The influence of compartmentalisation on flooding in Central Holland MSc Thesis E.P. de Bruine April 2006 Author: E.P. de Bruine Graduation Committee: Prof. Dr. Ir. H.H.G. Savenije (DUT) Dr. Ir. M.J. Baptist (DUT) Ir. S.N. Jonkman (DUT) Dr. A.W. Hesselink (RIZA WRE) Copyright:: E.P. de Bruine. 4 Compartmentalisation in Central Holland Preface . This report is the result of graduation research for the Master study Watermanagement at the faculty of Civil Engineering and Geosciences of Delft University of Technology. The graduation research consisted of a internship at Rijkswaterstaat RIZA-WRE. The author is solely responsible for the content of the report. I would like to thank Gerben Spaargaren of the water board Delfland, Rob Taffijn of the water board Schieland en de Krimpenerwaard, Frans van Kruiningen of the water board Rijnland and René Piek of the Province of South-Holland for providing me with the information I needed. I also would like to thank Paul Visser of DUT for giving me more insight in breach growth processes in dikes; Marcel van der Doef and Plony Cappendijk-de Bok of DWW for helping me with the calculations of damage and casualties; my colleagues at RIZA-WRE for creating the perfect atmosphere to work in and especially Sacha de Goederen for creating ‘boundary conditions’-files the NDB-model, Johan van Zetten for helping me with the schematisation in Sobek and Aad Fioole for teaching me all about GIS-applications; my girlfriend Angela Paulus for supporting me and reading all my drafts. Special thanks go to Alex Roos, Stephanie Holterman and Marcel van der Doef of DWW for supplying me with the model schematisation of the study area. This study would not have been possible without it. Special thanks also go to my graduation committee: Professor Savenije (DUT) for his sharp view on the matter; Martin Baptist (DUT) for his enthusiasm and indispensable knowledge of hydrodynamic modelling; Bas Jonkman (DUT,DWW) for his technical input and for constantly providing me with relevant reports and Annika Hesselink (RIZA-WRE) for reading all my drafts very carefully, helping me to improve the texts and for teaching me to create a network of interested and helpful people. Erik de Bruine, April 2006. 5 Compartmentalisation in Central Holland 6 Compartmentalisation in Central Holland Summary . The Dutch landscape is characterised by polders of low elevation next to the North Sea and the rivers Rhine and Meuse. This landscape makes it necessary for the Dutch to protect themselves against flooding by building dikes around the polders. These dikes are called primary dikes, the area that is protected a dike-ring area. The safety level for dike-ring area 14: Central Holland is 1/10.000 per year, meaning that a flood event statistically can occur every 10.000 years. The policy against flooding is based on taking measures to guaranty this low chance of occurrence of a flood event. However, after the flood event in New Orleans, USA, a discussion has arisen about the Dutch policy on water safety. This discussion is held in the project Waterveiligheid 21ste eeuw (WV21). Other options to reduce the flood risk are taken into consideration, not only measures to decrease the chance of flooding, but also measures that decrease the consequences of a flood event. Compartmentalisation is one of these options, but only little information is available about the ability of compartments to reduce the consequences of a flooding. In 2008, the (new) policy on water safety will be determined. This report focuses on the hydraulic effect of compartmentalisation to reduce the consequences of a flood event. Compartmentalisation stands for the division of the dike-ring area into smaller units (compartments) by the construction of dikes. Only a few of these compartments are supposed to flood in case of a large flooding. The floodwater is retained within these compartments, preventing flooding in the rest of the area. Investigations are also made into the influence of canals in the area and the possibility to redirect floodwater to a target location rather than retaining it. A hydrodynamic model schematisation in Sobek-rural for the study area Central Holland is developed in the project Veiligheid Nederland in Kaart (VNK). This schematisation is the basis of the schematisation used in this study. Boundary conditions differ from those used in VNK. A framework is developed to create different spatial layouts of the dike- ring area (figure 4-8). Most layouts are obtained by the raising or lowering of existing spatial line elements, such as roads, railroads and dikes. For some layouts new elements are added. Simulations are made for 4 breach locations and a total of 15 different spatial layouts. The Digital Elevation Model is adjusted to obtain the desired spatial layout. The effect of the canal system is tested by schematising the emergency barriers in the canals (noodboezemkeringen). These barriers are physically present in the canals, but were not yet schematised in the model. The simulations show the flood patterns for 10 days. The patterns differ in total area of flooding, water depth, flow velocity and the rise rate of the water. The discharge through the breach in the dike is also 7 Compartmentalisation in Central Holland influenced in some cases. For each simulation the total damage and the number of casualties are calculated by the program HIS-SSM. Several conclusions can be drawn from the simulation results. Firstly, the canal system functions as a network through which the floodwater can spread very quickly. Floodwater enters the canals near the breach location and is transported quickly to more distant parts of the canal system. Flooding will occur at locations where the canal embankments are relatively low. This effect can be countered by closing the emergency barriers that are present in the canals. The flooding at distant locations is prevented and more floodwater will remain near the breach location. Secondly, the use of small compartments (<10 km2) near the breach location effectively reduces the inflow of floodwater in the polder. The water level rises quickly in the compartment, approaches the water level in the river (or sea) and restrains further inflow of floodwater. The peak discharge in the polder can be reduced by 30%. When the water level in river decreases again, water flows from the polder back into the river. The floodwater volume that eventually has to be pumped out of the area is small because of this effect. On the other hand, a large number of casualties can be caused due to the quick rise rate of the water in the compartment. The use of small compartments therefore should only be used in sparsely populated areas, or measures should be taken to reduce the number of casualties, like an excellent evacuation plan and the presence of many evacuation routes. Thirdly, the redirection of floodwater can prevent damage and casualties if a high rise rate of the water level is not desirable. In this case, the inflow of floodwater remains the same, but the floodwater that enters the polder is redirected to a location with a relatively low value. Casualties because of a high rise rate are prevented. The effectiveness of the redirection depends on the route available. More casualties can be expected along this route because the floodwater arrives earlier than in the current situation. The number of casualties due to high flow velocities does not increase because the maximum flow velocities near the breach (where flow velocities are generally highest) remain approximately the same. The concepts developed in this study do not make an integral layout for Central Holland. They must be considered as building blocks that can be used to create a polder layout that reduces damage and casualties optimally for more than one breach location. These building blocks can also be used for areas similar to Central Holland: large flat areas below sea level with high concentrations of value and inhabitants. But compartmentalisation or redirection should be implemented with care. The impact of a measure on damage and casualties depends on local water depth, flow velocity and rise rate. A hydrodynamic calculation is therefore necessary to determine these effects. 8 Compartmentalisation in Central Holland Table of contents . Preface 5 Summary 7 1. Introduction 11 Flooding 11 Towards a new safety approach 11 Problem description and goals 12 2. Area description 17 The Rhine-Meuse Delta 17 Dike-ring area 14: Central Holland 18 3. Model development 23 Model schematisation 23 Boundary conditions: approach 24 Flood scenarios 26 Damage and casualties 31 4. Spatial design of the dike-ring area 33 Framework 33 Selection process 39 Designing the layouts 41 5. Simulation results 43 Ter Heijde 43 Maeslant barrier 47 Rotterdam Maasboulevard 51 North Sea Channel 60 6. Sensitivity analysis: Maasboulevard 65 Breach dimensions 65 Land roughness 70 7. Reflection 73 Model results 73 Sensitivity analysis 78 8. Conclusions and recommendations 81 Main conclusion 81 Conclusions by subject 83 Recommendations 85 Abbreviations and terms 87 References 90 9 Compartmentalisation in Central Holland 10 Compartmentalisation in Central Holland 1. Introduction . Flooding The Dutch have a long history in relation to water. The water is a blessing on the one hand, supplying drinking water and transport possibilities. On the other hand, there is always the threat of flooding, causing damage and taking lives of civilians. The Dutch have experienced several big flood events. Well known is the sea flood of 1953 when almost 200.000 acres of land flooded, more than 1800 people died and approximately 680 million euros of damage was caused (source: www.delta2003.nl, October 2005). Also the rivers cause trouble, for instance the big flooding of the river Rhine (1926) and the more recent nearly floods in the rivers Rhine and Meuse (1993 and 1995).