Effects of flexible water level management on flood risk in areas outside primary levees around the IJsselmeer and Markermeer

Abstract

Since 2019, Rijkswaterstaat has been applying flexible water level management in the IJsselmeer and Markermeer to make the Dutch water system more robust and future proof. The fixed target water level of -0.20 m NAP during summer is replaced by a flexible water level in which the water level can fluctuate between -0.10 and -0.30 m NAP to enlarge the fresh water supply and to anticipate on weather conditions. In the IJsselmeer region, 15,600 hectares of land is located outside primary levees. These areas are mainly pasturelands, recreation areas, nature areas and some buildings and infrastructure areas. By applying flexible water level management, the probability of high water levels increases and therefore flood risk in areas outside primary levees increases. However, the relation between applying flexible water level management and flood risk has not been investigated in detail yet. This research explores this relation further, on smaller scale as well as for the whole IJsselmeer region.

Flood risk is defined as the yearly probability of exceedance of hydraulic loads (lake water level, wind set up and wave run-up) multiplied by the consequences of inundation and is expressed in euros per year. Risk is a set of scenarios, with each a probability and a consequence, and therefore discretization is necessary. Hydraulic loads are calculated in Hydra-NL, a probabilistic model. By uploading hydraulic loads associated to their exceedance probability in the Waterschadeschatter, the consequences are defined and flood risk can be computed.
Flood risk depends on the lake water level, wind set up and wave run-up. The lake water level and wave run-up are in general higher when flexible water level management is applied and wind set up is lower. This observation is valid for all areas outside primary levees in the IJsselmeer region. About 90% of the total damage comes from water damage in buildings and infrastructure areas, and then especially from residential areas. Since less than 3% of areas outside primary levees consist of buildings and infrastructure areas and less than 10% of buildings and infrastructure areas consist of residential areas, the flood risk is reduced. Moreover, most computations show overestimations. Water levels computed in Hydra-NL are higher than occurred water levels and the upper boundary of -0.10 m NAP is used to calculate effects of flexible water level management. Because both overestimations are used and there are just a few residential areas, most areas located outside primary levees in the IJsselmeer region will hardly suffer from applying flexible water level management. This research shows that interests of areas outside primary levees in the IJsselmeer region and applying flexible water level management go quite well together. Therefore, the IJsselmeer region offers many opportunities.