5. Coastal defence

Coastal defences may give protection against:

o. flooding - termed sea defence or tidal defence,

o. erosion of the coast and encroachment by the sea - termed coastal protection.

This chapter deals with both types of defences.

Who does what?

o. The Ministry of Agriculture, Fisheries and Food (MAFF) and the Welsh Office (WO) are responsible for Government policy, guidance and provide grant aid towards capital works and preliminary studies. Their approval is necessary for coastal defences.

o. Local authorities have powers to undertake coastal protection works. Maritime local authorities are empowered to construct and improve coastal protection works with the benefit of government grant aid, although County Councils are expected to contribute financially.

o. Both local authorities and the Environment Agency have powers to undertake sea defences.

o. Coastal Cell Groups are developing Shoreline Management Plans.

o. Local planning authorities control development in areas of flood risk under the Town and Country Planning legislation with advice from the Environment Agency.

o. English Nature and the Countryside Council for Wales advise on conservation implications of flood defence strategies and schemes within SSSIs.

Stated Government aims

• To reduce the risks to people and the developed and natural environment from flooding and coastal erosion by:

-. encouraging provision of adequate and cost-effective flood warning systems;

-. promoting adequate defence measures which are technically, environmentally and economically sound;

-. discouraging development in areas at risk.

Background

The Severn Estuary presents a challenging environment for flood defence. As discussed in the introduction, the tidal range in the estuary is the second highest in the world. Adverse weather conditions can raise water levels by more than two metres above predicted levels. In addition, average sea levels have been rising since the last ice age and the rate of rise is now increasing due to global warming (see issue C6).

Examples of tide levels around the estuary are given in Table 5.1.

Table 5.1: Tidal data around the estuary

* mAOD = metres Above Ordnance Datum

Note: Predicted flood levels are stillwater levels i.e. they do not take wave action into account.

Coastal defences

The tidal forces, with their associated waves and currents, are constantly shaping the shoreline and sometimes threaten natural and man-made assets. In 1811 a tidal flood reached 20 km inland to Glastonbury, and as recently as December 1981 over 3500 ha of land and 1300 properties were flooded in Somerset alone. Further up the estuary, storm tidal floods reached as far as Gloucester in February 1990 and February 1995. In response to events like these, over hundreds of years, coastal defences have been constructed to reduce the risks from tidal flooding and coastal erosion.

Today thousands of people around the estuary depend upon artificial defences to protect coastal land and property from flooding and erosion. These defences may protect only a few properties, a seaside town or, in the case of Avonmouth, a major industrial area. In parts of the lowlands in the upper estuary, the Somerset Levels & Moors, and the Gwent Levels many square kilometres of land are more than 3 m below the highest tides. Nevertheless the majority of the shoreline remains naturally defended, as can be seen from Map 5.

The state of the coastal defences

A survey of all sea defences was undertaken by the former NRA between 1990 and 1993. Similar surveys of coastal protection works have been undertaken on behalf of MAFF and Welsh Office. This was done to establish the state of coastal defences, in order to assess risk and the need for investment.

Sea defence works are carried out to:
o. raise the level of defences,
o. increase the life of the defences,
o. protect the defences against erosion.

The Environment Agency and local authorities undertake programmes of maintenance and improvement works, and many of the defences in need of attention have already been addressed (see Table 5.2).

* It should be noted that the timing of Environment Agency schemes is reviewed twice a year and is dependent upon available funding and relative priorities.

Map 5: Coastal defences
Standards of protection from flooding

Complete protection against flooding and erosion cannot be provided, instead defences are built to protect against floods of specified sizes. MAFF and the Welsh Office have published indicative target standards of protection which can be used as an initial guide to appropriate levels of defence for different categories of land use (see Table 5.3). However, each scheme must be economically viable and cost effective. In some cases it is not economical to maintain the existing line of defence and with agreement the defence is set back to a more sustainable location. This is often known as 'managed retreat', set-back or coastal realignment.

Table 5.3: Target standards of flood defence for different land uses

* The return period for a flood is the average interval in years between the occurrence of floods equalling or exceeding that level.

The role of coastal internal drainage boards

Significant areas of low-lying land along the Estuary are controlled by internal drainage boards instead of the Environment Agency or local authorities. These boards have powers to raise money and undertake drainage works. The boards maintain an interest in ensuring the protection and free drainage of their land, although in general, sea and tidal defences which protect these areas are managed by the Environment Agency. Further information on internal drainage boards and land drainage can be found in chapter 3.

New development in floodable areas

The Environment Agency seeks to guide new development away from flood risk areas. This issue will become increasingly important given expected sea-level rises. New developments will need to take account of these risks, both present and future. Any proposal to provide mitigation, including the raising of land levels, should not increase risks elsewhere.

Flood warning

Flooding can occur any time that certain climatic conditions coincide with high tides but the risk of tidal flooding is highest from September to April. The Environment Agency undertakes flood warning for both river and sea flooding.

Flood forecasting and warning systems enable emergency services, operating authorities and individuals to take measures to lessen the impact of flooding where protection cannot be provided. A national Storm Tide Warning Service is operated by the Meteorological Office. The Environment Agency uses this information, together with meteorological forecasts and its own network of tide level gauges, to forecast flooding problems and to inform maritime local authorities or emergency services who may be affected. This service is not as comprehensive as the river flood warning schemes, such as that for the River Severn upstream of Gloucester. At present, on the estuary upstream of Avonmouth, only Severn Beach receives formal warnings of tidal flooding.

Coastal protection

Coastal erosion and accretion (the build-up of sediments) are continuous natural processes. Increases in sea level and increased storminess are expected to speed both these processes. Interference with the natural process of erosion can threaten the supply of new beach material and should be avoided unless important assets are at risk. When coastal protection is put into place it can shift the erosion problem onto another section of coast. It is therefore necessary to look at erosion problems (and sea defence) in a strategic context so that problems are not just shifted from one site to another. Such issues will be addressed by the Shoreline Management Plans produced by Coastal Cell Groups. The benefits of protection must exceed the cost.

Problems have been identified along the Severn at the following locations and are shown in Table 5.4. Most of those sites are associated with sea defences and there are only a few (notably Aust and Beachley) where the Coast Protection Act applies.

Table 5.4: Areas of erosion problems in the estuary

Issues

Many issues in this report are related to one another. Issues raised in this chapter have particular links with those in chapters 3, 4, 9, 12, 13, 14 and 15.

THE NEED FOR NEW AND IMPROVED COASTAL DEFENCES

C1 The need for strategic planning of coastal defence - Shoreline Management Plans

Coastal defences need to be planned within a strategic framework which recognises the many issues and interests which are affected. MAFF and the Welsh Office encourage the establishment of Coastal Groups representing the operating authorities within a 'coastal management cell'. These lengths of shoreline are selected so that any changes within a cell do not significantly affect those neighbouring. Within the Severn Estuary area there are three Coastal Groups covering:

1. Swansea Bay from Worms Head to Lavernock Point;
2. the Severn Estuary upstream from Lavernock Point and Brean Down; and
3. North Devon and Somerset from Hartland Point to Brean Down.

Strategic planning of coastal defences therefore encompasses all the issues in this chapter and will form a framework within which they can be resolved.

Who is involved: MAFF, the Welsh Office, Environment Agency, maritime local authorities.

What is happening: Each Coastal Group is preparing a Shoreline Management Plan in accordance with MAFF and Welsh Office guidance and funding. These plans will provide the basis for sustainable coastal defence policies and will set objectives for the future management of the shoreline. Plan preparation is being supported by investigative work to address the issues raised above.

C2 The need to consider providing new and improved sea and tidal defences where target standards of protection are not being met

The Environment Agency maintains a survey of the condition of all sea and tidal defences. MAFF and the Welsh Office have published indicative target standards of flood protection which can be used by the Environment Agency and local authorities as an initial guide to providing an appropriate level of defence for different categories of land use (see Table 5.3). Some defences in the estuary do not meet these current target standards and therefore improvement should be considered within the framework of Shoreline Management Plans.

Who is involved: MAFF, the Welsh Office, the Environment Agency, and local authorities.

What is happening: The Environment Agency has identified those parts of the coast requiring new defences and has drawn up a prioritised timetable for their construction (see Table 5.2). The priority and funding of new schemes is approved by an executive Flood Defence Committee. In some cases local authorities may choose to promote a scheme. MAFF/ Welsh Office require all new schemes to meet technical, economic and environmental standards and provide financial grant aid towards the cost.

The Shoreline Management Plans will eventually help to plan coastal defence work at a strategic level, see issue C1.

C3 The need for coastal protection to protect property

MAFF and the Welsh Office maintain surveys of the condition of coastal protection schemes. Coastal erosion and accretion are continuous natural processes. Increases in sea level and increased storminess are expected to speed both these processes. Interference with the natural process of erosion can threaten the supply of new beach material and should be avoided unless important assets are at risk. The benefits of protection must exceed the cost.

Problems have been identified on the estuary at a number of areas which are listed in Table 5.4. Often coastal protection issues are associated with sea defences, and the authorities work together, but there are a few where only the Coast Protection Act applies.

Who is involved: MAFF, Welsh Office, maritime local authorities, Environment Agency.

What is happening: Local authorities consider the need for coastal protection work and may promote a scheme provided the required technical, economic and environmental standards are met. The cost will generally be supported by grant aid from MAFF or Welsh Office and contributions from counties where two-tier local government applies. Shoreline Management Plans will help to take a strategic look at coastal defence. The Environment Agency acts as an advisor.

THE IMPACTS OF COASTAL EROSION

C4 Concern about the loss of salt-marsh, other intertidal habitats and beach habitats because of erosion

The natural physical response to a rise in sea level is for increased erosion to cause the shoreline to move inland. Soft shores, particularly salt-marshes can be 'squeezed' between the advancing sea and hard cliffs or coastal defences. This inter-tidal zone provides an important habitat and is also important to coastal defence because it provides a natural buffer against wave action. The Severn Estuary has significant salt-marsh and wetland areas, particularly in the upper reaches, which may be so affected.

Who is involved: Maritime local authorities, Environment Agency, coastal cell groups, English Nature, Countryside Council for Wales.

What is happening: In some cases the best action may be to allow the land to become inter-tidal by moving the defence line inland. This is sometimes known as managed retreat, setback, or coastal realignment. Sometimes the defences are deliberately drawn back to provide environmental gain such as an increase in salt-marsh. An example is at Porlock in Somerset where sea defences cannot be naturally sustained and local organisations are working together to manage the future of the site. Compensation may be payable and MAFF or the Welsh Office can make payments to create salt-marsh on suitable land under their Habitat Scheme. In the upper part of the estuary opportunities for 'managed retreat' are limited by the presence of residential property and by cost. The existing defences are usually built on the higher ground near the shore where salt-marsh accretion is greatest - it would be much more expensive to build defences on the lower ground further inland. In other parts of the estuary opportunities for managed retreat may be restricted by the existing areas of nature conservation value.

Some suggestions: Where 'coastal squeeze' occurs against coastal defences, 'managed retreat' may be an option (see issue C3). There is a need to assess those areas affected. Many salt-marshes are designated as Sites of Special Scientific Interest. The use of the Countryside Stewardship scheme may help.

THE IMPACT OF COASTAL DEFENCES ON OTHER USES

C5 The impact of coastal defences on nature conservation, landscape, archaeology and amenity

There is some public concern that sea defences and coast protection damage or change wildlife habitats and the landscape and can affect archaeology and amenity. Sea defences and coast protection may need to be improved to respond to changes such as rising sea level and development in areas at risk from flooding or erosion. At the local level these can lead to the loss of, or changes to gardens, natural vegetation and wildlife habitats, access to the shore and views of the estuary. At a larger scale they can intrude into the perceived naturalness of the estuary and influence use of adjoining land.

Who is involved: Environment Agency, local authorities, MAFF, Welsh Office, National Farmers Union, English Nature, Countryside Council for Wales, landowners, English Heritage, Cadw and other organisations involved with the preparation of Shoreline Management Plans.

What is happening: Defences are only constructed following a rigorous analysis of the need. The Environment Agency and local authorities ensure that environmental assessment is part of the early design of all sea defence and coast protection schemes and consult the statutory organisations, landowners and voluntary organisations. Coastal Cell Groups, which include the statutory organisations involved with the coast, are producing shoreline management plans for their coastlines which address all of these issues.

They carefully consider the best defence options. 'Hard' defences such as concrete wave return walls, gabions or rock armour are often the most appropriate in developed areas, such as to protect coastal towns. However, such defences eliminate the natural shoreline, removing wildlife habitats and detract from the landscape. So called 'soft engineering' solutions will be used where appropriate, with careful and sympathetic choice of materials following public consultation and consistent with the preferred strategic option defined in the Shoreline Management Plan.

Previous archaeological discoveries during work on sea defences such as Goldcliff highlight the need for an archaeological watching brief to be maintained through the period when earth moving activities are occurring. This has recently happened in relation to sea defence upgrading south of Brean Down.

Some suggestions: Improve information available to the public about flood defence and coast protection schemes, including the aim of such schemes, the benefits, the impacts on nature conservation, landscape and amenity and the reasons for the choice of design. Further consideration could be given to soft engineering options and managed retreat, whilst appropriate levels of compensation/ grant-aid could make these more viable alternatives.

THE IMPACTS OF GLOBAL WARMING ON COASTAL DEFENCES

C6 The need to improve defences because of raised sea levels resulting from global warming

It is likely that because of global warming sea levels worldwide will rise by more than 500 mm in the next 100 years, although the present rate is probably about 2 mm per year in the Severn Estuary. With a rise of, say, 3 mm per year, tide levels which have a statistical probability of occurring once every 200 years on average at Avonmouth will be twice as frequent (once in 100 years) by 2006 and twice as frequent again (once in 50 years) by 2016. As Table 5.3 shows the Environment Agency uses these probabilities to design target standards for different land uses. Hence, a scheme designed to meet the standard of protection for high density urban development would only meet that appropriate for rural communities, by 2016, if additional action was not taken. The improvement of defences will take place within the strategic framework of Shoreline Management Plans.

In addition it has been suggested by scientists that storms will become more frequent and more violent as a result of global warming although this effect has not yet been confirmed. Storms can raise sea levels above predicted levels and generate increased wave action, causing overtopping and increased erosion of existing defences.

Who is involved: Maritime local authorities and the Environment Agency.

What is happening: MAFF and the Welsh Office have agreed that a global warming allowance of 5 mm per year should be built into the design of all new sea defence schemes. This is above the current estimates of rise. Furthermore they require that all new schemes are designed so that further raising can be provided in future.

C7 Reduced drainage from coastal land because of sea level rise

Sea level rise will impede drainage from the land. A tide which today just reaches the mean high water level at Avonmouth can be expected to exceed that level for about an hour and Ordnance Datum for about half an hour longer, with a sea level rise of 300 mm. This increase in 'tide-lock' may increase land waterlogging and lead to increased dependence on stormwater storage and pumping. It may also have significant effects on the drainage and regime of rivers which outfall into the Estuary. Lengthening of tidal pills because of tidal accretion can also be a problem. Rivers affected are the Parrett, Avon, Wye, Usk, Rhymney, Taff, Little Avon, Frome, Dimore, Horsebere, Hatherley, Leadon, Cinderford Brook and the Lyd.

Lowland drainage is an important issue for coastal zone internal drainage boards. A number of internal drainage boards abut the coast or drain to rivers which are themselves 'tide-locked'. These include the Caldicot & Wentlooge Drainage Board, South Gloucestershire Drainage Board and a number of Boards within the Somerset Levels & Moors. Internal drainage boards wish to see lower water levels in their areas to increase flood storage to compensate for the longer period of tide-lock. This may affect nature conservation areas.

Who is involved: Internal drainage boards, the Environment Agency, local authorities, nature conservation agencies.

Some suggestions: The effects of sea level rise may identify a need for improved outfalls, stormwater storage and pumping arrangements. Water Level Management Plans should be prepared for sensitive sites. See also issue C4. Ensure development behind defences does not reduce flood storage volume or further erode defences.

THE NEED FOR TIDAL FLOOD WARNING

C8 The need to improve tidal flood warning system

Flooding can occur any time that certain climatic conditions coincide with high spring tides but the risk of flooding is highest from September to April.

Who is involved: The Environment Agency.

What is happening: Flood forecasting and warning systems enable emergency services, operating authorities and individuals to take measures to lessen the impact of flooding where protection cannot be provided. A national Storm Tide Warning Service is operated by the Meteorological Office. The Environment Agency uses this information, together with meteorological forecasts and its own network of tide level gauges, to forecast flooding problems and to inform maritime local authorities or emergency services who may be affected. This service is not as comprehensive as the river flood warning schemes, such as for the River Severn upstream of Gloucester. At present, on the estuary upstream of Avonmouth, only Severn Beach receives formal warnings. The Environment Agency has plans to improve tidal flood warning systems once improved forecasting systems have been developed.