As a consequence of city expansion, the conditions of catchments located in urban areas have been permanently changing. This leads to an increase of flood risk in an urban area resulting from overflows of small streams, which are not prepared to receive more water. Moreover, hydrological data from long-time period are usually not available for small streams in an urban area and are limited to the last few years or to the moment after introduced changes within catchment in relation to urbanization. Therefore, it becomes hardly possible to use direct methods of estimating flood flows for small streams, which needs a reach data set. For that reason, mathematical modelling is growing as a basic method of evaluation of flood flows in urban area with limited information of a catchment. The object of this paper is to identify parameters of a conceptual model of rainfall-runoff process within a small ungauged urban catchment and to verify how chosen characteristics of a model depend on chosen rainfall characteristics. The results of modelling of pluvial flooding, conducted in a small urban catchment – Służew Creek catchment (located in Warsaw, Poland), have been presented in the paper. This catchment has been monitored by Department of Water Engineering and Environmental Restoration (Warsaw University of Life Sciences – SGGW) for a few years. Next, the results present a comparison of estimation of instantaneous unit hydrograph (IUH) by the Rao, Delleur and Sarma equations with the one established on recorded data. This empirical method consists of the ratio of urbanized area within catchment as well as in the duration and amount of an effective rainfall during a storm event. Moreover, including ratio of urbanized area allows user to adopt this method in a simple way to new conditions of the catchment, which is important as far as urban areas are considered. Considered IUHs were based on the Nash model, in which catchment is depicted as a cascade of N linear reservoirs with the same retention parameter k. For separation of the effective rainfall from the recorded storm the CN-SCS method was applied. To compare two methods, IUHs were estimated basing on three hydrological years (2007–2009) for a Berensewicz Pond profile and the comparison was made on the base of an IUH characteristic value – Lag time. The results of identification and verification of characteristics values of IUH (Lag and k) for Służew Creek catchment, presented in this paper, have indicated a statistical correlation between lag time and the sum of total and effective precipitation, and between k parameter and the sum of total and effective precipitation as well. Moreover, the analysis has shown that IUH characteristics estimated with the use of Rao, Delleur and Sarma equation has given promising results in comparison with the characteristics of measured IUH. Therefore, this method could be useful to estimate and predict flood flows in ungauged urban catchments in situation of limited information. However, this research needs further investigation and verification.
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