In July 2016 in the Three-city agglomeration a rainfall episode of over a day duration and 150 mm summary rainfall height, occurred. This situation, extreme as for Polish conditions, caused significant freshets in rivers and streams running into Gdansk Bay, the Baltic Sea, and serving as collectors of rainfall waters for the sea-coast towns. In many areas of the Three-city flood phenomena and overflows took place. The flood also occurred in the catchment area of the Kacza river in Gdynia. The passing of flood water rise caused damage of many infrastructure objects located along the river valley. The estuary section of the river suffered most, especially sea-shore belt together with beach around the estuary which were significantly washed out. In this paper an approach was made to answer to the question on direct causes of the situation which occurred at the estuary of the Kacza river. To this end, there was worked out a hydrodynamic model of the considered section of the river, based on the solving of two-dimensional differential motion equations of free-surface liquid, and simulations of water flow propagation along the river’s valley were performed for a few variants of hydraulic engineering infrastructure of river bed. Numerical hydraulic analysis made it possible to determine a role of the hydraulic engineering objects in forming the flood water rise as well as their impact on location and range of washout zones of sea-shore belt.