Thursday, 13 February 2014

Longshore drift at Sidmouth

Following on from a graphic demonstration of the effect of tidal and wave energy on the coast at Sidmouth 
Futures Forum: The power of the sea - its tides and its waves
perhaps another aspect of the power of the sea can be explored - the effect of longshore drift:

Longshore Drift | gcse-revision, geography, coastal-landscapes, coastal-processes, longshore-drift | Revision World
gcsegeographyatheathfield: Coasts

Chesil Beach is a very good example of this phenomenon:
Futures Forum: "The storms that flayed the Dorset coast this week revealed a story not just of frail humanity humbled by nature’s power, but of foresight, preparedness – and elegant engineering solutions."

The formation of Chesil Beach
Long shore drift

It has long been recognised that waves hitting a beach at an angle drive the beach materials in the direction that the waves are moving.

The beach is exposed to the full force of the Atlantic and the long shore drift is predominantly to the east. This explains the occurrence of pebbles from Budleigh Salterton and more importantly, demonstrates that at least a proportion of the material that forms the beach came from the west.

An early theory to the pebble gradation was that the strong south westerly gales carried all of the material to the east and that less strong waves from other directions carried the smaller pebbles back to the west. More recently it has been suggested that the larger pebbles move more rapidly due to their larger surface area. The local variations in pebble size both along and across the beach are thought to be significant but no satisfactory theory can yet explain it.

Beaches - Jurassic Coast

Students from the College have been considering this subject along the coast at Dawlish Warren:

www.sidmouthcollege.devon.sch.uk/PDF/Newsletter 2013/Newsletter 251013.pdf

Sidmouth, along with other towns on the South coast, has been exposed to the potentially damaging effects of longshore drift - and measures were taken in the late 1990s to lessen this impact.

Here's a very impressive 360" view of Sidmouth's waterfront:
Sidmouth seafront west
gcsegeographyatheathfield: Coasts

And here's a useful overview of both the principles and the work undertaken in Sidmouth:

Offshore breakwaters

Offshore breakwaters, act as a direct barrier to waves, reducing the wave energy before a wave reaches the shoreline, therefore consequently reducing erosion potential at the shoreline. The gaps between segmented structures allow some wave energy to reach the shoreline, but this is dissipated by refraction and diffraction. Erosion may continue in the lee of the gaps leading to formation of an embayed shoreline as sand moves into the shelter of the structures.
The breakwaters are either constructed some distance away from the coast or built with one end linked to the coastline.
Breakwater construction materialsThe breakwater design and size, together with placing will be dependant on the beach type, morphology and wave patterns in the area.
When oncoming waves hit these breakwaters, their erosive power is absorbed and disapated around the breakwater.
This can cause an area of slack water behind the breakwaters where sand can build up (salients). these may grow seawards sufficiently to connect with the structure, forming an artificial tombolo.

If the salient is stable, the beach may grow as sediment is 'trapped'. However, as a result nearby unprotected sections of the beaches will not receive these fresh supplies of sediments and may gradually shrink due to erosion, namely longshore drift.
As breakwaters can have a such a strong influence on longshore drift they are generally not normally used on long expanses of open coast or within estuaries if strong wave or tidally induced currents are present.
In 1994 a breakwater was constructed as part of a coast defence scheme at Sidmouth in Devon. Sidmouth is a popular holiday destination which depends on it shingle beach for both protection from storms and as an am entity for tourists.

The severe storms in 1989 and 1990 caused substantial depletion of the beach and exposure of the foundations of the seawall.
A coastal protection scheme comprising of an offshore rock breakwater, rock groynes and beach replenishment scheme. The construction works were carried out in the winter months to minimise disruption.
The breakwaters at Sidmouth offer protection from the prevailing south-westerly seassidmouth breakwater

South West Coastal Group
Breakwater at Sidmouth


The best form of natural defence is a beach which efficiently absorbs the energy of the waves. However, along many coasts longshore drift causes the beach to thin out in places and erosion of the land behind becomes a problem.

Groynes are regularly used as part of sediment control systems, designed to slow down longshore drift and build up the beach.

As a result beach material builds up on the updrift side, where littoral drift is predominantly in one direction, creating a wider and a more plentiful beach, therefore enhancing the protection for the coast because the sand material filters and absorbs the wave energy. However, there is a corresponding loss of beach material on the downdrift side, requiring that another groyne to be built there.

However, groynes do not protect the beach against storm-driven waves and if placed too close together will create currents, which will carry sand material offshore.

Traditionally they were constructed from tropical hardwoods which are more resistant to marine borers and erosion. However rock groynes have been introduced at a number of sites since the 1980s. Considerable investment has been made at sites incorporating groynes and beach recharge simultaneously.
fThey are built at right angles to the shore and spaced about 50-100 metres apart.
Without regular maintenance of groyne systems they degrade and can become dysfunctional.Many experts consider groynes to be a "soft" solution to coastal erosion because of the enhancement of the existing beach.
Groynes are increasingly viewed as detrimental to the aesthetics of the coastline, and face strong opposition in many coastal communities.
In some areas a problem called Terminal Groyne Syndrome can occur. This is where the last groyne that has been built or the terminal groyne, prevents longshore drift from bringing material to other areas along the coast, effectively moving the problem further along the shoreline.

South West Coastal Group

From the same website - the groynes in 1947:

cc_groyne_sidmouth1947.gif (400×271)

A study from the British Geological Survey in 2011 suggested just that the new rock groynes have simply displaced the power of waves and tide.

Natural landslide mechanisms in the cliffs adjacent to Sidmouth include rock-block and toppling failures induced by marine undercutting, and hydraulic stoping along faults and major joints at the foot of the cliffs. The principal landslide mechanism is the collapse of unconsolidated Head deposits and deeply weathered mudstones in the highest (mostly 3 to 5 m) part of the cliff. The falling material commonly destabilises the underlying well-jointed sandstones and mudstones.

Artificial factors that have influenced erosion rates in the cliffs east of the River Sid outfall in the last 100 years have included the refraction of waves adjacent to the end of the river wall, and interference with the easterly longshore drift of the beach gravels.

Natural and artificial influences on coastal erosion at Sidmouth, Devon, UK - NERC Open Research Archive

A letter to the Herald a year ago drew attention to further professional study:

Breakwaters have caused erosion

Sunday, February 24, 2013 
SIR - David McCluskey (Opinion, February 15) is correct in stating that coastal erosion is driven by marine action and protection of the base of Sidmouth’s east cliffs is the only way to mitigate the increased erosion of the past 15 years.
He is also right in stating that I blame the breakwaters for the increased rate of erosion.
In this I am supported by the Shoreline Management Plan, written by Halcrow, an eminent firm of engineers, and published by the Environment Agency. The mechanism is very simple.

The breakwaters have stopped all longshore drift of shingle from the west, thus denuding the base of the cliffs, which before 1995 was protected by a shingle bank five metres high and is now bedrock.
This shingle protected the base of the cliffs from undercutting by the sea, which now has unfettered access to cause the mayhem we have recently witnessed.

Paul Griew
Leader, Cliff Road Action Group

Breakwaters have caused erosion - Letters - Sidmouth Herald

The East Beach Beach Management Plan Steering Group will be considering these issues over the coming months:
Futures Forum: Sidmouth Beach Management Plan Steering Group - background
Futures Forum: Sidmouth Beach Management Plan Steering Group - report

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