Sediment transport mechanisms and selective removal of soil particles under unsteady-state conditions in a sheet erosion system

摘要

Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies.Most agricultural soils are located on gentle slopes where fertility is high;however,until now,the main attention on sediment transport mechanisms was paid to high-slope gradients,where soil erosion is intense,but soils are less productive.Despite the importance of sediment size distribution(SSD)and transport mechanisms under unsteady-state conditions,few studies have been done on this issue.Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions.To address this issue,sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions.Six inflow rates(75,100,125,150,175,and 200 mL/s)were applied on two contrasting soils,including an adjacent agricultural soil and a not cultivated soil(namely Cropland soil and Control soil,respectively)at two low slope gradients(1.5%and 2%).No rill formation was observed during the experiments.The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions.The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size.The contribution of suspension-saltation(SS)to the total sediment load for the Cropland and Control soils varied from 22%to 68%and from 35%to 59%,respectively,while up to 78%and 65%of soil particles were transported by bed load(BL),respectively.However,SS and BL indicated a reverse trend with stream power.The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles(0.042 mm),whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine(≤0.084 mm)and coarse(1.5 mm)particles.The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers.The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils,where small stream powers can remove fine and fertile soil particles.