Influence of future zoning on flood risks

As the majority of urban areas in the western part of the Netherlands are well located beneath the sea surface, their safety for flooding depends heavily on the state of the retaining works of the Dutch flood defense system. Since the results of a major flooding might be catastrophic, every retaining work is statued every five years against the strict requirements as established by the Dutch law. The requirements for retaining works have been defined in Dutch law as a failure frequency (e.g. a flooding might appear once in 1250 years), and were based on the economic value of the area that was protected by the retaining work at that time. However, the economic value of many areas has grown rapidly ever after, and will be growing in future. As the economic value of the areas has risen, so the possible flood damage has. To counteract these flood damages, strengthening measures might be necessary on the retaining works to lower the failure frequency. Other possibilities might be to change the future zoning of the districts according to their economic value. In this paper, we take each relevant risk of flooding into account for the studied area. Hereby three main causes of flooding are distinguished: high water of seas and rivers, high water of regional water courses and extreme rainfall leading to flooding from canals and lakes. The High Water Information System (HIS) was used to set up risk maps of the area for each cause of flooding under certain design conditions, and were thereafter combined to one main risk map of the area. The procedure was carried out for the present situation as well as for possible future situations of the area (based on the ‘card of ‘The New Map of the Netherlands’, a continuously updated overview of the land use plans that are being produced in the Netherlands). The main result is a broad comparison between different measures (e.g. urban planning, dike strengthening, retention areas) that might be undertaken to maintain (or reduce) the risk level of the present protected area with the new land use plan. We show our results for a representative study area in the western part of the Netherlands.