23 October 2015
Far from being fixed in one place for ever and ever, beaches are constantly on the move under the influence of the weather. At Shoreham and Southwick, the ‘prevailing wind’- the wind we get most of the time, comes from the south west and because the wind builds waves on the sea, that’s where the waves mostly come from.
Waves landing on the beaches move shingle around and keep the sea foreshore – the littoral zone to engineers – in a constant state of dynamic motion. A beach that moves under the action of waves is the most efficient form of coastal defence as it absorbs the energy of the waves rather than trying to resist it like the Port harbour arms and the Brighton Marina seawalls have to do.
When fixed structures like harbour arms and seawalls are exposed to storm waves they have to resist enormous forces and are liable to suffer damage or even be breached. Shoreham Port’s goal is to maintain the beaches and keep them in the same place but without fixing them down in some way so that they couldn’t absorb all the wave energy.
However, because of the prevailing wind, there is a tendency for the beach movement to be predominately towards the east on our coast and regular surveys by Adur & Worthing Council show that an average of 16,000 cubic metres of beach shingle move eastwards past the frontage every year. This process is known as ‘longshore drift’ or ‘littoral drift’ and must be managed to ensure that the entire frontage remains properly protected from storm damage and flooding.
Tony Parker, Director of Engineering at Shoreham Port commented “This summer and autumn, with the help of Adur & Worthing Council, we have been rebuilding eight groynes on Southwick Beach which were badly damaged during the winter storms of 2013 - 2014. Now that the groynes have been rebuilt, the shingle can be replaced in between them to protect the seawall again. This process is known as a ‘beach feed’ and our team carry this out once a year to ensure our beaches are kept in the best possible condition.”
The purpose of groynes is to hold a beach in front of a solid structure, in this case a seawall, so that it can absorb storm wave energy. The groynes don’t prevent littoral drift; they just move it a little towards the sea and away from the seawall.
When the groynes were damaged, it allowed the beach shingle in front of them to escape so that the seawall behind was exposed to the danger of storm damage. The Port’s contractor, Matthews (Sussex) Ltd, are currently moving 35,000 tonnes of shingle that has accumulated against the western harbour arm over the last 10 months or so and depositing it between the new groynes.
