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.
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 originates from 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.
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.
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.
At the northern end of a sediment segmentation body (SSB), sandy material is deposited around the base of the cliff, raising the level of upper beach to form a salient.
Salients offer some protection for the cliff against wave action. Quite often, a high tide is prevented from reaching the cliff.
A salient diminishes as the sediment body pulls away southward to begin a long traverse of the lower beach to the low water line. Depletion of the upper beach cover increases exposure of the cliff base, and the potential for erosion becomes greater.
The word ‘ord’ may have passed into local dialect through Old English from Danish and relates to a pointed tip or end, though other origins are also suggested.
An ord system has characteristic elements. These include a sand bar or ridge at the lower beach, as described above, usually asymmetric in cross section, emerging from the salient as a tongue.
Another element is the runnel, a parallel channel or trough immediately landward of the sediment body, retaining at least for a while water from the last high tide.
There is normally a presence of mudballs, a sure indicator of active erosion.
The most depleted section of upper beach between salients is regarded as the ord centre. Here, sediment cover is thin or possibly absent altogether from the surface of the clay platform that underlies the beach.
Fine material released from a newly eroded cliff or platform at an ord centre adds little to the breach but may well contribute to a downdrift configuration.
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.
Pr55. The sequence starts with a salient (at left of image) providing a full upper beach above a clear transition (the point where beach slope changes) and a wide lower beach.
Pr56. A proto-runnel appears, accompanied by 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.
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 as a guide to the coast in general, while values decrease a little towards the northern and southern ends.
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 coast take well over a century to complete the journey.
The domain of a sediment segmentation body, or the area of beach occupied by the feature, is to all intents a function of the prevailing wave approach angle, the angle an incoming wave makes with the shoreline.
Cross-beach distance from the base of the cliff to the mean low water mark may, for convenience, be considered a constant. Therefore, as the angle of wave approach (Θ in the diagram) increases, contact with the shore (and cliff line) decreases, and vice versa.
Wave approach angles are influenced by the shape of the coastline. The shoreline extent of a sediment system will vary according to which part of the coast it is opposite.
Domain lengths can be estimated from intervals between salient centres, these being better defined than ord centres.
The following bar chart shows average wave approach angles between Bridlington and Neck of Spurn. The longer the bar, the greater the angle. Ranges change wherever the coast has curvature or a shift in alignment.
Positioned above the bar chart is an inverted graph trace which plots distances between the centres of salients as shown in the migration chart. The higher the point, the shorter the distance. A credible fit is evident.
The above exercise can be tabulated into zones. Monitoring profiles may be assumed to be 500 metres apart. Domain lengths, average and trend, are unrounded results of calculations – no precision is implied.
Within the shoreline environment, sediment features are not always easy – or indeed every time possible – to identify and delineate. Forever changing, configurations of sediment may be fragmented, incomplete, superimposed, or merged.
sediment segmentation and cliff loss
The model describes how a series of sediment segments migrate along the East Yorkshire coast, at an average pace in the order of 500 metres per year. There may be twenty or so such bodies at any one time, with more undergoing formation in the ‘nursery’ of Bridlington Bay, north of Barmston.
Where a segment lies close to the cliff, a sandy salient raises the level of the upper beach, providing a degree of protection from destructive wave energy.
Between salients, especially where the segment pulls away seaward from the cliff (in what is sometimes called an ord system), the upper beach is depleted of sediment cover, allowing waves greater access to the base of the cliff.
The result is a high probability of cliff erosion. As sediment shifts southward, the potential for significant cliff loss follows. (Examples of tracking cliff loss migration from data tables appear in the hotspots page.)
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.