| European Code of Conduct for Coastal Zones | ||
| V. COASTAL PROTECTION
5.1. Status and Trends Coastal defence is the general term which covers all aspects of
human initiated defence against coastal hazards such as flooding
and erosion. Coastal defence efforts may be small scale involving
relatively small structures or may involve extensive land claims,
e.g. by establishing buffer zones. Coastal defence structures are
generally concentrated on coastal plains around cities and harbours,
tourist areas, industrial complexes and infrastructure. In other
words, coastal defence is concerned primarily with the protection
of economic interests rather than natural habitats.
There are a wide range of engineering methods and techniques for
coastal defence which operate in different parts of the shoreline.
Offshore techniques operating away from the shoreline include offshore
breakwaters, stable bays, and barrages and lately nourishment. Techniques
operating on the lower shore between low and high tide include beach
nourishment, groynes, revetments, and sedimentation polders. Sea
walls, revetments and flood embankments are located at or just above
high tide on the upper shore as is the landward extent of the managed
retreat technique. Finally, supra shore techniques which operate
at or above spring tides include dune building, cliff strengthening,
and beach ridge restructuring (Ministry of Agr. Fish. and Food,
UK,1993).
There are two main types of approaches to coastal defence; Hard engineering: establishment of structures which aim to resist the energy of the waves and tides. Such structures include; breakwaters and seawalls designed to oppose wave energy inputs, groynes designed to increase sediment storage on the shore, and flood embankments and barrages designed as water tight barriers. Soft engineering: establishment of elements which aims to work with nature by manipulating natural systems which can adjust to the energy of the waves, tides and wind. This approach has economic benefits while minimising the environmental impact of traditional engineering structures. The methods which can be used include artificial nourishment, the set back of structures and plantations of osier hedges and marram grass. In practice most coastal defences incorporate aspects of both these approaches (Ibid). Current trends favour the concept of shoreline management, working with the dynamic nature of the coastal environment rather than fighting against the forces of the sea. This is best exemplified by the widespread move away from hard engineering methods of coastal defence which act to restrain coastal processes, towards soft engineering approaches which recognise the dynamic nature of the coastal environment by utilising these processes to advantage. Soft engineering methods usually have a lesser impact on the environment and may require less maintenance. Present and forecast sea level rise, and an increase in the frequency and force of coastal storms resulting from climate change is likely to mean that coastal defence efforts will become increasingly necessary to protect against erosion and flooding. According to the 1995 Assessment of the Intergovernmental Panel on Climate Change (IPCC), global sea level has risen 10-25 cm over the last 100 years, and may rise an additional 15-95 cm (with a best estimate of 50 cm) by 2100 (IPPC 1995) . The implications for shoreline management are clear. Shoreline retreat resulting from a 1 m rise in sea level has been estimated at 50-1000 m for some beaches in the United States for example (Titus et al 1993). UNEP predicts that "by the middle of the next century, damage to coastal settlements, harbours, coastal roads, and other infrastructural features could be considerable as most of these developments are only slightly above the present mean sea level." Given that approximately 30% of the EU member States beaches are already estimated to be eroding, serious attention to coastal defence strategies must be paid. According to a 1994 ENVIREG report (ENVIREG 1994), "Millions of ECU are being set aside for coastal protection and sea defences but the traditional approach of erecting ever larger embankments is looking less viable. If the conventional engineering approach is adopted then the area of wetlands is expected to be further and dramatically reduced, caught between a rising sea and strengthened embankments." Whilst it will still be worth protecting built-up areas, the large Community surplus of agricultural land means that there is no longer a clear case for continuing a policy of land retention and reclamation. Alternatives, according to ENVIREG, include soft-engineering methods such as managed retreat (see below). Coastal defence strategies vary according to the location. Where the cost-benefit analysis on coastal defence expenditure takes into account the potential to recreate natural wetlands, there will be places in which land will be allowed to return to its natural state, acting as a buffer zone. Wetlands provide effective natural storm defences, absorbing the energy of the waves and accumulating sediment that raises the level of the land. This idea of allowing certain agricultural areas to flood is under discussion in many countries in Europe, given the desired reduction in agricultural output and the high cost of traditional methods of coastal defence. Artificial nourishment is another coastal defence technique which
increase the volume of sand on a beach or in the foreshore by the
importation and emplacement of sediment to form a new foreshore
and/or beach profile (de Ruig 1996)
. Usually sediment is supplied directly from offshore through
hydraulic pumping, but sand can also be derived from other sources
such as dredging. Regular beach nourishment has been the main method
to combat erosion of the Dutch coastline since 1991 (Ibid).
5.2. Impacts The impacts of coastal defences vary widely according to the techniques used, their specific design and the characteristics of the local environment. Some generalised impacts of coastal defences include (Helcom 16/17, Annex 6) :
Defensive structures which are designed to reduce wave energy at the shore often result in the build up of sediment in the wave shadow of the structure. In some situations this may lead to covering or other changes to existing shoreline ecosystems. Hard defence techniques which reduce upper shore and cliff erosion also disrupt longshore sediment transport which often leads to the accelerated erosion of adjacent shorelines (Ibid) . Some structures can be visually intrusive or can limit access to the shore and sea. They often present serious navigational and/or safety hazards. Where low cost materials are used, such as motor cars, tyres or sunken ships, long-term breakdown presents pollution hazards (Ibid). Defence techniques located in estuaries to protect against flooding such as barrages, tidal surge barriers and flood embankments can seriously disrupt the natural processes of these ecologically rich environments (Ibid). Nourishment techniques, if not carefully designed and/or if improper fill material is used can result in increases in the turbidity of coastal waters, and the continued wash-out of fine material can have long term negative effects on adjacent benthic and inter-tidal ecosystems. Changes in beach grain distribution can lead to the incursion of coarse-grained material over supra-tidal ecosystems such as lower cliff or dune communities. Rapid sediment deposition can swamp inter-tidal invertebrate communities and have serious effects on feeding birds (Ibid). Sea walls and other upper shore structures, if placed too close to the waterline, reduce the active width of the beach and dune during storms. This significantly disrupts the sediment balance and causes erosion especially along downdrift stretches of coastline. They also result in wave reflection leading to a lowering of the foreshore and sometimes to the undermining of the toe of the seawall, which may ultimately cause it to collapse. Sea walls prevent sediment transport between beach and dune resulting in the deterioration of these environments (Ibid). Finally, afforestation of coastal dunes with non-native species,
primarily for the purposes of coastal defence, has disturbed the
natural dynamics of coastal systems. Impacts from such afforestation
are discussed in the Chapter on "Forest Management". 5.3. Opportunities With a rise in sea level, the width of the intertidal zone is reduced as low water levels rise and high water marks are held in place by hard defences. This results in a significant loss of habitats particularly salt marshes and mudflats, which in turn reduces the buffering protection afforded to hard defences such as sea walls. These then can become increasingly destabilised as the wall toe is exposed and undermined. This is increasingly likely to occur on a large scale and may require large capital expenditure in order to upgrade defensive works (English Nature 1995). The increasing recognition that global sea level is rising combined with the recognition of the coastal squeeze phenomenon has led to the development of the concept of managed retreat . Managed retreat involves setting back the line of actively maintained defence to a new line inland of the original, or preferably to rising ground, and promoting the creation of intertidal habitat on the land between the old and new defences. Depending on the situation and the requirements of the scheme the original front defence may be retained until it degrades naturally, or may be either wholly or partially removed. The intervening land then forms a new, wider intertidal profile which is more able to respond to coastal processes and reduces the effect of coastal squeeze (Ibid). Managed retreat is not a do-nothing option but requires active management. There are two main advantages of managed retreat. Firstly, set back defences have an additional element of wave absorption provided by the widened intertidal zone and therefore are of a lower specification and cost. Secondly, the creation of a widened intertidal zone reduces coastal squeeze (Ministry of Agr. Fish. and Food, UK, 1993). Environmental opportunities should be considered in all coastal
defence schemes. Such schemes may provide opportunities to enhance
the landscape and restore or create coastal habitats such as saltmarshes,
brackish lagoons, and artificial reefs. Sympathetic engineering
techniques for environmental improvement can be practised to provide
a diversity of habitats. Even the most unpromising environments
can often be improved, and apparently sterile habitats offer some
of the greatest challenges (Ibid). 5.4. Guidelines for Coastal Defence When considering coastal defence works the dynamic character and permanent change of the coast should be recognised and accepted as an integral factor of coastal zone management and planning. Natural processes should only be disrupted by coastal defence works when life or important assets are at risk. Development in areas at risk should take place only if it is accompanied by coastal defence programmes ensuring acceptable standards of safety. The whole project, i.e. the development and the coastal defence system together, should be subject to EIA and should be proven to be in the long term public interest. A risk assessment of all coastal areas should be carried out to determine the impact of sea level rise and coastal retreat so that planning strategies and development zones can be determined. In accordance with the principles of sustainable development, coastal defence schemes should not tie future generations into expensive or inflexible options. Defence measures should be part of a strategic plan for the relevant coastal area in which all defence works are based on a scientific understanding of natural coastal and river processes. Wherever possible coastal defence measures should be nationally or regionally incorporated into integrated coastal zone management plans which:
Certain designated areas require special consideration when determining coastal defence strategies. New coastal defence measures not designed to protect settlements or other exceptional values should normally not be executed as they are of questionable economic value and may impose negative impacts on the environment. In fact it is often both economically and environmentally beneficial for coastal areas outside settlements that have been subject to episodic flooding before they were dyked for land use purposes only, to be restored as coastal wetlands through the removal or relocation of dykes further inland. In order to avoid shifting the burden of coastal defence from one area to another it is important that cliffs as sediment supplier and natural coastal flood areas as potential nutrient traps, should not be subject to any new coastal defence measures. Techniques The managed retreat option should be considered in order to preserve coastal ecosystems, such as saltmarshes. Where coastal defences are necessary the use of natural materials such as stones, sand, soil, or wood is preferable to artificial materials such as concrete, asphalt, or plastic as these are foreign to the coastal landscape and may cause pollution upon disintegration. Soft engineering methods using natural materials are generally preferred over hard engineering methods as they typically have a lesser impact on the environment and better maintain the natural character of the coast. If hard measures are necessary, they should be located as far inland as possible. For beach/foreshore nourishment marine sand with proper grain sizes should be used, containing only a minor content of fines so that natural sediment transport processes are not excessively disturbed and in order to reduce the turbidity of coastal waters during extraction and deposition thus maintaining coastal water standards. Where dredge sediments are used they must be tested for pollutants and should not be used if pollutant levels are detected. (See guidelines on sand extraction.) Beach nourishment should not be carried out annually; beach flora and fauna should be given ample time to recover. Dune management techniques should encourage the preservation of dune slack areas and a diverse dune flora by avoiding planting regimes which concentrate on marram or trees where possible and allowing mobile sand areas to develop. Project construction The construction phase of any coastal defence scheme should be planned and carried out with special care and should include consideration of the:
A post-project appraisal is a recommended practice as it encourages the mitigation of any reported and unforeseen environmental problems and aids in the improvement of the design and implementation of future schemes. Such an appraisal should include:
Guidelines relating to dune afforestation
for the purposes of coastal defence are located in the Chapter on
"Forest Management". REFERENCES: Coastal Protection
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