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. Although the situation can vary considerably from place to place, from year to year, and within the year, the overall loss of land through cliff recession along this coast is often quoted as being one of the fastest in Europe.
On the chart, erosion rates are shown opposite the locations to which they relate.
In order to understand the causes for such rapid erosion it is necessary to travel back in time, to what is generally known as the ice age.
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, a lobe of ice crept south along the dry bed of the North Sea at the east of England (global sea levels were low because of the volume of water bound up in ice sheets). The coastline around the mouth of the Humber was determined at the time by the outcropping of a great thickness of chalk that underlies the North Sea basin.
The glacial body grew and expanded laterally, extending as far as the chalk cliffs, where the higher ground stopped a further spread of ice. Temperatures eventually rose and the ice melted. A vast amount of material picked up and carried within the ice, or pushed by it, was left behind. Geologists refer to material deposited in this way as 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.
A rising sea level created an inlet 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 consumed 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 will reach the former cliffs of chalk in about 10,000 years.
Coastal erosion is usually regarded as being about losses at the cliff top, where the impact on agricultural resources, on homes, businesses and amenities, is directly felt.
For the most part, cliff recession is an outcome of what is happening on the beach. Tides carry water in across the sands and out again, nominally twice a day. Sea current flows north to south. The result is an overall movement of beach material in the same southerly direction. This is called longshore drift.
Waves arrive at the beach at an angle such that there is a differential along their length in terms of distance travelled and energy released. The beach responds by forming into curved segments hundreds of metres long, each extending from near the cliff and crossing the beach as a sand bar or ridge to end at the low water line.
As a segment pulls away from the cliff, beach level is significantly reduced, perhaps down to the clay platform, allowing the sea easier access to the base of the cliff. Around two-thirds of cliff loss results from areas of depleted beach behind segments.
Because segments move, or migrate, southwards as a consequence of longshore drift, the potential for increased erosion at the cliff top moves at roughly the same pace, averaging about 500 metres per year.
Of the 63 kilometres of glacially deposited coast from Sewerby at the north of Bridlington to the Spurn peninsula, a little over 10 kilometres (about 16%) may be regarded as defended.
In the chart at the top of the page, it can be seen that rates of erosion are higher immediately to the south of Bridlington, Hornsea, and Withernsea, places protected by seawalls. The situation also applies downdrift of other types of defence, at Barmston, Ulrome, Skipsea, Mappleton and Easington.
Increased erosion occurring downdrift of a protective installation is widely referred to as the terminal groyne effect, a result of longshore drift. The relative softness of the clay and high rates of loss for the East Yorkshire coast offers some textbook examples of the terminal groyne effect.
Another consideration is that the cliffs themselves represent a supply of beach material, one that is effectively cut off behind a protective barrier.
Any proposal to limit cliff erosion at a particular location has to be viewed with a mind to cost, typically thousands of pounds per metre for rock armour, and the near certainty that loss of cliff will increase elsewhere as a consequence.
Throughout the centuries, land taken by the sea has meant the removal of numerous communities. The first ‘erosion map’ to name and propose sites for these appears in Thomas Sheppard (1912), The Lost Towns of the Yorkshire Coast. A modern rendering is presented at erosion map.
Three glacial tills make up the Holderness Formation.
Resting on the chalk bedrock is the Basement Till. This 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 much of the cliff. Towards the southern end, between Aldbrough and Easington, Withernsea Till sits above the Skipsea Till and for a distance is the dominant member.
The glacier which delivered the two main tills is considered composite or two-tier in that the ice originated from two sources: north, and north-west. Both flows are believed to have moved together, one partially overriding the other, though there could have been a readvance. Whichever the case, each flow carried its own defining till.
Until 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 and gravels – especially at Dimlington High Land – as well as small and occasionally larger boulders, known as erratics, together with fossil fragments.
The earliest Ordnance Survey maps record the coast as it was around 1852. Later surveys allowed for estimation of land loss by comparing changes in cliff line positions. An idea of the technique is possible from overlay maps.
Actual linear measurement, rather than approximations from maps, became possible in 1951 by the siting of a series of erosion monitoring posts along the coast, with a few more added later.
In 1999, GPS (Global Positioning System) technology was introduced, and readings began at some new locations. GPS was replaced in 2009 by LiDAR (Light Detection And Ranging).
Since 2003, monitoring has been conducted every six months from north of Bridlington down to the Spurn peninsula. Cliff recession is measured along notional lines, or profiles, located in the main at 500 metre intervals along the coastline. Beach contour modelling and continuous cliff line recording is also carried out.
The period 2003 to 2016 produced an overall average erosion rate of 1.45 metres per year for the entire coastline. When defenced stretches are removed from the total length, the loss of unprotected cliff is 1.70 metres per year.
As regards volume of material removed by the sea, a calculation based on length of unprotected cliff and cliff height suggests that roughly one million cubic metres, weighing perhaps 2.2 million metric tonnes, are lost for every metre of recession along the coast. In fact, the tides not only erode the cliff but the beach and sea bed, too. The question of where all the washed away material is deposited awaits a complete answer.
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