Experimental study on the wave overtopping performance of Xbloc+ armour unit

Abstract

Nowadays, the use of concrete armour units on rubble mound breakwaters designs has become a common practice. Concrete armour units can be placed in a single layer system or in a double layer system and can its placement can be either random or uniform. Recently, Delta Marine Consultants have designed a new armour unit called Xbloc+. This new block is applied as a one layer system and has a regular placement. As Xbloc+ is still under development, preliminary guidance on the performance of this new armour unit is required. This led this research to investigate how Xbloc+ behaves concerning wave overtopping. To analyse wave overtopping, small scale tests were performed in a 2D wave flume. Wave overtopping was measured at a 3Dn distance from the seaward edge of the crest, which is made of rock. In front of the slope there is a flat transition of 0.3m followed by a sloping foreshore of 1/30. In total, 10 series of tests were conducted. In this research, three wave steepness (Sop) were tested (0.02, 0.04 and 0.06) to see the effect of wind waves and swell conditions on the armour layer and tests were performed in two different slope angles (1/2 and 3/4). Each series is formed by several sub tests conducted with increasing wave heights (and wave period in order to maintain a constant wave steepness). Tests were carried out until the failure of the armour slope was reached. Besides, a smooth slope was tested in a 1/2 slope angle to be able to compare smooth and rough results and determine the roughness coefficient of the armour unit more accurately. The test results showed that wave overtopping rates increase exponentially with wave height (Hm0) and wave period (Tm-1,0). Moreover, waves with lower wave steepness (swell conditions) induce higher overtopping discharges as compared to larger wave steepness. Steeper slopes also lead to higher overtopping rates. Therefore, the larger the breaker parameter is, the larger the overtopping discharge results. The influence of the armour roughness and armour permeability is also analysed and it is observed that swell conditions are less affected by these properties. This can be explained by means of the breaker type. Swell conditions are characterized by large surging waves. These waves have a thicker water tongue as compared with other type of breaking (e.g. spilling or plunging) and therefore, they tend to feel the top layer “smoother”. This fact is supported by the analysis of the influence of the roughness coefficient since higher values are obtained for surging waves. In fact, the roughness coefficient ranges between 0.34 and 0.68 showing that, although empirical prediction consider this parameter as a constant, it is dependent on the wave conditions.
Concerning the comparison of dimensionless overtopping rate over Xbloc+ armour between test results and empirical prediction, it was found that the present formulae does not give a good estimation due its simplicity. In order to improve the representation of some parameters such as slope angle and wave steepness, correction factors are introduced in the empirical formulae leading to a better fit between prediction and measured data.