Velocity and water-layer thickness of overtopping flows on sea dikes: Report of measurements and formulas development
The study aims to establish a set of formulas estimating velocity and depth of overtopping flows at sea dikes including seaward-side slope, crest and landward-side slope. These formulas are applicable to flows which are generated by natural waves and the simulator giving associated characteristics of any arbitrary value of exceedance probability. Note that the probability is based on the total number of incident waves and not to be confused with the total number of overtopping events. The difference between the fictitious wave run-up level and the crest level (Ru2% - Rc) is widely considered as a measure of the (potential) energy of wave overtopping. This term is therefore used to estimate values of the mean overtopping discharges, ow velocities, water-layer thickness and overtopping volumes. In line with many existing research works and guidelines, the potential term is crucial to derive other parameters. Experimental data of Van Gent [2002] is reanalyzed with a slightly different perspective to achieve the study objective. The analysis procedure basically consists of three main steps:  from wave conditions and dike cross-section to calculate wave run-up levels;  from wave run-up levels to estimate flow parameters on the dike crest;  from flow parameters on the dike crest to further compute the corresponding parameters on the landward slope. The obtained equations are verified with flow measurements of the simulator on real dike slopes. Consequently, feasible values representing the roughness of different grass coverings are determined.
Design, construction, calibration and use of the wave overtopping simulator
The wave overtopping simulator is a device which is able to simulate overtopping waves at the crest of a dike and at the inner slope. The device has been used in the ComCoast project to test a traditional dike section with grass and the SGR, the Smart Grass Reinforcement, placed in May 2006. This report describes in the first part (Chapters 1-6) the design, construction and calibration of the 1 m wide prototype of the wave overtopping simulator, as it was performed from May July 2006, under the ComCoast programme. This first part has also been reported as a final version 2.4 in November 2006. The second part continues with the actual construction and testing of the real wave overtopping simulator and the use of it during testing at the dike at Delfzijl, all under the ComCoast programme. Further, the measurements itself and analysis of velocity and flow depth during this testing has been described. Also the work of Bosman, as performed in this project as a MSc-thesis, has been summarised. This second part describes the period from November 2006 to August 2007.
THE ERODIBILITY OF GRASSED INNER DIKE SLOPES AGAINST WAVE OVERTOPPING
INTRODUCTION Dutch river dikes, the core material of which usually consists of clay are covered by grass on both the inner and outer slopes. Sea dikes with hard revetments in the wave impact zone of the seaward slope also have a grass cover on the crest and the inner slope. The grass cover includes grassland vegetation rooted in soil, where two layers can be distinguished: the top soil and the subsoil (Fig. 1, Muijs 1999). The porous turf has a high root density and is elastic in moist conditions. The root structure holds the small clay aggregates together and prevents them from being washed out, whereas the underlying substrate, is stiff or plastic and less permeable. For clay, relatively large forces are necessary to break the aggregates, while relatively small forces are necessary to transport the aggregates.