Coastal Erosion

The cliffs of the East Riding of Yorkshire, where the plain of Holderness meets the North Sea, provide an example of coastal erosion on an observable scale. Rates of land loss can vary considerably from place to place along this coast, from decade to decade, year to year, and within the year. An approximate measurement of past recession is possible by comparing cliffline positions shown on Ordnance Survey maps, the first edition of which was published in the mid-nineteenth century. Systematic direct measurement began in 1951 when 114 erosion monitoring posts were sited along the coast, with a few more added later. In 1999, GPS (Global Positioning System) technology was introduced, replaced in 2009 by LIDAR (Light Detection And Ranging). Since 2003, measurements have been taken at six-monthly intervals.

Data collected from the monitoring positions is available online from the East Riding of Yorkshire Council. See profile details, results, summary. Values have been entered into a spreadsheet (OpenOffice). Because of the degree of variation according to location and in time, a meaningful average rate of erosion is never easy to propose. However, if the longest time span of measurement is considered, 1852 to 2010, then an average rate of recession for unprotected cliff is 1.27 metres per year. For the much shorter but most recent period 2003 to 2010, the average rate rises to 1.98 metres per year for unprotected cliff, though if areas of seaside frontages (Bridlington, Hornsea, Withernsea) and other defence works are included then the figure is 1.53 metres.

On the chart below, the trend line indicates that more erosion takes place lower down the Holderness coast. Clearly evident from the chart is that a higher rate of erosion occurs immediately to the south, and a lesser rate to the north, of a major defence structure.

Regarding the volume of material lost per year, a crude method of estimation would be to multiply the average erosion rate for unprotected cliffs (1.98 metres for the recent period) by the average height of the cliffs (14.7 metres – see below) by the length of unprotected coastline (52,650 metres, if the distance from Sewerby to Spurn north is taken as 62.94 kilometres, and 10.29 kilometres is deducted for protected sections). The result is 1.53 million cubic metres. Supposing a weight for the material of 2.2 metric tonnes per cubic metre, then some 3.37 million tonnes would be removed annually. In fact, the tides not only erode the cliff but the beach and the sea bed, too. The question of where all the washed away material is deposited awaits a complete answer.

The earliest Ordnance Survey map records the Holderness coast as it was around 1850. Two villages are marked which no longer exist in the same form, and which were already reduced then. Overlay maps tell the ultimate fate of Great Colden (also known as Great Cowden) and (Old) Kilnsea. Throughout the centuries, land taken by the sea has meant the removal of numerous communities. Perhaps the first ‘erosion map’ naming and depicting the sites of these is found in Thomas Sheppard (1912), The Lost Towns of the Yorkshire Coast. At the link, pages can be turned to read the contents of the book. A modern rendering of the erosion map is available here.

As a matter of note, the ending ‘-sea’ for a place name was originally ‘-sey’, meaning mere, or lake. The style is preserved in Woodmansey, a village between Hull and Beverley. There used to be a number of meres in Holderness, all products of the ice age. Only Hornsea Mere remains.

By back projection using an assumed approximate rate of erosion, the coastline in a position it might have been prior to the Ordnance Survey era may be estimated. An example would be for the time of the Domesday survey of land and settlements or, as Sheppard included, a coastline at the time of the Romans. It follows that there should be a limit to how far into the past the method can be applied, and to how far into the future it will work.

An ice age is not a single event that comes and goes but consists of major advances and retreats, with fluctuations, oscillations, and with prolonged periods of little change. Individual events are measured in thousands or tens of thousands of years. During the most recent advance, and from the specific standpoint of the area known today as the East Riding, a body of ice crept south along the dry bed of the North Sea (sea levels were low because of the volume of water bound up in ice sheets). The local coastline was determined at the time by the outcropping of a great thickness of chalk that underlies the North Sea basin. As the glacier grew, it expanded laterally, extending as far as the chalk cliffs, where the higher ground stopped a further spread of ice. In time, global temperatures rose and the ice melted. The vast amount of material picked up and carried within the ice was left behind. That intraglacial load became the ground of Holderness. Geologists refer to material deposited in this way as glacial till, once called boulder clay.

With the ice gone, the North Sea began to return. Between present-day Britain and mainland Europe lay Doggerland. Over the course of a few thousand years the post-glacial territory was eroded and inundated, a process doubtlessly hastened at one stage by a massive tsunami caused by an underwater landslide off the coast of Norway. As the sea level rose, an inlet appeared between the east coast and the Dogger Hills (now submerged as the Dogger Bank). If a start to the story of East Yorkshire’s coastal erosion is required, then the broadening of that sea inlet would serve. The North Sea continued to regain its earlier domain. In the soft tills of Holderness, an area essentially borrowed from the sea, the process goes on. Land will be lost at a significant rate for as long as the tides are able to wash against the foot of an unprotected cliff. At current erosion rates, and with no other influences, the sea would reach the former cliffs of chalk in around 10,000 years.

Three glacial tills (or ‘members’) make up the substance of Holderness. Resting on the chalk bedrock is the Basement Till. This almost certainly dates from a previous, more extensive glacial period of 350,000-128,000 years ago. At the coast, which to all intents provides a cross-section through material left by ice, the Basement Till is (sometimes) exposed north of Bridlington and at Dimlington High Land.

Skipsea Till, from the most recent glaciation, runs the entire coastline and accounts for most of the cliff. Towards the southern end, between Aldbrough and Easington, Withernsea Till sits above the Skipsea Till. The glacier which delivered these tills is considered composite or two-tier in that the ice originated from two sources: north and north-west. Both flows may have moved together, one overriding the other, or there could have been a readvance. Whichever the case, each flow carried its own defining till.

Until around the mid-1970s, Skipsea Till was referred to as Drab Till or Clay, and Withernsea Till as Purple. There had also been a Hessle Till but this was successfully argued to be the surface layers of Skipsea and Withernsea tills subjected to weathering over thousands of years. Within the tills and between them are pockets and beds of sands, silts (especially at Dimlington High Land), and gravels as well as small and occasionally larger boulders, or ‘erratics’. Differences in erosion rates of the tills of Holderness are slight.

wave action and cliff undercutting (1-6)
The North Sea at the Yorkshire coast flows from north to south. Waves approach the beach predominantly from a north-easterly direction and the water falls back at a lesser angle. Material is moved down the coastline in a zig-zag fashion by a process known as longshore drift. As tides come and go, the sea can be thought of as sawing away at the cliff foot. In high winds and storms, there is also a destructive pounding.

cavities and caves (7-10)
Where a cavity forms in the clay, a combination of surging water and trapped air may enlarge it to a temporary cave. Accumulations of pebbles and stones produce abrasive ‘milling’, which accelerates the process.

detachment (11-14)
At the top of the cliff, the material of the edge has nothing to contain it on the seaward side. It begins to detach, or separate. Rainwater enters the clay more easily through the cracks, weakening it further (and freezing in winter). Detachment may continue in the cliff face on a more massive scale.

slumping, sliding, crumbling... (15-20)
A failure at the bottom of the cliff will produce slumping above. Sections of the previous land surface drop vertically and are seen to descend the cliff as if on a downwards escalator. Elsewhere, material may slide like slurry, or crumble to fine pieces, or fracture along bedding planes.

...and tumbling (21-24)
In addition to slumping, which is commonplace along much of the coast, there are blocky falls when a large amount of material tumbles towards or on to the beach.

stacks (25-26)
Stack formation is rare in the Holderness clays although sometimes the potential is observed.

cliffs (27-28)
The till of East Yorkshire is generally considered to begin at the north end, where the pre-glacial chalk cliff turns inland at Sewerby. Clay also lies above the chalk where from time to time it slides or falls down the face.

(29-30)
Along the coast, the cliff edge is usually irregular in the way it breaks. For a short distance below the surface, the clay is weathered into soil – this layer was once thought to be a separate till member.

(31-34)
The Holderness cliffs display contrasts in angle of slope, also in height. A figure on the beach in the distance at the extreme right of picture 32 provides scale for Dimlington High Land (which stood higher in the past). Low cliffs are found at each end of the oast.

(35-36)
Heavy clay tills predominate, one member clearly overlying another in the cliffs to the south, but there are also sections where sandy material covers the clay.

on the beach (37-40)
As the cliffs erode, the sea tends quickly to dissolve and wash away the clay. Many pictures taken of East Yorkshire beaches show little evidence of the vast amount of material removed from the cliff. But ‘mud-balls’ will always be found somewhere. They may align parallel to the cliff or be randomly scattered. A close-up of a mud-ball reveals a pebbly content.

Another beach feature is the ord – a product of longshore drift characterised by a bank of shingle and sand with runnel (back channel). As ords creep slowly southwards, they cause an increase in the rate of local cliff erosion.

in the clay (41-44)
Erratics of many sizes are found in the clay. A few exhibit flattened faces and parallel scratches, distinctive evidence of having been transported within a glacier.

At certain locations, beds of silt, sand and gravel mark phases of glacial retreat, or boundaries between tills.

defences (45-50)
Some man-made structures found along the Holderness coastline were constructed to defend against enemy invasion. Defences to counter coastal erosion range from simple groynes, or breakwaters, to extensive sea walls. Gabions, which are metal enclosures filled with lumps of rock, have been used in places. Locations such as Mappleton and the gas terminal at Easington are protected by lines of granite boulders.

Skipsea
Kilnsea
Ulrome
Aldbrough

images 1 to 50 enlarged
supplementary gallery



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