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SOIL PHYSICS

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

^a State Key Lab. of Earth Surface Processes and Resource Ecology
^b School of Geography, Beijing Normal Univ., Beijing, 100875, China
^c USDA-ARS Grazinglands Research Lab., El Reno, OK 73036

^* Corresponding author (ghzhang{at}bnu.edu.cn).

Precise estimation of sediment transport capacity (T_c) is critical to the development of physically based erosion models. Few data are available for estimating T_c 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 T_c in shallow flows and to investigate the relationship between T_c 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 x 10^–3 to 5 x 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 T_c 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 T_c increased as S increased, with T_c being slightly more sensitive to q than to S. The T_c 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 T_c (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 T_c 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.

Abbreviations: ANSWERS, Areal Nonpoint Source Watershed Environment Response Simulation • NSE, coefficient of Nash–Sutcliffe model efficiency • RE, relative error • WEPP, Water Erosion Prediction Project