Aim of the study:
The paper presents the results of hydraulic calculations (numerical simulations), which were made to investigate the role of floodplain (polder) adjacent to the left bank of Cisowska Struga in Gdynia downstream Hutnicza St. in the process of retention and transformation of flood waves occurring in this river due to torrential rainfall. The paper also addressed the issue of the impact of potential changes in the development of the retention area under consideration for the flood risk in Rumia.

Material and methods:
In order to perform hydraulic calculations, mathematical modeling was used. All hydraulic calculations of flood wave transformation in channels and floodplains were performed using the HEC-RAS program and data on characteristic flows obtained from available hydrological calculations.

Results and conclusions:
In the light of the obtained results, it should be stated that the analyzed retention area between Hutnicza St. in Gdynia, and the railway line is very important hydrologically. Potential reduction of the retention capacity of this area will negatively affect the transformation process of flood waves in the Cisowska Struga on the analyzed section. At the outflow section in the area of Dębogórska St. and further in Rumia, elevations of the water level will be elevated, and flood waves will reach faster here. Due to the potential increase in the flood risk in Rumia, this is undesirable.