6-How has the geomorphic shoreline system at Porlock Marsh changed since October 1996?
“Porlock provides the only fully documented example of a nationally important coastal geomorphological system which has undergone catastrophic failure and subsequent evolution following sediment inhibition” (SSSI Citation for Porlock Ridge and Saltmarsh p 1).
The storms of October 1996 resulted in the over washing and demolition of the crest of a weakened managed section of Porlock Ridge approximately 100 m west of New Works. The changes which have occurred since then have significant increased the complexity and dynamism of the geomorphic shoreline system. Most notably:
- A permanent channel in the clay substratum through the ridge that cannot be sealed naturally by longshore drift of sediment has been created. The photograph in Resource 38 shows the extent of Holocene clay surface exposed at the breach. Refer back also to the image in Resource 36.
- As a consequence a new intertidal lagoon has been created on the lowlands behind the ridge with a strongly accreting and expanding saltmarsh which is replacing the predominantly fresh and brackish wetlands that existed here previously. The photographs in Resource 39 (1988) and Resource 40 and Resource 41 (2002) show the saltmarsh evolving with the establishment of saline tolerant grasses. Resource 42 illustrates one impact of the saline inundation of the marsh – the death of an oak tree beside the old decoy barn on the once freshwater marsh. In 2001 (p 1) a Defra/Environment Agency technical report identified that “rapid headwater erosion of the breach channel recently intercepted the main artificial drainage channel so that the lagoon now almost completely empties to 2.0 m at low water, whereas between Oct-96 and Dec-00 it retained water to a level of around 3.8 m” The extent of this tidal range can be seen in the time lapse film below taken during extreme spring tides in 2015.
- The pebble ridge was pushed landward and Orford (2004 p 3) notes that by February 1997, just four months after the breach, the beach ridge had reconsolidated at a new position 15-25 m farther landwards. Four years later a Defra/Environment Agency report identified that some parts of the ridge had already migrated inland by as much as 50 metres
- Both Orford (Ibid p 3) and Pethick (1996 p 2) observed how the ridge, as well as migrating inland, also quickly evolved a much broader and lower profile (reduced by as much as 7 m in places). Pethick (Ibid p 2) interprets this re-profiling not as damage but as the ridge rapidly regressing to its optimum pre-management morphology of several centuries previously – i.e. a time before attempts were made to maintain a continuous and static barrier by raising and steepening its crest. Whilst an immediate and understandable assumption might be that a wider and lower ridge profile might decrease the capability of the ridge to prevent flooding, Pethick (Ibid p 2) emphasises that the opposite is true and the ridge is now “a more stable structure which although allowing some over-topping (see image in Resource 44 shows over-topping during a storm in April 2002) and seepage will not fail catastrophically under storm conditions as did the more artificial ridge profile” The Defra/Environment Agency technical report (2001 p 1) highlighted how the ‘bowing’ of the pebble ridge as it migrated inland had increased its length by 6% since 1996 with the positive effect of “retaining its wave dissipation capacity by migrating inland to lengthen it dissipative profile”
- Shingle movement eastwards from Porlock Weir along the ridge is leading to substantial accretion of pebbles and shingle west of the breach and the formation of a shingle spit – see photograph in Resource 45 As a result the ridge here is reasonably wide with a measure of stability and able to absorb storm wave forces within the shingle. Conversely, shingle which manages to migrate as far along the ridge as the breach never has the chance to accumulate as it is quickly eroded and removed out to sea by the tidal water outflow from the salt marsh behind the barrier. There is little likelihood therefore of the breach being ‘healed’ by natural depositional processes in the foreseeable future. Similarly shingle carried along the ridge by longshore drift is unable to pass the breach and replenish the barrier on its eastward side. As a consequence the ridge immediately east of New Works is being starved of new material and becoming narrower and increasingly vulnerable to breaching and over-topping at high tides which coincide with cyclonic storm events (such as occurred in October 1996) during which tide level can increase by up to two metres. The image in Resource 46 shows the very limited extent of shingle along the ridge to the east of the breach (the far side of the channel in the photographs) compared with shingle accumulation in the foreground (west side of the breach).
Consolidate Your thinking
Before moving on, reflect for a moment on the different ways in which local stakeholders such as residents and local businesses in the settlements of Porlock Weir and Porlock might have reacted to the breach. Consider particularly the likely views of what should be done of the two main landowners of Porlock Bay – the National Trust and Porlock Manor Estate http://porlockmanorestate.org/ The National Trust is a conservation charity funded through subscriptions, entrance fees, bequests and legacies and has a very clear policy of how the 775 miles of UK coast it cares for should be managed. Read its policy document Shifting Shores . In the light of your understanding of this document what do you think the position of the National Trust would have been following the breach? How do you feel its view may have conflicted with the opinion of the other major landowner – Porlock Manor Estate – a commercial farming company with tenant farmers in Porlock Bay working the coastal grazing marsh? What response to the breach did they advocate: Link to Report
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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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