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.
Rear-Slope Revetment Stability Approached by the Wave Overtopping Simulator: Experiments Regarding the Stability of Single-Layer Cube Revetment on Breakwater Crests and Rear Slopes
Contrary to a breakwater outer slope, rear-slope revetment does not have to withstand breaking waves. However, generally, an equal dimensioned revetment is used on both the inner- and outer slope. Although with low-crested breakwaters severe overtopping occurs, the application of similarly sized armour on both sides still results in an over-dimensioned inner-slope revetment. An (economical) optimisation is possible in this regard. The tests executed by Hellinga (2016) in this topic, showed large scatter in the found stability results of single-layer cube revetment at the rear slope. This scatter was party introduced by the large number of inﬂuencing factors in the test scope (Hellinga, 2016). By developing an overtopping simulator, the scope of the model-tests is delimited to solely the breakwater crest and rear slope. Moreover, larger scale tests are feasible in the same ﬂume, reducing scaling errors. The main objective of this thesis is a) determine if the approach of a (semi-) small scale overtopping experiment using an overtopping simulator is increasing the reliability of the test results, and (b) to increase the understanding of failure mechanisms of single-layer cube revetment on the crest and rear side of low-crested breakwaters. To design and construct a simulator that is able to generate realistic waves, the relevant mechanisms of overtopping waves were determined and boundary conditions were composed. The behaviour of the simulator was tested in combination with a ‘mock’ breakwater. Seven types of realistic breakwater models were installed to investigate both the hydraulic behaviour of the overtopping volume and the stability behaviour of the revetment. The determination of the layout of a conﬁguration is every time well substantiated by the results from the former conﬁguration. The water level inside the Wave Overtopping Simulators (WOS) reservoir showed a skewed drop during the release of an overtopping volume. Consequently, the water level measured at one single point was not accurate to determine the volume (change) inside the reservoir, and hence the discharge and velocity. Diﬀerent test conﬁgurations failed, where others remained stable during exposure to simulated waves in the test range. Compared to stability numbers of the theoretical front side, the cubes are relatively stable. The transition of the crest and rear slope showed to be a weak point, and outwash of the rock material ﬁlling the gap is a signiﬁcant possibility. Improvements on the WOS are possible to enhance the ability to simulate the characteristics of a realistic overtopping wave volume and to give a more detailed evaluation of the magnitudes of these overtopping characteristics. The normal forces between the cubes at the slope are more important for the stability than the own weight of the cubes. The lack of interaction between the cubes at the crest results in a signiﬁcantly lower resistance to hydraulic loads compared to the cubes at the rear slope. The layout of a sharp inner crest-line transition, if well designed, showed a stable alternative for a rounded transition.
Wave overtopping resistance of grassed slopes in Viet Nam
The Simulator was applied to test the resistance against wave overtopping of grass covered dike slopes in Viet Nam. Observation and measurement during destructive tests were performed to investigate the development process of damage induced by overtopping flow. Damages were likely to be initiated at transition of either geometry or materials, at eroded holes and around objects existing on the slope. Grass cover could withstand a certain overtopping rate that varies widely and in order of 10 l/s per m.