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Where a sea current flows along a coast, tides move beach sediment – sand, shingle, cobbles, rocks – in the same general direction by a process known as longshore drift.
Any obstacle to longshore drift will have an affect on local sediment distribution. Groynes are widely used as a means of protecting the cliff line from coastal erosion. By design, a groyne traps and retains sediment at a particular location, in order to minimise wave action. However, the installation increases erosion elsewhere.
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 shape described as crenulate (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 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 detrimental interruption to longshore drift.
Various mathematical models are proposed to explain the development and planform of a crenulate bay – see under references.
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 as that of wave C, and 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.
Various examples of the terminal groyne effect are to be found along the coast of the East Riding of Yorkshire.
Bordering the North Sea, where the current flows north to south, the cliffs are formed from unconsolidated glacial tills. These soft clays allow processes of coastal erosion to be rapid and therefore observable over relatively short periods of time. For background, see some basics.
A little over 16% of the distance has some type of construction to counter the sea. Since a proportion of the material 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.
Ulrome and Skipsea
Substantial defences against the destructive power of the sea were built at Withernsea during the 1870s. Nine extensions have followed.
Rock armour was introduced to prevent outflanking and protect fixed properties lying south-west of the seawall.
An increased rate of erosion beyond the most recent extension, added in 2005, has resulted in crenulate embayment.
Evidence of earlier embayment episodes is visible behind the existing line of rock armour.
Part of the Golden Sands holiday park, occupying the lower centre of the map, sits between the coastal road and receding cliff.
Further extension has been proposed (currently as the South Withernsea Coastal Defence Scheme). Replacement of timber groynes and a strengthening of the sea wall were carried out at Withernsea's north end in 2017/2018.
View along the most recent rock armour extension.
Cliff recession starts where defences end. In the middle to greater distances are setback positions marking the end points of previous defence stages.
The protected cliff shows stability in the form of vegetation, though active erosion attempts to creep behind, or outflank, the granite barrier.
Distance shot – a crenulate bay characteristic of the terminal groyne effect.
An endangered lamp standard at the Golden Sands holiday park.
Another caravan base succumbs.
[All pictures 3 September 2012.]
The first significant defence against the sea at Hornsea, built 1870, lasted six years. 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 pass behind the structure.
Traditional timber groynes, constructed perpendicular to the shoreline, retain some sediment for the beach [15 September 2012].
Parallel to the shoreline, blocks of granite have been added to both sides of an earlier gabion (‘rock cage’) [17 September 2011].
Between the line of rock armour and a stable cliff, the beach level is raised [15 September 2012].
An incoming tide flows around the southern tip of the configuration [17 September 2011].
Advanced cliff setback is clear in this picture. The trowel in the foreground is pressed into platform clay, indicating a thin beach covering [15 September 2012].
Material that was once part of the Hornsea defences is transported down the coast. These pieces were observed at a distance of one-and-a-half kilometres from the main installation [16 May 2014].
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.
L-shape configuration of rock armour. In the foreground, part of the cliff has slumped [5 October 2013].
Wave refraction (change of direction), diffraction (spreading), breaking water and backwash behind the cliff-parallel limb [7 November 2013].
One component of the Mappleton defences is a rock revetment, positioned at the base of artificially profiled (or graded) cliffs [15 June 2015].
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 [26 May 2012].
In August 2015, a section of cliff dropped. The face was severely undercut by wave action in an example of outflanking [15 August 2015].
Separation at the cliff top [15 August 2015].
At Barmston, there are two examples of the terminal groyne effect.
Opposite the end of Sands Lane, a mass of recycled anti-tank blocks and other wartime remnants was installed around 1978. The concrete barrier offers some protection to the Barmston Beach holiday park.
About 750 metres to the south, Barmston Main Drain collects water from the land and discharges it through an outfall pipe into the sea. The cover has been in existence since the 1970s.
Cliff setback behind the revetment opposite the end of Sands Lane.
The pile of military debris is gradually being dispersed by the sea [12 July 2018].
Barmston Main Drain outfall cover. The drain itself is substantially armoured against tidal erosion and ingress by a more recent revetment.
Barmston outfall is responsible for deep embayment.
Viewed from the opposite direction.
[All pictures 23 November 2012 except where otherwise captioned.]
Ulrome and Skipsea
The map is from 2011. It shows two more examples of the terminal groyne effect, at Ulrome and Skipsea (the parish boundary runs across the centre of the map).
From the early 1990s a privately built and maintained seawall was a dominant feature at Galleon Beach, Ulrome. A ‘plug’ revetment was later installed to protect a property about 200 metres to the south at Skipsea. The construction at Ulrome was prone to repeated damage from the waves, especially at the southern end.
Ulrome and Skipsea defences were demolished and all material removed spring 2015.
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.
[Centre pictures 16 April 2014 and 1 June 2017.]
Sensitivity to the terminal groyne effect is such 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.
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.
The Google Earth image from 2007 shows cliff setback south of beach debris at Ringbrough. Three years later, the observation tower (below the point of pin Pr68) tumbled over the cliff. More recently, the entire site was cleared.
Godwin Battery at Kilnsea is mostly gone.
more on East Yorkshire coastal erosion
Page prepared by Brian Williams in August 2013. Last change March 2019.