Composite Beaches - Can we protect our coastlines by mimicking nature?

Composite beaches consist of a lower foreshore of sand and a naturally occurring permanent or semi- permanent backshore ridge of cobbles, and are ubiquitous beach environments in the UK, USA and NZ. These beaches are known to be highly stable because they combine a dissipative sand slope and a reflective cobble berm, which represent the two most stable end members in the classification of beach morphodynamics. Furthermore, when a permanent cobble ridge exists it can play a significant natural coastal protection role, stabilising the upper beach and providing overtopping protection to the hinterland. As a result, constructed cobble berms (or dynamic revetments) designed to mimic composite beach cobble ridges are being trialled to provide coastal defence at vulnerable beaches on the US West Coast with some success. Despite the common occurrence of composite beaches and their value as a natural form of coastal defence they have received very little attention in academic literature with almost no composite beach- specific studies being undertaken to date. As a result, understanding of composite beach processes such as cross- and longshore cobble transport, wave runup and overtopping of cobble ridges and response to sea level rise is minimal.This project aims to fill this identified research gap and undertake one of the first detailed studies into composite beach behaviour. This work will enhance understanding of this valuable beach type and gain insight which will enable the design nature-based coastal defence structures which mimic composite beach behaviour to provide sustainable and low-cost coastal protection.