Nowadays, flooding in the urbanized areas gets more of a problem. Taking this into consideration, there should be more weight put on the correct calculation of flow capacity. One of the factors affecting it is the hydraulic conditions at the outlet. In the case of seaside streams it depends strictly on the level of the sea. The paper attempts to designate the impact of sea level rise on the course of swellings in seaside surface water streams. For this purpose the numerical analysis of water flow were made for stream Strzyża in Gdańsk, where the receiver is Martwa Wisła, where the water level is equal to the level of the sea. The calculations were performed on a numerical model based on the de Saint-Venant equations, solved using the finite difference method (McCormack scheme). The analysis was performed for two flood waves of exceeding probability 1 and 10%, and average flow in stream. For each variant two Manning roughness factors were adopted. The results obtained are shown in the form of profiles of maximum water levels in the creek and the summary of the range of impact of sea level. Summing performed calculations, it turned out that the impact of the sea depends directly on the level of water in the creek. Much greater impact on the calculations have flow values. This is crucial information in determining the level of significance of the analyzed problem. This work may be a prelude to a more detailed diagnosis of the phenomenon.