Assessing Submerged Vegetation Roughness in Streambed under Clear Water Condition using Physical Modeling¹

Abstract—Roughness coefficient, also called Manning’s coefficient, is one of the most important hydraulic parameters in the rivers. This coefficient, in addition to the flow conditions, depends on streambed characteristics such as type and density of vegetation. In this study, a physical model in a flume with 7 m length, 0.25 m width and 0.25 m height was conducted to evaluate the streambed roughness coefficient and consequently the discharge passing from waterways. Flume bed was filled using uniform sediment with median grain diameter of 1.9 mm, variation coefficient of 1.4 mm and a thickness of 0.4 m. Roughness coefficient variation in the slopes of 0.2, 0.4 ,and 0.6%, discharges of 4, 6, and 8 L/s and vegetation cover densities as 0, 12, 25, and 50% were investigated. To simulate the covering of streambed, vegetation scrub was used in the experiments. The results showed that by increasing the density of vegetation, roughness coefficient increases while with increasing flow velocity, slope and Froude number decreases. By analyzing the data from this study, streambed roughness coefficient was obtained in terms of different variables as applicable relationships for different conditions of flow and streambed. The results of this study with quantifying the effects of various parameters on the roughness coefficient can be used by water engineers.