Evaluation of Image Processing Technique in Estimating the Manning's Roughness Coefficient in the Surface Layer of Riverbeds

Considering the importance of adequate roughness coefficient estimation in river engineering studies, evaluation of image processing technique in estimating Manning’s roughness coefficient in the surface layer of riverbeds carried out in this study. The mentioned approach evaluation conducted by implementing of sieving analysis and digital image processing methods simultaneously for a 7.5km reach of Shalmanrood River of Gilan. The processing of captured images signifies that this technique has an excellent accuracy in estimating the size of sediment particles (particles with a size of d50 or larger) and can be used to estimate Manning’s roughness coefficient of sediment particles of riverbed, utilizing the given empirical formulas. To evaluate the image processing results in estimating Manning’s coefficient values, one-dimensional modeling by HEC-RAS Hydraulic model was used and the model was conducted through different scenarios. Finally, on given cross sections, the comparison of output hydraulic properties with respect to Cowan’s method results showed that Bray’s empirical formula (d90) will have the best efficiency in estimating the Manning’s roughness coefficients in the surface of riverbed, with a maximum relative difference of 13.7% in top width.