Aim of the study:
Extreme flood events cause discussions about the effectiveness of flood protection. There are a questions about the capacity of the riverbed, as well as priorities in the protection of people's lives and the nature. Resolving these issues requires a new look with accordance to the principles of sustainable development. The study aims to check whether a potential conflict between flood and nature protection can be resolved using engineering methods without disturbing habitats.

Material and methods:
The study analyses the impact of islands located in the riverbed on flow capacity and determines the active zone of cross sections to ensure the minimum capacity of the flood with an attempt to reduce the impact on natural and valuable areas. The results of numerical modelling were used in combination with the Pasche method.

Results and conclusions:
Due to the threat to the community caused by the floods, the impact of islands was shown and areas designated for deforestation and for protection were indicated. The results of the Pasche method show that for different land cover the active cross-section has decreased up to 50% of the original value. The zone bII covers only a part of the cross-section mostly close to the river banks. The zone designated for intervention overlaps with the active flow area. This approach to increase the riverbed flow capacity is insufficient because it does not ensure minimization of interference and maximization of water level reduction. In addition, the implementation of the Pasche method for large rivers and long sections requires iteration method.