WTI Onderzoek en ontwikkeling landelijk toetsinstrumentarium : Product 5.12 Analyses grass erosion in wave run-up and wave overtopping conditions

Within the framework of WTI, Research and Development of Flood Defence Assessment Tools (Wettelijk Toetsinstrumentarium), which is called in Dutch: WTI-2017 “Onderzoek en ontwikkeling landelijk toetsinstrumentarium”, research was carried out to develop an erosion model for grass in the wave run-up zone consistent with the current model in use for the wave overtopping zone. In order to use the erosion model for wave overtopping in the wave run-up zone it was necessary to re-evaluate all wave overtopping tests and to incorporate the flow velocity increase on the slope. This led to a re-evaluation of previously measured critical velocities Uc and cumulative overload D. To validate the erosion model in the wave run-up zone a wave run-up simulator was constructed and tested. The wave run-up generated by the simulator was measured at a test location near the village of Colijnsplaat (Zeeland, The Netherlands). The simulator performed well and four 2 meter wide test strips on the dike were tested with an increasing wave overtopping load. Strom conditions were simulated with a Hm0 of 2 m and a (simulated) water level increasing to just below the transition between the hard revetment and the grass revetment. The grass proved to be very erosion resilient and (unfortunately) did not fail. The validation was therefore not optimal, however the proven strength of the grass has added to the confidence in the model. Apart from the large scale erosion tests small sod pull out tests were performed. The pull-out tests were carried out ‘in one go’ and fatigue tests were done at ca. 80% of the maximum stress with up to 100 repetitions. The small tests will hopefully link this measurable field parameter to the grass.