Aim of the study:
The efficiency of flood protection in a small urban basin which based on system of small reservoirs was analysed.

Material and methods:
To assess the flood routing and surge reduction mathematical model was implemented to the river basin. It was lumped hydrological model basing on SCS-CN method. The channel routing was done by using kinematic wave equation. The sub-basins have been determined based on the topographic maps and rain sewage systems. In the paper, the hydrologic model of the water system of Oruński Stream will be presented with particular effort paid to the new and renovated retention structures. Hydrological modelling system of HEC HMS was used to simulate the basin response on precipitation. Due to recently established monitoring system of the Oruński Stream, model could be calibrated and validated. Single rain episode from April 2018 was used to model calibration proving its high compatibility. The calibrated model was next used to reproduce flood surge of July 2001 flood with historical basin characteristics. The peak discharge shown a need for additional retention in a river basin. Theoretical scenarios was also preceded. The rain depth was calculated based on the IFS. As a worst case scenario the rain producing the most intense surface runoff was included. In all simulations historical conditions as well current state (2018) and further basin urbanisation was taken into consideration.

Results and conclusions:
Results of the study shown reduction of the historical flood surge from July 2001 by nearly 60% while all existing flood protection system was used.