coastal features - management

8/14/2019 Coastal Features - Management

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EROSION TRANSPORTATION - DEPOSITION


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Factors at work on coasts


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


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By ~ building sea walls, breakwaters,

harbours, groynes, and reclaiming land will allaffect the amount and rate of erosion anddeposition.

How can we defend our coast against the sea?

Note wider beach this

side of the groyne.


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Sea Wall- defence &erosion

Sea walls reflect a largeproportion of the wave energy,which can cause furthererosion of the beach andultimately the potentialundermining and collapse of

the wall.They need a lot ofexpensive maintenance, but areuseful in protecting coastaltowns and other built up areas.


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Managing a coastline withgabionsGabions

Gabions are generally galvanisedsteel wire mesh baskets filledwith rock (often local flint inNorth Norfolk). The size of thebasket vary but they are typically2m x 1m x 1m and arerectangular cubes . Gabions arenormally used on eroding clifffaces where their purpose is twofold. Firstly they provide stabilityto the cliff face, preventingfailure, and secondly they permitwater to drain away that might

otherwise contribute tolandsliding. Gabions are relativelycheap and easy to construct butthey are not suitable for use inlocations exposed to wave actionbecause they are too flexible and

are easily destroyed.

M i i ith


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Managing erosion withRevetments This section is made of wooden slats that

allow the water through, but break up thewave energy, not reflect it.

Sediment getsdeposited behind the

revetment to increasethe amount of beachand so the naturaldefense for the

cliffs. This has beenuse effectively on thecliffs below the BP

terminal at Easington

Timber Revetments were first constructed ascheaper alternative to a solid seawall. TheRevetments are constructed from timber pilesdriven into the beach and connected by asloping lattice of timber planks.

The Revetment is designed so that it willbreak the force of the wave without reflectingthe energy which might cause scouring of thebeach. A Timber Revetment reduces the waveenergy available to attack the cliffs but itdoes not prevent erosion from taking place.

In addition, sediment, which can be sand,shingle or pebbles, is carried through or overthe revetment structure by an incoming wave.However, the sediment cannot easily returnseaward once the force of the wave has beendissipated. This action results in a build up ofmaterial behind the revetment and theincreased depth of material protects the foot

of the cliff from being attacked by the sea.


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Photo of revetments

The wooden slats can

be clearly seen here.


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The result of using revetments.

New beach has clearly been depositedhere, eventually adding protection tothe cliffs behind. Once this becomesstabilised with plants it will be even

better.

Groynes havealso been used.


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Managing erosion with Rock Armour Rock or rock armour has become increasingly

important in coastal defence works over thelast 20 years or so. The rock used will

typically be a very hard igneous rock that isexceedingly resistant to erosion. When rockis placed on the beach it is arranged so thatlarge void spaces are left. When waves hitthe rock they are only partially reflected,

and can flow round the rocks, expendingtheir energy less quickly which reducesscour. Another advantage of using rock isthat if the rocks are moved during heavystorms it is a simple procedure to replacethem in the correct position.

Rock armour is often used to protect thebase of seawalls and prevents direct attackand damage of the wall. Additionally therock provides support for the wall, improvingits stability.


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Rock Armour

Sheringham Beach. Both rockgroynes and rock armour can clearly

be seen.


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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 addedwill need to be very similar to the material naturallyfound 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 alternativelythe sediment can be obtained from land based quarries.Beach management structures eg: groynes, may also be

necessary to ensure the extra material remains on thebeach, and even so some will probably be lost throughthe natural movement of coastal sediments. This maymean further nourishment is required in the future tokeep the beaches at the desired level.


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Beach Nourishment


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Beach Nourishment

h f


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Managing Longshore Drift


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Evidence ofLongshore Drift on

the Holderness Coast

Direction ofFETCH is

quite clear.


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Managing Longshore Drift


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Groynes

The purpose of a groyne is to create and maintain a healthybeach 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 sedimenttransport (sand) in the direction of longshore drift. This causesa build-up of the beach on the groyne's updrift side. Secondly,groynes interrupt the tidal flow forcing the tidal currentfurther offshore beyond the groyne end. This slows the tidal

current inshore causing the deposition of heavier sediments andencouraging the beach to grow in size.

Obviously, if a groyne is stopping material from moving alongthe coast then the down-drift side of the groyne will bestarved of material (which is why it is usually lower than theup-drift side). If a groyne is correctly designed then there willbe a limit to the amount of material it can hold and excesssediment is free to move on through the system. However, if agroyne is too large it may trap all sediment reaching it and thiscan cause severe beach erosion problems on the down-driftside, which in turn can result in cliff erosion problems.


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Permeable groynes

In an attempt to prevent problems with down-driftstarvation, a series of groynes were designed on thebasis that they would be "permeable" and instead of

trapping the total amount of sediment they would allowsome to pass through.


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Easington BP terminal seriously under threat fromerosion in need of coastal protection


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Sea defenses at Withersea, Holderness

h k d l


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The rock groyne and armour at Mappleton


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The effect of the rock groyne at Mappleton

The impact of the groyne isclear. New beach to the

North and cliff erosion tothe South

Fetch

Cliff Erosion at


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Cliff Erosion at

Happisburgh, Norfolk

1986

1997

1999

2001


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