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Grass Revetment Reinforcements a Study Into the Effectiveness of Measures Applied During Critical Conditions

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Grass Revetment Reinforcements a Study Into the Effectiveness of Measures Applied During Critical Conditions Grass revetment reinforcements A study into the effectiveness of measures applied during critical conditions G.P. van Rinsum April 2018 Delft University of Technology Photo front page: by G.P.van Rinsum, 16 September 2017. Training Drents Overijsselse Delta water board and NATRES (Military: National Reserve). Grass revetment reinforcements A study into the effectiveness of measures applied during critical conditions by G.P. van Rinsum to obtain the degree of Master of Science in Hydraulic Engineering at the faculty of Civil Engineering and Geosciences at the Delft University of Technology, to be defended publicly on Wednesday April 18, 2018 at 15:30. Status report: Final Specialization: Hydraulic Structures and Flood Risk Student number: 4213785 Email: [email protected] Project duration: September 1, 2017 – April 18, 2018 Thesis committee: Chairman Prof. dr. ir. S.N. Jonkman TU Delft Daily supervisor Dr. ing. M.Z. Voorendt TU Delft Technical supervisor Dr. ir. B. Kolen TU Delft / HKV Lijn in water Technical supervisor Ir. K.T. Lendering TU Delft / Horvat & Partners Company supervisor Ir. F.J.Havinga HKV Lijn in water / HvA Graduation work carried out at HKV Lijn in water An electronic version of this thesis is available at http://repository.tudelft.nl/ Preface This report is the final version of my master thesis on the effectiveness of grass revetment reinforcement mea- sures applied during critical conditions. This report is written as final product for completion of my Master of Science in Hydraulic Engineering at Delft University of Technology. The research has been carried out in col- laboration with HKV Lijn in water and the Wiki-Noodmaatregelen group. The request for the research subject came from the Wiki-Noodmaatregelen group. A work group, funded by STOWA, working on combining and extending the knowledge of water boards on reinforcement measures applied during critical conditions. I had close contact with the members of this group during the research by sharing the progress of my re- search, carrying out a workshop, interviews and observations of exercises. I thank the members of this work group for their feedback and input for my research. I want to thank two persons in particular. Eric Huijskes, the chairman of the group, for his support and feedback, using his network in organizing the workshop and nice conversations we had during the meetings and high water exercise. Secondly, I want to thank Wijnand Evers of the Drents Overijsselse Delta water board for his help, feedback and opportunities to observe the cri- sis response during high water exercises and even when the high water was reality in January 2018. It was important for this research to go out in the field and see how the crisis organization works to be able to im- plement this into the theoretical models. Wijnand Evers is always interested in improving the quality of the crisis organization and not afraid to share potential shortcomings of the water board as long as it leads to a better practise in the future. I want to thank my colleagues for the great time I had at HKV. It felt as a privilege to work at HKV during this period, both in Delft and Lelystad. The nice working environment and colleagues helped me to keep going and enjoy the research. At HKV work a lot of experts in specific fields of the hydraulic engineering world, who are more than willing to share their expertise. This was both inspiring and helpful for improving the quality of my thesis. Special thanks goes to my graduation committee: Bas Jonkman, Bas Kolen, Fred Havinga, Kasper Lendering and Mark Voorendt. The committee was well balanced in terms of expertise. I really appreciated the very constructive meetings where knowledge and feedback was shared. Every meeting gave me a new positive im- pulse in the right direction to improve my research. Individual meetings, for more specific discussion points, were also very helpful in steering in the right direction. Each committee member was more than willing to share their expertise in different phases of the research, despite the full agendas. Guido van Rinsum Delft, April 2018 iii Summary The Netherlands is prone to flooding both from the rivers and the sea. A large part of the country depends on the flood protection system. The permanent defences, like the river dikes, are the most important part of the Dutch flood protection strategy. Water boards maintain these flood defences regularly to prevent flooding. Water boards also install (temporary) reinforcement measures during extreme events at weak spots in the flood defence. Measures like the containment of sand boils, installation of supporting berms and reinforce- ment measures for the grass revetment. The effect of these measures is often not quantified in terms of risk or failure probability reduction. The objective of this research is to determine to what extent grass revetment reinforcement structures (Dutch: bekrammingen) contribute to the safety against flooding. Those reinforcement measures can be applied both at the inner and outer slope of the dike. This research focusses on measures applied at the outer slope to pre- vent erosion of the revetment during extreme conditions. There are two options: the reinforcement measure fails or is successful. Failure of the measure can be caused by: failure of detection of the weak spot, failure of placement of the measure or technical failure of the mea- sure itself. The flood defence does have its initial strength when the measure fails, nothing has changed. The failure probability of the flood defence changes with a successful applied reinforcement measure, this failure probability is not equal to zero. The description above is implemented in the model: • The failure probability of detection and placement is analysed by an event tree of these phases. The failure probabilities of the respective sub steps are quantified with the OPSCHEP model. This is a model to quantify the probability of failure caused by humans. This model is modified to apply it to the field of temporary reinforcement measures. The technical reliability is assessed by means of an analysis of the strength of, and load on the reinforcement structure. The detection, placement and technical reliability result in the failure probability of the measure. The second uncertainty parameter is the time required for installation, consisting of the detection and placement duration. • The reliability of the flood defence without reinforcement measure is quantified with the WBI-2017 erosion formulas for the grass revetment under wave impact. The models are modified to account for initial damage. Initial damage can be present at the grass revetment for various reasons, such as driving tracks and damage by animals. • The effect of a successful installed reinforcement measure is implemented in the model by means of an erosion modification factor. This factor reduces the erosion speed of the grass revetment due to the presence of the reinforcement measure. The components above are integrated into a crude Monte Carlo simulation. For each simulation step are the hydraulic conditions discretized per hour. The cumulative erosion is calculated per time step. The erosion speed is modified when the the measure is installed successful. This reduction in erosion speed is imple- mented in the model after the time required for installation. The calculation method results in the failure probability of the flood defence, taking into account the reinforcement measure with a certain reliability. The grass revetment reinforcement measures can be applied at known and unknown weak spots. These two options are assessed in two separate case studies. Case A, known weak spot, analyses a measure with a total length of 500 meter. Case B, unknown weak spot, examines a measure of 200 meter. Within these cases are four scenarios assessed: a pessimistic and optimistic scenario and the current practice at the Rivierenland and Drents Overijsselse Delta water board. The input parameters of the cases are based on observations dur- ing high water exercises, the high water of 2018, a workshop and interviews at water boards. The failure probability of the reinforcement measure pm is the most important parameter in the reliability assessment of the flood defence with reinforcement measures. This value is larger for unknown weak spots v vi Summary compared to known weak spots due to the detection part. In the case studies are the values for pm found according to Table1. Table 1: Failure probabilities of reinforcement measure pm of the case studies Known weak spot Unknown weak spot Optimistic 0.01 0.04 Rivierenland 0.02 0.14 Drents Overijsselse Delta 0.11 0.38 Pessimistic 0.16 0.65 Grass revetment reinforcement measures for known weak spots are only potentially effective if the material, personnel and equipment are prepared and the construction rate is such that the measure can be installed in approximately 10 hours. The reliability is dominated by the placement failure probability and the effective- ness is bound by the placement capacity. The resulting effectiveness is dependent on the level of prepared- ness, exercises, material availability and time constraints. In the case study are average reduction values for the conditional failure probability of the flood defence found of 32 for an optimistic and 4 for a pessimistic scenario. Be aware that the pessimistic scenario still assumes a certain amount of preparation. Hence, the factor 4 in failure probability reduction is not reached without preparation. The effectiveness for grass revetment reinforcement measures applied for unknown weak spots is less ef- fective due to a larger value for the failure probability of the reinforcement measure and time required for installation. The resulting effectiveness is dependent on, amongst others, the detection phase, prioritization, knowledge level and training. In the case study are average reduction values for the conditional failure prob- ability of the flood defence found of 14 for an optimistic and 1.0 (no reduction) for a pessimistic scenario.
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