Low Cost Rock Structures for Beach Control and Coast Protection

Abstract

Coastal rock structures are widely used in coastal engineering for a variety of purposes, including controlling the morphological development of beaches and providing protection against coastal erosion or flooding by wave overtopping. Strict adherence to existing design guidance has resulted in many of these structures being built using multiple layers of different rock sizes, high quality imported rock and carefully prepared foundations. Some innovative structures have, however, used locally available rock with simpler cross-sections placed on unprepared foundations, apparently without significant reduction to the overall performance of the scheme. This report gives guidance from a short research project which examined practical experience on rock structures from around the UK, with particularly emphasis on those that depart from conventional design rules. The report demonstrates that there are opportunities for lower cost rock structures for beach control and coast protection. Established design guidance provides a good degree of confidence in predictions of performance of coastal structures, but it is widely perceived that simple design rules can be overly prescriptive, particularly for nearshore structures in shallow water depths. The opportunities for lower cost structures principally relate to improved assessment of armour size for depth-limited waves, reduction in armour size for closer armour packing, and the need for complex underlayer / filters. The report emphasises the need to understand the performance of individual structures in the context of the overall scheme and ultimately national objectives, which provides an incentive to re-explore the balance between cost and structure performance. It also encourages the consideration of cost issues during the design of rock structures. Although the use of lower cost structures may also provide safety and environmental advantages, the structures described are envisaged to be of greatest benefit in locations where conventional structures would not be economically justified.