Ministerie van Verkeer en Waterstaat Flood Risks and Safety in the Netherlands (Floris) Floris study - Full report November 2005 DWW-2006-014 ISBN 90-369-5604-9 Flood Risks and Safety in the Netherlands – Full report 2 Flood Risks and Safety in the Netherlands – Full report Table of contents Table of contents ..................................................................................3 Foreword...............................................................................................5 Summary...............................................................................................7 1. Introduction .................................................................................11 1.1 Risks demand attention ................................................................11 1.2 Background to the study...............................................................15 1.3 Goal of the Floris project...............................................................16 1.4 Approach taken by the Floris project.............................................17 1.5 The parties involved......................................................................18 1.6 Testing the Water Defences every five years .................................19 1.7 Projects aimed at flooding risks.....................................................19 1.8 Report structure ...........................................................................19 2. Description of the method ............................................................. 21 2.1 The essence of the method ............................................................21 2.2 Determining the consequences of a flood .....................................22 2.2.1. Outline ..................................................................................22 2.2.2. Flooding scenarios .................................................................23 2.2.3. Determining the economic damage due to flooding ...............25 2.2.4. Determining the number of victims ........................................26 2.2.5. Damage to natural features....................................................29 2.2.6. Environmental damage...........................................................29 2.3 Determining the probability of flooding ........................................ 30 2.3.1. Process ..................................................................................30 2.3.2. Data collection.......................................................................31 2.3.3. Categories of flood defences..................................................31 2.3.4. Dikes .....................................................................................33 2.3.5. Dunes ....................................................................................40 2.3.6. Hydraulic structures ...............................................................41 2.3.7. Weak locations analysis..........................................................44 2.3.8. Influence of human intervention ............................................45 2.4 Perception of risk .........................................................................47 2.5 Cost/benefit analysis ....................................................................48 2.6 Uncertainties ................................................................................48 2.7 Step-by-step analysis of the re sults...............................................50 3. Results for 16 dike rings...............................................................54 3.1 The dike rings considered .............................................................54 3.2 Risks on the basis of detailed calculation of consequences.............57 3.3 Risks on the basis of global calculation of consequences ...............59 3.4 Additional information obtained from the risk calculations ............61 3.4.1. Contribution of the individual failure mechanism ...................62 3.4.2. Contribution of the individual sections ...................................65 3.4.3. Flooding scenarios .................................................................67 3.4.4. Economic damage per flooding scenario ................................71 3.4.5. Number of victims per flooding scenario ................................72 3 Flood Risks and Safety in the Netherlands – Full report 3.4.6. Overview of cost/benefit analysis for dike ring 14..................75 3.4.7. Damage to natural features and the environment ..................77 4. Analysis and utility of the method................................................78 4.1 Are the calculated probabilities not too large?...............................78 4.2 Analysis of the consequences of flooding......................................80 4.3 Risk assessment essential ..............................................................80 4.4 Comparison with other studies......................................................81 4.5 Assessment of risks.......................................................................82 5. Conclusions and Recommendations .............................................85 5.1 Conclusions ..................................................................................85 5.2 Recommendations ........................................................................87 References ..........................................................................................89 Appendix A. Glossary of terms……………………………………………B-1 Appendix B. Area description and results for the 16 dike rings………B-9 4 Flood Risks and Safety in the Netherlands – Full report Foreword This is the full report of Flood Risks and Safety in the Netherlands (Floris) project. This report gives the results of the study into the risk of flooding in 16 dike ring areas in the Netherlands. This report presents both the method and the results. The "Flood Risks and Safety in the Netherlands: Interim Report of the Floris study" has also recently been published. This interim report describes the main findings of the Floris study. This Full Report describes the results in more detail than in the Interim Report. The Full Report also describes the method for calculating the consequences of flooding, the probability of flooding and the flooding risks. The conclusions and recommendations are identical in both reports. Besides the Interim Report and the Full Report, other detailed technical reports were also drawn up during the Floris study, including three reports describing the 'risk cases' and 16 dike ring reports. The risk case reports provide a description of the detailed method for determining the consequences and risks of flooding. The 16 dike ring reports provide a description of the results for the global consequences, probabilities of flooding and the risks for all 16 dike rings investigated. Many companies and organisations have contributed to the Floris research project. I would like to thank everyone for their constructive contributions and congenial cooperation. C.J. van Westen Floris Study Project Manager 5 Flood Risks and Safety in the Netherlands – Full report 6 Flood Risks and Safety in the Netherlands – Full report Summary The purpose of the Floris Project The government considers it important that the public has a better understanding of the probability of their area being hit by a flood. The government also wants to have a clear view of the relatively weaker areas in flood protection. Other experts have, moreover, indicated that the protection against flooding may no longer be properly in proportion to the consequences of flooding. The Flood Risks and Safety in the Netherlands (Floris) Project was therefore initiated in 2001 at the request of the State Secretary of Transport, Public Works and Water Management. The purpose of the Floris project is to gain an understanding of the consequences and the probability of flooding in the Netherlands. The project was conducted by the Road and Hydraulic Engineering Institute of the Netherlands Public Works Department (Rijkswaterstaat), in close cooperation with the Water Boards and Provinces. The results were released in the summer of 2005. Essence of the method The Flood Risks and Safety in the Netherlands Project has resulted in the further development and application of a new method which can be used to calculate the consequences of flooding. Detailed calculations have been made of the number of victims and the economic losses resulting from various flooding scenarios for three dike rings. The consequences have been determined more globally for the remaining dike rings. A new method has also been applied for determining the probability of flooding. The essence of the method is that various 'failure mechanisms', as they are known, can initiate a flood: not only extremely high water levels, but also instability in a dike or failure to close a hydraulic structure on time. Any failure mechanism carries a risk of flooding. The probability of all failure mechanisms together determines the risk of flooding in a dike ring. This method has been used to determine the flood risks of 16 of the 53 dike rings. The 16 dike rings were selected to give a representative view of safety in the Netherlands related to flooding. The calculations also show where the relatively weak locations in the water defences lie. A great deal of data was needed to be able to apply the new methods, including information about the subsoil under the dikes and hydraulic structures. In some cases, this data is surrounded by many uncertainties. An essential element in the probability