The paper presents the problem of designing an open reservoirs with a capacity of about 100 000 m³ in heavily urbanized areas of anthropogenically transformed natural watercourses with numerous outlets drainage and large sealed surfaces runoff. In particular the issues of the hydrological basics design based on the meteorological data in the catchment uncontrolled water level checkpoints with particular emphasis on the tools of computer-aided modeling of hydrological processes (HEC-HMS, HEC-RAS) and available for use mathematical models and statistical rainfall-runoff, this practical the method SCS-CN and linear model cascade tanks Nash to transform the effective precipitation in surface runoff.