Coastal Features - Management

EROSION – TRANSPORTATION - DEPOSITION

Factors at work on coasts

There are two different approaches to defend against coastal erosion and flooding HARD ENGINEERING and SOFT ENGINEERING

How can we defend our coast against the sea?

• By ~ building sea walls, breakwaters, harbours, groynes, and reclaiming land will all affect the amount and rate of erosion and deposition.

Note wider beach this side of the groyne.

Sea walls reflect a large proportion of the wave energy, which can cause further erosion of the beach and ultimately the potential undermining and collapse of the wall. They need a lot of expensive maintenance, but are useful in protecting coastal towns and other built up areas.

Sea Wall- defence & erosion

Gabions Gabions are generally galvanised steel wire mesh baskets filled with rock (often local flint in North Norfolk). The size of the basket vary but they are typically 2m x 1m x 1m and are rectangular cubes . Gabions are normally used on eroding cliff faces where their purpose is two fold. Firstly they provide stability to the cliff face, preventing failure, and secondly they permit water to drain away that might otherwise contribute to landsliding. Gabions are relatively cheap and easy to construct but they are not suitable for use in locations exposed to wave action because they are too flexible and are easily destroyed.

Managing a coastline with gabions

Managing erosion with Revetments

This section is made of wooden slats that allow the water through, but break up the wave energy, not reflect it.

•Timber Revetments were first constructed as cheaper alternative to a solid seawall. The Revetments are constructed from timber piles driven into the beach and connected by a sloping lattice of timber planks. Sediment gets deposited behind the revetment to increase the amount of beach and so the natural defense for the cliffs. This has been use effectively on the cliffs below the BP terminal at Easington

•The Revetment is designed so that it will break the force of the wave without reflecting the energy which might cause scouring of the beach. A Timber Revetment reduces the wave energy available to attack the cliffs but it does not prevent erosion from taking place. •In addition, sediment, which can be sand, shingle or pebbles, is carried through or over the revetment structure by an incoming wave. However, the sediment cannot easily return seaward once the force of the wave has been dissipated. This action results in a build up of material behind the revetment and the increased depth of material protects the foot of the cliff from being attacked by the sea.

Photo of revetments

The wooden slats can be clearly seen here.

The result of using revetments.

New beach has clearly been deposited here, eventually adding protection to the cliffs behind. Once this becomes stabilised with plants it will be even better.

Groynes have also been used.

Managing erosion with Rock Armour • Rock or rock armour has become increasingly important in coastal defence works over the last 20 years or so. The rock used will typically be a very hard igneous rock that is exceedingly resistant to erosion. When rock is placed on the beach it is arranged so that large void spaces are left. When waves hit the rock they are only partially reflected, and can flow round the rocks, expending their energy less quickly which reduces scour. Another advantage of using rock is that if the rocks are moved during heavy storms it is a simple procedure to replace them in the correct position. • Rock armour is often used to protect the base of seawalls and prevents direct attack and damage of the wall. Additionally the rock provides support for the wall, improving its stability.

Rock Armour

• Sheringham Beach. Both rock groynes and rock armour can clearly be seen.

Beach Nourishment

• A beach

is the perfect defence against wave action and, if a beach is poor, one option may be to undertake beach nourishment. • This basically involves adding large quantities of material to a beach in order to build it up. The material added will need to be very similar to the material naturally found on the beach and will probably come from remote sources. • Offshore dredging can provide a good source of suitable material for beach nourishment schemes or alternatively the sediment can be obtained from land based quarries. Beach management structures eg: groynes, may also be necessary to ensure the extra material remains on the beach, and even so some will probably be lost through the natural movement of coastal sediments. This may mean further nourishment is required in the future to keep the beaches at the desired level.

Beach Nourishment

Beach Nourishment

Managing Longshore Drift

Evidence of Longshore Drift on the Holderness Coast

Direction of FETCH is quite clear.

Managing Longshore Drift

Groynes • The purpose of a groyne is to create and maintain a healthy beach on its updrift side, which in turn provides protection to the land behind. This is achieved through two main processes. • Firstly, groynes act as a barrier to physically stop sediment transport (sand) in the direction of longshore drift. This causes a build-up of the beach on the groyne's updrift side. Secondly, groynes interrupt the tidal flow forcing the tidal current further offshore beyond the groyne end. This slows the tidal current inshore causing the deposition of heavier sediments and encouraging the beach to grow in size. • Obviously, if a groyne is stopping material from moving along the coast then the down-drift side of the groyne will be starved of material (which is why it is usually lower than the up-drift side). If a groyne is correctly designed then there will be a limit to the amount of material it can hold and excess sediment is free to move on through the system. However, if a groyne is too large it may trap all sediment reaching it and this can cause severe beach erosion problems on the down-drift side, which in turn can result in cliff erosion problems.

Permeable groynes

In an attempt to prevent problems with down-drift starvation, a series of groynes were designed on the basis that they would be "permeable" and instead of trapping the total amount of sediment they would allow some to pass through.

Easington BP terminal

seriously under threat from erosion – in need of coastal protection

Sea defenses at Withersea, Holderness

The rock groyne and armour at Mappleton

The effect of the rock groyne at Mappleton Fetch

The impact of the groyne is clear. New beach to the North and cliff erosion to the South

Cliff Erosion at Happisburgh, Norfolk 1986 1997 1999 2001

Impact of Sea Level Change