Transport Capacity Of Overland Flow Research Articles

Effects of sediment characteristics on the sediment transport capacity of overland flow

The transport of sediments is a crucial part of soil erosion. Accurately calculating the sediment transport capacity is key to the construction of soil erosion process models. Research on Tc has focused mainly on the dynamics of a single particle of sediment and hydraulic variables. There have been few studies of the impact of soil aggregates on the Tc. To clarify how sediment characteristics, including those for single particles and aggregates, affect the Tc of overland flow with no raindrop import, flume experiments were implemented at slope gradients varying from 5.24% to 26.80% and flow discharges ranging from 0.68 to 5.41 × 10−3 m2 s−1. The experimental materials were five typical soils in China. The results indicated that the correlation between the measured Tc and sediment mechanical composition indexes of the five soils was indistinctive in this study. The sediment settling velocity with aggregates has a significant correlation with the measured Tc. New equations, including for the sediment settling velocity with aggregates ωud75, were established to calculate the Tc. The empirical equation that included ωud75, slope gradient and unit discharge performed greatly in predicting Tc (R2 = 0.93, NSE = 0.90). ωud75 can effectively improve the calculation accuracy of Tc. The new equation including flow and sediment properties obtained through dimensional analysis performed well in predicting Tc (R2 = 0.99, NSE = 0.91), and the calculation accuracy was better than that of the empirical model derived in this study. These findings indicate that the sediment settling velocity is an important variable in the equation for predicting sediment transport capacity of overland flow.

Prediction of sediment transport capacity based on slope gradients and flow discharge.

Sediment transport capacity (Tc) is an essential parameter in the establishment of the slope soil erosion model. Slope type is an important crucial factor affecting sediment transport capacity of overland flow, and vegetation can effectively inhibit soil loss. Two new formulae of sediment transport capacity (Tc) are proposed of brown soil slope and vegetation slope in this study and evaluate the influence of slope gradient (S) and flow discharge (Q) on sediment transport capacity of different slope types. Laboratory experiments conducted using four flow discharges (0.35, 0.45, 0.55, and 0.65 L s-1), four slope gradients (3, 6, 9, and 12°), and two kinds of underlying surface (Brown soil slope, Vegetation slope). The soil particle size range is 0.05–0.5mm. The vegetation stems were 2mm in diameter and randomly arranged. The results show that the sediment transport capacity was positively correlated with the flow discharge and slope gradient. The vegetation slope’s average sediment transport capacity is 11.80% higher than the brown soil slope that same discharge and slope gradient conditions. The sensitivity of sediment transport capacity to flow discharge on brown soil slope is higher than that of slope gradient. The sensitivity of sediment transport capacity of vegetation slope to slope gradient is more heightened than flow discharge. The sediment transport capacity was well predicted by discharge and slope gradient on brown soil slope (R2 = 0.982) and vegetation slope (R2 = 0.993). This method is helpful to promote the study of the sediment transport process on overland flow.

Effects of different types of undecomposed surface litter on the sediment transport capacity

Plant litter cover affects the sediment transport capacity of overland flow, which, in turn, influences the prediction of the soil erosion. In previous studies, researchers focused on the differences in the hydraulic variables and soil erosion for different types of litter incorporated in the soil or present on the surface. The quantitative relationship between the litter morphology and sediment transport capacity on no-till systems with litter covered surfaces has not been examined yet. Therefore, in this study, the effects of different types of litter on the hydraulic variables and sediment transport capacity were examined, along with the quantitative relationships among the three aspects. To this end, one discharge magnitude (2.632 × 10−3 m2 s−1), two slope gradients (8.8% and 26.8%), and five types of undecomposed surface litter (Populus simonii, Robinia pseudoacacia, Hippophae rhamnoides, Pinus tabuliformis, and Bothriochloaischaemum) were considered to conduct experiments in a steel flume bed. The coverage of each type of litter ranged from 0% to 70%. The results showed that the flow regime, hydraulic variables and sediment transport capacity were affected by the types of litter. The length, width-to-length ratio and roundness of the individual litter leaves could suitably reflect the impact of the litter morphology on the sediment transport capacity, as indicated by the high Pearson correlation coefficients (> 0.5). Long and narrow litter with a small roundness more notably reduced the sediment transport capacity than short and wide litter with a large roundness. The litter morphology did not influence the relationships between the sediment transport capacity and hydraulic variables. In particular, the hydraulic variables such as the unit stream power, flow velocity combined with slope gradient, and stream power combined with flow depth could accurately predict the sediment transport capacity (R2 > 0.94), and the characteristic variables of the litter morphology and cover were not required to be considered in the prediction equation. The presented findings can be used to develop a simple algorithm to predict the sediment transport capacity of litter covered surfaces.

Effects of Pinus tabulaeformis litter cover on the sediment transport capacity of overland flow

Sediment transport capacity (Tc) plays a pivotal role in process-based soil erosion models. Surface cover is an important factor that affects Tc; investigation into the potential effect of litter cover on Tc is limited. The objectives of this study were to quantify the effects of Pinus tabulaeformis litter cover on Tc; to explore various relationships between hydraulic variables, litter cover, and Tc; and to compare the similarities and differences of the effect of litter and stem cover on Tc. Experiments were conducted in a 5×0.38 m flume. The unit flow discharge varied from 1.316 × 10−3 to 3.947 × 10−3 m2 s−1, and the slope gradient varied from 8.8 to 26.8 %. Pine needles of Pinus tabulaeformis were selected to simulate litter cover. Seven levels of litter cover were considered: 0%, 5%, 10 %, 20 %, 30 %, 50 %, and 70 %. The median diameter of the test sediment was 0.35 mm. The results showed that Tc decreased exponentially as litter cover increased. Measured Tc could be predicted adequately (R2 = 0.96) by unit flow discharge, slope gradient and litter cover, or the stream power and litter cover. Flow velocity or unit stream power could predict the Tc equally well (R2 = 0.95) without including the cover term because the effect of litter cover on the Tc manifests itself in the increased resistance and reduced flow velocity. Tc is most sensitive to the slope gradient when the level of cover is low. The effect of litter cover on Tc could be greater than the unit flow discharge and slope gradient in relative terms when the level of cover exceeded 38 % and 81 %, respectively. The effect of stem cover on Tc is greater than that of litter cover, other variables being the same.

Modeling study on the time-varying process of sediment trapping in vegetative filter strips

Vegetative filter strips (VFS) play an important role in reducing erosion, retaining runoff, and trapping sediment. The bed surface of a VFS is dynamic during continuous sediment deposition, which makes it difficult to determine the sediment transport capacity of overland flow in real time. This study described this process from a systematic perspective. Based on self-feedback theory, an ordinary differential equation was established with a self-inhibition correction factor and its analytical solution was obtained. A sediment-trapping-process model of VFS (STPMOD-VFS) was proposed. Then, the sediment trapping processes of three experiments under different conditions were predicted using the STPMOD-VFS, and the prediction was considered highly satisfactory. The uncertainty estimation results showed the STPMOD-VFS had significant ‘equifinality’ performance. The sensitivity of different parameters of the STPMOD-VFS was found significantly different. The parameters related to the sediment delivery rate of silt-laden inflow and those that control the attenuation coefficients of the instantaneous sediment trapping efficiency of the VFS were sensitive, while those parameters that control the initial sediment outflow rate were less sensitive. Finally, an equation describing the sediment trapping process by a VFS under a dynamic sediment inflow rate was built and solved, and a general expression for the VFS sediment trapping time-varying process was given. The findings of this study could help in evaluation of sediment processes on grassed hillslopes.

Influence of soil aggregate characteristics on the sediment transport capacity of overland flow

The sediment transport capacity of overland flow is the core input variable of a process-based soil erosion model. Many studies have focused on the sediment transport capacity for overland flow; however, few studies have explored the relationship between sediment transport capacity and soil aggregate characteristics. The objective of this study was to investigate the effects of soil aggregate characteristics on the sediment transport capacity of overland flow. The unit flow discharge ranged from 0.68 × 10−3 m2 s−1 to 5.41 × 10−3 m2 s−1, and the slope gradient varied from 5.24% to 26.80%. Five types of typical Chinese soil were investigated. The results showed that the best correlation was the relationship between the sediment transport capacity and the mass percentage of aggregates greater than 0.25 mm (WSA0.25) under the Le Bissonnais method of wetting stirring conditions. Sediment transport capacity was not correlated to the other soil aggregate characteristics, including the mean weight diameter (MWD) under the Le Bissonnais method of fast wetting and slow wetting conditions and the Yoder method, the degree of aggregation (A), and the fractal dimension (D) under the Yoder method. New equations including WSA0.25 were developed to predict the sediment transport capacity. The equation including flow discharge, slope gradient and WSA0.25 provided the best accuracy for predicting sediment transport capacity. The sediment transport capacity increased linearly with the mean flow velocity. Between the hydraulic variables of shear stress and stream power, our results showed that stream power was an optimal predictor for calculating sediment transport capacity. These findings offer a new approach for predicting the sediment transport capacity of overland flow.

Effect of gravel content on the sediment transport capacity of overland flow

The sediment transport capacity (Tc) is an important parameter for analysing and controlling soil erosion processes. However, few studies have investigated the Tc of soils containing gravel (grain sizes of >2 mm) on steep slopes. Colluvial deposits on steep slopes in benggang are composed of loose materials with large amounts of gravel. This study aimed to investigate the effects of gravel content on the Tc of overland flow using colluvial deposits with a range of slope gradients and flow discharges in a nonerodible flume (4 m long, 0.12 m wide and 0.1 m high). The experiments were carried out using six gravel mass contents (0, 10, 20, 30, 40, and 50%), four slope gradients (18, 36, 57, and 84%), and four flow discharges (0. 56, 1.11, 2.22, and 4.44 × 10-3 m2 s−1). The experimental results revealed that the measured Tc increased linearly as the gravel content increased. The effects of the slope gradient, flow discharge and gravel content on Tc were significant (p < 0.01). Tc increased following power functions either with slope gradient, flow discharge and gravel content (coefficient of model determination (r2) = 0.96 and Nash-Sutcliffe model efficiency (NSE) = 0.96), or with slope gradient, flow discharge and sediment median grain size (d50) (r2 = 0.98 and NSE = 0.98). The two established power functions facilitated the prediction of the Tc of colluvial deposits in this study. However, the power functions overestimated the predicted Tc values when the measured Tc was less than 1 kg m−1 s−1 and underestimated the values when the measured Tc ranged from 2 to 4 kg m−1 s−1. These findings are helpful for improving the simulation of erosion processes in colluvial deposits and deepening the understanding of erosion mechanisms in soil and rock mixtures. Moreover, the relationship of hydrodynamic variables with Tc on colluvial deposits remains to be investigated in additional in-depth studies.

Nondimensional sediment transport capacity of sand soils and its response to parameter in the Loess Plateau of China

AbstractSoil erosion is a major contributor to land degradation in the Loess Plateau in China. To clarify the sediment transport capacity of overland flow influenced by hydraulic parameters, such as shear stress, sand shear stress (hydraulic gradient partition method and hydraulic radius partition method), mean flow velocity, Froude number, stream power, and unit stream power, indoor experiments with eight‐unit‐width flow discharges from 0.0667 × 10−3 to 0.3333 × 10−3 m2·s−1, six slope gradients from 3.49 to 20.79%, and two kinds of sand soils (d50 = 0.17 and 0.53 mm) were systematically investigated. A nondimensional method was adopted in data processing. Results showed that there was a partition phenomenon of relation curves because of the different median grain diameters. The correlation between the nondimensional stream power and nondimensional sediment transport capacity was the highest, followed by the correlation between the nondimensional unit stream power and nondimensional sediment transport capacity. However, there was a poor correlation between the flow intensity indices of velocity category and nondimensional sediment transport capacity. Nondimensional stream power, nondimensional unit stream power, and nondimensional shear stress could predict sediment transport capacity well. Ignoring the partition phenomenon of the relation curves, stream power could be used to predict sediment transport capacity, with a coefficient of determination of .85. Furthermore, a general flow intensity index was obtained to predict sediment transport capacity of overland flow. Finally, an empirical formula for predicting sediment transport capacity with a coefficient of determination of .90 was established by multiple regression analyses based on the general flow intensity index. During the analysis between measured sediment transport capacities in present study and predicted values based on Zhang model, Mahmoodabadi model, and Wu model, it was found that these three models could not accurately predict sediment transport capacities of this study because different models are estimated on the basis of different experimental conditions.

Assessment of Soil Erosion Risk in Mubi South Watershed, Adamawa State, Nigeria

The study aims to assessment soil erosion risk in Mubi South watershed with the aid of RUSEL model and Geospatial techniques. RUSLE model parameters such as rainfall, soil map, topography map, cover management and conservation practice factor map were derived. The method employed includes the use of RUSLE model and Geospatial techniques using ArcGIS 10.3 Software, for analysis and presentation of result. It was found that sandy soil are the dominant soil of the watershed which covered about 65%, 18% silt and 17% clay. The landuse landcover has about 29 % of area covered by agricultural activities, 19% were covered by forest, and 25% were not cultivated and covered by bare land. The study area has about 0.58 to -0.07 normalized difference vegetation index (NDVI) with majority of the area within the lower topography of 570 m above sea level. The Soil cover management factor ranges from the higher value of 0.5 to the lower value of 0.01 in the watershed, 15.8 mm to 15.7 mm occurrences of daily rainfall and 492.34 mm rainfall and runoff covered when rainfall per-day is greater than 15 mm rainfall. The results of the study also show that average rate of soil detachment is 1t ha -1 yr -1 . The average transport capacity of overland flow is 1.5 t ha -1 yr -1 . Average soil per detachability by raindrop is 69.6 t ha -1 yr -1 total soil particle detachments is 69.66 t ha -1 yr -1 and average estimated soil erosion of 3.52 t ha -1 yr -1 . It is recommended that other soil erosion model to be applied in the study area for further comparative analysis of soil erosion risk.

Estimation of Soil Erosion Risk in Mubi South Watershed, Adamawa State, Nigeria

The study aims to estimation soil erosion risk in Mubi South watershed with the aid of RUSEL model and Geospatial techniques. RUSLE model parameters such as rainfall, soil map, topography map, cover management and conservation practice factor map were derived. The method employed includes the use of RUSLE model and Geospatial techniques using ArcGIS 10.3 Software, for analysis and presentation of result. It was found that sandy soil are the dominant soil of the watershed which covered about 65%, 18% silt and 17% clay. The land use landcover has about 29% of area covered by agricultural activities, 19% were covered by forest and 25% were not cultivated and covered by bare land. The study area has about 0.58 to -0.07 normalized difference vegetation index (NDVI) with majority of the area within the lower topography of 570 m above sea level. The Soil cover management factor ranges from the higher value of 0.5 to the lower value of 0.01 in the watershed, 15.8 mm to 15.7 mm occurrences of daily rainfall and 492.34 mm rainfall and runoff covered when rainfall per-day is greater than 15 mm rainfall. The results of the study also show that average rate of soil detachment is 1 t ha-1 yr-1. The average transport capacity of overland flow is 1.5 t ha-1 yr-1. Average soil per detachability by raindrop is 69.6 t ha-1 yr-1 total soil particle detachments is 69.66 t ha-1 yr-1 and average estimated soil erosion of 3.52 t ha-1 yr-1. It is recommended that other soil erosion model to be applied in the study area for further comparative analysis of soil erosion risk.

Transport Capacity of Overland Flow for Sediment Mixtures

AbstractSediment transport capacity of overland flows is crucial to understanding and modeling the erosion and deposition processes. The transport capacity for sediment mixtures is particularly complex because of the need for an algorithm to allocate the total available shear stress or stream power to individual size classes. The transport capacity for sediment mixtures may not even be unique, depending on whether the flow is eroding or depositing sediments unless all sediments are of a single size class. Using data from a set of flume experiments involving steep slopes and high sediment concentrations, a formula was proposed for the sediment transport capacity limited by available stream power and available accommodation space for sediments. Sediments used for the experiment were further sieved into five distinct size classes. The sediment transport capacity was measured for 25 flow-slope combinations to validate a new formula for individual size classes, and to propose and test a method to compute the s...

Special Issue on Soil Erosion and Sediment Yield Modeling

Soil erosion is one of the most serious environmental problems because it removes fertile soil that is rich in nutrients and, in turn, increases the natural level of sedimentation in rivers and reservoirs, reducing their storage capacity and life span. Land degradation stems from a combination of factors related to land use, agricultural intensification, and intense rainstorms. For maintaining and improving soil productivity, soil resources need to be conserved for optimal land use. In other words, to restore the productivity of soil and prevent further damage, planning for conservation and management of watersheds become vital. For assessing soil erosion and sediment yield from watersheds, several empirical models on the basis of geomorphological parameters have been developed and are available in the literature. Among several methods, the sediment yield index (SYI) method and universal soil loss equation (USLE) are extensively used for the estimation of soil erosion and prioritization of watersheds for their treatment. Research in hydrological modeling and related watershed planning issues form a significant component of the prevailing environmental activities. During the last three decades, a number of empirical and conceptual hydrological models have been developed for the prediction of soil erosion and sediment yield modeling. Thus, there is a need to have another look at different aspects of soil erosion and sediment yield modeling and enhance their understanding for improved practical applications. The present special issue of the Journal of Hydrologic Engineering focuses on soil erosion and sediment yield modeling, including the effects of land use/cover changes, extended applications using empirical and physically based soil erosion and sediment yield models, and watershed management coupling remote sensing and the geographic information system (GIS). Within this framework, the papers of this special issue have been grouped into the following: studies on the basis of physically based, conceptual, and empirical models; advancement and review; watershed management; and experimental work. Papers titled “Identification of Critical Erosion Watersheds for Control Management in Data Scarce Condition Using the SWAT Model,” “Use of Caesium-137 Measurements and Long-Term Records of Sediment Load to Calibrate the Sediment Delivery Component of the SEDD Model and Explore Scale Effect: Examples from Southern Italy,” “Application of SWAT Model and Geospatial Techniques for Sediment-Yield Modeling in Ungauged Watersheds,” “Soil Erosion and Sediment-Yield Prediction at Basin Scale in Upstream Watershed of Miyun Reservoir,” “Improved Hillslope Erosion Module for the Digital Yellow-River Model,” and “Evaluation of GIS-Based Watershed Model for Streamflow and Sediment-Yield Simulation in the Upper Baitarani River Basin of Eastern India” fall in the category of physically based soil erosion and sediment yield modeling. Papers titled “Inductive Group Method of Data Handling Neural Network Approach to Model Basin Sediment Yield,” “Modeling Suspended Sediment Using Artificial Neural Networks and TRMM-3B42 Version 7 Rainfall Dataset,” and “Accounting for Conceptual Soil Erosion and Sediment Yield Modeling Uncertainty in the APEX Model Using Bayesian Model Averaging” deal with conceptual modeling for soil erosion and sediment yield. Papers titled “Application of GISCoupled Modified MMF Model to Estimate Sediment Yield on a Watershed Scale,” “Impact of Climate Change on Future Soil Erosion in Different Slope, Land Use, and Soil-Type Conditions in a Part of the Narmada River Basin, India,” and “Spatially Distributed Sheet, Rill, and Ephemeral Gully Erosion” employ empirical models for the estimation of sediment yield. The paper “Sediment Graphs Based on Entropy Theory,” which derives an instantaneous unit sediment graph (IUSG or USG) to determine sediment discharge and establishes a relation between sediment yield and runoff volume, exhibits an advancement in the field of sediment yield modeling. Conversely, the paper “Geographic Variation of USLE/RUSLE Erosivity and Erodibility Factors” provides a comprehensive review on the subject. The papers “Integrated Modeling Approach to the Response of Soil Erosion and Sediment Export to Land-Use Change at the Basin Scale,” “Evaluating the Impact of the Spatial Distribution of Land Management Practices on Water Erosion: Case Study of a Mediterranean Catchment,” and “Sediment Fingerprinting for Calibrating a Soil Erosion and Sediment-Yield Model in Mixed Land-Use Watersheds” deal with watershed management. The papers “Establishing a Soil Loss Threshold for Limiting Rilling,” “Transport Capacity of Overland Flow with High Sediment Concentration,” “Modeling Rainfall Erosivity by Measured Drop-Size Distributions,” “Modeling Rill Erosion at the Sparacia Experimental Area,” “Rapid Weathering and Erosion of Mudstone Induced by Saltwater Migration near a Slope Surface,” and “Rain Microstructure and Erosivity Relationships under Pressurized Rainfall Simulator” are on the basis of experimental work. The aim of this issue is to present high-quality technical articles representing the most recent developments in the field of soil erosion and sediment yield modeling, including practical applications. This special issue is expected to become the first in a sequence of future issues on this topic.

Transport Capacity of Overland Flow with High Sediment Concentration

The concept and estimation of sediment transport capacity of overland flows are pivotal to soil erosion, sediment transport, and deposition modeling. There is a limited understanding of the effect of high sediment concentration on the transport capacity of overland flow, although sediments in suspension are known to affect turbulent mixing and settling velocity in rivers. A new functional relationship between a dimensionless parameter involving stream power and settling velocity and the volumetric concentration at the transport limit was developed using a set of flume experiments with slope up to 46.6%, unit discharge up to 50 cm2·s−1, median particle size of 0.326 mm, and sediment concentration up to 1,140 kg·m−3. The new relationship has two theoretical limits on sediment concentration at the transport limit. Under low flow conditions, the sediment concentration is limited by the available stream power. At high stream power, the sediment concentration is limited by the space available in flow to accommodate sediments in motion. As a predictor of the sediment concentration at the transport limit, the new relationship worked very well with the Nash-Sutcliffe coefficient of efficiency of 0.95 and was shown to be superior to empirical relationships based on stream power and other commonly used predictors of the transport capacity for rivers. The paper also shows that formulas for the transport capacity which have been validated and widely used for rivers with high sediment concentrations are inaccurate and should not be used to predict the transport capacity of overland flow.

Soil Erosion Prediction Using Morgan-Morgan-Finney Model in a GIS Environment in Northern Ethiopia Catchment

Even though scientific information on spatial distribution of hydrophysical parameters is critical for understanding erosion processes and designing suitable technologies, little is known in Geographical Information System (GIS) application in developing spatial hydrophysical data inputs and their application in Morgan-Morgan-Finney (MMF) erosion model. This study was aimed to derive spatial distribution of hydrophysical parameters and apply them in the Morgan-Morgan-Finney (MMF) model for estimating soil erosion in the Mai-Negus catchment, northern Ethiopia. Major data input for the model include climate, topography, land use, and soil data. This study demonstrated using MMF model that the rate of soil detachment varied from &lt;20 t ha−1y−1to &gt;170 t ha−1y−1, whereas the soil transport capacity of overland flow (TC) ranged from 5 t ha−1y−1to &gt;42 t ha−1y−1. The average soil loss estimated by TC using MMF model at catchment level was 26 t ha−1y−1. In most parts of the catchment (&gt;80%), the model predicted soil loss rates higher than the maximum tolerable rate (18 t ha−1y−1) estimated for Ethiopia. Hence, introducing appropriate interventions based on the erosion severity predicted by MMF model in the catchment is crucial for sustainable natural resources management.

A unit stream power based sediment transport function for overland flow

Soil erosion is a serious global problem requiring effective modeling for accurate assessment of sensitive areas and related erosion rates. The outcome of soil erosion models depends strongly on the estimation of sediment transport capacity. In most of the existing spatially distributed soil erosion models sediment transport capacity of overland flow is often estimated using stream flow transport capacity functions. The applicability of stream flow functions to overland flow conditions is questionable because hydraulic conditions like flow depth, slope steepness and surface roughness under overland flow are substantially different from stream flow conditions. Hence, the main objectives of this study were i) to check the suitability of five existing well known and widely used transport capacity functions (Yalin 1963; Low, 1989; Govers, 1990; modified Engelund and Hansen (Smith et al., 1995); and Abrahams et al., 2001) for use under overland flow conditions, and ii) to derive a new function based on unit stream power by dimensional analysis to quantify transport capacity for overland flow. To accomplish the objectives, experiments in a 3.0 m long and 0.5 m wide flume were carried out using four different sands (0.230, 0.536, 0.719, and 1.022 mm). The unit discharges used for experimentation ranged from 0.07 to 2.07 x 10(-3) m(2) s(-1) and slopes ranged from 5.2 to 17.6%. In this study, none of the predictions with the existing functions was in good agreement with measured results over the whole range of experimental conditions, especially at low flow intensities. The percentages of observations in which the discrepancy ratio ranged between 0.5 and 2.0 were: 65% (Yalin 1963), 74% (Low, 1989), 57% (Govers, 1990), 54% (modified Engelund and Hansen (Smith et al., 1995)), and 25% (Abrahams et al., 2001). The results showed that the selected functions reasonably estimate transport capacities only under those ranges of conditions for which they were formulated. Although the excess.shear stress concept based function (i.e. Low's function) produced excellent results, the degree of accuracy of the results varied substantially with grain size (P.O.(0.5-2.0): 53-100%). In contrast, the performance of the Govers' function, which is based on the unit stream power concept, was quite similar for all the selected sands (P.O.(0.5-2.0): 50-63%). Based on the unit stream power concept, a new function for low flow intensities was derived by dimensional analysis using the data gained from the flume experiments. (c) 2012 Elsevier B.V. All rights reserved.

Effects of sediment size on transport capacity of overland flow on steep slopes

Sediment transport capacity is a key concept in determining rates of detachment and deposition in process-based erosion models, yet limited studies have been conducted on steep slopes. We investigated the effects of sediment size on transport capacity of overland flow in a flume. Unit flow discharge ranged from 0.66 to 5.26 × 10-3 m2 s-1, and slope gradient varied from 8.7 to 42.3%. Five sediment size classes (median diameter, d 50, of 0.10, 0.22, 0.41, 0.69 and 1.16 mm) were used. Sediment size was inversely related to transport capacity. The ratios of average transport capacity of the finest class to those of the 0.22, 0.41, 0.69 and 1.16 mm classes were 1.09, 1.30, 1.55 and 1.92, respectively. Sediment transport capacity increased as a power function of flow discharge and slope gradient (R2 = 0.98), shear stress (R2 = 0.95), stream power (R2 = 0.94), or unit stream power (R2 = 0.76). Transport capacity generally decreased as a power function of sediment size (exponent = −0.35). Shear stress and stream power predicted transport capacity better than unit stream power on steep slopes when transport capacity was <7 kg m-1 s-1. Sediment transport capacity increased linearly with mean flow velocity. Critical or threshold velocity increased as a power function of sediment size (R2 = 0.93). Further studies with fine soil particles are needed to quantify the effects of sediment size on transport capacity of overland flow on steep slopes. Citation Zhang, G.-H., Wang, L.-L., Tang, K.-M., Luo, R.-T. & Zhang, X.C. (2011) Effects of sediment size on transport capacity of overland flow on steep slopes. Hydrol. Sci. J. 56(7), 1289–1299.

An approach to estimating sediment transport capacity of overland flow

Estimating sediment transport capacity of overland flow is essential to the development of physically based soil erosion models. Correlation analysis indicates that stream power is a dominant factor for sediment transport in overland flows and a new sediment transport capacity equation is proposed based on dimensional analysis. The coefficients of the new equation are calibrated using the published laboratory data, and rainfall impact is taken into consideration by adding an empirical factor on the dimensionless critical stream power. The new sediment transport capacity equation is a function of stream power, rainfall impacted critical stream power and slope. The new equation is applied in a one-dimensional soil erosion model to simulate field data of a runoff plot and the simulation results are reliable.

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.

Prescribed‐fire effects on rill and interrill runoff and erosion in a mountainous sagebrush landscape

AbstractChanging fire regimes and prescribed‐fire use in invasive species management on rangelands require improved understanding of fire effects on runoff and erosion from steeply sloping sagebrush‐steppe. Small (0·5 m2) and large (32·5 m2) plot rainfall simulations (85 mm h–1, 1 h) and concentrated flow methodologies were employed immediately following burning and 1 and 2 years post‐fire to investigate infiltration, runoff and erosion from interrill (rainsplash, sheetwash) and rill (concentrated flow) processes on unburned and burned areas of a steeply sloped sagebrush site on coarse‐textured soils. Soil water repellency and vegetation were assessed to infer relationships in soil and vegetation factors that influence runoff and erosion. Runoff and erosion from rainfall simulations and concentrated flow experiments increased immediately following burning. Runoff returned to near pre‐burn levels and sediment yield was greatly reduced with ground cover recovery to 40 per cent 1 year post‐fire. Erosion remained above pre‐burn levels on large rainfall simulation and concentrated flow plots until ground cover reached 60 per cent two growing seasons post‐fire. The greatest impact of the fire was the threefold reduction of ground cover. Removal of vegetation and ground cover and the influence of pre‐existing strong soil‐water repellency increased the spatial continuity of overland flow, reduced runoff and sediment filtering effects of vegetation and ground cover, and facilitated increased velocity and transport capacity of overland flow. Small plot rainfall simulations suggest ground cover recovery to 40 per cent probably protected the site from low‐return‐interval storms, large plot rainfall and concentrated flow experiments indicate the site remained susceptible to elevated erosion rates during high‐intensity or long duration events until ground cover levels reached 60 per cent. The data demonstrate that the persistence of fire effects on steeply‐sloped, sandy sagebrush sites depends on the time period required for ground cover to recover to near 60 per cent and on the strength and persistence of ‘background’ or fire‐induced soil water repellency. Published in 2009 by John Wiley &amp; Sons, Ltd.

Investigating erosion rates within a Japanese cypress plantation using Cs‐137 and Pb‐210ex measurements

In Japan, soil erosion represents an important threat to the longer‐term sustainability of Japanese cypress plantations. Although it has been demonstrated that raindrop impact, rather than sheet or rill/gully erosion, is the dominant process in both sediment detachment and sediment transport at the point or hillslope scale in such plantations, the role of overland flow at the catchment scale has received little attention to date. To confirm the dominant erosion process operating in a Japanese cypress plantation catchment, we predicted the sediment transport capacities of both overland flow, expressed as a function of the upslope contributing area and the local slope gradient, and rainsplash, expressed as a topographic curvature, for 45 sampling points across the catchment. In addition, we compared the predicted values of sediment transport capacity and rainsplash with the erosion rates estimated using Cs‐137 and Pb‐210ex measurements. The results indicated a close correspondence between the local transport capacity of overland flow and the local erosion rates estimated using the radionuclide measurements. In contrast, there was no significant correlation between the topographic curvature and the erosion rates. Furthermore, the average radionuclide inventories for the catchment were much smaller than the local reference inventories, indicating net loss of eroded soil and radionuclides from the catchment. These results suggest that overland flow makes a significant contribution to surface soil erosion even on forested hillslopes. In addition, the results provide a useful demonstration of the potential for using radionuclide measurements to produce independent information on long‐term erosion rates, which can be used for investigating erosion processes and validating erosion models.