Sediment Transport and Soil Detachment on Steep Slopes: I. Transport Capacity Estimation

Abstract

Precise estimation of sediment transport capacity (Tc) is critical to the development of physically based erosion models. Few data are available for estimating Tc on steep slopes. The objectives of this study were to evaluate the effects of unit flow discharge (q), slope gradient (S), and mean flow velocity on Tc in shallow flows and to investigate the relationship between Tc and shear stress, stream power, and unit stream power on steep slopes using a 5‐m‐long and 0.4‐m‐wide nonerodible flume bed. Unit flow discharge ranged from 0.625 × 10−3 to 5 × 10−3 m2 s−1 and slope gradient from 8.8 to 46.6%. The diameter of the test riverbed sediment varied from 20 to 2000 μm, with a median diameter of 280 μm. The results showed that Tc increased as a power function with discharge and slope gradient with a coefficient of Nash–Sutcliffe model efficiency (NSE) of 0.95. The influences of S on Tc increased as S increased, with Tc being slightly more sensitive to q than to S The Tc was well predicted by shear stress (NSE = 0.97) and stream power (NSE = 0.98) but less satisfactorily by unit stream power (NSE = 0.92) for the slope range of 8.8 to 46.6%. Mean flow velocity was also a good predictor of Tc (NSE = 0.95). Mean flow velocity increased as q and S increased in this study. Overall, stream power seems to be the preferred predictor for estimating Tc for steep slopes; however, the predictive relationships derived in this study need to be evaluated further in eroding beds using a range of soil materials under various slopes.

Effect of hydraulic parameters on sediment transport capacity in overland flow over erodible beds

“…Govers (1990) and Everaert (1991) found, in different studies, that for erodible beds the effect of slope on transport capacity is higher than the effect of unit discharge. Contrasting with this, the nonerodible bed experiments of Zhang et al (2009) revealed that transport capacity is more susceptible to unit discharge as compared to slope. This raises questions about the applicability of information obtained from non-erodible beds for the development of sediment transport equations to be used in soil erosion models.…”

“…Sediment transport capacity is defined as the maximum sediment load that a particular discharge can transport at a certain slope (Merten et al, 2001). During the last three decades, several efforts have been made to analyze the influence of different hydraulic parameters on transport capacity, such as unit discharge, mean flow velocity, and slope gradient (Beasley and Huggins, 1982;Julien and Simons, 1985;Govers and Rauws, 1986;Finkner et al, 1989;Govers, 1990Govers, , 1992Guy et al, 1990;Everaert, 1991;Abrahams and Li, 1998;Jayawardena and Bhuiyan, 1999;Prosser and Rustomji, 2000;Abrahams et al, 2001;Zhang et al, 2009). These variables were mostly considered for the quantification of sediment transport capacity under overland flow conditions because they can be easily measured in the field and also have a pronounced impact on transport capacity.…”

“…These variables were mostly considered for the quantification of sediment transport capacity under overland flow conditions because they can be easily measured in the field and also have a pronounced impact on transport capacity. It is generally known that transport capacity increases with the increase of unit discharge, slope gradient, and mean flow velocity, since the energy exerted by a certain discharge on the bed increases with these variables (Beasley and Huggins, 1982;Govers, 1990Govers, , 1992Everaert, 1991;Zhang et al, 2009Zhang et al, , 2010a. In comparison to river hydraulics, far less research has been done to quantify sediment transport under shallow flow on an eroding hillslope.…”

“…The relationship between transport capacity and unit discharge has often been studied, and previous research has made it clear that this relationship is always dependent on slope (Beasley and Huggins, 1982;Julien and Simons, 1985;Govers and Rauws, 1986;Govers, 1990;Everaert, 1991;Jayawardena and Bhuiyan, 1999;Prosser and Rustomji, 2000;Lei et al, 2001;Zhang et al, 2009). However, the effect of unit discharge and slope on transport capacity varies from erodible to non-erodible bed conditions (Gover, 1990;Everaert, 1991;Zhang et al, 2009), probably due to the fact that, for the same hydraulic and sediment conditions, the roughness of erodible beds is always higher than that of nonerodible beds (Hu and Abrahams, 2006).…”

“…However, the effect of unit discharge and slope on transport capacity varies from erodible to non-erodible bed conditions (Gover, 1990;Everaert, 1991;Zhang et al, 2009), probably due to the fact that, for the same hydraulic and sediment conditions, the roughness of erodible beds is always higher than that of nonerodible beds (Hu and Abrahams, 2006). Govers (1990) and Everaert (1991) found, in different studies, that for erodible beds the effect of slope on transport capacity is higher than the effect of unit discharge.…”