Numerical modeling of flow in riverine basins using an improved dynamic roughness coefficient

Abstract

Various parameters such as bed and bank materials, shape and irregularity of the section, vegetation, river meanders, plan of the river path etc. affect the flow hydraulic resistance. In open channel hydraulics the effects of all these parameters are generally considered as the roughness coefficient. The Manning’s equation is one of the most practical equations to flow resistance analysis, in which the surface roughness is defined by Manning coefficient. Since many parameters are effective on the value of this coefficient, in this research study it was tried to define the roughness coefficient somehow that it be able to dynamically change with different river and hydraulic conditions. The collected data in Karun River (Iran) for two periods were used as the case study. It is shown that the accuracy of model predictions for water surface elevations were improved more than 13% in error estimation in comparison with the corresponding results obtained for a constant roughness coefficient. The roughness coefficient (n) for Karun River was also estimated using the empirical method proposed by Cowan for two different dry and wet periods. These values were then successfully compared with the average corresponding roughness coefficients calculated by the numerical model for those periods.