Annex 01: Porlock 

Location Description Geomorphological Biological Site issues Relevance Monitoring Lessons in restoration Comment Additional issues References	The Shingle bar and developing saltmarsh, August 2002

Site Name: - Porlock Ridge and Salt Marsh

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Location

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Description

This site comprises a shingle ridge and associated saltmarsh hinterland extending for a distance of approximately 4 kilometres along the west Somerset coast, immediately north of Porlock village. Originally scheduled as Porlock Marsh Site of Special Scientific Interest in 1990 for its shingle ridge, saltmarsh and coastal grazing marsh the interest today centres on its geomorphological interest, shingle ridge vegetation and developing saltmarsh. The following is taken from the citation provided by English Nature for the site as revised in 2001.

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Geomorphological setting

"Porlock shingle ridge was formed, as sea level rose in the middle Holocene period, from shingle eroded from Head deposits which masked the sea cliffs to the west after the last glacial period. This major source of shingle has long since disappeared leaving only a relatively insignificant input of sediment from occasional cliff falls. The rate of increase in volume of sediment in the beach ridge from this modem source is considerably less than the rate of increase in the length of the ridge as it continues to rollback, in a lengthening curve, landward into Porlock bay. This means that the ridge has been growing steadily thinner ever since it was formed, a condition exacerbated by the further reduction in shingle inputs caused by the construction of groynes at Gore Point at the western extremity of the ridge. The modem ridge was therefore unable to withstand extreme storm events and a breach opened during the storm of October 1996 which flooded the low-lying marsh hinterland. Rapid evolution of the beach following the breach has provided a unique opportunity to study the development of a coarse sediment barrier system in an open coast location.

The type of geomorphological development seen at Porlock has been noted for coastal shingle systems elsewhere (e.g. west coast of Newfoundland, Canada) but, although other UK shingle systems probably developed and are developing in a similar manner, 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."

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Biological setting

"A large part of this site is lower saltmarsh dominated by glasswort Salicornia europaea and annual sea-blite Suaeda maritima. Other plant species associated with this habitat include sea aster Aster tripolium, sea purslane Atriplex portulacoides, common saltmarsh-grass Puccinellia maritima and spear-leaved orache Atriplex hastata. On areas of slightly higher saltmarsh, sea plantain Plantago maritima, sea arrowgrass Triglochin maritima and sea milkwort Glaux maritima also occur. On shingle areas which are not inundated by salt water a variety of vegetation communities have established. Where the shingle ridge itself is most stable, saxicolous lichens cover the pebbles. Amongst the species which occur here are Rinodina aspersa which is nationally rare and three other species which are nationally scarce: Buellia subdikcifonnis, Caloplaca arnoldii and Lecanora subcarnea. On the back face of the ridge and on shingle deposited to the landward side of it communities of higher plants are found. These include swards with coastal species such as upright chickweed Moenchia erecta, sea storksbill Erodium maritimum, bird's-foot clover Trifolium ornithopodioides and subterranean clover Trifolium subterraneum. Also found here is the nationally scarce Babington's leek Allium ampeloprasum ssp. Babingtonfi and Geranium robertianum."(see Figure, Porlock 01 below).

Figure, Porlock 01: Geranium robertianum growing on shingle at Porlock Bay in Somerset.

The site is visited regularly by grey heron Ardea cinerea, little egret Egretta garzetta and shelduck Tadorna tadorna. Small winter flocks of lapwing Vanellus vanellus, curlew Numenius arquata, teal Anas crecca and shelduck occur on the site as a whole. The site is also visited by a very wide range of migratory species."

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Site issues

The apparent lack of new source material to ‘feed’ the Porlock shingle ridge has resulted in attempts to ‘protect’ the ridge and site from erosion and flooding since 1824 when the first groynes were built. Up to 1985 a series of coastal protection measures were undertaken to maintain the ridge in situ with varying degrees of success.

Because problems of erosion and instability remained Halcrow (consulting engineers) were commissioned to produce a report on how these might be overcome. This report recommended four interventionist options of which the last - "a beach nourishment programme" was accepted. Although proposals were put in hand the owners of the site where the source shingle was to be removed refused permission. In 1992 a further study (by Posford Duvivier) suggested a further four alternatives including: "do nothing"; "managed retreat"; "sustaining existing standard of defence" and providing an "improved standard of defence". In the event, in 1993 the National Rivers Authority, which had previously maintained the ridge, indicated that they would no longer do so.

In 1994 because of this decision the National Park Authority chaired a working group (the Porlock Bay and Marsh Working Group) to look at the issues and produce a management plan for the area. The group, after long deliberation, recommended that a ‘do-nothing or managed retreat’ option was adopted following a 1990 breach. This recommendation was rejected by the National Park Authority who suggested that the owner concerned should be allowed to maintain a sea defence if he wished and offered money towards his costs effectively adopting Option 3 of the Posford Duvivier report. (The report itself did not recommend this option). Funding for this was agreed by the local councils and one of the owners, but not by the National Trust or English Nature.

[Any protection would have been contrary to the "Management Statement" issued by English Nature as part of the SSSI notification package under the Wildlife & Countryside Act 1981 Section 28(4) as amended. A scheme of working was, however, agreed with English Nature and shingle was moved from the harbour to the weak point of the ridge at Porlockford for about 2 years. Then the 1996 event occurred following which nothing has been done, though the owner concerned tried to persuade the National Park Committee that he should be allowed to dump tons of spoil from a new sewage work construction, in the breach.]

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Relevance to shingle restoration/re-creation

In the event, the scheme promoted by the National Park Authority was pre-empted by the storm in October 1996, which caused a major breach in the ridge. No major remedial work has been undertaken on the site and regular inundation of the land behind the ridge is helping to re-create tidal habitats and transitions to brackish, freshwater and terrestrial habitats. The policy of allowing nature to take its course is allowing the system to readjust to changing climatic patterns and to sea level change. This policy means that some habitats have been lost (notably coastal grazing marsh) and replaced by others. In particular saltmarsh continues to develop at the expense of coastal grassland, as flooding becomes more extensive and more frequent. These changes have and are being monitored.

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Monitoring

An important consideration at this site is also the opportunity to monitor the way in which a breach in a barrier beach responds in the absence of human intervention. This will provide information applicable to other sites in the UK and elsewhere, where policies associated with allowing erosion, breaching and natural ‘roll-over’ to occur on shingle beaches is contemplated.

The vegetation of the inundated area has been studied both pre and post breach (Figure, Porlock 02). In particular it has looked at the way in which some habitats have been lost and replaced by others. The results show how saltmarsh has and is continuing to develop at the expense of coastal grassland, as flooding becomes more extensive and frequent.

Figure, Porlock 02. Overview of the vegetation prior to breaching in 1993, derived from the Report of the Porlock Bay and Marsh Working Group, Exmoor National Park Authority. Numbers represent areas resurveyed in 1997 and again in 1999 (see table below).

Table, Porlock 01. Change in vegetation pre and post inundation. Surveys using National Vegetation Classification communities: MG Mesotrophic grassland; SM Saltmarsh; U Upland grassland; MC Maritime cliff grassland, (Pers. Comm. MJ Edgington, English Nature).

Figure, Porlock 03. Semi-improved formerly grazed grassland inundated by seawater. Aster tripolium and Puccinellia maritima in the foreground, August 2002.

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Lessons in restoration

The way in which this site has responded to the cessation of artificial protection and reprofiling of the shingle ridge provides an important example of how natural processes can restore habitat. In this case a natural breach of an artificially maintained shingle ridge in 1996 has resulted in the creation of intertidal habitat in the low-lying flooded areas behind it. The development of saltmarsh, mudflat and reedbed has been of sufficient conservation value to meet the criteria for designation as a Site of Special Scientific Interest which was revised in 2001 (see above).

This case study also provides an insight into the way in which attitudes to coastal erosion and flooding have changed over the last 20 years or so. In the early days the ascendancy of the ‘hard’ engineering solution to coastal defence offered few other options. (This is manifest in the 1985 Halcrow report). Later considerations, notably as a result of expert geomorphologist advice from the late, Prof. Bill Carter, helped to establish that purely engineering solutions may not be sustainable. By 1992 options, including allowing the shingle beach to breach and develop more ‘naturally’ in response to tides and waves were beginning to be more acceptable. Although there was still resistance to a wholly non-interventionist policy, the conservation requirements are seen as being met by such a policy.

Although there was resistance to a wholly noninterventionist policy, especially locally, the conservation requirements were seen as being met by such a policy. This included allowing the system to readjust to changing climatic patterns and to sea level change. The fact that some of the local peoples’ worst fears namely that "the shingle bank will be allowed to disappear into the sea..."; "land will be covered with detritus... from the Bristol Channel" have not been realised is important. In fact the situation in 2002 had every appearance of developing a sequence of new habitats (mudflats, saltmarshes and transitions to grazing marsh and reedbed) of some significance.

Figure, Porlock 04. The breach in the Porlock shingle ridge, August 2002

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Comment

This case study provides an excellent example of the dilemmas posed by changing sea levels and the incidence of storms. It highlights the variety of organisations concerned with developing policy and the problems often encountered by ‘locals’ in accepting what is seen as a capitulation in our ‘battle with the sea’ in the face of cost considerations. It also points to alternative approaches, which may be less damaging to perceived threats than might at first be supposed. The breach in the shingle bank is substantial (see Figure, Porlock 04 opposite) and the tide now regularly flows into and out of the site. However this change has not caused catastrophic failure of the whole system. It has on the other hand, created a new and developing habitat, itself of considerable wildlife value. It has also become a case study for A-Level and GCSE Geographers (McTernan & Wilson 1999)

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