Development of a New Flow-dependent Scheme for Calculating Grain and Form Roughness Coefficients

Estimating channel roughness is crucial for whatever engineering plans that have been in mind for any reach under consideration. Not only is resistance coefficient not a measurable quantity, but also various factors affecting on its value make its estimation a challenge. Despite of numerous methods available for roughness estimation, the complexity of some of available iterative schemes particularly with no mechanism for modifying initial guess in each iteration restrain numerical modelers to apply merely outdated resistance equations in practice. In order to improve the estimation of hydraulic resistance, a new straightforward flow-dependent scheme, which is capable of estimating Manning's coefficient due to grain and form roughness, is introduced. A large data is utilized to calibrate and testify the new scheme. The results of comparing the new scheme with that of different models available in the literature show that it achieves the best estimation results and yields to more than 0.87 and 0.67 for R-2, 0.15 and 0.17 for mean absolute relative error for estimating grain and form Manning's coefficients, respectively. This comparison demonstrates that the results achieved by the new scheme are acceptably accurate in favor of roughness estimation.