Coastal Environments - CaseStudy Flashcards
South Devon and Dorset
A regional Scale Study of Coastal Management
The Geographical Setting
The complex geology and the variable orientation of the shore along the south coast of England creates a wide diversity of environments, habitats and landforms.
Part of this coast is the newly designated SMP2 cell of South Devon and Dorset, stretching from Durlston Head in the east to Rame Head in the west, a distance of over 300 km.
The coastal environments in this region include sea cliffs and slopes; estuaries: sand dune systems; spits; and both fresh and salt water lagoons.
Much of this coastline is highly valued for its distinctive rocks, including rich fossil deposits, and its historic role in the development of the science of geology. A large section of the shore is now protected as England’s first natural UNESCO World Heritage Site - the Jurassic Coast.
While landforms formed by both erosion and deposition are found in this region, the long-term pattern of change is slow coastal retreat, called transgression. One suggested rate, calculated for the last 2000 years, is an average retreat of 1 mm annually.
Climate change, producing more severe storms and a rising global sea level, is set to continue and accelerate this process of erosion with serious consequences for the future. In many coastline cells sediment is produced, moved and deposited by natural processes.
In Devon and Dorset many of the deposition features, including the shingle beaches and bars, are now understood to be relict features. This means that they were formed from material left, possibly at the end of the last Ice Age, but are no longer being actively formed.
This long-term sequence of change can be disrupted by short-term events. In recent decades storm events have not only damaged the local communities but in places caused the equivalent of a century’s worth of erosion in one night. Add to this scenario the impact of people, both in their attempts to defend the coast and their other activities, and a complex picture emerges.
At best, human action may slow down the erosion and retreat of the shore; at worst it may accelerate the natural change. People make high demands on the coast both from land and sea based resources.
One consequence of the many demands made on this coastline is that, along some sections, it is heavily defended. In low lying areas the defence is to prevent flooding, whereas along cliffs it is designed to reduce or prevent erosion. The most common defence structures are linear sea walls and groynes. In the long-term, if the sea walls were retained, as the unprotected areas eroded landward, then these sea walls would form promontories along the areas of coast highly exposed to waves in deep water.
In short, given a future of eroding shorelines, managed retreat in all but a few key locations is the only sustainable option. The story goes that almost one thousand years ago, Cut, King of England, commanded the tide to stop in order to demonstrate to his subjects that he was not all powerful.
The Planning Framework and the Proposed Shoreline Management Plan
Under the current policy of Shoreline Management Plans (SMP2) coastline planning has four options:
- Hold the existing defence line
- Advance the existing defence line
- Managed realignment
- No active intervention
The procedure also requires that the plan addresses the future at three timescales: short-term (next 20 years), medium-term (20-50 years) and long-term (50-100 years).
The South Devon and Dorset SMP2 initially divides the coast into 17 sub-cells, these are further divided so that proposals are stated for 190 sections of coast (management units). For each of these, the preferred policy for each timescale is stated. The SMP2 was implemented from 2010. Finally, it should be remembered that SMPs are drawn up as guidelines for the regions and that numerous groups are involved in the decision making process for any one scheme or management unit
Management of the Dorset and East Devon Coast
The section of the Dorset shoreline between Portland Bill and Thorncombe Beacon includes 13 of the region’s 190 coastal management units.
Facing south west this coastline is open to a potential wave fetch over 8000 km, stretching from the Caribbean. Indeed, as the prevailing winds are from the south west this is clearly an example of a high energy coast. High cliffs are found at Portland Bill and West Bay but the dominant feature is Chesil Beach.
This narrow bar of shingle defines and protects the freshwater lagoon called The Fleet and as a tombolo connects the Isle of Portland to the mainland. The Isle of Portland is a small but steep island made of Portland stone which has been used for many important buildings including Westminster Palace and St Paul’s cathedral in London as well as Belfast’s City Hall.
At the northern end of this stretch of coast is the dramatic red East Cliff at West Bay made famous by the ITV detective series ‘Broadchurch’.
Since the implementation of the SMP2 in 2006 this coastline has been impacted by some of the severest storms on record, in particular during the winter of 2013-2014.
These not only tested the existing defences but also the planned responses along the various sections of the coastline.
Three Examples of SMP2 Management Units on the Dorsey and East Devon Coast
• Example A: Chiswell village and south Chesil Beach (6a02-03)
• Example B: Chesil Beach, The Fleet to East Cliff
• Example C: The town, harbour and tourist amenity at West Bay
Example A: Chiswell village and south Chesil Beach (6a02-03)
At the southern end of Chesil Beach on the edge of Portland Bill lies the small low-lying fishing village of Chiswell. Here the threat of flooding has been met by a series of hard engineering structures. These include a 300 m long sea wall with a recurved upper edge constructed in 1959. This was modified in the 1980s with steel piles driven into the foundation to prevent undermining by storm waves. Steel gabions filled with pebbles were used to form a 550 m long mattress under the beach to prevent flooding. Behind these defensive features is a 900 m long drainage channel designed to remove wave water breaking over the defences during storms. The total capital cost of these defensive structures was £5 m. At the time of construction these schemes met with opposition from environmentalists who were concerned with both its visual impact and interference with the natural processes of the beach.
A series of storms between December 2013 and March 2014 were the worst recorded for 60 years. Huge Atlantic breakers removed 150,000 m’ of the shingle from the beach in front of the built sea defences at Chiswell. The height of the beach was reduced by 3 m and its width by 6 m. In places the base of the sea wall was exposed and gabion cages in the mattress were ripped open and their pebbles scattered.
Several times storm waves breached the beach causing the A354 Portland Beach Road, which links Chiswell and the Isle of Portland to Weymouth and the mainland, to be closed. Nevertheless, the existing defences reduced the potential damage with only 6 of Chiswell’s 110 properties reporting severe flooding.
As the SMP2 policy (Figure C64) indicates this section of coast has a ‘hold the line policy over all three time periods. Consequently repair work was undertaken immediately after the storms and completed by October 2014 before the next winter season. Concrete repairs were made to the damaged wall, the gabion castle and mattress were replaced and rebuilt, and the flood drainage system improved.
At the same time, using soft engineering methods, shingle was restored to the shore by beach recycling and the shingle beach slope was re-profiled ready for any
future storms.
The section of Chesil Beach to the north of Chiswell (6a03) is designated as a section for ‘managed retreat? Here, intervention will only take place when flooding impacts on the A354 between Portland to Weymouth, to re-establish this vital commercial and domestic road link.
The Village of Chiswell, Chesil Beach and The Fleet
The Hard Engineering Strategies at Chiswell Village
Example B: Chesil Beach, The Fleet to East Cliff
Chesil Beach is a world famous shingle beach that stretches 29 km from West Bay in the north west to the Isle of Portland to the south east. The beach has a steep storm beach profile and the shingle is graded in size with coarser stones nearer to Portland. Local fishermen claim to be able to tell their location on the beach from pebble size alone.
The tombolo section of Chesil Beach connects the Isle of Portland, a limestone island in the English Channel to Abbotsbury and forms a lagoon behind it, The Fleet. The shingle beach itself continues north westwards against the cliff to West Bay. The small settlements of West Bay in the north and Chiswell in the south currently have coastal protection schemes that will be maintained for the next
50-100 years. However, the plan for almost the entire shingle beach between these two settlements is to allow the expected coastline retreat due to locally sinking land and rising eustatic sea levels.
Chesil Beach itself is therefore the coastal protection along this shoreline and it is expected that natural processes will continue to push the beach eastwards and inland. As a result The Fleet lagoon is likely to become narrower over time and it is probable that storms will overtop the beach itself more often. In the longer-term (100 years) there are threats to some car parks at the beach, a caravan park, the swannery at Abbotsbury and one listed monument (St Peter’s Abbey), as well as potential habitat loss. Nevertheless the SMP2 policy, short, medium and long-term, is ‘no active intervention’.
Example C: The town, harbour and tourist amenity at West Bay
The harbour at West Bay lies at the mouth of the River Brit and in the centre of the Jurassic heritage coast. To the south east lies the start of Chesil Beach, dramatically backed by the East Cliff, a layered wall of vertical orange brown sandstone. To the north west the geology of West Cliff is very different and the highly faulted and weak clay cliffs are in constant danger of slump and collapse.
A wide range of hard and soft management strategies have been implemented at West Bay village. These were designed to address a range of management issues including the need to:
• prevent flooding by storm waves of the low-lying land to the east of the harbour.
• deal with erosion rates averaging 0.5 m per year that threaten houses on top of West Cliff.
• prevent river flooding by storm waves flow causing the River Brit to back-up and flood the town.
• maintain a sheltered harbour for fishing and pleasure craft.
• conserve protected land and marine habitats.
• maintain the aesthetic appearance of the coast for the residents and the tourist industry.
Bridport which lies 1.5 miles inland. While Bridport is home to 13,000 people only 1000 live in West Bay and economically it depends on fishing and, increasingly, tourism.
Beach and boat based activities in the summer months have grown, in part encouraged by the filming of the TV drama ‘Broadchurch’, since 2013. The settlement has over 60 small businesses that depend on the tourists, from ice cream and fast food stalls to hotels and up-market restaurants.
In 2002 a major review of the defence structures at West
Bay concluded there was a 50% chance that the existing sea walls and the ancient piers at the mouth of the harbour would fail within five years. A €15 million new defence scheme was constructed by March 2005 designed to address the different issues faced along this section of coast.
The West Cliff is nearly vertical and rock falls onto the beach are common. Consequently, as the cliff recedes the cliff-top walkway is periodically moved inland.
East Beach, the short stretch of shingle beach between the harbour and the edge of East Cliff is the start of Chesil Beach itself and is designated as a Site of Special Scientific Interest (SSSI). This is why no hard engineering structure
has been constructed there even though the village area behind the beach is very low-lying and prone to flooding. Instead, since 1986 the area has been protected by a purpose-built shingle ridge. Constructed by the Environment Agency this barrier was 7.5 m high and the shingle was sourced from Burton Bradstock, 3 km to the east along Chesil Beach. This is an example of beach recycling. As the material was added the beach was re-profiled to improve its resistance to storm events. In this high energy environment shingle is rapidly transported and as it appears that nature no longer provides new beach material to this section, the shingle ridge has to be replenished every 5 years. The cost of an alternative hard defence scheme is each estimated at £2m but the beach’s SSSI status precludes this approach.
In the centre of West Bay village the ancient stone piers of the town harbour faced directly into the most common direction of storms and storm waves. This made boat access so hazardous that, on average, it could be used on only 200 days a year. A second issue was that twin piers tend to cause sediment to gather at their entrance. Annually 300 tonnes of sediment had to be removed to maintain a deep water channel for boats. The 2005 scheme strengthened the East Pier with concrete rock armour and boulders, and extended the West Pier (now named Jurassic Pier) to 230 m, realigning it to shelter the entrance. At the same time a steel flood barrier was installed across the river near the harbour to be closed when there is a risk of high tide or storm flooding.
To the west of the town harbour the small West Beach was backed by a sea wall and promenade, and both structures had been severely undermined by erosion, including the loss of steel piling at the wall’s foundation. The 2005 scheme addressed these issues creating a new wall fronted with rip-rap rock armour and a single stone groyne designed to trap sediment on the 120 m long West Beach. Beach nourishment ensured that there was sand on West Beach to protect. The only defence measure taken along part of West Cliff was cliff drainage.
The Situation of West Bay Village
West Bay’s West and East Cliffs
Map of the Coastal Defences Constructed at West Bay, Dorset
Fishing and Tourism form the Basis of West Bay’s Economy
Images of the Shingle Ridge Soft Engineering Beach Recycling at East Beach
Images of the Realigned, Extended and Reinforced Harbour Piers and River Flood Gates
Coastal Defence at West Beach, West Bay
Future Planning at West Bat (SMP2)
As the region’s Shoreline Management Plan (Figure C64) suggests, the strategy for the East and West Cliff sections on each side of West Bay village is for ‘No active intervention. Nevertheless these cliffs are expected to retreat over time and consequently the harbour and village may be left more prominent and exposed to coastal erosion.
In the short and medium-term (until
2055) the defences in the town will be retained. Beach recycling and re-profiling will continue on the East Beach and if necessary beach nourishment with material from Freshwater
Beach, 6 km along the coast, will be undertaken. Maintenance will continue for the harbour piers, the sea wall, promenade, flood gates, drainage and rock groyne, and in addition beach replenishment of West Beach as required.
However in the longer-term
(2055-2105) is will be necessary to retreat back from the present position of the East Beach and construct a new flood defence line. To the west of the town the existing defences will also need to be raised and reinforced in response to sea level rise and erosion. The primary reasoning for the hold the line policy is the threat to the existing and developing facilities in West Bay. The harbour, car parks, beach access, housing and tourist amenities ensure that there is a favourable cost-benefit to maintain the current defensive structures for the foreseeable future.
