Modelling wave overtopping for flood defence reliability, the outline of a research project.

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(1)MODELLING WAVE OVERTOPPING FOR FLOOD DEFENCE RELIABILITY, THE OUTLINE OF THE RESEARCH PROJECT Jord J. Warmink, University of Twente, j.j.warmink@utwente.nl Vera M. Van Bergeijk, University of Twente, v.m.vanbergeijk@utwente.nl Weiqiu Chen, University of Twente, w.chen-6@utwente.nl Suzanne J. M. H. Hulscher, University of Twente, s.j.m.h.hulscher@utwente.nl ABSTRACT Critical erosion of the dike cover due to overtopping waves is currently defined using conservative estimates. To assess failure probability, the actual erosion rate during overtopping is required, however the effects of dike geometry, vegetation type or objects on a dike are largely unknown. In this research project, a numerical model will be developed to evaluate measures to minimize erosion, and provide insight in the effect of transitions between grass cover and embedded objects. INTRODUCTION Failure of a dike cover due to wave overtopping erosion may initiate dike breach (Figure 1). Surface transitions in the grass cover, such as cure points, height difference, roughness difference and objects are often weak spots (Dean et al. 2010, Van der Meer et al. 2014), but the effects on the location and evolution of dike cover erosion is highly uncertain. Dike cover erosion is dominated by the turbulence-dominated shear stress at the jet front (Aguilar-Lopez 2016). Therefore, a detailed FEM model is required with a sufficiently accurate turbulence model. In this project the model of Bomers et al. (2016) will be further developed, tested and applied to evaluate protection measures to provide insight in the effect of hard constructions on dike cover erosion. At the University of Twente, two PhD students recently started on the challenge of quantifying the effect of transitions in grass covered dikes on dike erosion. In this presentation, we present their research plans. RESEARCH OBJECTIVES Both the PhDs work in the framework of the recently granted All-Risk program: implementation the new riskbased standards in the Dutch Flood Protection Program (HWBP). This program is a joint effort of 6 Universities in the Netherlands to generate knowledge to support the implementation of the fully probabilistic risk-based flood protection strategy. The topic of PhD1 is to assess the impact of wave overtopping using field and flume experiments. PhD2 focusses on the numerical modelling of this phenomenon. The objective of this research project is to develop tools for probabilistic safety assessment due to dike cover erosion for a range of grass types and transitions: RQ1: How well can a detailed numerical model predict dike cover erosion for a range of grass cover states and transition configurations? RQ2: How do vegetation and transitions affect the probability of failure of the dike cover? RQ3: To what extend can innovative technical measures and maintenance strategies reduce the probability of failure of dike cover: locally around transitions, and for the dike as a whole?. RESEARCH METHODS The FEM model will be validated using the flume and field experiments and subsequently applied to evaluate measures to minimize erosion, provide maintenance guidelines and provide a tool for probabilistic dike safety assessment. Currently, the failure probability of transitions cannot be assessed, because the required tools do not exist. Furthermore, the effect of different maintenance strategies, such as mowing, mixing vegetation types, grazing (cattle trampling) that all affect the grass cover quality and soil compaction are assessed.. Figure 1 – Picture of Wave overtopping of a grass covered dike at Hartlepool, UK (Source: HR Wallingford).. ACKNOWLEDGEMENTS This work is part of the research programme All-Risk with project number P15-21 which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO) REFERENCES Aguilar-Lopez, J.-P. (2016) Probabilisitc safety assessment of Multi-functional flood defences. PhD thesis, University of Twente. Bomers et al. (2016). Modelling erosion development during wave overtopping of an asphalt road covered dike. Proc. FloodRisk 2016, Lyon, France Dean et al. (2010). Erosional equivalences of levees: steady and intermittent wave overtopping. Ocean Engineering 37:104-113 Van der Meer, J. et al (2014) Wave overtopping simulator tests on transitions and obstacles at grass covered slopes of dikes, Coastal Engineering Proceedings 01/2014.

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