9-Twenty years on what have the costs and benefits of the limited intervention policy been?

After almost two decades of a largely non- interventionist management strategy which has sought to allow the natural evolution of the coast, Porlock Bay is now the focus of research by geographers from all over the world.  This is because it represents arguably the best example in the United Kingdom of how natural geomorphological processes can shape the coastline and provide more resilience to flooding.  The importance of the nationally significant active geomorphological processes and features at Porlock Bay is emphasised by Orford (2004 p.5):

The Porlock barrier is central to geomorphological studies into how a freestanding gravel barrier responds to relative sea-level rise and storminess changes. The barrier is likely to remain a centre of coastal interest for its combination of evolving swash-aligned and drift aligned longshore sections; its post-management adjustment to a stable cross-barrier profile in relation to relative sea-level rise and storm-wave climate; its rollover dynamics and washover response; its breaching behaviour; its developing tidal inlet control on barrier segmentation and longshore transport impedance; its back-barrier fine-sediment deposition and saltmarsh development. It represents one of the best UK examples of how managed ‘stabilized' barriers are non-sustainable at the decade-timescale. It also exemplifies the likely mode of barrier failure, if coastal gravel-dominated barriers are not allowed to adjust freely to changing relative sea level.

The non-interventionist strategy exemplifies the policy adopted by the National Trust for all of the 775 miles of UK coastline it manages detailed in its Shifting Shores policy At Porlock Bay this approach has been supported over the years firstly by English Nature and latterly by Natural England, the government’s adviser for the natural environment, which was created by the merger of English Nature and the Countryside Agency in 2006 Central to the National Trust’s Shifting Shores policy is the importance of adaptation to natural geomorphological processes through planning for change and then ‘rolling back’ buildings, infrastructure, shoreline and coastal habitats to accommodate natural processes rather than attempting to prevent them through constructing ‘defences’.  To underline its commitment to this approach the National Trust has identified Porlock Bay as one of its ‘coastal hotspots’ across England, Wales and Northern Ireland where coastal adaptation strategies will be in place by 2020

Some of the impacts of the non-interventionist approach to coastal management at Porlock Bay over the past twenty years include the following:

Habitat change

Since the inundation of 1996 there have been significant changes in marsh vegetation.  Before the breach Porlock Marsh displayed a diversity of habitats including brackish and saline pools (former duck decoys), saline ditches, small areas of saltmarsh and swamp, drainage ditches and managed reedbeds. Beyond the marshy boundary, arable and grassland (used for grazing) was divided by hedgerows rich in plants and animal life including species of moth and flowering plants found nowhere else within Exmoor National Park.  In 1990 Porlock Marsh was designated as a Site of Special Scientific Interest by English Nature (which was integrated into Natural England in 2006).  Even before the flood of 1996 there were small areas of saltmarsh on Porlock Marsh, probably originating from the previous floods in 1981 and 1990.  However, since 1996 Porlock Marsh has flooded regularly at high tide with salt water. This has killed off all of the plants unable to survive submergence in salt-water conditions and encouraged the growth and expansion of species better adapted halophytic species.

Since 1996 the marsh vegetation has undergone a dramatic transition from a coastal grazing marsh habitat to saltmarsh.  Significant changes to saltmarsh habitat were apparent in less than a year following the breach when English Nature carried out habitat surveys of the Porlock Marsh SSSI. It used the National Vegetation Classification (NVC) scheme to identify the major habitats present on the marsh compared with the pre breach year of 1993 summarised in the table below:

In the 1993 survey English Nature identified seventeen different NVC communities.  In 1997 there were just five and the number of plant and animal species was recorded as having fallen from 51 to 32 over the same time frame.  Since 1996 there has been a net gain of about 38 hectares of saltmarsh and 8 hectares of lagoon and a loss of approximately 31 hectares of grassland (all types) and four hectares of swamp within the SSSI.  An important factor contributing to this rapid transition to saltmarsh habitat is the location of Porlock Marsh behind the shingle ridge which protects the colonising plants from direct wave attack and allows them to quickly establish root in the mud.

Consolidate Your thinking

In 2002 Natural England reviewed the SSSI status for Porlock Marsh.  The site is notified for both its nationally important active coastal processes and features together with its nationally significant saltmarsh and coastal vegetated shingle habitats.  Review the SSSI citation here  and summarise the main geomorphological and biological reasons for its designation.

Saltmarsh and coastal vegetated shingle are both priority habitats in the UK Biodiversity Action Plan and Porlock Marsh is the only saltmarsh habitat within Exmoor National Park. Saltmarsh is one of the most threatened habitats in Britain today.  There is just 44 000 hectares of saltmarsh remaining in Britain today and so Porlock Marsh, however small an area, is considered significant given the decline elsewhere in the country of this threatened habitat.  Only 56% of coastal saltwater marshes in the UK are assessed as being in a ‘favourable condition’.  Read the survey report at http://jncc.defra.gov.uk/page-3573 and summarise the main reasons responsible for the unfavourable condition of the remaining 44% of this habitat.

The birds of Porlock Marsh

Habitat change over the past twenty years especially the large increase in the extent of the saltmarsh and intertidal mud flats at the expense of the brackish lagoon, the small copse adjacent to it and the area of reedbed has had a detrimental effect on the population of waterfowl (birds that are ecologically dependent on wetlands) at Porlock Marsh.  This decline of bird life has been documented by Ballance (2015) and brought about he argues by the gradual conversion from fresh to brackish marsh to tidal creek

Porlock Marsh was classified originally as an important regional centre for its breeding populations of lapwing and redshank together with significant numbers of other breeding waders including oystercatcher on and round the shallow lagoon.  In addition reed warblers, reed buntings and water rail were recorded as breeding in the reed bed.  There was also a small heronry in the now dead trees on the south side of the marsh.  The records also indicate small pre breach wintering populations of shelduck, teal, mallard, lapwing, snipe and curlew and a variety of wading birds such as whimbrel passing through in spring and autumn.

By 2003 lapwing had ceased to breed on the marsh and the number of breeding redshank had declined by at least half.  Between 1990 and 1996 Ballance records an average annual highest count of lapwing as 49 compared with an average highest count of just one bird 2008-2014.  Failure to breed successfully was the result of nests being lost to tidal flooding.  Oystercatchers started to breed on the beach rather than on the marsh but failed to rear young mainly as a consequence of human disturbance.  Between the years 1990-1996 the average annual highest account of oystercatcher is documented by Ballance as 64.  Between the years 2008-2014 this number had dropped to 27.  Breeding numbers of mallard and shelduck declined and herons abandoned the small heronry, the trees having died as a result of tidal flooding.  Reed warblers associated with this cover and the adjacent reed bed declined or disappeared.  Following the loss of the permanent open water of the former lagoon and the cover vegetation which grew to its south, populations of many winter and passage waterfowl declined including little grebe, cormorant, grey heron, mute swan, teal, mallard and snipe.

It is only possible to speculate on future waterfowl population trends.  If the current trends continue then a further decline in winter and passage species which have declined since the loss of the lagoon will occur with perhaps the eventual loss of breeding redshank also.  However, the establishment of larger and more regular winter wader bird populations such as dunlin on the expanding mud banks is likely. With the development of higher saltmarsh Chown (2004) considers that the reappearance of breeding lapwing and redshank is possible in time as well as breeding skylark, meadow pipit and reed bunting.  The establishment of environmental stewardship across approximately 90 hectares of former lowland grazing land within Porlock Marsh may contribute to increasing numbers of farmland birds (see Environmental Stewardship).

Consolidating your thinking

Although the shingle ridge at Porlock has been breached it continues to migrate inland.  A gradual ‘roll back’ of the ridge is occurring at an annual average rate of about 0.1m per year as it naturally migrates landward in response to rising sea levels via over washing.  Consequently the ridge has moved inland about 30m at the mouth of the breach since 1996.  Over time this will result in a reduction in the area of the saltmarsh and intertidal mud. This will occur because although the ridge is mobile the field boundaries inland (see O.S map Resource 4 and artist’s picture of Porlock Bay Resource 8) are fixed and immobile.  A reduction in the extent of the wetland will certainly have serious implications for numbers of waterfowl as available habitat is reduced through such compression or ‘coastal squeeze’.  Bray and Duane recommended as long ago as 2001 that the potential loss of wetland by compression should be counter-acted by shifting the field boundaries inland and allowing the lower lying areas of fields to flood.  This process is known as creating compensatory habitat to replace that lost as the shingle ridge covers existing saltmarsh as it migrates inland.  Allowing this to occur would be entirely in line with the agreed policy of non-intervention but how do you feel local landowners and farmers might react?  How might these stakeholders might be persuaded to adapt to rather than resist coastal processes in this way?  One argument is that allowing such natural migration of the saltmarsh will in fact reduce rather than increase the risk of flooding in Porlock village as it will be able to absorb and dissipate tidal surges far more effectively than a solid defence structure like a sea wall behind which flood water builds up and then can overtop in a sudden catastrophic event. What other arguments in favour of a non-interventionist and adaptive approach here can you think of?

Porlock Weir

The implications of a non-interventionist coastal management policy for the settlement of Porlock Weir to the west of Porlock Bay (see Ordnance Survey map Resource 4, satellite image Resource 43 and photographs Resource 51 , Resource 52 and Resource 53) are major.  With rising sea levels and increased frequency of severe weather events resulting from climate change combined with no active intervention to maintain existing coastal defences, then more frequent defence failure leading to flooding and erosion of residential and infrastructure in the village can be anticipated especially when strong onshore winds accompany high tides – see the reports of recent flooding events here in the Somerset County Gazette. 

The  Exmoor National Park Local Plan recognises that it is unlikely that there will be public funding for the maintenance or enhancement of coastal defences at Porlock Weir in the future in line with the Shoreline management Plan policy of non-intervention.  In response to this, it designates the settlement and the surrounding area a Coastal Change Management Area (CCMA and see Policies CC-S3, CC-S4 and map 5.1 on page 99-100  with a strategy with sufficient flexibility to enable:

• Existing localised defences to be maintained or replaced if private funding is made available provided there would be only very limited impact on sediment transport along the coastline;
• The replacement or relocation of buildings at risk of being lost to coastal change to ‘safer’ inland areas including the relocation further inland or to neighbouring settlements with which socio-economic links already exist.  There is also provision to enable alternative uses e.g. residential to commercial storage of properties increasingly impacted upon by coastal flooding over time and obligations to ensure that abandoned buildings are rendered safe or cleared entirely.

Consolidating your thinking

The potential impact of coastal change at Porlock Weir was further studied by the Somerset Coastal Change Pathfinder Project (Resource 54).  Given that the coast at Porlock Weir would be allowed to evolve naturally in the future with no intervention to influence natural processes of erosion, the major aim of this project was to identify a way forward for helping the community of Porlock Weir and the surrounding area adapt and increase its resilience to coastal change in the future.  Following discussions with a very wide range of stakeholders in the area, the main outcome of the project was the Porlock Weir Adaptation Action Plan (pages 100-115 of the report in Resource 54).  This plan covers the following issues:

• Flood Warnings
• Emergency Action Plans
• Flood Resistance Measures – to stop flood water entering properties
• Flood Resilience Measures - to minimise damage caused by flood water
• Other Measures

Read through and consider carefully the separate objectives and actions under each of these headings in the plan.  From each choose the action which you feel is likely to have the greatest impact in the short, medium and long term and justify your choice.

South West Coast Path

The South West Coast Path is England's longest waymarked long-distance footpath and a National Trail. It stretches for 630 miles (1,014 km), running from Minehead in Somerset, along the coasts of Devon and Cornwall, to Poole Harbour in Dorset. Prior to the breach in the shingle ridge the route of the coast path at Porlock Bay followed the line of the ridge between Hurlestone Point and the village of Porlock Weir (see Resource 6 and Resource 7).  The breach resulted in the severance of the statutory route of the coast path and Exmoor National Park Authority (ENPA) were therefore required by law to identify and put in place as soon as possible, an alternative route.  ENPA held negotiations with the two land owners (Porlock Manor Estate and National Trust) with a view to identifying, signposting and way marking the route. Consequently the route shown on the Ordnance Survey map in Resource 4 for the provision of public access on foot around the back of the marsh area on a temporary basis as a permitted path was identified in 1999 with the two landowners in consultation with their tenants.  An  interim decision was taken in recognition of the uncertainty which existed at the time as to future coastal management strategies and the need to evaluate the possible impact of people (and dogs) using the path on farming and wildlife interests.  This route has since become the established route of the South West Coast Path although sections of it are flooded at very high tides.

Consolidating your thinking

On a copy of the sketch of Porlock Bay in Resource 8 draw on and label the route of the redirected South West Coast Path. What recommendations would you make to ENPA regarding how the new section of footpath might be managed to ensure that people have access to and enjoy the beauty of the area on the one hand whilst the interests of farmers and wildlife (particularly bird life) are protected on the other?  Think about things such as signage, interpretation, screening, hides etc. as well as safety considerations given that it is a public path increasingly prone to flooding at high tides.

Environmental stewardship

Since the breach there have been very significant changes to the way in which land is farmed at Porlock Marsh.  Approximately 90 hectares is now managed by tenant farmers under Environmental Stewardship which is an agricultural-environment scheme run by DEFRA.  This means that in order to achieve environmental benefits such as encouraging biodiversity to develop on the evolving saltmarsh behind the shingle ridge, traditional cattle grazing can no longer be carried out.  In return farmers receive payments through the scheme for managing the land to benefit wildlife.  In 2016 A Vision for Porlock Marsh observed that the environment stewardship schemes:

“have enabled positive environmental management of the surrounding fields as well as encouraging breeding skylarks and other farmland birds, butterflies, dragonflies and bees, and even a few hares. The hedgerows and stone lined hedgebanks are characteristic landscape features, and positive traditional management of these would help to enhance the landscape character and visitor experience, as well as benefitting wildlife” (p. 21)

see https://www.gov.uk/guidance/environmental-stewardship

Consolidating your thinking

The same vision document does not rule out in the medium to longer term the reintroduction of some extensive livestock grazing on Porlock Marsh.  What could the possible environmental, cultural and economic benefits of such as reintroduction policy be and what are the practical considerations that would need to be taken into account to enable it to become a reality? Whilst the restriction on livestock grazing across the marsh could well have environmental benefits how might the local community and landowners feel about the social and cultural impact of the loss of farming as an integral part of a  ‘living landscape’ across the marsh?  Think about the practical implications of reintroducing stock onto the marsh, such as the need for stock proof barriers, access to fresh water, compatibility with people walking on the marsh, and the daily inundation of the marsh by tidal water.

Go to Question 10 -  How has the management of change at Porlock marsh been affected by its location within the protected landscape of Exmoor National Park?