The underscour of rubble mound breakwaters by wave action

Master Thesis


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University of Cape Town

Rubble mound seawalls, groins and breakwaters are still the most common type of shore-protection structures currently in use. Major reasons include: easiness to construct and repair, flexible with respect to settlement, favourable wave energy dissipation, fitness for any water depth and foundation, and because of their economical nature if rubble-stones are readily available. A complete failure of the rubble mound structure can be expected if the stone gradation is improper or if filters are not provided, or are improperly constructed to specification. Many failures have been attributed to internal erosion whereby beach materials are removed by percolating water, such as that due to water waves and surface run-off. Washing-out of the backfills, settlement of the main structure and overtopping of the subsequent waves will follow eventually. Two-dimensional model experiments have been conducted in order to clarify the basic characteristics of sand scour under dolos-protected rubble mound breakwaters, and to investigate some preventive measures against the scouring process. The inter-relationships of parameters associated with the scouring process have been investigated. Sand scour in a three-dimensional model has been discussed, and design procedures associated with Rubble Mound Breakwater and Dolos concrete armour units have also been included. This thesis is also intended to call the attention of researchers to consider foundation erosion, and protection by filter systems, in their future experimental studies on stability of rubble mound structures.

Bibliography: pages 123-125.