Improving Flood Fatality Risk Assessment for River Flooding in The
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Improving flood fatality risk assessment for river flooding in the Netherlands Implications of alternative functions and model resolution variations on mortality and fatalities in the Bommelerwaard A.R. Brussee Improving flood fatality risk assessment for river flooding in the Netherlands Implications of alternative functions and model resolution variations on mortality and fatalities in the Bommelerwaard by A.R. Brussee in partial fulfilment of the requirements to obtain the degree of Master of Science in Civil Engineering at the Delft University of Technology, to be defended publicly on Monday June 15, 2020 at 2:00 PM. Faculty of Civil Engineering and Geosciences Department of Hydraulic Engineering Date of report: June 7, 2020 Project duration: September 2, 2019 – June 15, 2020 Thesis committee: Dr. ir. J. D. Bricker, TU Delft (Chair) Prof. dr. ir. S. N. Jonkman, TU Delft Dr. H. C. Winsemius, TU Delft & Deltares Dr. ir. K. M. de Bruijn, Deltares Ir. G. Verhoeven, Deltares An electronic version of this thesis is available at http://repository.tudelft.nl/. Preface This thesis finalizes my master’s study Hydraulic Engineering at the Delft University of Technology with the specialization Flood Risk. This topic matched very well with my interest in and curiosity about flood risk in the Netherlands. Since the last Dutch flood event was many years ago, we sometimes tend to forget that we live for a large part below sea level and that we have to deal with high peak river discharges at times. The study on the consequences of a potential breach in one of our dike rings is for that matter of great importance. This thesis was a collaboration between the Delft University of Technology and the research institute Deltares. I would like to express my gratitude to the members of my graduation committee for their time, guidance, and support during my graduation project. The feedback and discussions during the meetings have always led to improvements. I would like to thank Jeremy Bricker for initiating this project, chairing the committee, and providing valuable input (practically and academically). My thanks go out to Bas Jonkman for joining the graduation committee because of his expertise on loss of life. His guidance along with many suggestions proved to be of great importance and increased the quality of my work. I would also like to thank Hessel Winsemius for his critical view, he contributed to this study from the moment of joining and helped me form my research questions and approach. Special thanks to the committee members and supervisors from Deltares. Karin de Bruijn was my daily supervisor and helped me a lot in asking the right questions and by giving constructive feedback. We had regular talks about my thesis and flood risk management overall; I enjoyed these conversations and they were very valuable to me. Govert Verhoeven introduced me to the new software program D-Flow FM and helped me from the start with the case study set-up and the modelling assumptions. Besides my committee members, I would like to thank Rinske Hutten who I could go to for all the struggles with the ‘interacter’ of D-Flow FM. I thank Rolf van Buren for helping and showing me the possibilities within ArcGIS for quick operations of the grids. In addition, I would like to thank Dennis Wagenaar for providing me knowledge about SSM2017 and FIAT. Furthermore, I would like to thank the entire flood risk management department of Deltares for welcoming me, for the nice conversations during lunch or coffee breaks, and for making me realize, even more, the relevance of flood risk management. My thesis was planned from September till June, but unfortunately, due to the COVID- 19 circumstances, I have worked from home for the last three months. My thanks go out to everyone for their flexibility and for helping me graduate online. I would like to thank in particular my family and friends for supporting me during the process and keeping me motivated in these strange times. Finally, I truly enjoyed my time as a student at the Delft University of Technology. I hope that this final piece of my study time contributes to the broader discussion on water safety in the Netherlands. I wish you a pleasant read. Anneroos Brussee Delft, June 2020 iii Summary The number of fatalities due to a potential flood event is traditionally determined utilizing ‘mortality functions’. Data of recent large-scale flooding in the Netherlands are not available since the Netherlands was successful in flood prevention. Therefore, only data from the last coastal flood event in 1953 with 1795 direct fatalities are available. The mortality functions are empirical relationships to provide mortality as a function of three explicit flood characteristics, namely water depth, flow velocity, and water level rise rate. Many more factors are included implicitly since the functions were derived from 1953 data. These underlying factors are thus based on the circumstances of the coastal flooding in 1953 and might not be representative anymore for future flood events elsewhere in the Netherlands. For example, the quality of the built environment, spatial planning, communication systems, transport means, and also the socio-economic conditions have changed since 1953. Moreover, the mortality functions are derived from coastal flooding and some underlying aspects can have a different effect in river flooding. The three flood characteristics in current flood risk assessments are determined by means of coarse flood simulations. Since modern software is becoming more advanced, more detailed flood simulations are becoming possible. Therefore, the applicability of the mortality functions needs to be studied if finer model resolutions are used. This report presents the case study of river area the ‘Bommelerwaard’ in which the validity of the 1953-based functions, possibilities for alternative functions, and finer model resolutions in hydrodynamic models are tested and analyzed with regards to their impact on flood fatality risk. A hydrodynamic model is developed to simulate the flood characteristics for the mortality calculations and to analyze the impact of using finer model resolutions. The new flood simulation program D-Flow Flexible Mesh is used which is able to apply finer resolutions at locations that require more detail. The different model resolutions that are tested are 100m and 25m, and 5m for the area close to the breach. The flood simulations with these different model resolutions resulted in similar outcomes for the number of estimated fatalities in this case study. Overall, the 100m model is preferred because it is sufficiently able to indicate the dangerous locations, provide the order of magnitude of the flood characteristics, while it demands short computation times and matches the level of detail of the data of 1953. However, it is recommended to model the area around the breach (‘breach zone’) with finer model resolutions because the resulting higher local peak velocities are relevant for potential building collapse. For the areas around obstacles and underpasses, it is also recommended to use finer resolutions or to make use of 1D objects or fixed weirs. This study concluded that finer model resolutions at dangerous locations have an impact on the individual risk value of the neighbourhood and this can have consequences for the maximum individual risk value and thus the overall safety standard of a large dike ring. Furthermore, the case study illustrated that compartment dikes have a significant impact on the local mortality because of the high water level rise rates just upstream. It is recommended to look into possibilities to reduce this high local mortality rate and hence, individual risk, for example by optimizing the location and number of compartment dikes or exploring the effects of openings in the dikes. This study identified the discussion points in the current Dutch loss of life approach by a literature study, knowledge of recent flood events abroad, and loss of life approaches internationally. The most important factors for loss of life that came forward are water arrival time, people vulnerability, building characteristics (collapse), and human behaviour. Based on these factors, preliminary alternative functions or adaptations to the current v vi Summary parameters are proposed. These adaptations have been further investigated through sensitivity analyses. Moreover, the impact on the individual risk has also been assessed since this is the decisive risk criterion for dike trajectory 38-1 in the Bommelerwaard. It is recommended to substantiate and take into account the factors water arrival time, improved building characteristics, and age in the loss of life approach. Preventive evacuation is already taken into account in this approach, but in addition, water arrival time can be included by means of fleeing. This study shows that water arrival time has a great effect on the number of fatalities because some areas have relatively large arrival times and this enables inhabitants to flee the area. Emergency response is thereby of crucial importance. Also in 1953 this factor proved to be relevant. The improved building characteristics compared to 1953 are shown to have a limited impact on the absolute number of fatalities in this case study but it reduced the maximum value of the individual risk and is thus of relevance, especially for dike ring areas with large water depths (>2.1 m) and high rise rates (>0.5 m/h). Moreover, this study underlines the vulnerability of the elderly during flood events. Since the age distribution has shifted since 1953 and significantly more elderly are present in society nowadays, it is relevant to take this explicitly into account. This case study shows that correcting for age can have a significant impact on the number of fatalities. The impact on the individual risk is limited, but this depends on the spatial distribution of the elderly and should be further analyzed.