Southeast Regional Coastal Monitoring Programme

Seawalls

Beaches fronting seawalls can respond in a number of ways. When the beach is broad and high, dynamic equilibrium beach profiles may be able to form under virtually all wave conditions and there may be little or no interaction at all with the seawall. As the beach becomes smaller, the frequency of beach/wave/structure interaction increases. Overtopping of sea walls may occur, resulting on flooding and beach material may also be thrown over the seawall. Coastal flood forecasting models are unable to deal with beach morphodynamics and beach structure interaction. Data is required to develop these models, particularly following severe storm events. Regular data is needed for beach management to establish trend analyses, to ensure that the performance of the beach and seawall system is capable of providing the necessary standard of service and continuing to do so. Beaches fronting seawalls are generally lower risk sites than barrier beaches, but can be high-risk sites where flood risk is a significant problem. The required spatial data frequency depends upon changes in orientation and consistency of the beach system fronting the seawall.

Overtopping of beach fronted seawall with narrow beach

Beaches tend to be more volatile when they form part of a system that includes a vertical seawall structure; these are highly reflective and can result in (at least) short term lowering of the beach adjacent to the seawall.

Seawall - Totland Bay
Volatile beach profile response in front of vertical wall

Although stepped seawalls are slightly more dissipative than vertical walls, they are still largely reliant upon a beach to reduce overtopping or scour and similar levels of information are required as for vertical seawall systems.

Stepped seawall at Wheelers Bay IOW

As the size of the beach fronting a seawall decreases, the possibility of beach scour increases; this can eventually result in undermining and collapse of the seawall; this is the most common mode of seawall failure. This particular problem is becoming increasingly evident at sites where hard defences have been constructed, without beach recharge or beach management measures. Monitoring of beaches fronting seawalls needs to be sufficiently frequent to identify scour trends and responses of the dynamic system to storm events.

Scour exposing foundations of seawall

In some instances scour in front of vertical seawalls has resulted in reduction of the seawall stability; this may be critical when the wall forms toe weighting and restraint from sliding as part of a cliff stabilisation system. Rock armour is often used to improve stability in front of these seawalls. Whilst this approach can be very effective rock armour is expensive. Adequate monitoring of the beach response in front of the seawall can provide the necessary information required to intervene prior to the need for major rehabilitation.

Rock armour and toe weighting - Folkestone Warren