Stability of Single Layer Cubes on Breakwater Rear Slopes

Abstract

Breakwaters with single layer concrete armour are very commonly applied nowadays. Much research is available on the stability of single layer armour units on the seaward slope. However, rear slopes design methods for single layer armour units are rare. On rear side slopes usually similar sized units as on the seaward slope are applied. Limited design methods are available for the stability of concrete armour units on the rear side of a breakwater. To obtain stability information on single layer cubes on the rear side, scale model tests have been performed in the wave flume of the University of Technology Delft. The purpose of the model tests is to obtain a relation between the overtopping wave characteristics (overtopping volume of an individual wave event and the overtopping wave front velocity) and the stability of the cubes at the rear slope of low crested breakwaters. This relation can lead to a more optimal rear side breakwater design. The test programme consists of different configurations, in which the following parameters have been varied: - Wave steepness: three typical values for steepness have been tested s_op=0.015, 0.027 and 0.039; - Relative crest width: relative crest width ranges were W_c/H_s= 1.1 6.3; - Relative free board: relative free board ranges were R_c/H_s= 0.4 1.3; - Rear slope angle: two slope angles of 1:2 and 1:1.5 have been tested; - Packing density; packing densities of 73% and 69% have been tested A total of 11 breakwater configurations have been tested. Each configuration was tested with a sequence of waves with increasing wave height until failure of the armour layer occurred. The focus was on the determination of start of damage of the rear armour cubes, but also on filter material wash out. Together with the stability tests, wave overtopping volume tests have been performed. The overtopping volume per wave was measured in a special designed box. The overtopping wave front velocity at the crest is measured by correlating the signals of two wave gauges. The number of displaced cubes in the rear slope armour layer has been determined with image analyses and expressed in the damage number N_od. The influences of the various parameters and overtopping characteristics on the rear slope damage are presented in stability graphs. Due to lack of sufficient data points, no trend line could be established. However, the graphs can be used as a guideline for conceptual breakwater design. The critical overtopping characteristics are in a wide range. Therefore, the quantitative influence of the overtopping velocity and -volume on the rear slope damage is unclear. This suggest that other aspects may influence the stability of the rear slope, for example the transmission through the breakwater or the shape of the overtopping flow around the rear slope. The analysis of the test data shows that damage on the single layer cubes on breakwater rear sides occurs later than expected. The expected cube dimensions were calculated for various configurations. First, the armour dimension of randomly placed rock is calculated with the formula of van Gent and Pozueta [2004]. Secondly, the difference in stability of randomly placed rocks and regular placed cubes was estimated applying a factor obtained from the known stability numbers on seaward slopes of both armour layers. The estimated cube dimension was used in a first test series. During this test series (almost) no damage was observed. Therefore, the remainder of the experiment was carried out with a 37% smaller cube diameter. The damage seems to be limited to first rows below the water level, which may indicate that cubes on the rear side below this level could be lighter.