Managing coastal erosion

Managing coastal erosion

1 Introduction Nowhere can natural processes and human activities come into greater conflict than in coastal areas. Despite extensive attempts to reduce the effects of coastal erosion, coastal flooding and cliff falls occur on a regular basis. With the threat of a rise in the global sea level due to the 'greenhouse effect' and global warming, interest in new management strategies awakens.

Some of the broad strategies adopted to deal with coastal problems within Britain and the USA are identified and discussed in Geofile no. 122, Coastal management (January 1989). In this Geofile, current ways of thinking are identified, together with several detailed case studies.

2 Why manage coastal erosion? Over the past two centuries coastal areas have undergone a dramatic increase in industrial, urban and recreational use. Early attempts to combat erosion were made during the nineteenth and early twentieth centuries in this country, when large-scale engineering works were carried out on many parts of the British coast. Piers, groynes, breakwaters and sea walls appeared off coastal towns such as Brighton, Eastbourne and Margate. Old maps reveal that coastal erosion has been affecting human use of the coast for much longer; large areas of the old town of Brighton were lost to the sea between 1292 and 1705. In 1723 the sum of fl,700 was raised to construct two huge groynes to save the rest of the town (Hansom, 1988).

Erosion of soft-rock coasts, such as the sandy till cliffs of Norfolk has been documented for over 800 years. Old maps show that land was being lost at rates of up to four metres per year on the stretch of coast between Cromer and Mundesley during the period 1 8 3 8 to 1 8 6 1 (Younger, 1990). During a recent 15-year period, 50 metres of coast has been eroded at Sidestrand, south-east of Cromer.

Coastal erosion continues to be a major threat to human occupation of the coastal zone. On unprotected stretches of the British coast, erosion continues at an alarming rate, often exceeding two metres per year in some

Figure 1: Areas of Britain ar greatest risk from rising sea levels

areas. It is estimated that over 40% of the English coast is currently protected by some form of sea defence. In addition, 21% of the coast of Wales, and 7% of the coast of Scotland, is also protected. With sea level predicted to rise by up to 1.10 metres by 21 00, many other parts of the British coast will be in need of protection (Figure 1).

3 Why does coastal erosion occur? The most obvious reason is that coastal erosion is a natural process; that it is part of a cycle of erosion, transportation and deposition of sediments by wave action. It is only when human activity is affected that attempts are made to challenge these natural processes and to try to control them. There are other reasons, however, which might help explain why coastal erosion has increased over the recent past and is likely to increase further in the future.

(a) Decrease in sediment supply (i) Reduction in available beach material. Sea level has risen by over 100 metres since the coldest phase of the last glaciation. The advancing sea transported large volumes of unconsolidated sediment from the exposed continental shelves to form many of our present-day beaches. Since that time, processes of attrition and longshore drift have depleted this sediment, and our beaches are not being fully replenished by new material. At present, British rivers do not supply a great deal of pebble-size material, as they might have done during the later stages of the last glaciation when meltwaters contained coarse glacial and periglacial debris.

(ii) Colonisation of beach material by vegetation. Many sand and silt beaches have been colonised by plants, thus stabilising the sediment and removing it from the system (albeit on a temporary basis in some cases).

(iii) Human interference. The building of many coastal defences such as breakwaters and groynes has led to a reduction in the availability of beach material further down-drift (the 'knock-on effect') as longshore drift is impeded. This was seen clearly in Seaford in East Sussex after the construction of a harbour wall at Newhaven in 1890. On the Black Sea coast of Romania, the building of jetties at the mouth of the Sulina, one of the distributaries of the Danube, has prevented the southward drift of sand and silt. This has resulted in considerable erosion to the south of the Sulina.

Figure 2: Predicted changes in global sea level to AD 2100

100

50

0 2000 2050 21 00

Source: Daily Telegraph, 28 February 1990

l Source: Dawson (1992)

September 1992 no. 201 Managing coastal erosion

(iv) Gravel extraction. The extraction of large volumes of gravel from offshore zones for commercial purposes has led to a number of erosional problems. Not only is the availability of sediment reduced, but the geometry of the sea floor is changed, thus setting up new wave patterns and affecting longshore drift. Following the dredging of 650,000 tonnes of gravel near Hallsands in Devon between 1897 and 1901, the village was destroyed by storms during 1903 and 1917. Recent research in Japan has shown that the dredging of over 20 million cubic metres of sand from the Genkai Sea has been largely to blame for rapid erosion of nearby beaches. Aerial photos reveal that beaches have retreated by up to 45 metres since 1945.

(b) Global warming and sea level rise Estimates of sea level rise vary. Recent estimates suggest that UK sea level rises will be between 0.2 and 1 .l0 metres by 2100 (Figure 2). This will clearly imperil low-lying areas such as the Fens and the estuaries of rivers such as the Mersey, the Thames and the Severn. On a global scale, countries such as Bangladesh and island communities such as the Maldives face an ever- increasing risk to human life from coastal inundation. It is thought that rates of erosion are also likely to increase as a.result of increasingly high tides and storm surges.

4 How should coastal erosion be managed? The best method of reducing coastal erosion is to establish a beach. Beaches are generally very effective at absorbing the energy of waves that hit them. Many traditional methods of coastal management rely upon the creation or the simulation of a beach. Increasingly, however, there is a belief that we should not fight nature; that we should come to terms with coastal processes and work with them, rather than against them.

(a) Structural responses: the 'hard engineering approach' Many coastlines have traditionally been protected by sea walls and other forms of basal structure, or by groynes (Figure 3). In many cases, a combination of methods is used:

1 SEA WALLS Aim: to absorb wave energy in place of a beach and to protect the bottom of cliffs from wave attack. Designs include bull-nose concrete walls, slatted revetments and stone and concrete blocks.

2 GROYNES Aim: to trap and then stabilise shingle by slowing down longshore drift. Groynes are usually placed at an angle of five to ten degrees to the perpendicular in order to prevent scouring on the leeward (downdrift) side of the groyne. The exact angle will depend upon the direction of the prevailing waves. The length and spacing of groynes are also related to wave direction; the relationship between the length and spacing usually varies between 1 :4 and 1 :10. Groynes are usually constructed out of hardwood timber, although concrete, stone and steel sheeting are used in some places.

figure 3: Structural responses to coastal erosion

Source: Knapp, Ross and McCrae (1989)

(b) Non-structural response: the 'soft engineering approach' This approach is based largely upon the philosophy that human interference should be minimised, and that natural processes of beach renewal should be maximised.

(i) Beach nourishment. This is the replacement of beach material that has been removed by longshore drift and by attrition. Successful schemes have been in operation since the 1930s. The Copacabana Beach in Rio de Janeiro, Brazil, was rebuilt in 1970 with beach sand from a nearby area. In Bexhill, Sussex, erosion accelerated after extensive sea defences were constructed at Eastbourne in the early 1940s. Groynes alone did not solve the problem, so between 1975 and 1985, in addition to the construction of over 100 new groynes, over 150,000 cubic metres of shingle was placed on the beach. The shingle came from three main sources: nearby land-based gravel pits, dredged from offshore, and recycled from nearby Hastings beach, where gravel accumulation was becoming a problem. At nearby Seaford, major engineering works were completed in 1987 at a cost of €13 million. Every year about 160,000 tonnes of shingle is moved from the eastern part of the beach back to the west, at a cost (in 1987 prices) of €60,000 per annum. The defences were completed just in time to save Seaford from severe flooding during the 'hurricane' of October 1987.

(ii) Beach stabilisation by reducing slope angle, providing drainage of cliffs and revegetating vulnerable coastal areas. This has been carried out successfully at Whitby in Yorkshire, and in several locations in Cornwall.

(iii) Wave alteration; this involves trying to reduce the erosional power of the waves. Plans were considered to build a submerged barrier off the Norfolk coast. The aim is to change the rhythm of the waves and thus reduce their ability to erode the cliffs.

(c) Other recent initiatives In 1991 plans were submitted to build a €30 million scheme off the Holderness coast. This involved building an 'armour-plated' reef of mine and quarry waste, bound together by recycled fuel waste from nearby furnaces. In early 1991 Lincolnshire tried a scheme to combat dune erosion which involved the planting of discarded, rooted Christmas trees!

(d) Do nothing! Instead of asking the question 'how should we protect the coast from eroding?', perhaps we should ask ourselves whether we should protect the coast at all. Millions of pounds of taxpayers' money are spent annually on protecting coastal land from natural forces (in 1991 over €26 million, and in 1992 an estimated €35 million was spent by central government alone on coastal defence work). At Barton-on-Sea a €1.3 million project was destroyed within four years. Sea defences need constant attention and lots of money; perhaps it would be cheaper to let nature take its course and pay compensation to those affected instead.

(e) Retreat from the coast! Norfolk became the first British county to propose a ban on development upon coastal areas. In their current structure plan they propose a 75m 'setback line' from the sea. Erosion in Norfolk is currently over 1 metre per year, but reaches far higher levels on a local level. In parts of the USA 'retreat' policies have been in operation for some time. In Northern California small properties have been required to be built at least 30 times the annual rate of erosion away from the shore since legislation was passed in 1979. Larger properties must be at more than double this distance.

5 Where now? How should we be managing coastal erosion? Dr Robert Kay of the School of Environmental Studies at the University of East Anglia claims that hard engineering methods 'push us into a spiral of spending more and more money to mess up our shorelines to build bigger and bigger walls'. Certainly there have been notable failures and doubts about expensive schemes: defence work at Peacehaven in Sussex cost €4.5 million between 1976 and 1982 for a 2.2 km stretch of coastline. Even in 1986 it was evident that the groynes were not fully effective; they were too widely spaced in relation to their height and were lightly constructed. They were being overtopped by shingle, and the leading groyne was badly damaged in winter storms. Undercutting of the shore platform upon which the groynes and sea wall were built is actively occurring at a rate of several millimetres per year. The knock-on effect down the coast is now beginning to emerge, According to Cleeve and Williams (1985), the total 'benefit' of the scheme was a mere €0.4 million per kilometre, and yet the scheme went ahead. To make matters worse, planning permission has not been restricted within the coastal strip.

September 1992 no. 201 Managing coastal erosion

One of the main problems with the 'soft engineering' approach is that people have yet to accept that some coastal areas are beyond saving, and should be left to the sea. A second problem is that coastal management is the responsibility of several bodies. In East Sussex, for instance, a 60-km stretch of coastline is administered by 12 different authorities.

Many authorities would like to see a national approach to coastal management; a national body which would oversee all aspects of coastal management, from pollution and erosion to flooding and recreational use. The adoption of 'setback lines' for planning is also favoured by many (Figure 4). With government spending on coastal erosion reaching record levels of f35 million in 1992, it may be time to consider alternative approaches.

figure 4: 'Sefback' from the coast - a non-structural response to coastal erosion

6 Case studies (a) The chalk cliffs of Sussex The chalk cliffs of Sussex lie between Brighton and Eastbourne (Figure 5). They rise vertically, reaching a maximum height of 165 metres at Beachy Head. Retreat is rapid; from studies of old maps they are estimated to be retreating by between 30 and 50 centimetres per annum, a rate which reaches over 1.25 metres in places. Retreat is most common in cold winters (freeze-thaw) and dry summers (shrinkage). Retreat tends to be intermittent, and follows a cycle (Figure 6). In addition to basal erosion and cliff fall, the shore platform is being lowered at rates of up to lOmm per annum.

figure 5: Coastal defences in East Sussex

Source: Robnison and Williams (1983)

figure 6: Retreat of chalk sea cliff..

A B Fissure g m downwards c I

Source: Cleeve and Wlliams (1985)

Erosion has been exacerbated by the construction of major structures such as Brighton Marina and the harbour breakwater at Newhaven. The latter was completed in 1890, and over 60,000 cubic metres of shingle was reported to have accumulated within the first three years of the life of the 3,000 feet long breakwater (Joliffe, 1964). Today, the accumulation of shingle starts over 800 metres west of the breakwater. The problems faced by nearby Seaford are considerable. Along the coast between Newhaven and Splash Point at Seaford groynes were first constructed in 1836 and a major sea wall was built

in 1881. In 1898 the Newhaven and Seaford Sea Defence Act enabled extensive coastal works to be carried out. Neglect during and after the First World War led to accelerated erosion once again, with beach levels falling by over three metres and the low water mark moving over 60 metres inshore. The seawall was increasingly vulnerable to storm waves. Shingle nourishment was first attempted in 1936, when over 6,000 cubic metres of shingle was placed on the beach at Seaford. Between 1945 and 1966 further work on the sea wall was carried out. The importance of retaining an adequate shingle beach to act as a first line of defence has always been recognised. Early groynes proved too short to be fully effective, so longer groynes were constructed during the 1950s. Despite this further work, however, the area remained vulnerable. In 1982 a report was published by the Southern Water Authority outlining possible options for new defence works. All methods of management were considered, and some rejected immediately; artificial offshore barriers, for example, were rejected on the basis of expense and the difficulty they would cause for navigation. Four options were eventually proposed for the 2.4 km stretch of coastline. All are based upon 1982 costs:

(i) A new sea wall, able to withstand storm waves independently of beach

(ii) Interlocking precast concrete wave breaks on the beach designed to

(iii) Beach restoration and nourishment (initially 1 million cubic metres)

(iv) Repair existing defences and continue to feed the beach; this would

protection.

dissipate wave forces away from the existing wall.

together with new groynes.

eventually lead to option (i).

Figure 7 summarises the costs of construction and maintenance of the four options.

figure 7: Estimated costs of options for new coastal defences at Seaford

Option Estimated Estimated

ance cost: cost (1 ) mainten- capital annual gross

pletion (2) on com-

f f (i)

10,000 26.1111 (iv) 165,000 8.4m (iii) 10,000 16.8m (ii) 10,000 24.0111

>onstruc- tion

period

Years 15 10 4 50

Net we: Capital costs

f 15.lm 11 .Em 6.8m 8.9m

nt value Mainten-

costs (3) ance

f 0.5m 0.4m 2.2m 1.6m

Total com-

parative net

present value

f 15.6111 12.2m 9.0m

10.5m

Notes: (1) All estimates at 1982 prices. (2) Initial annual maintenance costs will vary. Figures shown are estimated

(3) Maintenance costs over 50 years discounted to net present value. annual expenditure at end of construction period.

Source: Southem Water Authority, 1982

All four options were designed to provide a similar standard of protection. Options (i) and (iv) would have created the most adverse conditions on the promenade and highway, with the need to close the road as at present. Options (i) and (ii) would have restricted the recreational use of the beach to the greatest extent. Option (iii) appeared to have the least impact and the lowest cost, although it would need thorough modelling and testing before adoption. On 1 April 1987, work commenced on Option (iii), and was completed in October of the same year. Over three million tonnes of shingle was placed on the beach, much of it having been dredged from 800 metres offshore. Favourable reports of the beach continue to be published in the local press. What remains to be investigated is the 'knock-on effect' further down the coast.

(b) Coastal erosion in New Zealand New Zealand has about 10,000 km of coastline. Approximately one quarter of New Zealand's coast is considered to be eroding at the present time. The 'Kapiti Coast' of New Zealand is on Wellington's west coast (Figure 8). The beach is made up mostly of sand, although there is gravel nearby. Sand dunes lie behind the beach. Between 1906 and 1923 housing was built on the seaward side of the dunes at Paekakariki and Raumati. Although subsequent developments left a natural buffer zone between houses and the

September 1992 no. 201 Managing coastal erosion

Figure 8: Coastal erosion on New Zealand's Kapiti Coast

Source: Macauley and Clay

beach, new properties have been encroaching on to the fragile foredunes in recent years. Despite historical evidence of erosion, settlement continued. In 1954 beaches at Paekakariki and Raumati lost two to three metres of sand dunes to erosion through heavy waves. Action in the form of setting up pole and brush fences was taken, but still the erosion continued. In September 1976, storm waves eroded 15 metres of dunes and took the coastal protection works with them. Thirty houses were threatend and one was lost. The response to this was to construct a 4.5 km timber sea wall from Raumati to Paekakariki in 1978. Despite the fact that some of it was destroyed during construction, it was completed. Maintenance is expensive and continuous.

There are several reasons for the erosion of the Kapiti coast.

(i) The growth of a cuspate foreland at Paraparaumu beach is reducing the supply of material by longshore drift.

(ii) Construction of the main railway and state highway along the coast restricts the supply of sediment from the south.

(iii) The seawall constructed to help reduce erosion is actually increasing it by reflecting wave energy, increasing turbulence and keeping fine sand in suspension. This prevents deposition and causes beach material to be lost from in front of the sea wall.

(iv) Storm frequency is unpredictable.

The best solution for the Kapiti coast would be to allow erosion to occur naturally and to set future development back from the edge of the dunes.

7 Conclusions Clearly there are a number of options open to those who seek to protect human use of the coastal zone. It should be remembered, however, that coastal erosion is a natural process: it only becomes a problem when it threatens human activities. The main problem would therefore seem not to be coastal erosion, but development of human activities within the zone of natural beach movement. Over the past two hundred years, human activity has increased in this zone; removal of vegetation, urbanisation and sea defences which prevent natural regeneration of beaches by longshore drift have all contributed to the current situation.

There are three main responses to coastal erosion:

0 to prevent erosion from becoming more of a problem 0 to protect selected parts of the coast from erosion 0 to take no action and to accept the losses

Prevention 0 draw up policies for coastal land use and restrict development by

0 map coastal hazard regions; study old maps, photographs and use local

0 protect highly vulnerable zones such as sand dunes with no-build

0 Consider paying compensation to threatened home owners 0 restrict sand and gravel extraction in offshore zones unless thorough

creating 'setback lines

knowledge and old records

policies.

research has been carried out.

Protection 0 Hard engineering solutions, such as groynes and breakwaters 0 Soft engineering options, such as beach nourishment.

Take no action 0 Accept losses; pay compensation; set up disaster funds.

As sea levels threaten to rise, it is evident that new policies must be found to cope with the increased risk to human activities and lives. At present there appears to be a movement towards the 'soft' approach such as beach nourishment, combined perhaps with a policy of 'setback from the coast. According to the Natural Environmental Research Council it will cost over f5 billion to protect Britain's coast against a sea level rise of 1.5 metres. Wholesale abandonment of low-lying areas is unlikely to occur; areas such as the Fens are too valuable agriculturally and ecologically to be left to the ravages of the sea. According to Dr Michael Tooley of Durham University's Geography Department, the future will be decided by simple economic principles: cost-benefit analysis.

Bibliography Southern Water Authority Report on Seaford sea defences (1982)

Various articles from Coastal Engineering, New Scientist, The Independent on Sunday, The Daily Telegraph, The Guardian, Geography Review

Cleeve and Williams (1 985) Cliff Erosion in Sussex, Sussex University

Dawson, A. (1992) Global climatic change, Oxford University Press

Hansom, J.D. (1988) Coasts, Cambridge University Press

Knapp, Ross and McCrae (1 989) Challenge of the Natural Environment, Longman.

Macauley, J. and Clay, A. (1987) 'Nature fights back: A Geography of New Zealand's Natural Hazards, Longman.

Robinson, D. A. and Williams, R. B. G. (1 983) 'The Sussex Coast Past and Present', in Sussex: Environment, Landscape and Sociefy, Sussex University Younger (1990) 'Will the Sea always win?' Geography Review

Acknowledgements GEOFILE researched and written by Anne Fielding Smith and Roger Smith Maps and diagrams: Mike Ormond, Micrographics, Llandudno, Gwynedd Printed by Hollen Street Press, Slough, Berkshire

0 1992 Mary Glasgow Publications Ltd, Avenue House, 131-133 Holland Park Avenue, London W11 4UT

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