MOMENTS OF RUNOFF COEFFICIENT AND PEAK DISCHARGE ESTIMATION IN URBAN CATCHMENTS

A study of hydrologic losses in urban catchments is presented, on the basis of rainfall-runoff data of events recorded in 21 urban experimental catchments. Interest was mainly focused on the runoff coefficient, usually used in simple conceptual models for discharge estimation in the design of drainage networks. This coefficient shows characteristics that are typical of random variates and that are usually neglected. From analysis of experimental data, its probability distribution function was found to be approximately normal, and simple relationships for estimation of main moments were developed. These relationships can be used in problems concerning reliability and risk design of drainage networks. A modification of the rational method, taking into account the randomness of the runoff coefficient to achieve a correct estimation of risk level of design discharges, is then proposed for urban catchments.