Roughness Coefficient in Mountain Rivers

River discharge estimations require an evaluation of the river flow resistance that generally is related with Manning's roughness coefficient. The literature suggests different ways of estimation of Manning's roughness, but in general they can be applied to channels in normal conditions such as uniform flow. But gravel bed rivers have a much larger roughness coefficient than people may think and the literature indicates. The use of a traditional estimation of roughness then would lead us to an underestimation of the roughness, resulting in an overestimate of the flow velocity and an underestimate of the river depth. The purpose of this paper is to recommend some formulas to estimate the roughness coefficient in this kind of flood. These empirical formulas were obtained using data from rivers in the United States, United Kingdom, and Chile. The range of error of the estimation of the roughness using the formulas recommended in this paper has been reduced considerably compared with earlier research. The average error ranged around 6% for rivers with large roughness-scale and 3% for rivers with intermediate scale.