East Yorkshire coastal erosion

sediment segmentation

Content created by Brian Williams in May 2015.
Revised July 2016.





 Holderness location

This page presents a model to explain the movement of beach sediment and associated cliff erosion along the glacially deposited coastline of the East Riding of Yorkshire, the greater section of which is known as the Holderness coast.




summary
Material which forms the beach is called beach sediment, a broad term for sand of all grades, for shingle, pebbles, cobbles, and rocks. Sediment is gradually driven down the coast by the action of sea current and tides. It moves not as a whole but in segments, or bodies. Depletion of sediment as one body gives way to another allows waves to reach the base of the cliff more frequently and with greater energy, often creating erosion hotspots.


sources of sediment
A main supply of sediment for the East Yorkshire coast enters at the ‘top’, arriving into Bridlington Bay having been brought around Flamborough Head by a sea current flowing north to south. Within the bay are sand banks known as the Smithic Sands, which act as a sediment depository. The bay is subject to a circulatory tidal regime.

Another supply is delivered by the very processes of erosion. The substance of the cliffs was for a time the load of a composite glacier which endured for four thousand years or so (see some basics). At each surface that the ice passed over in its distant journey, grains to rocks of every size would be picked up. Part of the glacial course included the sediment-layered dry bed of the North Sea.

Nearly a third of eroded cliff material consists of sand and coarse particles. Stones and boulders, called erratics and found in abundance within the tills, are reduced eventually to finer grades once on the beach. The rest is mostly clay, becoming mud and soon washed out to sea.


beach movement
Sediment spills from the southern end of Bridlington Bay, north of Barmston, where it enters the realm of longshore drift.

Longshore drift term describes the action by which beach sediment is transported in the direction of the sea current when waves arrive at an angle to the shore. See the terminal groyne effect page.

The impression may be one of an entire beach slowly working down the coastline as a single entity. In the East Yorkshire situation, this is not altogether the case. As well as longshore drift there is an additional disturbance to sediment.



 sediment body formation

An incoming wave, arriving oblique to the shoreline, travels an increasing distance relative to the length of its crest before breaking. As a result, some energy is lost, and sediment is deposited a little lower on the beach.

(Refraction, or reduction of a wave’s angle of approach as it enters shallow water, is ignored for present purposes since it may be considered to affect wave behaviour equally at all points.)

The alignment of released sediment develops a seaward tendency, or ‘pull’. Subsequent waves, influenced by the curvature, add more sediment in a manner of positive feedback sometimes referred to as self-organising. A body of sediment builds, variously identifiable along its length as a bank, bar, brow, or ridge.

Although the sediment body pulls towards the sea, it does not continue into deeper water. Wherever such a feature comes to an end, waves are able to carry sediment higher up the beach, and the process begins again.

The beach therefore should not be thought of as a continuous longitudinal expanse but more in the light of segments, each hundreds of metres in length, moving in sequence.

Behind the curve of a sediment body can be found a shoreline starved of sediment. Beach level is reduced, allowing almost every high tide to reach the cliff foot regardless of time of year. In stormy conditions, wave energy has a facilitated pathway to the soft clay material.


ords
Previous researchers, notably Ada Pringle and Anne Scott, referred to the sites of reduced beach as ord systems, or ords.

The word ‘ord’ may have passed through Old English from Danish and relates to a pointed tip or end, though other origins are also suggested.

An ord system has characteristic features. These include a sand bar or ridge at the lower beach, as mentioned above, usually asymmetric in cross section and parts of it sometimes called a tongue. Another is the runnel, a channel or trough between the sediment body and cliff, often retaining water from the last high tide.

Because of a sediment deficiency behind the bar, the upper beach (nearest the cliff) greatly diminishes, possibly to expose the underlying clay platform. It is said to be a thin beach (the opposite would be a full beach). The depleted stretch, which typically extends quite some metres until a resumption of full upper beach, is regarded as the ord centre.

Ords, representing weakness in beach protection, provide a means of beach strengthening, too. Input of sediment from the newly eroded cliff does not fill the break that enabled it to become available, but contributes to the downdrift configuration.


pace of movement
The early work on ords crucially included estimates of pace of movement. Calculated from a number of surveys conducted between 1969 and 1983, particularly 1977 to 1983, average migration was put at 496 metres per year.

Pace is essentially determined by wave and wind conditions. During a quiet season there may be little or no discernible movement. Northerly storms tend to produce the most repositioning. In the surveys, annual variation ranged from 5 to 775 metres.

It follows that estimates for the rate of ord movement can be applied to the sediment segments that define ords. Perusal of beach level data collected since 2008 would support the figures.

If an approximation of 500 metres a year is accepted, then sediment segmentation bodies and their associated ord systems migrating south along the East Yorkshire take well over a century to complete the journey.

Progress of sediment movement can be tracked from seasonally updated Beach Level Contours selected from the Coastal Explorer Interactive Map (open Layer List at right of green toolbar). Areas of full upper beach, sometimes referred to as salients, and stretches of sediment depletion are clearly depicted using colour grading.




In the aerial map below [Google Earth: imagery date May 2007], an ord system passes Long Lane, Atwick. For some 285 metres, the upper beach is much reduced or absent at the ord centre, exposing the cliff base to increased erosion and creating a hotspot.



 ord system (aerial imagery)

 sand step, Tunstall: 29 September 2012
A sand step marks the approach of an erosion cycle. The trowel is pressed into an earlier beach surface [Tunstall: 29 September 2012].
 sediment body – view south, Atwick: 23 April 2013
Waters of an incoming tide provide an outline for the southern end of a sediment body. Beyond the tongue, which 'pulls' seaward, the lower beach is flooded. Visible at upper middle picture is an associated runnel [Atwick: 23 April 2013].
 stones in gutter runnel, Hornsea north: 23 April 2013
Here, a 'gutter' runnel has formed at the cliff base. The stones will act as milling agents, adding to the erosion process [Hornsea north: 23 April 2013].
 sediment body – view north, Atwick: 26 May 2013
Beach visitors stroll along a sand bar. Again, a seaward pull and runnel are evident [Atwick: 26 May 2013].
 erosion of high platform, Grimston: 14 July 2014
Breaking of a high platform during an erosion event. The scaling rod is one metre overall [Grimston: 14 July 2014].
 breaking sand brow, Hilston: 7 May 2013
Hilston beach pillbox on a sand bar soon to be covered by a rising tide. The picture shows mudballs and a fall [Hilston: 7 May 2013].
 sand bar and runnel, Tunstall: 24 September 2011




on the beach

Within the shoreline environment, sediment features are not always easy – or indeed every time possible – to identify and delineate. One reliable indicator of an ord system, and of active erosion, is the sight of a swarm of mudballs.

A frame of reference, or domain, for a sediment segmentation body (SSB) in terms of length along the shore depends on the prevailing angle of wave approach, which varies for different sections according to curvature of the coastline and other factors.

Typical domain lengths are proposed as follows. A little over 1400 metres if between Barmston and Hornsea. From Hornsea to Cowden, about 2100 metres, and from Cowden to Holmpton increasing to perhaps 3000 metres. At the southern end of the coast, sediment body length becomes progressively reduced in the approach to Spurn (chart).

Width, across the shore at low tide as averaged from aerial imagery, is some 180 metres. In height, from low water mark to high water mark, five metres or so. That is, the vertical dimension compares to less than one per cent of longitudinal.

Changes of configuration have little effect on overall pace of migration and total volume of material shifted.

There may be nineteen or twenty SSB domains present along the coast at any one time. See the migration chart.

Below is a chart composed of nine consecutive cross-beach profiles representing a distance along the beach of four kilometres, from Cowden to Aldbrough, in units of 500 metres. The vertical scale is approximately twice that of the horizontal.

 cross-beach profiles Prs55-63, October 2010

Pr55. The sequence starts with a full upper beach (at left of image) above a clear transition (the point where beach slope changes) and a wide lower beach.

Pr56. A proto-runnel appears, and there is a drop in upper beach level, signifying sediment depletion behind the main sediment mass.

Pr57, Pr58. Asymmetrical in section, the body of sediment pulls seaward. A fully formed runnel becomes wider, and the upper beach reduces to a local minimum.

Pr59. The runnel is at its deepest and contains water from the last high tide which interrupts the scan trace. However, sediment is deposited at the upper beach, increasing the level there.

Pr60. Features characteristic of ords begin to disappear as the sediment body reaches the low water mark.

Pr61, Pr62, Pr63. Another ord system develops.

Although it is perhaps reasonable to think of the beach as affording a degree of natural protection, and breaks in sediment as a major cause of erosion, there is also the view that erosion should be regarded as the norm with sediment a transitory means of defence.




at the cliff

Depletion of sediment results in exceptional cliff erosion, which creeps along the coast at essentially the same pace as sediment migration.

Since it is the loss of land at the cliff top which impacts on communities and businesses, emphasis tends to be placed on this aspect of coastal erosion rather than what is happening on the beach. See the data-in-detail page for a background to cliff loss measurement.

A relationship between beach sediment and cliff loss migrations is set out at the following link.


 link to migration chart

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