River Bed Resistance Coefficient Variation of Different Sediment Particle Shapes

Abstract

In gravel bed rivers, the flow resistance is mostly due to grain roughness. Strickler was the first who determined the grain roughness coefficient in terms of sediment particle size. In the so called Strickler’s formula, the Manning’s coefficient has been related to only the median particle size ( 6 / 1 S cD n = ). Therefore it is the purpose of this study to conduct experimental test to find out if the same sediment size but in different shapes, can produce different flow resistance and if so by how much. To reach such goals four different sizes of round sediment particles taken from natural river bed and four crushed sediment particles with the same sizes as river bed sediment were tested under constant flow conditions. The experimental tests were conducted in a 12 meter long flume of 25 centimeter wide. The energy gradient between the upstream and downstream stations was calculated by Bernoulli’s equation. Then the roughness coefficient was computed using the Manning’s formula. Comparison of the data shows that the particle shape can affect the particle roughness. The results also have been compared with the Strickler’s type of equation and the best relations have been developed. Introduction In many river engineering activity determination of river flow characteristics is the first step which usually is done by solving the governing equations of the flow which is known as Saint ve Nant equations. In solving these set of two equations, the energy gradient should be known which is calculated from one the flow resistance equations such as Manning’ formula (Eq.1): 2 1 3 2 1 f S R n V = (1) Where V is the flow velocity, R is hydraulic radius of river, Sf is the energy grade line and n is the Manning’s coefficient. During past decades many researchers have been trying to present criteria for predicting the n values. Any parameter which causes 1585 World Environmental and Water Resources Congress 2012: Crossing Boundaries © ASCE 2012