The theoretical analysis and comparison of the nonlinear reservoir method and the sector method of calculating the stormwater runoff hydrographs from the urbanized catchments are carried out in the paper. The key advantages of the sector method are in taking into account the variable depth and flow velocity of the surface flow. A comparison of stormwater surface runoff hydrographs for completely impervious square drainage subcatchments, obtained by the nonlinear reservoir method and by the sector method, is performed. The accuracy of the numerical implementation of the sector method was controlled by the deviation of the calculated volume of the stormwater runoff comparing to the corresponding theoretical value. The relative difference between the two indicated volumes for the entire investigated range of values of the input parameters did not exceed 0.002 %. It was established that the stormwater runoff hydrographs, obtained by the sector method, comparing with similar hydrographs for the nonlinear reservoir method are generally
characterized by considerably more intensive growing of the flow rate at the first stage of the hydrograph and the same quick reduction of the discharge after the rainfall event. The maximum calculation flow rates of the stormwater surface runoff hydrographs using the sector method for completely impervious square drainage subcatchments for the climatic conditions of the city of Lviv at the return period P = 1 year are 1.29 times higher than the corresponding volume discharges obtained using the nonlinear reservoir method. The invariance of generalized non-dimensional stormwater runoff hydrographs, modelled by the nonlinear reservoir method and by the sector method, was obtained for the following input parameters: absolute dimensions of square subcatchments, longitudinal slope of the territory and the roughness coefficient of the subcatchment’s surface.
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