5-Why did the storm of October 28th and 29th 1996 cause a catastrophic breach in Porlock Ridge?
As a result of the factors explored in Ancillary Question 4 the highly unstable pebble ridge was unable to withstand the extreme storm event of October 28th – 29th 1996 which opened a breach and allowed the sea to flood the low lying SSSI marsh hinterland. The coastal geomorphological system at Porlock Bay underwent a catastrophic failure at a location which the National Rivers Authority et al (1992 p 2) had predicted from mathematically modelling to be particularly vulnerable four years previously:
“Two parts of the ridge are markedly weaker than the rest. These are at New Works and at the end of the groyned section close to Sparkhayes Lane, where failure is likely within a year. Such an event could of course happen at any time, given the correct tidal and meteorological conditions”. The New Works location can be seen approximately a third of the way along the pebble ridge from the west in the photograph in Resource 31 This photograph was taken a year before the breach occurred.
Orford (2004 p 3) describes the breaching event in this way:
“During late October 1996 Hurricane Lili moved across the North Atlantic Ocean and degenerated into a deep depression and south westerly gale before moving across southern Ireland and the UK. Storm surge levels superimposed on the high tide exceeded the height of the managed barrier between Porlockford Cliffs and New Works on October 28th. Massive over washing of saline water demolished the barrier crest and moved gravel onto the back barrier area. The volume of over washing saline water was sufficient to fill the back barrier area and during the falling tide forced a breach west of the New Works” The breach was therefore caused by the effect of the water trying to empty from the marsh. The two photographs in Resource 32 and Resource 33 show the early stages of the breach in the ridge occurring whilst the images in Resource 34 , Resource 35, Resource 36, and Resource 37 reveal the extent of the gap in the barrier created.
Later Orford (Ibid p 4) concluded that “the breach is unlikely to be sealed by the existing low longshore transport rates”. This prediction proved entirely accurate as ten years after the storm there was no indication of depositional processes ‘healing’ the breach. In fact a report by DEFRA and the Environment Agency (2001 p 4) just five years after the storm, identified that the breach had already been significantly enlarged by the rapid erosion of the clay substratum, to such an extent that it ‘is unlikely to seal naturally’.
On a site visit to Porlock Bay immediately after the breach Pethick (1996 p 2) reported that in part, the failure of the ridge was a consequence of many years of poor ridge management measures, “including the repeated re-structuring of the profile which resulted in an over steepened and therefore unstable beach face slope with low permeability caused by the lack of sorting of the remobilised sediment. These factors meant that, under storm conditions, the breach suffered both draw-down on the beach face and wash-over on the beach crest, a combination of processes which resulted in saline flooding of the marshlands behind the ridge and led to the breach”.
Consolidate Your Thinking
Bray and Duane who authored the Defra/Environment Agency report of 2000 stated that the 1996 Porlock Ridge breach was the result of several ‘co-incident factors’. In your own words write a paragraph of no more than 150 words that summarises concisely all of these inter-related factors which together contributed to the catastrophic failure of the pebble ridge.
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Coastal Management In Porlock Bay
- Introduction and Background
- 1- How have geomorphological processes shaped the coastal landforms at Porlock Bay?
- 2- How did the Porlock pebble ridge alter the morphology of the coast?
- 3- How did human activity alter the marsh?
- 4- Why has the pebble and shingle ridge at Porlock Bay become progressively more vulnerable to breaching?
- 5-Why did the storm of October 28th and 29th 1996 cause a catastrophic breach in Porlock Ridge?
- 6-How has the geomorphic shoreline system at Porlock Marsh changed since October 1996?
- 7-Who actually determines how coastal environments such as Porlock Bay should be managed in the future?
- 8-Why was a policy of limited intervention adopted following the ridge failure?
- 9-Twenty years on what have the costs and benefits of the limited intervention policy been?
- 10-How has the management of change at Porlock Marsh been affected by its location within the protected landscape of Exmoor National Park?
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