Determining Manning Roughness Coefficient in the Gol-Gol River of Ilam

Roughness coefficient is one of the main factors that lead to energy loss in channels and rivers which have a very important role in water level in cross-sections of rivers. There are several methods that suggested by various researchers for estimating roughness coefficients which are mostly based on experimental tests. In river hydraulic, flow resistance or roughness coefficient is changing because of bed mobility. In this case, a flow resistance formula cannot be used directly without any information about changes of flow resistance coefficient, in different situations of flow and sediment processes. Gol-Gol River of Ilam dam selected for this study. So, results of various empirical methods obtained to find out this coefficient using field survey and characterization of rivers such as cross-sections, soil materials of river bed and banks, then results compared with observed and simulated data of river flow. The HEC-RAS software used to determine the water level in different cross-sections for various discharges and roughness coefficients. Results showed that among various methods, results of Chow method was closer to the observed data, and this method had the lowest mean relative error, so it recommended as the best method to estimate the manning roughness coefficient of this river.