Sediment Load Measurements Research Articles

Measurement and Calculation of Sediment Transport on an Ephemeral Stream

Sediment transport remains a significant challenge for researchers due to the intricate nature of the physical processes involved and the diverse characteristics of watercourses worldwide. A type of watercourse that is of particular interest for study is the ephemeral streams, found primarily in semiarid and arid regions. Due to their unique nature, a new measurement algorithm was created and a modified bed load sampler was built. Measurement of the bed load transport rate and calculation of the water discharge were conducted in an ephemeral stream in Northeastern Greece, where the mean calculated streamflow rate ranged from 0.019 to 0.314 m3/s, and the measured sediment load transport rates per unit width varied from 0.00001 to 0.00213 kg/m/s. The sediment concentration was determined through various methods, including nonlinear regression equations and formulas developed by Yang, with the coefficients of these formulas calibrated accordingly. The results demonstrated that the equations derived from Yang’s multiple regression analysis offered a superior fit compared to the original equations. As a result, two modified versions of Yang’s stream sediment transport formulas were developed and are presented to the readership. To assess the accuracy of the modified formulas, a comparison was conducted between the calculated total sediment concentrations and the measured total sediment concentrations based on various statistical criteria. The analysis shows that none of Yang’s original formulas fit the available data well, but after optimization, both modified formulas can be applied to the specific ephemeral stream. The results indicate also that the formulas derived from the nonlinear regression can be successfully used for the determination of the total sediment concentration in the ephemeral stream and have a better fit compared to Yang’s formulas. The correlation from the nonlinear regression equations suggests that total sediment transport is primarily influenced by water discharge and rainfall intensity, with the latter showing a high correlation coefficient of 0.998.

Estimating stream sediment loads to assess management options for a Southern Appalachian mountain lake

Enka Lake is a 25-ha reservoir in the Southern Appalachian mountains surrounded by the Biltmore Lake residential neighborhood. The aesthetic and recreational value of Enka Lake can be negatively affected by turbid water and sediment buildup, resulting from stormwater originating upstream in its 15.38 km2 watershed. In this study, we monitored streamflow and sediment loads for nearly 2 years to better understand sedimentation dynamics and evaluate management options. We collected nearly 500 water samples throughout the watershed, focusing on two tributaries that provide the majority of streamflow into Enka Lake. During baseflow conditions, those tributaries are very clear (turbidity ranging from 1 to 20 NTU), but sediment-laden stormflow (turbidity > 1000 NTU) from dirt roads, gravel driveways, and poorly vegetated yards is common and, during extreme storm events, causes lake discoloration. Discharge-sediment data pairs were used to construct sediment rating curves and estimate sediment fluxes through these tributaries. The predicted sediment fluxes were tested with high-frequency sampling during and after three storms in different seasons; predicted values underestimated the peak sediment fluxes (0–400%) but generally matched total measured sediment loads. Estimates of annual sediment load from the two tributaries (540–900 tonnes/year) suggest that a dredging effort commissioned by the neighborhood association in 2016 may have removed only about 1–3 years’ worth of sediment. Installing sediment forebays or other retention structures is likely the preferred sediment management strategy moving forward (over a regular dredging schedule) since they can be maintained more easily without affecting lake levels. Designing these retention structures will benefit from the estimates of stormflow and sediment fluxes, and the study design presented here—including the help of community-based volunteer samplers—provides a model that could be used at other sites in the Southern Appalachians and elsewhere.

A strategy for modelling sediment distribution along slope lengths of an ephemeral gully through the simulated scouring experiments

Accurately describing the soil erosion process is crucial for improving the accuracy of existing simulation models of ephemeral gully erosion. However, a laboratory experimental strategy for simulating sediment distribution along slope lengths of an ephemeral gully is lacking. This limitation greatly hinders the development of simulation models for ephemeral gully erosion. In this study, we constructed six soil flumes with 12 m in length, 0.1 m in width, and 0.5 m in depth, which were filled with cultivated loess soils. Four flow rates (5.33, 10.67, 21.33, and 42.67 × 10−3 m3 s−1 m−1), four slope gradients (5°, 10°, 15°, and 20°), and six gully lengths (2, 4, 6, 8, 10 and 12 m) were adopted to perform a series of laboratory scouring experiments. The measured sediment concentrations at the outlet ranged from 23.39 kg m−3 to 534.60 kg m−3. The trend of exponential increase in sediment concentrations along slope lengths of an ephemeral gully was similar to that observed in a rill. According to the fitting equation, the maximum sediment concentrations (Ae) ranged from 358.05 kg m−3 to 910.88 kg m−3. The attenuation coefficients (Be) indicated the rate of increase in sediment load with slope length, which ranged from 0.042 m−1 to 0.185 m−1. By combining the measured sediment loads along the slope length (8 m) with the sediment transport capacity, the sediment distribution along slope lengths of an ephemeral gully could be accurately quantified. Moreover, the results confirmed that a flume with a short slope length (4/6 m) could effectively determine the sediment distribution along slope lengths at gentle slope of 5° and 10°. However, a minimum length of 8 m was advisable at steep slope of 15° and 20°. In summary, our experimental strategy provides an efficient method for determining the sediment distribution along slope lengths of an ephemeral gully.

Quantifying the contribution of bank erosion to a suspended sediment budget using boat‐mounted LiDAR and high‐frequency suspended sediment monitoring

AbstractUnderstanding the physical processes as well as the hydrological and morphological factors that influence channel bank erosion is important for river restoration and the management of the floodplain and associated ecosystems. In this study, we introduced an innovative approach to quantify river bank erosion and its contribution to a reach fine sediment budget by combining repeat bank erosion surveys using a jetboat‐mounted LiDAR scanner with concurrent high‐frequency suspended sediment load measurements into and out of the surveyed reach. Using this information, we established a sediment budget for a 5.5‐km‐long study reach of the lower Ōreti River, Southland, New Zealand. A total of three surveys were conducted along the study reach to understand changes in the bank erosion contribution to suspended sediment load at different time scales. The first two surveys were separated by a short period of 8 weeks, and the third survey followed 2.5 years later. The measured volumes of fine sediment rendered from bank erosion equated to 25% and 29% of the measured outflowing suspended load over these two inter‐survey epochs, respectively. By comparison, the net contribution of measured bank erosion and derived fine sediment deposition on the riverbed to the outflowing suspended load was 12% over the first, shorter epoch and 25% for the second, 2.5‐year epoch. These results highlight the important role of in‐channel sediment deposition in the variability of net suspended sediment exports from channel reaches experiencing bank erosion. The approach used in this study has a unique capability to accurately monitor bank erosion and obtain high‐resolution topography data capturing changes in river banks over different time periods.

Effects of climate variability and anthropogenic factors on sediment load reduction in the Weihe River basin, China

AbstractSediment load of China's Yellow River has dramatically decreased due to the interplay of climate variability and human activities in the past decades. However, the contribution of individual driving factors and the detailed spatial distribution of sediment load at a catchment scale were not well understood. In the study, we quantitatively assessed the relative contributions of individual climatic (i.e. precipitation) and anthropogenic (i.e. vegetation restoration, terrace construction) driving factors to sediment load variations in the largest tributary of the Yellow River, Weihe River. The employed annual sediment load measurements and rainfall data for 1961–2015 were collected at 15 hydrological and 60 meteorological stations that were widely distributed in the basin. Changing trends, periodicity, and abrupt changes of hydro‐climatic variables were detected using the non‐parametric Mann–Kendall test, the empirical mode decomposition method, and Pettitt's test. Results showed that annual sediment load presented significantly decreasing trends at 12 hydrological stations. Annual rainfall erosivity showed a mixed tendency, with the changing trends varying among different regions. A 3‐year periodicity was identified in the annual sediment load at three main stream gauging stations. Abrupt changing points in sediment load primarily occurred in the 1990s and early 2000s at all of the 15 hydrological stations, which were largely attributed to human activities. The increase in vegetation cover was responsible for approximately 40.6%, 37.4%, and 54.7% of the sediment load reduction in the main stream of the Weihe River, the Jinghe River, and the Beiluohe River, respectively. Meanwhile, terrace construction accounted for 19.7%, 43.8%, and 9.9% of the sediment variation, respectively. Conversely, precipitation change accounted for 5.7%, 12.4%, and 8.3% of sediment load reduction, respectively. Overall, vegetation restoration and terrace construction were found to be the two dominating factors for sediment load reduction in the study area.

ASSESSMENT OF SEDIMENT REPLENISHMENT VOLUME IN LANGAT RIVER SYSTEM

The river sand mining activity has taken place in a sand-bed river system in Selangor, Malaysia for decades even before the legalisation was initiated in 2008. This study focuses on the determination of optimum sand extraction for low-flow and high-flow seasons. The sand replenishment rate was used as the benchmark in determining the threshold level of the extractable rate in the Langat River, Selangor system. The total sediment load was computed using Yang (1973) equation due to the high percentage of agreement between the predicted sediment load and measured sediment load. Almost 41.6% of the predicted data fall within the allowable discrepancy ratio test between the predicted value and measured value. The comparison of sand replenishment rate in high and low flow seasons proved that the river system has quicker capabilities in sand replenishment rate at the extraction point during the high-flow season compared to the latter by 83%. The quantifiable volume of the extractable sand rate is proposed specifically during low-flow months (May to September) whereby the slower replenishment rate is critical and riskier. The optimal sand mining volume during the low-flow months is calculated by reducing 10% from the total replenishment volume and the recommended optimum extraction load has been delivered by the number of 25 tons lorries for easier observation by the contractor and authority’s body. The monthly optimum extraction in Langat River during low-flow months is calculated at a minimum of 437 trucks to a maximum of 20,114 trucks per month.

SEASONAL VARIATION OF THE RELATIONSHIP BETWEEN SUSPENDED SEDIMENT AND RIVER DISCHARGE

Information on measured sediment load is important in calibrating hydrologic models used in sediment yield prediction. The purpose of this study was to relate sediment load to discharge at a river section. An appropriate section was selected at the Lotanyanda river, Limpopo, South Africa. Depth integrated sediment samples were collected on various dates at an appropriate interval once a day and analyzed to determine the sediment concentration and turbidity. The water level and temperature readings were also taken at the time of sediment sampling. Information on sediment concentration and discharge was used to determine the sediment load on the dates when sampling commenced autumn through winter in 2016. Based on the values of measures of performance that included R2, NSE, RMSE, PBIAS and RSR, performance of predicted sediment concentration based on sediment rating curves was very good and could beused for predicting sediment load. The measures of performance, however varied from season to season. The relationship between sediment load and discharge varied from one season to another and the quality of fit (R2) based on regression analysis differed from one season to another. Knowledge of sediment load discharge relationship can be used in sediment load prediction based on discharge information.

Use of Monitoring Approaches to Verify the Predictive Accuracy of the Modeling of Particle-Bound Solid Inputs to Surface Waters

For particle-bound substances such as phosphorus, erosion is an important input pathway to surface waters. Therefore, knowledge of soil erosion by water and sediment inputs to water bodies at high spatial resolution is essential to derive mitigation measures at the regional scale. Models are used to calculate soil erosion and associated sediment inputs to estimate the resulting loads. However, validation of these models is often not sufficiently possible. In this study, sediment input was modeled on a 10 × 10 m grid for a subcatchment of the Kraichbach river in Baden-Wuerttemberg (Germany). In parallel, large-volume samplers (LVS) were operated at the catchment outlet, which allowed a plausibility check of the modeled sediment inputs. The LVS produced long-term composite samples (2 to 4 weeks) over a period of 4 years. The comparison shows a very good agreement between the modeled and measured sediment loads. In addition, the monitoring concept of the LVS offers the possibility to identify the sources of the sediment inputs to the water body. In the case of the Kraichbach river, it was found that around 67% of the annual sediment load in the water body is contributed by rainfall events and up to 33% represents dry-weather load. This study shows that the modeling approaches for calculating the sediment input provide good results for the test area Kraichbach and the transfer for a German wide modeling will produce plausible values.

Detailed bed topography and sediment load measurements for two stepdown flows in a laboratory flume

Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate, a series of flume experiments were performed where the flow was reduced rapidly from equilibrium conditions. Sediment transport rates and bed elevation data across the flume and over a 15-meter-long test section were collected during the experiments to allow detailed examination of evolving bedform dynamics. It was found that relict bedforms stopped moving completely after flow reductions, and the mode of sediment transport was shifted to small bedforms that arose rapidly over dune stoss sections throughout the test section. The changes in sediment transport with time as the sand bed adjusted to the new flow rate was found to agree with predictions based on the relations proposed in Wren et al. (2020). Wavelet analysis is used to visualize changes in length and amplitude scales during the bed transition process.

Variation of runoff and sediment inflows to the Three Gorges Reservoir: Impact of upstream cascade reservoirs

The construction of cascade reservoirs upstream of the Three Gorges Reservoir (TGR) has been greatly intensified in the 21st century. While research attention is dedicated to the downstream effects of the TGR, few studies have been focused on the radical change of upstream inflows to the TGR. In this study, we reveal the impact of upstream cascade reservoirs on the annual runoff and sediment load inflows to the TGR based on the data in 1956–2016. The measured sediment load at control hydrological stations shows a drastic decrease (59%) while the runoff only had a slight decrease (6%). The similarity of these characteristics for that measured at Yichang indicates the leading effects of TGR inflows on its outflow. We develop a macroscopic quantification method utilizing double mass curves to separate the contribution from climate effects and human activities. Results show that precipitation (climate factors) dominated the runoff change, while human activities were the main driving factors for sediment load reduction in the studied area. In the four sub-basins upstream of the TGR, the sediment retention induced by large reservoirs was the major reason for sediment load reduction, especially in the Jinsha River Basin and the Jialing River Basin, which contributed 70% (or 347 Mt/yr) of the total sediment retention in the whole upper Yangtze River Basin (YRB) since 2015, much larger than that of the TGR (19% or 94 Mt/yr). As the total storage capacity of the cascade reservoirs increased to more than twice the TGR’s capacity in the 2010s, the regression relationship between runoff and sediment load broke down. The unprecedented situation in the upper YRB may have profound impacts on the morphology and ecology of the TGR as well as the downstream Yangtze River including its delta.

Enrichment of soil organic carbon in the eroded sediments due to the simultaneous occurrence of rain and wind

Study of the simultaneous effect of wind and rain is of great importance in the environmental studies of erosion for more accurate measurement and modeling of soil loss. There is little information on the combined effects of rain and wind on soil erosion as well as soil organic carbon (SOC) loss. The purpose of this study was to investigate interrill erosion in relation to SOC loss and enrichment ratio (ERoc) as influenced by rain intensity and wind velocity linking to flow hydraulic characteristics. Three rain intensities of 20, 40 and 60 mm h−1 and four wind velocities of 0, 6, 9 and 12 m s−1 were generated on three different soil samples (sandy, sandy loam and loam), each at three replicates (totally 108 runs). The SOC loss was determined through the concentration of organic carbon in sediment and the measured sediment load. The values of SOC loss and ERoc ranged from 0.04 to 4.04 mg m−1 s−1 and from 0.52 to 9.40, respectively. The measured interrill erosion rates and SOC losses increased at higher wind velocities and rain intensities, especially when wind velocity became more than a threshold value of 9 m s−1. For each soil, a direct linear relationship was distinguished between the losses of soil and SOC. Accordingly, SOC loss accounted for 0.36%, 0.17%, and 2.19% of the total soil loss in the sandy, sandy loam and loam soils, respectively. The ERoc was nearly constant for each soil and not changed with the increased soil erosion rate. Finally, flow velocity and unit stream power were found as the best predictors of soil and SOC losses in a wind-driven interrill erosion system.

A New Method for Computing the Sediment Delivery Ratio for the Hyper-Concentrated Flow Areas of the Loess Plateau, China

The sediment delivery ratio (SDR) is an important index for understanding sediment erosion, transportation and deposition features in a river basin. Based on the commonly accepted definition of SDR and the characteristics of the sediment delivery process in hyper-concentrated flow areas of the Loess Plateau, China, a new model for computing the SDR is proposed. The model is a functional relation of fractional form, in which the denominator is the surface runoff of the basin, and the numerator is the water volume needed for saturated sediment discharge in the controlling hydrological station at the exit of the basin when the saturated sediment load is equal to the measured sediment load. Using the proposed SDR equation, the long-term series of SDRs in the Yanhe River basin, a typical hyper-concentrated flow basin in the Loess Plateau, were calculated from 1952 to 2010. The results show that the long-term annual average SDR in the Yanhe River basin was 0.92, which is consistent with the results of previous studies; this finding confirms the validity and effectiveness of the method. The proposed method only requires gauge data of sediment concentration, sediment delivery rate, sediment delivery volume, and runoff ratio from hydrological stations, which makes it easy to use, and it can be used to easily estimate soil erosion in the basin.

Field methods of a near-bed suspended sediment experiment in the Yangtze River, China

Using experimental data of near-bed suspended sediment concentrations at five typical hydrometric stations of the Three Gorges Reservoir at the early reserving stage, the differences were investigated between the common method and improved method during flood seasons and non-flood seasons. The impact of taking measurements below 0.2 times the water depth on the results was discussed. The results show that the average discharges and velocities at each station calculated by the common method were slightly larger than those calculated by the improved method. Regarding the suspended sediment concentration at each station, the errors in the reservoir and downstream channels in dynamic equilibrium state were small, and the largest errors occurred where the river bed was strongly scoured in the downstream reach below the large dam. There was no significant relationship between water discharge and flow velocity, and the missed measurement phenomenon also occurred. The sediment discharge error was affected by the suspended sediment concentration, implying that errors usually occurred in channels with serious erosion during flood seasons. The correction coefficients (R2) of sediment discharge at each station were given during the experiment, which showed that the sediment discharges at the hydrometric stations where a large amount of sediment transport occurred near the river bottom, needed to be modified. Furthermore, the test methods proposed in this study were applied to calculate the sediment discharges of three rivers, and the results indicate that this method can narrow the gap between bathymetric comparisons and sediment load measurements.

Hydrological Modelling of Sediment Yield of Agr0 -Forestry Watershed

For this analysis, the Manot sub-basin region of Upper Narmada Basin located in Madhya Pradesh, India, is designated for estimation of runoff, sediment load, sediment concentration and sediment yield using SWAT. The model was implemented during 1989 and 2008. NBSSLUP soil data, Digital Elevation Model (DEM), slope, Land use Ground cover (LULC) and climate records of temperature and rainfall were used as inputs for the Manot Gauge Station. The forecasted model discharge was estimated statistically and compared the real daily flow data from 1989 to 2008, which corresponds to a major number. The R2 value (coefficient of determination) for discharge, sediment concentration, sediment load and sediment yield values for the period 1989 to 2008 were found to be 0.95, 0.94, 0.96 and 0.96 respectively. RMSE for discharge, sediment concentration and sediment yield were found to be 0.32, 0.27 and 0.17, respectively. The expected sediment load at the Manot Gauge Station for the duration (1989–2008) is 6670918 tons per year and the observed sediment load is 6255565 tons per year. Simulated sediment load and measured sediment load at the Manot Gauge Station are 14.13 tons / year / hectare and 15.02 tons / year / hectare, respectively. Predicted sediment concentrations and measured sediment concentrations at the Manot gauging station were found to be 8.81 ppm and 7.46 ppm, respectively. Similarly, the expected and observed Sediment Yield was 1.513 mm and 1.704 mm respectively.

Application of Soft Computing Models with Input Vectors of Snow Cover Area in Addition to Hydro-Climatic Data to Predict the Sediment Loads

The accurate estimate of sediment load is important for management of the river ecosystem, designing of water infrastructures, and planning of reservoir operations. The direct measurement of sediment is the most credible method to estimate the sediments. However, this requires a lot of time and resources. Because of these two constraints, most often, it is not possible to continuously measure the daily sediments for most of the gauging sites. Nowadays, data-based sediment prediction models are famous for bridging the data gaps in the estimation of sediment loads. In data-driven sediment predictions models, the selection of input vectors is critical in determining the best structure of models for the accurate estimation of sediment yields. In this study, time series inputs of snow cover area, basin effective rainfall, mean basin average temperature, and mean basin evapotranspiration in addition to the flows were assessed for the prediction of sediment loads. The input vectors were assessed with artificial neural network (ANN), adaptive neuro-fuzzy logic inference system with grid partition (ANFIS-GP), adaptive neuro-fuzzy logic inference system with subtractive clustering (ANFIS-SC), adaptive neuro-fuzzy logic inference system with fuzzy c-means clustering (ANFIS-FCM), multiple adaptive regression splines (MARS), and sediment rating curve (SRC) models for the Gilgit River, the tributary of the Indus River in Pakistan. The comparison of different input vectors showed improvements in the prediction of sediments by using the snow cover area in addition to flows, effective rainfall, temperature, and evapotranspiration. Overall, the ANN model performed better than all other models. However, as regards sediment load peak time series, the sediment loads predicted using the ANN, ANFIS-FCM, and MARS models were found to be closer to the measured sediment loads. The ANFIS-FCM performed better in the estimation of peak sediment yields with a relative accuracy of 81.31% in comparison to the ANN and MARS models with 80.17% and 80.16% of relative accuracies, respectively. The developed multiple linear regression equation of all models show an R2 value of 0.85 and 0.74 during the training and testing period, respectively.

A Computational Fluid Dynamics Simulation Model of Sediment Deposition in a Storage Reservoir Subject to Water Withdrawal

Siltation is one of the most common problems in storage projects and attached structures around the world, due to its effects on a project’s life span and operational efficiency. A three-dimensional computational fluid dynamics (CFD) model was applied to study the flow and sediment deposition in a multipurpose reservoir (Mosul Dam Reservoir, Iraq) subject to water withdrawal via a pumping station. A suitable control code was developed for the sediment simulation in intakes with multiblock option (SSIIM) model, in order to simulate a study case and achieve the study aims. The measured total deposited load in the reservoir after 25 years of operation and the measured sediment load concentration at different points near the pumping station intake were considered to validate the model results. The sediment load concentrations at several points near the water intake were compared; the percent bias (PBIAS) value was 3.6%, while the t-test value was 0.43, less than the tabulated value, indicating fair model performance. The model sensitivity to grid size and time steps was also tested. Four selected bed level sections along the reservoir were compared with the simulated values and indicate good performance of the model in predicting the sediment load deposition. The PBIAS ranged between 4.8% and 80.7%, and the paired t-test values indicate good model performance for most of the sections.

An event-based model of soil erosion and sediment transport at the catchment scale

Deforestation in New Zealand has led to increased soil erosion and sediment loads in rivers, adversely affecting aquatic ecosystems. Soil conservation actions over large areas are expensive and need to be targeted to obtain maximum benefit for least cost. Medium-complexity models of soil erosion have been used to target soil conservation, but they involve long-term averages and do not involve time variation. In this paper, an event-based model of soil erosion and sediment transport is developed. It has moderate complexity and is able to be calibrated with a modicum of data. The spatial framework is based on small subcatchments and stream links. For each storm event, the sediment yield for each subcatchment is estimated from peak storm discharge with a power function and routed through the stream network. We run the model on the Manawatū catchment in New Zealand for which all the major tributaries are monitored for water and sediment discharge. Spatial maps of sediment yield are produced for each storm event in a 5-year period from model inputs of maximum storm discharge at the tributaries. To test the model, predicted sediment loads of the storm events in the main channel of the Manawatū, at Teachers College, Palmerston North, are compared with measured sediment loads. The strong agreement between predictions and measurements suggests that the method for converting maximum storm discharge to sediment load at tributaries is robust. Analysis of the sediment concentration and discharge records for storm events revealed that sediment from channel resuspension and bank erosion was much smaller (typically <5%) than total storm sediment loads and could thus be ignored when routing storm sediment loads. The demonstrably small channel resuspension and bank erosion, relative to storm sediment loads, enabled simple routing from subcatchments to construct an event-based model at the catchment scale.

Modelling soil erosion in a Mediterranean watershed: Comparison between SWAT and AnnAGNPS models

In this study, the simulations generated by two of the most widely used hydrological basin-scale models, the Annualized Agricultural Non-Point Source (AnnAGNPS) and the Soil and Water Assessment Tool (SWAT), were compared in a Mediterranean watershed, the Carapelle (Apulia, Southern Italy). Input data requirements, time and efforts needed for input preparation, strength and weakness points of each model, ease of use and limitations were evaluated in order to give information to users. Models were calibrated and validated at monthly time scale for hydrology and sediment load using a four year period of observations (streamflow and suspended sediment concentrations). In the driest year, the specific sediment load measured at the outlet was 0.89 t ha−1 yr−1, while the simulated values were 0.83 t ha−1 yr−1 and 1.99 t ha−1 yr−1 for SWAT and AnnAGNPS, respectively. In the wettest year, the specific measured sediment load was 7.45 t ha−1 yr−1, and the simulated values were 8.27 t ha−1 yr−1 and 6.23 t ha−1 yr−1 for SWAT and AnnAGNPS, respectively. Both models showed from fair to a very good correlation between observed and simulated streamflow and satisfactory for sediment load. Results showed that most of the basin is under moderate (1.4–10 t ha−1 yr−1) and high-risk erosion (> 10 t ha−1 yr−1). The sediment yield predicted by the SWAT and AnnAGNPS models were compared with estimates of soil erosion simulated by models for Europe (PESERA and RUSLE2015). The average gross erosion estimated by the RUSLE2015 model (12.5 t ha−1 yr−1) resulted comparable with the average specific sediment yield estimated by SWAT (8.8 t ha−1 yr−1) and AnnAGNPS (5.6 t ha−1 yr−1), while it was found that the average soil erosion estimated by PESERA is lower than the other estimates (1.2 t ha−1 yr−1).

A Simplification of the Einstein Equation for the Calculation of Suspended Sediment Transport in Rivers

In this study, a simplified equation for estimating suspended sediment load is proposed, based on regression analysis and the concept presented by Einstein, Ackers and White, and Shields. Field data for 11 rivers located at different parts of the world were used for the calibration and validation of the proposed equation. The results show that the equation gives a good performance compared with Einstein's and Bagnold's equations when tested using field data for the Atchafalaya, Red, South American, Rio Grande and Al-Garraf rivers. The minimum and maximum values of mean absolute error (MAE) and root mean square error (RMSE) between the output of the proposed equation and field records were 0.005 to 0.007 and 0.09 to 0.289, respectively. For the above rivers, graphical comparisons show that the predicted values of suspended sediment load had a range of D50 0.2 gave a good prediction compared with the measured sediment load. The main advantages of the proposed equation are that it is easy to apply and the results obtained from its application have reasonable accuracy.

Estimation of historic flows and sediment loads to San Francisco Bay, 1849–2011

Summary River flow and sediment transport in estuaries influence morphological development over decadal and century time scales, but hydrological and sedimentological records are typically too short to adequately characterize long-term trends. In this study, we recover archival records and apply a rating curve approach to develop the first instrumental estimates of daily delta inflow and sediment loads to San Francisco Bay (1849–1929). The total sediment load is constrained using sedimentation/erosion estimated from bathymetric survey data to produce continuous daily sediment transport estimates from 1849 to 1955, the time period prior to sediment load measurements. We estimate that ∼55% (45–75%) of the ∼1500 ± 400 million tons (Mt) of sediment delivered to the estuary between 1849 and 2011 was the result of anthropogenic alteration in the watershed that increased sediment supply. Also, the seasonal timing of sediment flux events has shifted because significant spring-melt floods have decreased, causing estimated springtime transport (April 1st to June 30th) to decrease from ∼25% to ∼15% of the annual total. By contrast, wintertime sediment loads (December 1st to March 31st) have increased from ∼70% to ∼80%. A ∼35% reduction of annual flow since the 19th century along with decreased sediment supply has resulted in a ∼50% reduction in annual sediment delivery. The methods developed in this study can be applied to other systems for which unanalyzed historic data exist.