Rear-Slope Revetment Stability Approached by the Wave Overtopping Simulator

Abstract

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.