Stability of single layer armour units on low-crested structures

Abstract

Two dimensional physical model tests are executed with Xbloc armour units on low crested structures to answer the objective. On the basis of findings in the literature study it can be expected that the stability of the Xbloc elements on low-crested breakwaters is a function of crest freeboard and crest width. The crest freeboard (Rc/Dn) varied from -0.8 to 0.8 in steps of 0.4 and the tested crest widths (Wc) are 3 and 9 armour units wide. Additionally all tests series are executed with a wave steepness of 2 and 4%. All test series are executed once except the reference test series (Rc/Dn = 0, Wc = 3) which are repeated four times to acquire insight into the reliability of the test results. The number of rocking and displaced armour units is registered for the total breakwater, seaside slope and crest (also termed breakwaters sections). Settlements at both the sea- and leeward slope leads to openings in the armour layer at the transition from the seaside slope to the crest. As a consequence the interlocking properties of the upper part of the seaside slope and crest decreases and the area of the Xbloc crest elements normal to the wave induced flow increases. Moreover due to settlement the distance between two succeeding rows at the upper part of the slope increases whereas for the lower part of the slope it decreases. This together with the already decreased interlocking properties of the upper rows at the slopes and crest rocking results at both the upper part of the seaside slope and the outer seaward rows of the crest.