Low Crested Reef Breakwaters - Theory Illustrated by Arabian Gulf Projects

Abstract

Low crested reef type breakwaters are becoming increasingly popular for coastal protection. As they are entirely offshore, there are no structures on the beach to impact the amenity and safety of the beach itself. They may also offer attractive marine habitat and recreational amenity benefits. Their effectiveness increases with wave height. Fully submerged and semi-submerged low crested structures such as “reef” type breakwaters behave differently from conventional fully emerged structures and require appropriate design methods. This has restricted the use of low crested structures by designers without the design expertise in this field. Design parameters include crest level, length, width and distance offshore. Typical emerged breakwaters have a wave transmission coefficient, however, reef breakwaters reduce the wave height [1> Kt >0] rather than stopping all of the wave energy in the lee of the breakwater. To add to the complexity, Kt for a specific structure varies with both wave and tide conditions. However, design formulae have been developed from physical modelling and as well as monitoring the behaviour of artificial reef breakwaters and investigations of naturally occurring nearshore reefs found along coastlines and around atolls worldwide Submerged and semi-submerged “reef” breakwaters have proven to be a simple and robust coastal management tool for Arabian Gulf wave and tide conditions. Construction of low crested structures such as reef breakwaters has been assisted by the development of durable mega geo-containers as they are stable even with the high wave forces on the crest. Two coastal protection reef breakwater designs for application in the Arabian Gulf are reviewed.