Correlation between the Manning roughness coefficient and the representative diameter of the drag material in high mountain river beds, Cajamarca – Peru

Abstract

The choice of the Manning roughness coefficient is essential when determining a flow rate and thereby performing a hydrological and hydraulic modeling to later design a hydraulic structure. In general, it is very difficult to choose the Manning roughness coefficient due to the variability of the sediments that make up the bed of a natural channel such as a river, since in the absence of field data, very given in tables, which causes some uncertainty in the hydraulic designs and modeling carried out. Therefore, the objective of this work was to find the correlation between the Manning roughness coefficient and the representative diameter of the gravelly sand material that constitutes the channel of the Chonta River, located in the Andean zone of Peru. Speed and depth measurements were made, using a hydrometric windlass, as well as sedimentological sampling in the river bed in order to obtain numerical values of the Manning roughness coefficient (n) in the field. After the field and laboratory stages, the following equations were found: n = 0, 0438 · k(^1/6) and n = 0, 2399 · k² − 0, 0083 · k + 0, 0276, where n is the Manning roughness coefficient and k corresponds to the representative diameter of the river bed, whose value has been determined in the field by means of a granulometric analysis and corresponds to diameter 30 (d₃₀).