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Walton-on-the-Naze - Cliff Erosion Study Notes
Coastal Cliff Hertitage - Walton-on-the-Naze - Essex, England
The Naze Cliffs of Walton
~ Erosion Study Notes ~
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Erosion Rate
Informed sources put this at between 1.5 and 2.5 metres per year for the Naze cliffs. It is the measure of the speed at which the sea is advancing and the cliff top edge is retreating – and it is by no means an exact science. The underlying geology will speed things up if there is more soft rock exposed (Red Crag) and slow things down if there is more dense rock exposed (London Clay). However, this cycle could itself be modified, depending on the state of the weather, the height of the waves and the amount of rainfall in a given period. Dry cliffs fracture, producing lines of weakness, wet cliffs are prone to mud slides, and easterly gales at spring tides do the most damage at the cliff ‘toe’, thus encouraging the weight above to slump. In January 2001, after a wintry, stormy night, nearly a third of an acre was lost. More recently, in 2005, just 20 square metres was the most land lost at one go. Erosion rate is essentially an average. For instance, the Naze Tower is now just 48 metres from the cliff edge. That means that – in theory – in 25 years time it will be gone if nothing is done in the meantime. However, it may last for far longer, depending on the density of the geology at the base of the cliff – or a sudden bad spell might produce another 2001- style disaster, putting its safety in imminent danger. When it was first built three hundred years ago, the Tower was a whole kilometre inland! Cliffs are dangerous land formations, always on the move. We ignore this movement at our peril.
  
Mudflow
The underlying unstable geology, plus rain, produces lobe-like mudflows - or slides. Characteristically, a mudflow involves a weathered mass of rock or sediment that flows down the cliff slope as a jumbled mass, forming a hummocky topography of ridges and swales. They are common in unconsolidated glacial sediments – the type that make up the top strata of the Naze cliffs. The rate of movement is generally quite slow. During the sixties the pillboxes that are now on the beach made a slow, 'dignified' descent from their cliff top wartime defensive positions to their final resting place.
  
Rotational Slump
A rotational slump is characterized by the movement of a mass of weak rock or sediment as a block unit along a curved slip plane describing the arc of a circle and therefore tending to rotate. The Naze cliffs are characterised by the shell fossil structure of the crag and the glacial deposits of the sedimentary top. As such, they are highly vulnerable to this method of land loss. Looking along the line of cliffs reveals their easily recognized, characteristic form. The upper part (crown or head) consists of one or more transversely oriented zones of rupture (scarps) that form a stair-step pattern of displaced blocks. The upper surface of these blocks commonly is rotated backward (reverse slope), forming depressions along which water may accumulate to create small ponds or swampy areas. This is often seen amid the Naze cliffs in wet weather. The lower, down-slope end (toe) of a rotational slump is a fan-shaped, bulging mass of material characterized by radial ridges and cracks. At this point the once majestic cliffs are barely recognisable. Rainfall helps to disperse the material. The sea does the rest.
  
Soft Defence
Modern thinking about the protection of our coastline involves tremendous forces of wave action confronting a resistant surface. Water is incompressible, so the force has to go somewhere. Many existing concrete sea walls curl inwards (concave) to allow this force to be dissipated upwards. The first picture below shows what happens in an easterly gale at Southcliff, just north of Walton Pier. Unfortunately this method doesn’t get rid of all the wave energy and the sea rebounds only to hit the next incoming wave forming a pyramidal ‘wall of water’ that causes considerable vertical erosive damage to the beach material directly beneath in the shallow water. There is only a thin layer of sand covering the London Clay substrata on most of Walton’s beaches and for this reason they are know as ‘veneer’ beaches. Walton’s veneer beaches are particularly vulnerable to wave action, so, in order to preserve them, Tendring District Council are starting to use ‘soft’ defences. These are essentially long piles of rocks placed in what is know as a ‘progression filter’ whereby the air spaces between the rocks allows the force of the waves to be progressively absorbed as the wave penetrates the rock filter. It is now becoming evident that such soft defensive structures can actually encourage veneer beaches to re-establish themselves. When this happens, the beach itself can act as an energy absorption medium, thus strengthening the coastal defensive structure. Tower Groyne, in the second picture, is an example of a soft defence.
NAZE
2007
Tower Groyne - the rocks are inside the 'A' frame
Southcliff - northeasterly gale force nine...
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Undercutting
It doesn’t take much to systematically wear away at the cliff toe – the foundations of the whole cliff structure – and bring the whole lot crashing down some dark and stormy night. The sea is a patient and methodical excavator that does its destructive work almost unseen at the critical high water mark around spring tides. Even a gently lapping wave can do the required damage over an extended period of time – but once a North Sea storm breaks, the life of the exposed cliff is severely shortened. Just a few centimetres of wave-eroded material is enough to destabilise vast chunks the soft, unprotected red crag cliff face and cause it to vanish forever. Even on coastal protection constructions, the sea doesn’t give up. It will find ways of getting round the back of the most robust of concrete barriers to undermine carefully laid foundations. Coastal defence designers call the problem ‘outflanking’. Tendring District Council spends thousands every year just to maintain existing defence lines from being breached.
      
Wave-Cut Platform
Wave power at an exposed coast can erode a steep coastal slope through processes like ‘hydraulic action’ and ‘abrasion’. Hydraulic action is the wearing away of coastal rocks by the impact of crashing waves against the coast. Water surges into the lines of weaknesses and tightly compresses the air inside. When the waves retreat, the explosive effects of expanding air shatters coastal rocks. Although the Naze cliffs are relatively soft, this process does demonstrate how waves can erode even the strongest of rock. The process is helped by abrasion. This is the wearing away of coastal rocks by rock debris - sand, pebbles and boulders - hurled against the coast by waves. In storm conditions, boulders several inches in diameter are flung around. The sea can be likened to a huge sheet of sandpaper gradually wearing down the coastline. The waves erode along lines of weakness in the rock face. In the Naze cliffs, these can be within the clay itself or it can be the line between the relatively hard clay and the relatively soft slumped material above it. Eventually a groove is formed between the two. Further undercutting at the cliff toe results in an overhang, which ultimately collapses. As the Naze cliff retreats landwards, a flat clay terrace at the foot of the cliff is exposed – a wave-cut platform. Because of the degree of exposure and lack of protection, the residual beach material that lies upon this platform is thin and patchy. Much of the eroded material is transported along the coast by the prevailing longshore drift - the rest is carried out to sea to form offshore sandbanks.
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