Design of rubble-mound foundations for vertical seawalls: scour, screed layer and berm width

ENGLISH ABSTRACT: Coastal erosion has always been a challenging problem to solve for coastal engineers since it is such a complex process that threatens valuable environments and properties along the coastline. Structural erosion, or scour, develops due to the presence of a marine structure that interrupts the natural sediment transport processes. Scour is one of the most common mechanisms of failure of marine structures and therefore it is vital to gain a better understanding of the scour process in order to design marine structures that can withstand the adverse effects of scour. The objective of this study is to gain more knowledge of the scour process that develops in specifically the screed layer directly underneath the concrete elements of a vertical seawall that is protected by a rubble-mound berm. This was accomplished by investigating existing literature on marine structures, their failure mechanisms, the processes of sand and granular scour as well as research on physical modelling. A physical experiment was then set up in a 2D wave flume in the hydraulic laboratory of the CSIR in Stellenbosch, South Africa to test the influences of different aspects of the scour process in the screed layer underneath a model seawall. The influence of the wave period, the rubble-mound berm width, the screed layer thickness, the armour rock stability and the reflection coefficient was investigated. The scour damage in the model screed layer was measured with wooden dowels, as was done in previous research, as well as with a new method developed by the author that uses sonar technology to create a submerged image of the scour pattern that developed in the screed layer. The scour measurements for each different test set-up were analysed. Firstly, it was found that a shorter wave period resulted in more scour damage since the interaction between the incident and reflected waves was more significant near the seawall and occasionally superimposed due to the rapid change in wave direction and orbital velocities, which disturbs the screed material. The rubble-mound berm width, however, did not have as significant an influence on screed layer scour as expected. A wider berm did provide more scour protection, but the optimal berm design must balance protection and construction and material costs and therefore a narrower berm of 4Dn50 is recommended. A minimised screed layer thickness of 100 mm is recommended since it resulted in the least amount of scour whilst still being able to be constructed at an adequate accuracy. As expected, a more stable armour layer resulted in less scour damage in the screed layer. Lastly, it was interesting to observe that a test that resulted in less scour, had a higher reflection coefficient. This is probably due to less wave energy that was absorbed by the berm (causing scour in the screed layer) but reflected back seaward instead.