Sediment Transport in Unsteady Flow Conditions

Sediment transport under unsteady flow condition is studied experimentally. In the first step, sediment transport under different steady flow conditions was measured and an empirical equation was derived for its calculation. In the next step, two continuous and three stepwise hydrographs were generated in the flume, and their sediment transport rate was measured. The continuous hydrographs were then approximated by different number of steps. Sediment transport for the hydrographs was then calculated by assuming a steady state condition in each step employing the empirical equation derived in the first step of the study. Results showed that in continuous as well as the stepped hydrographs the difference between the calculated and measured sediment rates is less than 10%. This shows that in the range of the tested hydrographs which conforms to many rivers in Iran, approximating the hydrograph with steady state steps for sediment transport calculations leads to acceptable results. In the next step, the flow and sediment transport in the flume under the tested hydrographs were simulated by using HEC-RAS software. Various sediment transport equations were used and calculated results were then compared with experimental measurements. Results showed that by increasing the number of steps in stepped hydrograph in HEC-RAS, calculated sediment transport rates by each equation tend to constant values.