Sediment Transport Coefficient Research Articles

Effects of Erosion Control Works: Case Study–Reservoir Celije, Rasina River Basin, the Zapadna Morava River (Serbia)

The aim of this research was to analyze the impact of implemented erosion control works (ECW) on soil erosion intensity in the watershed of the Ćelije reservoir (Rasina River) in the period between 1968 and 2022. The Erosion Potential Method was used to calculate the annual gross erosion (W), sediment transport (G), and erosion coefficient (Z) in the study area. As a result of the performed ECW there was a general decreasing trend in the intensity of soil erosion processes in the last 54 years. The specific annual gross erosion was 1189.12 m3/km−2/year−1 in 1968, while in 2022 it was 554.20 m3/km−2/year−1. The specific sediment transport was 540.18 m3/km−2/year−1 in 1968 and 253.55 m3/km−2/year−1 in 2022. Due to the changes in the intensity of erosion processes, the specific annual gross erosion decreased by 634.92 m3/km−2/year−1 and the specific sediment transport decreased by 286.63 m3/km−2/year−1. The erosion coefficient was reduced from Z = 0.62 to Z = 0.35. A dependence between the slope of siltation and the natural bed slope was defined. The results show a significant correlation between erosion intensity and performed ECW, providing a basis for future watershed management and defining a strategy for soil erosion control in the Ćelije reservoir watershed.

Uncertainty in Nearshore Sand Bar Migration

AbstractA coupled nearshore hydrodynamics and sediment transport model with adjoint‐based uncertainty quantification has been applied to sand bar morphodynamics in the Duck94 field experiment (Duck, NC). The model, which is based on previously established physical process formulas, is capable of producing accurate predictions of sand bar migration during the experiment. However, reasonable perturbations to the model inputs—namely initial conditions, boundary conditions, and calibration coefficients—can produce a range of model predictions. This range is taken to represent the potential for uncertainty of the model, for scenarios where one or more model inputs are poorly constrained. Experiments are presented that quantify the relative impact of different input types on uncertainty, and the results are contrasted for accretive versus erosive wave conditions. During accretive conditions (shoreward bar migration), uncertainty is dominated by the growth of errors in initial conditions, owing to positive feedbacks in the model. In contrast, during erosive conditions (seaward bar migration), there is a smaller impact of errors in initial conditions, and uncertainty is instead dominated by errors in sediment transport coefficients. The results thus point to qualitative differences in the nature of nearshore morphodynamic uncertainty under different wave conditions, with a greater role of internal error growth (positive feedbacks) during accretive conditions and errors in model physics being more important during erosive conditions.

Transient hillslope erosion in slow evolution landscapes

SummaryTransient evolution and adjustment to changing tectonic and climatic boundary conditions is an essential attribute of landscapes, and characterizing transient behavior is a key to understanding their dynamics and history. Developing new approaches to detect such transience has been explored by various methods, in particular to identify landscape response to Late Cenozoic and Quaternary climatic changes. Such studies have often focused on regions of high relief and/or active tectonic activity where interferences between tectonic and climatic signals might complicate the interpretation of the observations. We investigated the case of the hillslopes of the Serra do Cipó quartzitic range in SE Brazil in order to detect and quantify transience in a tectonically quiescent landscape over 100‐ka timescales. We determined hilltop curvature from a high‐resolution digital surface model derived from Pléiades imagery and measured cosmogenic nuclide (10Be and 26Al) concentrations at these hilltop sites. We compare both observations with predictions of hillslope diffusion theory, observing a distinctive signature of an acceleration of denudation. We performed a joint inversion of topographic and isotopic data to retrieve an evolution of the hillslope sediment transport coefficient through time. The timing of the increase in denudation cannot be unequivocally associated with a single climatic event but is consistent with important, climatically modulated fluctuations in precipitation and erosion in this area during the Middle and Late Pleistocene.

Variable scale effects on hillslope soil erosion during rainfall-runoff processes

The variation of soil erosion across scales remains a controversial issue. A theoretical framework, coupling the normalized Green-Ampt equation for infiltration, 1D kinematic wave model for overland flow, and WEPP erosion modeling approaches for soil erosion, was used to explain and quantify the direct effect of scale on the soil erosion process. The results show that the relation between interrill erosion and slope length accords with a power-law decreasing trend, while the relation of rill erosion versus slope length shows a power-law increasing trend. Moreover, the power-law scaling of interrill erosion becomes more prominent with an increase of rainfall duration and intensity but is nearly independent of the interrill erodibility factor. The rainfall duration, soil type, soil critical shear stress, sediment transport coefficient, and rill erodibility factor substantially influence the magnitude of scale effect on rill erosion, while rainfall intensity, slope gradient, and Manning's roughness have minor effects. It suggests that the scale effect on soil erosion is variable and dynamic for different erosion types, rainfall and soil characteristics, which can explain why the controversial scaling relationships of soil erosion exist in previous studies. The physically based soil erosion model WEPP is compared to the empirical models USLE and RUSLE at various scales. Only when rill erosion plays a dominant role in soil loss do all these models exhibit consistent scale effect on erosion. These results provide insights into the essence of scaling in the hillslope soil erosion process, which are expected to benefit the development of upscaling techniques for the large-scale hydrological, geomorphological, and ecological processes.

Sediment transport and roughness coefficients generated by flexible vegetation patches in the emergent and submerged conditions in a semiarid alluvial open-channel

In the semiarid environment, low vegetation cover and deforestation on the banks of rivers represent a strong impact on the flow resistance process, altering the interaction of the flow with the banks, one of the conditioning factors of the morphological balance of the river channel. In extreme hydrological events, turbulence and high speeds can cause a marked erosion process at the banks, increasing the sediment yield. This study aimed to monitor water and solid discharges through direct hydrosedimentometric measurements, as well as to estimate, using a simplified model based on force balance, the hydraulic roughness coefficients generated by flexible vegetation element of the Ipomoea pes-caprae species under emergent and submerged conditions. The total annual rainfall for 2019 was 748.5 mm year−1, above the historical mean (642.80 mm year−1). The mean depth (h = 0.24 m) and flow (Ql = 0.15 m3 s−1) rates were highly correlated as a power function (R2 = 0.8041). In general, without specifically considering the effect of vegetation, the flow regime was characterized as turbulent and subcritical flow (Re = 66,634.65 dim. and Fr = 0.16 dim.), respectively. The suspended sediment concentration (CSS), suspended sediment (QSS) and bedload (QBed) discharges presented mean values of 409.41 mg L−1, 6.23 t day−1 and 0.069 t day−1, respectively. The total sediment yield (Yt) was 1.90 t km−2 year−1. The vegetation effect was analyzed on the right (RB) and left (LB) banks, where the stems of I. pes-caprae were concentrated between 0.22 and 0.40 m and 0.66–1.20 m from the banks, respectively. On the RB, the shear (1.0 < Sr < 1.95), free flow (0.27 < Sr < 1.0) and the transition (Sr = 1) zones were identified. On the LB, just emergent condition (Sr < 1.0) was indetified. The following relationship were found between vegetation drag coefficient (C′D), stem Reynolds number (Red), plant density Reynolds number (Rev) and Froude number (Fr) on the RB: C′D (1.22–12.46 m−1); Red (567 < Red < 2224), Rev (1305 < Rev < 35,011) and Fr = 0.14; on the LB: C′D (2.10–1168.29 m−1); Red (57 < Red < 1427), Rev (2615 < Rev < 167,534) and Fr = 0.08. These results demonstrate that the aquatic vegetation directly influences the river dynamics; the density vegetation having a hydraulic impact on the flow transport capacity, that is, as the C′D increases, it increases the resistance of the vegetation to the flow and decreases the capacity of transport of sediments.

Meandering-anabranching river channel change in response to flow-sediment regulation: Data analysis and model validation

Anabranching river has been recognized as an independent river pattern. A variety of investigations have attempted to disclose the formation of anabranching rivers, however, most previous studies mainly focused on the controls of anabranching channels formation. The mechanism of transition among sub-channels with varying flow-sediment regime in anabranching rivers has been poorly understood. In this study, we used multi-source data sets to investigate the changes in river channels and island (Wugui Island, about 7 km2) along the 27-km long Jianli Reach (Tashiyi-Tianziyihao) in the middle Yangtze River. Nine Landsat images (1987, 1990, 1994, 1996, 2000, 2003, 2006, 2010 and 2014) with 30 m spatial resolution were used to map the channel planform dynamics. Obvious changes in channel width and morphology of the island were observed during 1987–2014. The width of the left branch had reduced from 1041 m to 340 m, while the width of the right branch had increased from 632 m to 1531 m. During 1994–1996, the main river channel shifted from the left branch to the right. During the period of 1987–2014, the total width of the compound cross-section increased, and the mean length-width ratio of the island increased from 3.59 to 4.32. The mean annual flow discharge increased about 9%, and mean annual suspended sediment concentration and sediment transport coefficient (the ratio of mean annual suspended sediment concentration to mean annul flow discharge) decreased about 75% and 76% respectively from 1951 to 2009, in response to the operation of Gezhouba Dam and Three Gorges Dam. The meandering-anabranching channel planform adjusted by narrowing the island width and enlarging compound cross-section in response to upstream water-sediment regime. The shift of the main river channel from left branch to the right may resulted from the increase in flow discharge and decrease in sediment transport coefficient since 1980s. The adjustment has been intensified by the changes in the sediment transport rate after 1995. After the main channel was switched from left branch to right branch, the flow-sediment diversion capacity was kept stable with a diversion percentage of 90% in right branch. Due to the persistent influence of the Three Gorges Dam, the right branch will function as the main channel for a long time before the meandering-anabranching system achieves equilibrium. The shift and stability of main channel was closely related with the relative magnitude of flow-sediment partition in each branch. The significance of this study is that we successfully manifested the main channel shift process of anabranching river with the support of direct hydromorphological observation and theoretical verification.

Prediction of Parameters which Affect Beach Nourishment Performance Using MARS, TLBO, and Conventional Regression Techniques

Artificial beach nourishment is one of the most important environmentally friendly coastal protection methods since it protects the aesthetic and recreational values of the beach and increases its protective properties. Therefore, the main aim of the current study is to assess the accuracy of multivariate adaptive regression splines (MARS) in predicting the parameters, namely sediment transport coefficients (K) and the diffusion rate (Ω), which affect beach nourishment performance. The performance of the MARS was determined by comparison of the models using exponential, linear, and power regression equations trained by conventional regression analyses (CRA) and the teaching-learning based optimization (TLBO) algorithm. In all models, two different input data obtained from the experimental study were used, one dimensional and one non-dimensional. The results presented that the MARS models gave lower error values than the CRA and TLBO models according to the root mean square error, mean absolute error, and scattering index criteria. When the models were evaluated, it was revealed that dimensional and non-dimensional models gave approximate results. We proved that the dimensional and non-dimensional MARS models can be used to estimate the (K) and (Ω) values.

Effects of Erosion Control Works: Case Study—Grdelica Gorge, the South Morava River (Serbia)

The aim of this research was to analyse the changes in the soil erosion intensity caused by erosion control works (ECW) in Grdelica Gorge (The South Morava River) in the period between 1953 and 2016. For the purpose of quantifying the erosion intensity changes, the erosion potential model (EPM) was used to calculate the annual gross erosion (W), sediment transport (G), and erosion coefficient (Z) in the study area. As a result of the performed technical and biotechnical erosion control works, there was a general decreasing trend in the intensity of soil erosion processes in the last 63 years. The specific annual gross erosion in Grdelica Gorge was 1920.34 m3/km−2/year−1 in 1953, while in 2016 it was 492.42 m3/km−2/year−1. The specific sediment transport was 1421.05 m3/km−2/year−1 in 1953 and 364.39 m3/km−2/year−1 in 2016. Due to the changes in the intensity of erosion processes, the specific annual gross erosion in the study area decreased by 1427.92 m3/km−2/year−1 and the specific sediment transport by 1056.66 m3/km−2/year−1. The value of the erosion coefficient was reduced from Z = 0.84 in 1953 to Z = 0.32 in 2016. The results show that there is a significant correlation between the soil erosion intensity (erosion coefficient) and ECW (biotechnical works) performed in Grdelica Gorge. The permanent control of erosion processes in Grdelica Gorge is very important for torrential flood prevention and protection of two very important traffic routes (Belgrade-Skopje-Athens railway and motorway—Corridor X), as well as settlements, local roads, and other facilities in this area. Furthermore, these results are the basis for future water mana­gement projects, soil and environmental protection, spatial planning, agriculture, and other human activities.

Development of a numerical model for simulating bed-level changes in front of a sea wall

Anticipating the trend of seabed changes and scouring phenomena is one of the most important issues in coastal engineering, and several numerical models have already been presented to simulate coastal seabed evolutions and estimate the depth of scouring in front of coastal structures. The sea wall as a reflective structure plays an important role in protecting the coast, and how the presence or absence of these structures influences the sediment transport rate and consequently seabed changes should be studied more carefully. In this study, bed-profile changes and the potential for scouring were calculated by a numerical model at different locations in the vicinity of a sea wall. Unlike previous models in which the velocity of sediment particles was assumed to be equal to the velocity of water particles, a more detailed process was considered here to calculate the velocity of water and sediment particles. Finally, it became clear that using the sediment transport coefficient parameter did not significantly improve the seabed changes that were predicted by the modified model in the present study. This achievement shows the rationality of considering the velocity of sediment particles as being equal to the velocity of water particles in predicting seabed changes.

Sedimentation of overbank floods in the confined complex channel–floodplain system of the Lower Yellow River, China

AbstractThe channel boundary conditions along the Lower Yellow River (LYR) have been altered significantly since the 1950s with the continual reinforcement and construction of both main and secondary dykes and river training works. To evaluate how the confined complex channel–floodplain system of the LYR responds to floods, this study presents a detailed investigation of the relationship between the tempo‐spatial distribution of sedimentation/erosion and overbank floods occurred in the LYR. For large overbank floods, we found that when the sediment transport coefficient (ratio of sediment concentration of flow to flow discharge) is less than 0.034, the bankfull channel is subject to significant erosion, whereas the main and secondary floodplains both accumulate sediment. The amount of sediment deposited on the main and secondary floodplains is closely related to the ratio of peak discharge to bankfull discharge, volume of water flowing over the floodplains, and sediment concentration of overbank flow, whereas the degree of erosion in the bankfull channel is related to the amount of sediment deposited on the main and secondary floodplains, water volume, and sediment load in flood season. The significant increase in erosion in the bankfull channel is due to the construction of the main and secondary dykes and river training works, which are largely in a wide and narrow alternated pattern along the LYR such that the water flowing over wider floodplains returns to the channel downstream after it drops sediment. For small overbank floods, the bankfull channel is subject to erosion when the sediment transport coefficient is less than 0.028, whereas the amount of sediment deposited on the secondary floodplain is associated closely with the sediment concentration of flow. Over the entire length of the LYR, the situation of erosion in the bankfull channel and sediment deposition on the main and secondary floodplains occurred mainly in the upper reach of the LYR, in which a channel wandering in planform has been well developed.

How does grid-resolution modulate the topographic expression of geomorphic processes?

Abstract. In many locations, our ability to study the processes which shape the Earth are greatly enhanced through the use of high-resolution digital topographic data. However, although the availability of such datasets has markedly increased in recent years, many locations of significant geomorphic interest still do not have high-resolution topographic data available. Here, we aim to constrain how well we can understand surface processes through topographic analysis performed on lower-resolution data. We generate digital elevation models from point clouds at a range of grid resolutions from 1 to 30 m, which covers the range of widely used data resolutions available globally, at three locations in the United States. Using these data, the relationship between curvature and grid resolution is explored, alongside the estimation of the hillslope sediment transport coefficient (D, in m2 yr−1) for each landscape. Curvature, and consequently D, values are shown to be generally insensitive to grid resolution, particularly in landscapes with broad hilltops and valleys. Curvature distributions, however, become increasingly condensed around the mean, and theoretical considerations suggest caution should be used when extracting curvature from landscapes with sharp ridges. The sensitivity of curvature and topographic gradient to grid resolution are also explored through analysis of one-dimensional approximations of curvature and gradient, providing a theoretical basis for the results generated using two-dimensional topographic data. Two methods of extracting channels from topographic data are tested. A geometric method of channel extraction that finds channels by detecting threshold values of planform curvature is shown to perform well at resolutions up to 30 m in all three landscapes. The landscape parameters of hillslope length and relief are both successfully extracted at the same range of resolutions. These parameters can be used to detect landscape transience and our results suggest that such work need not be confined to high-resolution topographic data. A synthesis of the results presented in this work indicates that although high-resolution (e.g., 1 m) topographic data do yield exciting possibilities for geomorphic research, many key parameters can be understood in lower-resolution data, given careful consideration of how analyses are performed.

Detection of transience in eroding landscapes

AbstractPast variations in climate and tectonics have led to spatially and temporally varying erosion rates across many landscapes. In this contribution I examine methods for detecting and quantifying the nature and timing of transience in eroding landscapes. At a single location, cosmogenic nuclides can detect the instantaneous removal of material or acceleration of erosion rates over millennial timescales using paired nuclides. Detection is possible only if one of the nuclides has a significantly shorter half‐life than the other. Currently, the only practical way of doing this is to use cosmogenicin situcarbon‐14 (14C) alongside a longer lived nuclide, such as beryllium‐10 (10Be). Hillslope information can complement or be used in lieu of cosmogenic information: in soil mantled landscapes, increased erosion rates can be detected for millennia after the increase by comparing relief and ridgetop curvature. This technique will work as long as the final erosion rate is greater than twice the initial rate. On a landscape scale, transience may be detected based upon disequilibria in channel profiles or ridgetops, but transience can be sensitive to the nature of transient forcing. Where forcing is periodic, landscapes display differing behavior if forcing is driven by changes in base level lowering rates versus changes in the efficiency of either channel or hillslope erosion (e.g. driven by climate change). Oscillations in base level lowering lead to basin averaged erosion rates that reflect a long term average erosion rate despite strong spatial heterogeneity in local erosion rates. This averaging is reflected in10Be concentrations in stream sediments. Changes in hillslope sediment transport coefficients can lead to large fluctuations in basin averaged erosion rates, which again are reflected in10Be concentrations. The variability of erosion rates in landscapes where both the sediment transport and channel erodibility coefficients vary is dominated by changes to the hillslope transport coefficient. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Evaluating fluvial terrace riser degradation using LiDAR-derived topography: An example from the northern Tian Shan, China

The morphological degradation of fluvial terrace risers provides a constraint to terrace chronology. In this study, we morphologically date the terrace risers along the Kuitun River on the north flank of the Tian Shan, China and subsequently discuss possible relationships between terrace formation and the past regional climate changes and tectonic activity of the Dushanzi fault-related fold. To do this, 159 topographic profile swaths of terrace risers were extracted from LiDAR-derived DEM and were analysed to determine a range of best fitting morphological ages. Through Monte Carlo simulation, a locally applicable sediment transport coefficient (diffusivity) was calibrated as 5.5±1.6m2/ky given the morphological age of the T1/T2 riser and its independently known age. Taking this calibrated coefficient, we estimate age ranges of 11.6±3.4ka, 6.5±1.4ka, 5.3±1.1ka, and 4.2±1.2ka for terraces T3, T4, T5, and T6, respectively, under the assumption that the age of the riser is close to the abandonment age of the lower surface. These new terrace ages, combining climate proxy records from the oxygen isotope curve from the Guliya ice cap and paleoearthquake events in the Dushanzi fault related fold, suggest that tectonic activity may be an important factor in the formation of lower terraces within the growing anticlines, while in more extensive areas beyond anticlines, climate changes controlled the main deposition and incision events in the present study area, and thus terrace formation of T1–T3.

Influence of lithology on hillslope morphology and response to tectonic forcing in the northern Sierra Nevada of California

Many geomorphic studies assume that bedrock geology is not a first‐order control on landscape form in order to isolate drivers of geomorphic change (e.g., climate or tectonics). Yet underlying geology may influence the efficacy of soil production and sediment transport on hillslopes. We performed quantitative analysis of LiDAR digital terrain models to examine the topographic form of hillslopes in two distinct lithologies in the Feather River catchment in northern California, a granodiorite pluton and metamorphosed volcanics. The two sites, separated by &lt;2 km and spanning similar elevations, were assumed to have similar climatic histories and are experiencing a transience in landscape evolution characterized by a propagating incision wave in response to accelerated surface uplift c. 5 Ma. Responding to increased incision rates, hillslopes in granodiorite tend to have morphology similar to model predictions for steady state hillslopes, suggesting that they adjust rapidly to keep pace with the incision wave. By contrast, hillslopes in metavolcanics exhibit high gradients but lower hilltop curvature indicative of ongoing transient adjustment to incision. We used existing erosion rate data and the curvature of hilltops proximal to the main channels (where hillslopes have most likely adjusted to accelerated erosion rates) to demonstrate that the sediment transport coefficient is higher in granodiorite (8.8 m2ka−1) than in metavolcanics (4.8 m2ka−1). Hillslopes in both lithologies get shorter (i.e., drainage density increases) with increasing erosion rates.

Prediction formula for longshore sediment transport rate with M5' algorithm

ABSTRACT Mafi, S., Yeganeh-Bakhtiary, A., Kazeminezhad, M.H., 2013. Prediction formula for longshore sediment transport rate with M5' algorithm One of the most vital tasks for coastal engineers is to calculate the gross longshore sediment transport rate (LSTR) to control the shoreline erosion and beach evolution. In the past decades, several empirical formulas or parametric models have been proposed for predicting the gross LSTR as a function of the breaking wave characteristics, bed materials and beach slope. The main downside with these formulas is that they give wide range of different rate of predictions; consequently their reliability under the changing hydrodynamic conditions is very uncertain. In the present study, an alternative approach based on the Regression Trees (M5') was applied to present a new formula for prediction of LSTR in terms of the longshore sediment transport coefficient (e). Several high-quality data sets were employed, which comprise of the wave parameters, sediment characterist...

Using hilltop curvature to derive the spatial distribution of erosion rates

Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high‐resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr−1). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient‐limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

Equilibrium form of horizontally retreating, soil-mantled hillslopes: Model development and application to a groundwater sapping landscape

[1] We present analytical solutions for the steady state topographic profile of a soil-mantled hillslope retreating into a level plain in response to a horizontally migrating base level. This model applies to several scenarios that commonly arise in landscapes, including widening valleys, eroding channel banks, and retreating scarps. For a sediment transport law in which sediment flux is linearly proportional to the topographic slope, the steady state profile is exponential, with an e-folding length, L, proportional to the ratio of the sediment transport coefficient to the base level migration speed. For the case in which sediment flux increases nonlinearly with slope, the solution has a similar form that converges to the linear case as L increases. We use a numerical model to explore the effects of different base level geometries and find that the one-dimensional analytical solution is a close approximation for the hillslope profile above an advancing channel tip. We then compare the analytical model with hillslope profiles above the tips of a groundwater sapping channel network in the Florida Panhandle. The model agrees closely with hillslope profiles measured from airborne laser altimetry, and we use a predicted log linear relationship between topographic slope and horizontal distance to estimate L for the measured profiles. Mapping 1/L over channel tips throughout the landscape reveals that adjacent channel networks may be growing at different rates and that south facing slopes experience more efficient hillslope transport.

Impact of a uniform bore on an erodible beach

The impact of a uniform bore on an erodible beach is investigated using a shallow water description and a sediment conservation equation. The solution, which is obtained using the method of characteristics, employs a cubic bed-load formula and a crudely calibrated sediment transport coefficient. It is found that, as with the fixed bed case of Hibberd and Peregrine (1979), a backwash bore forms, which for the mobile bed also comprises a bed step. It is found that this bed step achieves a significant height. The volume of sand deposited above the final still water level is consistent with that observed under certain events on some sandy beaches.

Channel adjustments in response to the operation of large dams: The upper reach of the lower Yellow River

The Yellow River in China carries an extremely large sediment load. River channel-form and lateral shifting in a dynamic, partly meandering and partly braided reach of the lower Yellow River, have been significantly influenced by construction of Sanmenxia Dam in 1960, Liujiaxia Dam in 1968, Longyangxia Dam in 1985 and Xiaolangdi Dam in 1997. Using observations from Huayuankou Station, 128km downstream of Xiaolangdi Dam, this study examines changes in the river before and after construction of the dams. The temporal changes in the mean annual flow discharge and mean annual suspended sediment concentration have been strongly influenced by operation of theses dams. Observations of sediment transport coefficient (ratio of sediment concentration to flow discharge), at-a-station hydraulic geometry and bankfull channel form observed from 1951 to 2006 have shown that, although variations in flow and sediment load correspond to different periods of dam operation, changes in channel form are not entirely synchronous with these. The channel has been subject to substantial deposition due to the flushing of sediment from Sanmenxia Dam, resulting in a marked reduction in bankfull cross-sectional area. Flows below bankfull had a greater impact on channel form than higher flows because of very high sediment load. At-a-station hydraulic geometry shows that the variation of channel cross-sectional area below bankfull in this wide and relatively shallow system largely depends on changes in width. Such at-a-station changes are significantly influenced by (1) events below bankfull and (2) overbank floods. Bankfull depth is the main component of channel adjustment in that depth adjusts synchronously with channel area. The channel adjusts its size by relatively uniform changes in depth and width since 1981. Channel morphology is not the product of single channel-forming flow frequency. It is determined by the combination of relatively low flows that play an important role in fine sediment transport and bed configuration as with relatively high flows that are effective at modifying the channel's morphology. The sediment transport coefficient is a useful index for efficiently guiding the operation of the dams in a way that would minimize channel changes downstream. Sedimentation over the nearly 60years of study period caused the lower Yellow River to aggrade progressively, the only significant exception being the two years following completion of Sanmenxia Dam.

Relationship between Sediment Particle Size and Transport Coefficient on Steep Slopes

Transport capacity of overland flow is related to sediment particle size. The WEPP (Water Erosion Prediction Project) model considers soil particle size in the sediment transport capacity equation; however, the equation does not work well for steep slopes. This study introduces sediment particle size into the determination of transport capacity on steep slopes by quantifying the relationship between sediment particle size and transport coefficient using the modified sediment transport capacity equation for steep slopes from the WEPP model. The relationship is based on the measured sediment transport coefficient associated with different sediment particle sizes in a hydraulic flume. The experiment used five sediment particle sizes (0.10, 0.22, 0.41, 0.69, and 1.16 mm), on five slopes (8.7%, 17.4%, 35.9%, 34.2%, and 42.3%), and with five flow rates (0.66, 1.32, 2.63, 3.95, and 5.26 10-3 m2 s-1). The results showed that the sediment transport coefficient decreased significantly with increasing sediment size. The relationship was well described by a power function with transport coefficient as dependent variable and sediment particle size as independent variable. The sediment transport capacity equation could be modified using two independent terms: the sediment particle size power function, representing sediment properties, and flow shear stress, representing flow hydraulics. However, both the WEPP transport capacity equation and the modified equation did not predict the measured transport capacity well for steep slopes. Further studies are needed to improve the performance of the modified equation using fine sediment particles beyond the range of this study.