This master thesis is about the positive effects of river system behaviour along the river IJssel. Due to failure of the flood defences along the IJssel, the protected areas will flood and water will flow out of the river. The discharge in the river will decrease and thus the water level at the downstream ends of the river will also decrease. This will have an impact on the required dimensions of the flood defences downstream as the loads will be lowered due to the decrease in water level. The design of the dimensions will change and the costs of the flood defences downstream are lower. In this research, flood simulations are done in the dike-rings along the river IJssel due to overtopping with and without breach formation. The simulations are done in a developed HEC-RAS model and eventually the decrease in water level is generated within the model computations. The tool OKADER is used for the calculations of the required dimensions of the flood defences in 2050 and the associated costs when changes in water level is incorporated. At the end the interest is to quantify the positive effects of system behaviour in terms of differences in required dimensions of the flood defences along the river IJssel and the amount of saved costs.