The significance of flood duration for flood damage assessment

Introduction Flood risks can be reduced by either reducing the probability or the consequences of a flooding. These consequences can be quantified with flood damage models. Such models determine flood damage based on the water depth and the land use. This thesis will investigate the need to also use the flood duration as input parameter. Problem definition Besides the water depth, also other factors determine the resulting flood damages. These factors are often not taken into account in flood damage models. One of these influences is the flood duration. The longer a flooding lasts, the larger the material damage, and especially damage due to interruption will be. Flood duration causes interruptions and extra material damages. Taking into account flood duration can, therefore, theoretically make flood damage models more accurate. Flood duration predictions are, however, at the moment rarely done. This thesis aims to get both a qualitative and quantitative understanding of flood duration and the importance of flood duration for damage assessments. Research This thesis aims to explore the possibilities of assessing flood duration for flood risk management. This is approached by the following steps. 1. Development of a better understanding of flood duration. By looking at different areas and flood threats, a flood type categorization was developed and durations were estimated for each flood type 2. Exploration of the influence of flood duration on damage. A modeling method to roughly estimate the duration-dependent damage was developed. The framework of this method may also be useful for future duration dependent flood damage models. 3. Two case studies were carried out to study flood duration and its influence on damage in more detail: First the Betuwe and Tieler & Culemburgerwaard area was studied and secondly the area threatened by a breach at the Parksluizen in Rotterdam was focused on. Different scenarios were used with varying breach locations, measures and use of outlet and drainage structures. Results 1. The most important factors which determine the flood duration are duration necessary to repair the breaches, the possibilities for drainage by gravity, the elevation and elevation variation in the area and the magnitude of the flood event. Flooding durations in the Netherlands vary between hours and about one year. 2. Adding flood duration as input to flood damage models adds a little extra accuracy. This is limited because flood duration is correlated with the water depth. With the current flood damage accuracy, incorporating flood duration is only useful for specific cost benefit analysis related to measures that aim to change the flood duration. Conclusions and recommendations Flood duration can be significant for large floods in low and endyked areas. In these cases flood duration can also have a significant impact on the damage. However, a complex economic model is necessary to quantify this. Therefore, flood duration can only reach its full value as an input, in combination with better economic modeling.