Study on sediment transport mechanisms due to sheet erosion using flume experiment

Soil erosion is a dynamic process with high variability in sediment size distribution. Dynamic recognition of sediment size distribution is as a base for modeling of soil erodibility and movement of nutrients، pollutants and fine particles in consequence of erosion. This study focused on sediment transport mechanisms due to sheet erosion on two different soils (cropland and sandy) using flume experiment. The experiment was performed applying different flume slopes (1. 5 and 2 %) and flow rates (75، 100، 125، 150، 175 and 200 ml/s). The result showed that hydraulic condition as well as soil type، played an important role in selective transport of sediment particles so that، in the sediment size distribution of cropland soil two peaks were observed. For the first (fine particles with size of 0. 042 mm)، the dominant transport mechanism was attributed to suspension- saltation، while for the second peak (large particles with diameter of 1. 5 mm)، bed load was known as the main transport mechanism. Moreover، increasing stream power، the selectivity of fine particles declined therefore، both fine and large particles were transported by the flow. For sandy soil، only one peak for the range of 0. 109 to 0. 175 mm was observed. In this soil، saltation was the main mechanism in transporting sediment. Increasing flow rate، the frequency of particles smaller than 0. 015 diminished، whereas particles larger than 0. 218 mm increased. For both soils at higher stream powers، the relative importance of transport as suspension and bed load decreased and increased، respectively. The findings of this research reveal that sediment size distribution and also transport mechanisms depend on hydraulic condition such as slope and flow rate.