Estimation of Sediment Load in Arun River Basin

The single sediment rating equation is often used to estimate the suspended load in river, The sediment concentration is assumed as a function of mean daily discharge using a power equation. The single equation represents the average relationship between the sediment concentration and daily discharge which leads to the underestimate of suspended load. This paper proposed the seasonal sediment rating curves to meet with spatial variation of sediment load among different seasons. The single sediment rating equation is developed for the average sediment condition and the seasonal sediment rating equation is developed for the high sediment condition. The observed 2413 sediment concentration samples are used to develop the above two sediment equations. The least square method is used to optimize the coefficients in the sediment rating curves by minimizing the total sediment load. The results show that the annual average suspended sediment load is about 13.81 million tons using the seasonal sediment rating curve and the annual average suspended sediment load is about 7.54 million tons using the single rating curve. The seasonal sediment rating curves result in a high estimation of sediment yield, and can be used as the starting point for the modeling to ensure that the project configuration could handle the high sediment loads that could possibly occur, but which do not represent the normal or average condition. The annual average suspended sediment load derived from the single sediment rating curve can be used as the normal or average condition. The Einstein H.A. method and fixed ratio method are used to estimate the bed load. The total annual average sediment load is about 16.24 million tons for the high load scenario and the total annual average sediment load is about 8.294 million tons for the average load scenario. The proposed seasonal sediment rating equation follow the variation of runoff and can overcome the underestimation of suspended sediment load in design of hydropower station.