Shadowlands. Matthew Green
The penultimate discussion in the series to consider whether large events or small gradual processes contribute the most to coastal erosion, will look at the beach and cliffs of Thorpeness. This area has been subject to gradual long-term erosion, but it is also a dynamic high-energy location where several metres of cliff can be lost in a single storm event. The following features cliff, beach, waves, sand banks, sediment, storms and erosion, will be explored, as they are part of the constant taking apart and altering that occurs at Thorpeness.
To begin with the cliffs, it is useful to consider the geographical make-up, to get a sense of structure. At the top can be seen, glacial deposits of sand, gravel and clays, thought to have been deposited by the Anglian Ice Sheet, around 450,00 years ago. Underneath is a layer of Norwich crag, comprised of marine deposits of fossil shells, bands of yellow and brown sands and clay. The penultimate layer is Red Crag, mostly courser sand and gravel, and this lies on harder Coraline Crag.
The cliff composition gives a picture of how they could be manipulated by waves. Two papers’ Deriving mechanisms and thresholds for cliff retreat in soft-rock cliffs under changing climates: Rapidly retreating cliffs of the Suffolk coast, UK and the Shoreface Dynamics on the Suffolk Coast Marine Research Report find North Easterly winds produce higher waves with greater propensity to scour beaches. Damaging events can occur when NE waves record Significant Wave heights (SWH) that reach or exceed 3.11 metres, as waves of this height have the potential to move substantial amounts of beach material. For example, the event on 31st March 2022 where SWH exceeded 4 metres at Lowestoft caused considerable damage. Storm Surges can produce conditions capable of generating such waves, but they tend to cause damage in different places, in different ways.
Sediment transport is a significant feature at Thorpeness. The paper above to discuss cliff retreat, suggests low-level southerly waves, transport sediment in a northerly direction, possibly nourishing the Sizewell-Dunwich sandbanks. But NE waves scour sediment and transport it in a southerly direction. If we think of erosion as the removing of sediment and the depositing of this sediment at a separate location, then this is the constant process at Thorpeness. This could be why the width of the beach is so narrow, particularly where the most acute erosion occurs. At times the beach appears to be stripped of shingle, whilst at other times, shingle is piled up in ridges at the back of the beach. However, whatever the sediment situation, there appears to be hardly any distance between Mean High Water Spring (MHWS) and the base of the cliff.
Features of erosion include notches in the beach and base of the cliff and removal of substantial sections of cliff frontage. There is considerable slumping of debris from the top and the bottom of the cliff deposited on the beach, along with trees, turf and concrete slabs from gardens above.
Each feature discussed above contains complex properties that interact and evolve on this coastline. High wave events could weaken cliffs and cause cliff retreat, in the years that follow storm events. But perhaps it is too simplistic to identify large erosion events as the cause of erosion. Fragile composition of the cliffs could mean they will inevitably erode over time. The beach constantly changes, when shingle accretes, this could mitigate erosion.
New notching into the beach, widening of notching at the cliff base and new desiccation cracks in August 2022, occurred during summer months when very few storm conditions were recorded.
But it is the severity of the erosion during high wave events and the significant way they change the cliffs and beach, that suggest these events contribute most to coastal erosion. Particularly as they make mechanisms of recovery hard to envisage at Thorpeness. Complexity of the erosion and the characteristics and frequency of high wave events set the scene for the playthings of the North Sea.
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