East Yorkshire coastal erosion

terminal groyne effect

Created by Brian Williams in August 2013.
Extended in April 2014 and February 2015.



Where a sea current flows along a coast, tides move beach material – sand, shingle, cobbles, rocks – in the same general direction by a process known as longshore drift.

If transfer of material is at any point interrupted, either naturally or by a protective structure such as a groyne or seawall, then the local sediment budget is adversely affected. Material tends to be washed out to sea leaving adjacent beach levels reduced and allowing waves to deliver greater destructive energy at the base of the cliffs.

Shortly after a defensive structure is put in place, a section of cliff immediately downdrift of the work begins to erode at an increased rate, often dramatically. The resulting indentation develops a characteristic crenulate shape (from the Latin word for notch), which eventually extends some distance along the coast. Part of the embayment process attempts to creep behind or outflank the defence.

In time, the rate of loss will stabilise as the system adjusts to a position of equilibrium, or pattern of erosion similar to the coast in general, though the distinctive crenulate indentation remains.

The phenomenon of crenulate bay formation is increasingly referred to as the terminal groyne effect (TGE) or syndrome (TGS). ‘Terminal’ in this sense means the last of what might be a series of groynes in a groyne field.

It is not necessary for a groyne as such to be present. A simple seawall may result in increased downdrift erosion when reflected wave energy removes sediment. For convenience, the phrase terminal groyne effect is suggested for use with any impediment to longshore drift.

Various mathematical models are proposed to describe the development and planform of a crenulate bay – see under references.

 terminal groyne effect (figure)

 longshore drift (figure)

To take a closer look at the consequences of impeding longshore drift, consider first a beach with no barrier.

Wave A approaches the shore at an angle. It breaks and water runs up the beach (swash). Under the influence of gravity, the water returns to the body of the sea by the most direct route (backwash). Some water may seep into the beach (percolation).

Because backwash typically possesses less energy than swash and cannot sustain the sediment load, a quantity of the material brought by the wave is deposited.

The swash of subsequent wave B runs across the sediment dropped in backwash A and adds to wave B’s load, part of which is released during backwash B.

As the action is repeated, wave after wave, tide after tide, material is progessively transported, or drifted, along the shore – longshore drift.

The insertion of a groyne has two impacts.

Wave C represents the last complete cycle that is possible updrift of the groyne. Some of the sediment carried by wave D is deposited in the same backwash path, where the beach cover gradually accumulates (accretion).

Backwashed material does not necessarily round the groyne but continues out to sea.

Downdrift of the groyne, wave E is required to flood a greater area. Part of the incoming wave will change direction (refraction) and spread out (diffraction).

Sediment transported by wave E is insufficient to balance removal by drainage, marked as backwash E. Beach level is lowered, and the cliff near the groyne is exposed to greater high tide erosion.

More complex water movement, such as circulation, is possible especially in the case of larger structures.


The rest of this page presents examples of the terminal groyne effect in its relevance to the coast of East Yorkshire, itself a large crenulate bay, formed downdrift of the chalk of Flamborough Head.

A little over 16% of the distance has some type of construction to counter the sea. Since a proportion of the glacial till that constitutes the cliffs is made up of sand and coarse particles, a major defence work not only disrupts longshore movement but cuts off a source of beach material.

The softness of the material allows processes of coastal erosion to be rapid and therefore observable over relatively short periods.



click on name of location

 East Yorkshire TGE sites  Barmston  Ulrome/Skipsea  Hornsea south  Mappleton  Withernsea south  Easington




Withernsea south

 Withernsea south terminal groyne effect

Substantial defences against the destructive power of the sea were built at Withernsea during the 1870s. Nine extensions have followed.

Rock armour was introduced in 1968 to protect fixed properties lying south-west of the seawall, and more added in 2005.

An increased rate of erosion beyond the most recent extension has resulted in crenulate embayment.

Evidence of earlier embayment episodes is visible behind the line of rock armour.

Part of the Golden Sands holiday park, occupying the lower centre of the map, sits between a road and receding cliff.

Further extension has been discussed. In the meantime, timber groynes are being replacement and the sea wall strengthened at Withernsea's north end (2017).



 Withernsea south (1): 3 September 2012
View along the most recent rock armour extension.
 Withernsea south (2): 3 September 2012
Cliff recession starts where defences end. In the middle to greater distances are setback positions marking the end points of previous defence stages.
 Withernsea south (3): 3 September 2012
The protected cliff shows stability in the form of vegetation, though active erosion attempts to creep behind, or outflank, the granite barrier.
 Withernsea south (4): 3 September 2012
Distance shot – a crenulate bay characteristic of the terminal groyne effect.
 Withernsea south (5): 3 September 2012
An endangered lamp standard at the Golden Sands holiday park.
 Withernsea south (6): 3 September 2012
Another caravan base succumbs.


Hornsea south

 Hornsea south terminal groyne effect

The first significant defence against the sea at Hornsea, built 1870, lasted six years. Even at that time, signs of consequential downdrift erosion were noted.

In 1906 a stronger seawall was constructed, which has been extended five times since.

At the southern end, defences were specially reconfigured in 1977. A cliff-perpendicular limb retains sediment beyond the seawall and a new cliff-parallel extension is meant to address the problem of outflanking.

The arrangement is visible in the aerial picture as a T-shape, purposely separated from the seawall so that beach sediment may accumulate and pass behind the structure.



 Hornsea south (1): 15 September 2012
Traditional timber groynes, constructed perpendicular to the shoreline, retain some sediment for the beach.
 Hornsea south (2): 17 September 2011
Parallel to the shoreline, blocks of granite have been added to both sides of an earlier gabion (‘rock cage’).
 Hornsea south (3): 15 September 2012
Between the line of rock armour and a stable cliff, the beach level is raised.
 Hornsea south (4): 17 September 2011
An incoming tide flows around the southern tip of the configuration.
 Hornsea south (5): 15 September 2012
Advanced cliff setback is clear in this picture. The trowel in the foreground is pressed into platform clay, indicating a thin beach covering at the time.
 Hornsea south (6b): 16 May 2014
Material that was once part of the Hornsea defences can be found further down the coast. These pieces are typical at a distance of one-and-a-half kilometres.


Mappleton

 Mappleton terminal groyne effect

The village of Mappleton is where the B1242 coastal road runs close to the cliff line.

As early as the eighteenth century there was an awareness of how erosion was advancing on the village. In 1786 the church was measured to be 630 yards (576 metres) from the cliff. By 1990, the distance was 223 metres. The figures produce an average loss of 1.73 metres per year.

To protect both village and road, in 1991 a major defence scheme incorporating a prominent L-shape (that is, cliff-perpendicular and cliff-parallel) rock armour arrangement was constructed at a cost of about £2m.

Prior to the work, the rate of retreat along this section of coast was essentially the same for all points.

The aerial picture indicates that sediment has been retained north of the defence structure to create a beach more able to withstand wave erosion of the cliffs.

South of the structure, the terminal groyne effect has left its crenulate imprint on the coastline.



 Mappleton (1a): 5 October 2013
L-shape configuration of rock armour. In the foreground, part of the cliff has slumped.
 Mappleton (0): 7 November 2013
Wave refraction (change of direction), diffraction (spreading), breaking water and backwash behind the cliff-parallel limb.
 Mappleton (2): 17 September 2011
One component of the Mappleton defences is a rock revetment, almost hidden here by a full beach at the base of artificially profiled (or graded) cliffs.
 Mappleton (3): 26 May 2012
Distance shot – the camera is aligned with the true cliff line at middle picture, accentuating the degree of cliff setback (to the left) at the south of the defences. The seaward limb of rock armour stretches to the right.
 Mappleton (5): 5 October 2013
A cluster of encrusted rocks and debris in the lee of the seaward limb.
 Mappleton (6): 7 November 2013
Forward one month (5th October to 7th November 2013) and dispersal is apparent. These few rocks are not encrusted.
 Mappleton (7): 15 August 2015
In August 2015, a section of cliff dropped. The face was severely undercut by wave action in an example of outflanking.
 Mappleton (8): 15 August 2015
Separation at the cliff top.


Barmston

 Barmston terminal groyne effect

At Barmston, there are two examples of the terminal groyne effect.

 Sands Lane, Barmston

A V-shape distribution of rocks, installed by 1978 at the end of Sands Lane, has afforded some protection to the Barmston Beach holiday park.

Downdrift of the formation, a crenulate bay extends to the next defence structure.

 Barmston Outfall

Barmston Main Drain collects water from the land and discharges it through an outfall pipe into the sea.

The picture above was taken from the end of the outfall cover, in existence since the 1970s.

Not visible from the beach, the drain itself is substantially armoured against tidal erosion and ingress by a more recent revetment.



 Barmston (1): 23 November 2012
The sands at Sands Lane. A pronounced sand step (berm) separates different types of beach cover.
 Barmston (2): 23 November 2012
Distance shot.
 Barmston (3): 23 November 2012
Barmston outfall is responsible for deep embayment.
 Barmston (4): 23 November 2012
Viewed from the opposite direction.


Ulrome and Skipsea
 link to the Ulrome page
 link to the Skipsea page

From the early 1990s a privately built and maintained seawall has been a dominant feature at Ulrome beach. A ‘plug’ revetment was later erected to protect a property about 200 metres to the south at Skipsea.

Below, aerial imagery depicts the Ulrome and Skipsea configuration in late 2011 (Bing Maps) and, some eight years earlier, at the end of 2003 (Google Earth historical). The parish boundary runs across the centre of the pictures. Enlargements follow.

Crenulate embayment is clear at both sites. The defences at Ulrome have suffered repeated damage from the waves, especially at the southern end. Further along at Skipsea, the originally aligned revetment is breaking up.

 Ulrome and Skipsea terminal groyne effect: 2011
 Ulrome and Skipsea terminal groyne effect: 2003


 Ulrome terminal groyne effect: 2011
 Ulrome terminal groyne effect: 2003


 Skipsea terminal groyne effect: 2011
 Skipsea terminal groyne effect: 2003


 Skipsea revetment [Helen Wilkinson: 3 August 2007]

Three photogaphs, spanning a period of a little over six years, give an idea of the rate of cliff recession behind the revetment at Skipsea.

Taken on 3rd August 2007 by Helen Wilkinson, the first picture shows the defences positioned at the base of the cliff (the structure protruding from the upper cliff is a sunken container).

The second picture is dated 4th April 2010, and the third is 15th December 2013. Beach level between revetment and cliff is higher than the surrounding sands.

 Skipsea revetment [4 April 2010]
 Skipsea revetment [15 December 2013]


Easington gas terminal

 Easington defences northern end
 Easington defences [16/04/14]
 Easington defences southern end

When the North Sea gas terminal at Easington was opened in March 1967 the belief was that the useful life of the installation would be over before coastal erosion became an imminent threat.

Gas reserves proved greater than first expected and a revised duration for the facility meant that protection was necessary. A kilometre long revetment using over 130,000 tons of rock was constructed in 1999.

The design stage of Easington gas terminal defences had to consider two nearby Sites of Special Scientific Interest. To the north lies Dimlington High Land and to the south are the Easington lagoons.

There is no cross-beach component. The boulders lie at the base of an artificially profiled bank-like cliff which is covered by mesh, and vegetated. Beach sediment is able to pass freely along the defenced section.

Both ends of the revetment are rounded into the cliff behind tapered extensions in order to reduce outflanking.

Terminal groyne effect on the scale of other defence structures on the East Yorkshire coast is not manifest at Easington.

Monitoring south of the revetment indicates some increase in the rate of cliff loss but a crenulate bay as would be characteristic of the full TGE process is so far absent.



minor examples

 Ringbrough terminal groyne effect
 Kilnsea terminal groyne effect

 link to the Ringbrough page

 link to the Kilnsea page

Sensitivity to the terminal groyne effect means that even comparatively small interruptions to the movement of sediment can leave a tell-tale imprint in the cliff line.

Along the Holderness coast, two wartime defence installations are reduced to debris on the beach.

At Ringbrough, the process is complete. The Google Earth image (left) dates from 2007, before the tower had tumbled over the cliff (it is below the point of pin Pr68). Godwin Battery at Kilnsea is mostly gone.

Although remnants are best seen at beach level, their impact in geomorphological terms is clearer from above.

On the images, monitoring profile markers provide scale – intervals represent approximately 500 metres.




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