Abstrakt

In recent years Gdańsk had sustained economic and social losses due to severe flash floods coming down from moraine hills. The first flood occurred in July 2001 and the second in July 2016. Both events were caused by intense and long rainfall characterized by different from each other rain intensity in time. Among other Gdansk’s streams the Oliwski Stream has the most extended flood protection system consist of 15 small retention reservoirs. Due to its extensive flood protection system and lack of severe flood events in last years, Oliwski was not considered to require attention form the city authorities. Most of the reservoirs originate from ancient mill pounds operated in place for centuries. As a consequence of little attention, majority of reservoirs were significantly filled with sediment and in some cases shrunk considerably and become wetlands. One of the reservoirs was recently rebuild by cleaning the sediment and remodeling its artificial island, but the others were intact. During the 2016 flood, when in the Oliwski stream catchment an extraordinary daily sum of rain precipitation was recorded (170,2 mm in 16 hours, 140 mm in 8 hours), two of 15 reservoirs were breached and other two were severely damaged. It was the turning point for the city authorities, who decide to look closer on the flood management in Oliwski stream catchment. The aim of research projects was to establish technical solutions and procedures to be applied in case of risk of flooding due to heavy rain. One part of this study was bathymetric survey of all reservoirs to be included in hydraulic modeling system. The survey take place short after the flood and consist number of cross sections measured in each reservoir by using total station. In a paper the comparison of current and design bathymetry of two reservoirs is presented from which one was renovated in 2014 (Spacerowa reservoir) and the other was not cleared over the last 35 years (Kwietna reservoir). Base on the surveyed data some conclusions on sediment deposition could be stated.

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