a reconstructed surface of the pond bed and an aerial lidar digital terrain model (DTM). Immediately downstream from the study reach, the channel is floored by bedrock and immediately upstream the rate of channel erosion approximately doubled along the longitudinal profile of Richland Creek, which indicate that the study reach spans the transition from a channel dominated by vertical incision in the upstream direction to horizontal widening in the downstream direction. The combined hydrometeorological conditions and dominant processes causing reach‐scale cut bank erosion were investigated with analyses of stream stage, precipitation, and streambank volumetric and surfaces change that was measured during nine terrestrial lidar surveys in 2010–2012. The spatial variability of erosion during a simulated precipitation event was examined in a field‐based experiment. Erosion was greatest where mill pond sediment columns detached along vertical desiccation and horizontal seepage cracks. This sediment accumulated on the bank toe throughout the study and was a source of readily‐entrained fine sediment contrary to the upper reaches where depositional accommodation space is more limited. Findings suggest that hotspots of sediment excavation progress upstream, indicating that restoration efforts should focus upon stabilizing banks at these locations. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessing the vulnerability to soil erosion of the Ukai Dam catchments using remote sensing and GIS

Abstract

The investigation of basins for planning soil conservation requires a selective approach to identify smaller hydrological units, which would be suitable for more efficient and targeted conservation management programmes. One criterion, generally used to determine the vulnerability of catchments to erosion, is the sediment yield of a basin. In India, sediment yield data are generally not collected for smaller sub-catchments and it becomes difficult to identify the most vulnerable areas for erosion that can be treated on a priority basis. An index-based approach, based on the surface factors mainly responsible for soil erosion, is suggested in this study. These factors include soil type, vegetation, slope and various catchment properties such as drainage density, form factor, etc. The method is illustrated with a case study of sub-catchments immediately upstream of the Ukai Reservoir located on the River Tapi in Gujarat State, India. The area is divided into 16 watersheds and different soil, vegetation, topography and morphology-related parameters are estimated separately for each watershed. Satellite data are used to evaluate the soil and vegetation indices, while a GIS system is used to evaluate the topography and morphology-related indices. The integrated effect of all the parameters is evaluated to find different areas vulnerable to soil erosion. Two watersheds were identified as being most susceptible to soil erosion. Based on the integrated index, a priority rating of the watersheds for soil conservation planning is recommended.     

Interaction of wind and cold-season hydrologic processes on erosion from complex topography following wildfire in sagebrush steppe

Wildfire is a natural component of sagebrush (Artemisia spp.) steppe rangelands that induces temporal shifts in plant community physiognomy, ground surface conditions, and erosion rates. Fire alteration of the vegetation structure and ground cover in these ecosystems commonly amplifies soil losses by wind- and water-driven erosion. Much of the fire-related erosion research for sagebrush steppe has focused on either erosion by wind over gentle terrain or water-driven erosion under high-intensity rainfall on complex topography. However, many sagebrush rangelands are geographically positioned in snow-dominated uplands with complex terrain in which runoff and sediment delivery occur primarily in winter months associated with cold-season hydrology. Current understanding is limited regarding fire effects on the interaction of wind- and cold-season hydrologic-driven erosion processes for these ecosystems. In this study, we evaluated fire impacts on vegetation, ground cover, soils, and erosion across spatial scales at a snow-dominated mountainous sagebrush site over a 2-year period post-fire. Vegetation, ground cover, and soil conditions were assessed at various plot scales (8 m² to 3.42 ha) through standard field measures. Erosion was quantified through a network of silt fences (n = 24) spanning hillslope and side channel or swale areas, ranging from 0.003 to 3.42 ha in size. Sediment delivery at the watershed scale (129 ha) was assessed by suspended sediment samples of streamflow through a drop-box v-notch weir. Wildfire consumed nearly all above-ground live vegetation at the site and resulted in more than 60% bare ground (bare soil, ash, and rock) in the immediate post-fire period. Widespread wind-driven sediment loading of swales was observed over the first month post-fire and extensive snow drifts were formed in these swales each winter season during the study. In the first year, sediment yields from north- and south-facing aspects averaged 0.99–8.62 t ha⁻¹ at the short-hillslope scale (~0.004 ha), 0.02–1.65 t ha⁻¹ at the long-hillslope scale (0.02–0.46 ha), and 0.24–0.71 t ha⁻¹ at the swale scale (0.65–3.42 ha), and watershed scale sediment yield was 2.47 t ha⁻¹. By the second year post fire, foliar cover exceeded 120% across the site, but bare ground remained more than 60%. Sediment yield in the second year was greatly reduced across short- to long-hillslope scales (0.02–0.04 t ha⁻¹), but was similar to first-year measures for swale plots (0.24–0.61 t ha⁻¹) and at the watershed scale (3.05 t ha⁻¹). Nearly all the sediment collected across all spatial scales was delivered during runoff events associated with cold-season hydrologic processes, including rain-on-snow, rain-on-frozen soils, and snowmelt runoff. Approximately 85–99% of annual sediment collected across all silt fence plots each year was from swales. The high levels of sediment delivered across hillslope to watershed scales in this study are attributed to observed preferential loading of fine sediments into swale channels by aeolian processes in the immediate post-fire period and subsequent flushing of these sediments by runoff from cold-season hydrologic processes. Our results suggest that the interaction of aeolian and cold-season hydrologic-driven erosion processes is an important component for consideration in post-fire erosion assessment and prediction and can have profound implications for soil loss from these ecosystems. © 2019 John Wiley & Sons, Ltd.     

Using LM‐OSL of quartz to distinguish sediments derived from surface‐soil and channel erosion

This study describes the use of linearly modulated optically stimulated luminescence (LM‐OSL) to distinguish surface‐soil derived sediments from those derived from channel bank erosion. LM‐OSL signals from quartz extracted from 15 surface‐soil and five channel bank samples were analysed and compared to signals from samples collected from two downstream river sites. Discriminant analysis showed that the detrapping probabilities of fast, first slow and second slow components of the LM‐OSL signal can be used to differentiate between the samples collected from the channel bank and surface‐soil sources. We show that for each of these source end members these components are all normally distributed. These distributions are then used to estimate the relative contribution of surface‐soil derived and channel bank derived sediment to the river bed sediments. The results indicate that channel bank derived sediments dominate the sediment sources at both sites, with 90.1 ± 3% and 91.9 ± 1.9% contributions. These results are in agreement with a previous study which used measurements of ¹³⁷Cs and ²¹⁰Pb_ex fallout radionuclides to estimate the relative contribution from these two sources. This result shows that LM‐OSL may be a useful method, at least in the studied catchment, to estimate the relative contribution of surface soil and channel erosion to river sediments. However, further research in different settings is required to test the difference of OSL signals in distinguishing these sediment sources. And if generally acceptable, this technique may provide an alternative to the use of fallout radionuclides for source tracing. Copyright © 2015 John Wiley & Sons, Ltd.     

Erosion modelling for land management in the Tahoe basin,USA: scaling from plots to forest catchments

Abstract

Recent developments in hydrological modelling of river basins are focused on prediction in ungauged basins, which implies the need to improve relationships between model parameters and easily-obtainable information, such as satellite images, and to test the transferability of model parameters. A large-scale distributed hydrological model is described, which has been used in several large river basins in Brazil. The model parameters are related to classes of physical characteristics, such as soil type, land use, geology and vegetation. The model uses two basin space units: square grids for flow direction along the basin and GRU—group response units—which are hydrological classes of the basin physical characteristics for water balance. Expected ranges of parameter values are associated with each of these classes during the model calibration. Results are presented of the model fitting in the Taquari-Antas River basin in Brazil (26 000 km² and 11 flow gauges). Based on this fitting, the model was then applied to the Upper Uruguay River basin (52 000 km²), having similar physical conditions, without any further calibration, in order to test the transferability of the model. The results in the Uruguay basin were compared with recorded flow data and showed relatively small errors, although a tendency to underestimate mean flows was found.     

Use of magnetic and radiometric measurements to investigate erosion and sedimentation in a British upland catchment

In response to the potential shortcomings of single-technique strategies in the investigation of erosion and sedimentation, a combined magnetic and radiometric (¹³Cs and ²¹⁰Pb) approach has been undertaken in the upland, watershed-lake system of Howden Reservoir, Derbyshire. By combining these techniques, some assessment of sediment sources and the erosion status of the catchment has been achieved. Alone, each approach would have been unable to determine unequivocally sediment provenance. Furthermore, the parallel use of these measurements has highlighted limitations and/or uncertainties in both the magnetic and ¹³⁷Cs techniques. These problems reflect the particular soil characteristics and drainage conditions of this upland catchment. Despite the documented severity of peat erosion in the region, Howden Reservoir has a mixture of sediment sources and a relatively moderate rate of sedimentation. Sediment yields (total 127·7 t km^?2 yr^?1 including organic fraction 31·3 t km^?2 yr^?1) are, however, higher than in other British upland areas.     

The contribution of aeolian processes to fluvial sediment yield from a desert watershed in the Ordos Plateau,China

In arid zones, many active aeolian dunes terminate at ephemeral and perennial desert rivers. The desert rivers show very high rates of sediment transport that cause deleterious downstream effects on the river system and ecology. High sediment loading has been attributed to severe water erosion of sparsely covered watersheds during infrequent but heavy rainfall. Although aeolian erosion is known to lead to high rates of wind‐blown sand transport, direct confirmation of whether the aeolian processes accelerate or inhibit fluvial sediment loss is lacking. Here, we show that an aeolian‐fluvial cycling process is responsible for the high rate of suspended sediment transport in a Sudalaer ephemeral desert channel in the Ordos Plateau of China. Frequent aeolian processes, but low frequency (once every 3 years on average) flooding, occur in this region. Wind‐blown saltating grains appeared to be unable to cross the desert channel because of interruption of channel‐induced recirculating air flow, and therefore tended to settle in the channel during the windy seasons, leading to channel narrowing. During flooding, this narrowed channel was found to yield a threefold increase in suspended sediment loading and a 3.4‐fold increase in the weight percentage of the 0.08–0.2 mm sediment fraction on 18 July 2012. Loss of stored aeolian sand due to channel erosion accounted for about half of the total sediment yield in this watershed. These findings show that aeolian processes play an essential role in accelerating the sediment yield from a watershed characterized by aeolian‐fluvial interplay and also suggest that the drier the region and the greater the aeolian process, the more the aeolian process contributes to fluvial sediment yield. Copyright © 2013 John Wiley & Sons, Ltd.