Quantification of Grass Erosion Due to Wave Overtopping at the Afsluitdijk
Total Page:16
File Type:pdf, Size:1020Kb
Quantification of Grass Erosion Due to Wave Overtopping at the Afsluitdijk Martijn Kriebel July 2019 Cover photo: the Afsluitdijk in the direction of Friesland, as seen from Den Oever (Ministerie van Economische Zaken, 2015). Quantification of Grass Erosion Due to Wave Overtopping at the Afsluitdijk Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Water Engineering and Management at the University of Twente. To be publicly defended on 12 July 2019. Author: Martijn Kriebel [email protected] Graduation committee: Prof. dr. S.J.M.H. Hulscher University of Twente Marine and Fluvial Systems Dr. J.J. Warmink University of Twente Marine and Fluvial Systems V.M. van Bergeijk MSc. University of Twente Marine and Fluvial Systems Ing. C. Kuiper Witteveen+Bos Coasts, Rivers and Land Reclamation Ir. H. Trul Witteveen+Bos Flood Protection and Land Development SUMMARY Grass erosion on the crest and inner slope due to wave overtopping is one of the mechanisms that can cause a flood defense to fail according to the Dutch WBI 2017 safety standards. The probability of failure of the grass cover layer for a certain grass quality is determined based on the combination of significant wave height and average overtopping discharge. However, this relationship is only established for wave heights up to 3 meter. At the time of writing, a new design is being finalized for the Afsluitdijk, which is one of the primary flood defenses in the Netherlands. The normative significant wave height for this new design is 3.38 meter, thus exceeding the maximum wave height for which the probability of failure due to grass erosion is defined in the WBI 2017. Previous studies have used the cumulative overload method to quantify the combinations of wave height and critical overtopping discharge for wave heights up to 4 meter, but the effects of the geometrical and cover material transitions present on the Afsluitdijk have not been studied in detail. The goal of this study is to find the relationship between the significant wave height and the critical average overtopping discharge for wave heights larger than 3 meter. This is done using two approaches: (1) the cumulative overload method (COM) and (2) a combination of the coupled crest- inner slope velocity equations (VE) and the transition model (TM). However, the basic modelling approach that is used in this study could be applied to other models that can predict the amount of erosion due to wave overtopping. Simulations are carried out with significant wave heights up to 4 meter for the new design of dike section 17a of the Afsluitdijk. Several different cross-sectional locations are included in order to study the effects of transitions. Additionally, sensitivity analyses are performed to study towards which parameters the COM and the VE-TM are most sensitive and which could cause the largest variation in the found relationships. Both the COM and the VE-TM predict an increasing critical average overtopping discharge for a decreasing significant wave height for all cross-sectional locations. In all simulations the inner toe is predicted as the weakest cross-sectional location, with a minimum critical average overtopping discharge of qcrit. = 3.4 L/s/m (COM) and qcrit. = 1.4 L/s/m (VE-TM) for a significant wave height of approximately 4 meter. For locations on the crest, both modelling approaches predict a similar development of the flow velocity for a changing wave height, resulting in approximately equal relationships between the critical discharge and the wave height. For the other cross-sectional locations the predicted relationships are not the same, as the effects of a changing inner slope length on the hydraulic load at these locations are predicted differently in the two approaches. Furthermore, the COM predicts that a grass-to-asphalt transition is able to withstand a larger average overtopping discharge than an asphalt-to-grass transition. However, the VE-TM results show the opposite. This can be attributed to the fact that the local turbulence at these transitions is not accurately calculated in the VE-TM. Based on the findings in this study, it is recommended to carry out (scaled) wave overtopping experiments for wave heights larger than 3 meter that focus on finding the critical average overtopping discharges for the inner slope and the inner berm. The obtained data from these experiments can be used to validate the simulation results of this study. Additionally, this data may result in a better estimation of the flow velocity on the crest, which is subject to large uncertainties and towards which the COM and the VE-TM are sensitive. Furthermore, it is recommended to apply a more detailed turbulence model in the VE-TM in order to obtain more realistic critical discharges for cover material transitions. Finally, in order for the simulation results to be included in the WBI 2017, the modelling approach in this study should be modified so that a distribution for the probability of failure can be found for waves larger than 3 meter. MSc. Thesis – Martijn Kriebel i PREFACE In front of you lies my master thesis “Quantification of Grass Erosion Due to Wave Overtopping at the Afsluitdijk”, which is the final result of the research that I carried out in the last five months at Witteveen+Bos and marks the completion of my study Water Engineering and Management at the University of Twente. I would like to thank Hizkia Trul for his daily guidance and for giving me the opportunity to write my thesis at Witteveen+Bos, and Coen Kuiper for providing me with new knowledge and information regarding the Afsluitdijk. Furthermore, I grateful to thank Suzanne Hulscher, Jord Warmink and Vera van Bergeijk for their valuable feedback and their supervision throughout my graduation period. Finally, I want to thank my family and friends who have supported me throughout my study. Martijn Kriebel Enschede, July 5, 2019 MSc. Thesis – Martijn Kriebel ii TABLE OF CONTENTS Summary .................................................................................................................................................. i Preface .................................................................................................................................................... ii Table of Contents ................................................................................................................................... iii 1. Introduction .................................................................................................................................... 1 1.1 Problem Context ..................................................................................................................... 1 1.2 Study Objective ....................................................................................................................... 3 1.3 Study Area ............................................................................................................................... 4 1.4 Thesis Outline .......................................................................................................................... 5 2. Theoretical Background .................................................................................................................. 6 2.1 Wave Overtopping .................................................................................................................. 6 2.2 Grass Erosion........................................................................................................................... 7 2.3 Transitions ............................................................................................................................... 8 3. Methodology ................................................................................................................................. 10 3.1 General Approach ................................................................................................................. 11 3.2 Hydraulic Boundary Conditions ............................................................................................ 12 3.3 COM Modelling Approach..................................................................................................... 20 3.4 VE-TM Modelling Approach .................................................................................................. 25 3.5 Comparison of Modelling Results ......................................................................................... 31 3.6 Sensitivity Analyses ............................................................................................................... 31 4. Results ........................................................................................................................................... 36 4.1 COM ...................................................................................................................................... 36 4.2 VE-TM .................................................................................................................................... 38 4.3 Comparison COM & VE-TM ................................................................................................... 40 4.4 Sensitivity Analyses ............................................................................................................... 42 5. Discussion .....................................................................................................................................