浏览： 日期：2020-06-10
Coastlines display enormous diversity. This is due to the variety and complexity of the factors influencing coastal morphology. In very general Davisian terms, any shoreline is a product of “structure, stage and process”. In other words, in analysing any shoreline, one must take into account structural factors such as the arrangement of different rock types and their resistance to wave attack and solution, as well as other geological considerations such as the angle of dip and pattern of bedding and jointing of sedimentary strata, The form of any shoreline is also a product of its age and the stage reached in its evolution; that is to say, one must take into account former geological processes and earlier changes of climate and sea-level which may have produced particular features of the present coastline. Finally, the contemporary processes of coastal erosion and deposition operating on the shoreline are obviously important in determining its form, as are various other physical, chemical and biological processes operating above the tidal zone, together with human activity which is a rather specialized but important cause of coastal change. The multiplicity of factors involved and their local variations result in a wide variety of coastal landforms. Thus, “even within the small compass of Britain, one can contrast sinuous inlets of the South-West, the great sea lochs of Scotland, the low glacial coastline of East Anglia, the marshes of the Thames Estuary and the imposing chalk and limestone cliffs of the south coast” (Goudie, 1993).
It has frequently been argued that systems of classifications are simply aids to description and understanding. By reducing large bodies of information down to a relatively small number of categories, order is imposed on apparent chaos, complexity is reduced to relative simplicity, and description and analysis are thereby facilitated. For these reasons, geomorphologists have long been interested in reducing the variety and complexity of coastal landforms to a relatively small number of distinctive types. C.A.M. King (1980) has suggested that systems of coastal classification are of two types, descriptive and genetic, and argues that the genetic type is preferred, as it is important to know something of the origin of present coasts. She has also suggested that systems of coastal classification ought to take three factors into account; first, the form of the land surface against which the sea is resting; secondly, the direction of the long-term movement of sea-level relative to the land; thirdly, the modifying effects of contemporary marine processes.
Probably the earliest system of coastal classification was that proposed by E. Suess in his book “The Face of the Earth” (1888). This was based on the form of the land surface against which the sea is resting, and simply divided the world’s shorelines into Atlantic and Pacific types. In the former, structural trends are supposed to run at right angles to the coastline, while in the latter, structural trends are supposed to run parallel to the coast. Such a scheme is obviously very generalised and of no value for application to small areas. A less generalised system of classification was that proposed by the American geomorphologist, D.W. Johnson, in his book, “Shoreline Processes and Shoreline Development” (1919). This was based on the second of the factors mentioned by C.A.M. King (1980); namely, the movement of sea-level relative to the land. Thus, Johnson identified four main categories of coastline: submerged coasts, emergent coasts, neutral coasts, and compound coasts. These main categories are then sub-divided where appropriate (Figure 1).
1) Submerged Coasts
a) Ria coasts
b) Fjord coasts
2) Emergent Coasts
3) Neutral Coasts
a) Delta coasts
b) Alluvial plain coasts
c) Volcanic coasts
d) Coral reef coasts
e) Fault-line coasts
4) Compound Coasts (any combination of the above types) e.g. Ria coast with fronting offshore bars (considered by Johnson to be evidence of a coast which had first undergone submergence followed by emergence).
Figure 1. D.W. Johnson’s System of Coastal Classification
Johnson’s system of classification was criticised for its inadequate treatment of emergent coasts. In this group Johnson only recognised coasts with a broad flat coastal plain, such as that of the Eastern United States, and made no reference to steep emergent coasts of the type found in Western Scotland. It has also been suggested that another problem with Johnson’s scheme is that, if strictly applied, virtually all coasts are of the compound type found in Western Scotland. It has also been suggested that another problem with Johnson’s scheme is that, if strictly applied, virtually all coasts are of the compound type. That is to say, at some time in their history, almost all coasts will have been affected by both positive and negative movements of base level.
Another approach is that of F.P. Shepard whose “Revised Classification of Marine Shorelines” (1945) placed greater emphasis on contemporary shoreline processes rather that the evidence of former emergence or submergence. Thus, Shepard makes a broad distinction between Primary or Youthful Coasts shaped primarily by non-marine processes, and Secondary or Mature Coasts shaped primarily by marine processes (Figure 2).
1) Primary or Youthful Coast (shaped primarily by non-marine processes)
a) Shaped by terrestrial erosion and then drowned
e.g. Ria coast, Dalmatian coast, fjord coast, etc.
b) Shaped by terrestrial deposition
e.g. Delta coast, dune coast, mangrove coast, etc.
c) Shaped by volcanic activity
e.g. Coast of volcanic deposition, volcanic explosion coast.
d) Shaped by diastrophism
e.g. Fault scarp coast, fold mountain coast
2) Secondary or Mature Coast (Shaped primarily by Marine processes)
a) Shaped by marine erosion
e.g. Coasts made more regular by erosion, coasts made less regular by erosion.
b) Shaped by marine deposition
e.g. Sand spits, cuspate forelands, barrier reef coasts, etc.
Figure 2. F.P. Shepard’s System for Coastal Classification
Shepard’s scheme has been criticised for failing to include a category for emergent coasts. Another problem is that of knowing when a coast moves from the Primary to the Secondary category. Often it is no easy matter to assess whether a particular section of coast is predominantly a product of marine or non-marine processes. In any case, coasts are subject to short-term changes, and a period of erosion and recession may be followed by one of deposition and advance.
In order to deal with these problems and the limitations of earlier schemes of classification, a more sophisticated proposal was made by H. Valentin in “Die Kuste Der Erde” (1952). His system was based on the contemporary advance or retreat of the coast. Each of these two main categories was then subdivided according to the causes of advance (i.e. emergence or deposition) and the causes of retreat (i.e. submergence or erosion).
This brief review of a selection of schemes of coastal classification has served to highlight some of the problems involved. Early proposals tended to be descriptive, inflexible and incomplete. In contrast, Valentin’s scheme emphasises the dynamic nature of coastlines, and provided data are available for rates of change, allows for a more precise and scientific classification. It is probably the most useful system of classification proposed to date. Before moving onto to some of the specifics of coastal defence in the UK it is useful at this conjunction to examine the institutions and policies which have been, and still are, tackling the complexities surrounding coastal management in terms of coastal defence.
Land Drainage is defined under the Water Resources Act 1991 (amended by the Environment Agency Act 1995) to include defence against water, warping, irrigation, and the continuation of any other practice that involves the management of the level of water in a watercourse (Clark, 1996). The term has evolved alongside the divisions that manage its policy and operations. The Land Drainage Branch of the Board of Agriculture was responsible for land drainage and sea defence works from 1889, when it assumed control of the work carried out by the Land Commission of England that had been established under the Land Drainage Act 1861. The Land Drainage Act 1926 re-allocated certain powers in relation to land drainage from the Ministry of Agriculture and Fisheries, to county councils and county boroughs. The Land Drainage Act 1930 unified the various river catchment boards and drainage boards concerned with land drainage and sea defence, established a Land Drainage code of law, and increased funding available to the land drainage authorities. The Commercial, Land Drainage and Rural Life Division of the Ministry changed to become the Land Drainage, Publicity and Rural Life Division in 1935, and then the Land Drainage Division in 1938. This division focused solely on land drainage and sea defence issues, before it expanded to become the Land Drainage and Water Supply Division in 1944, and again in 1959, when it became the Land Drainage, Water Supply and Machinery Division.
Responsibility for flood protection and land drainage continued to shift, and in 1984 was held by the Land Drainage Division; in 1986 by the Flood Defence and Land Sales Division; and in 1989 by the Flood Defence Division. A Flood and Coastal Defence Division was established in 1993 within the Environment Policy Group of MAFF's Countryside, Marine Environment and Fisheries Directorate. By 1997, as a result of reorganisation within MAFF, it had been transferred to the Regional Services and Defence Group of the Agricultural, Crops and Commodities Directorate. Around 1995 it absorbed an Emergency Unit dealing with emergency planning in relation to national food supplies, and became known as Flood and Coastal Defence with Emergencies Division (FCDE) (Defra, 2005).
Until MAFF's replacement by DEFRA in 2001 (the Department for Environment, Food and Rural Affairs was created in June 2001 from the then Ministry of Agriculture, Fisheries and Food (MAFF) and from the environmental and countryside business areas of the then Department of the Environment, Transport and the Regions (DETR)), FCDE ran MAFF's flood and coastal defence programme. The National Assembly for Wales (exercising powers formerly held by the Welsh Office) worked with MAFF to monitor the progress made towards reaching policy objectives. These aimed to minimise flooding and coastal erosion in England and Wales, and to reduce the associated risks to people and the developed and natural environment. The Division was responsible for the following in England:
(www.defra.gov.uk)
The evolving understanding of coastal dynamics together with a political framework for action aided local authorities in the UK to undertake appropriate schemes of work where necessary. How these interactions have manifested historically in coastal defence schemes can be traced in Havant Borough Council. There was an increasing popularity in the Eastoke Peninsula as a residential area from the early 1920's. This popularity manifested itself in the building of beach huts and bungalows, which were developed from the 1930s along the backshore of the wide shingle beach. Due to the forces of natural erosion of the foreshore, it soon became necessary to build defences to protect these properties. By the end of the 1930s a concrete seawall had to be constructed in front of the Beach Club, with a timber revetment (sloping surface) and groynes adjacent to it. These defences had to be extended a few decades later, to the east and west, for a total of 2.6 kilometres. Unfortunately it was this seawall which made the natural erosion of the foreshore worse, by increasing the levels of stress. Consequently, repairs to the seawall were required in 1978.
The southern Eastoke Peninsula frontage regularly overtopped, causing flooding damage. This resulted in the Beach Replenishment Scheme (1985). Coupled with this was the fact that the concrete seawall was reaching the end of its serviceable life. Any failure of this coastal defence could have led “to erosion of up to 3 metres per annum and subsequent loss of properties. The frequency and severity of overtopping (water carried over the top of a coastal defence) events was increasing annually” (). A “rear splash wall” was subsequently constructed adjacent to the entire length of the seawall in an effort to reduce the damage. Unfortunately however, these measures failed to prevent “regular overtopping or storm damage to properties”.
Increasing scientific understanding has led to new schemes of costal defence. A programme, MAFF commissioned studies on the managed realignment of sea defences. The purpose of these studies was twofold. Firstly, to investigate the biotic and abiotic changes that would occur as a direct result of seawater inundation within areas of realignment and secondly, to investigate the potential impact of changes in ebb and flow rates within existing creek systems immediately outside sites of realignment. The site chosen for the experiment was at Tollesbury in Essex. During 1995, the old sea wall was breached and, for the first time in more than 150 years, the sea was allowed to flood low-lying agricultural farmland adjacent to Tollesbury Creek on every high tide. Soil stability and strength were investigated by the Silsoe Research Institute. Soil stability is strongly influenced by changes in its physical chemistry brought about by regular flooding by salt water.
They found that where sediment accretion was greatest, the material became more stable. They also found that the soil strength within the site was still significantly stronger than on the adjacent saltmarsh. The presence and density of intertidal invertebrates was assessed annually by Institute of Terrestrial Ecology (ITE, now CEH-Dorset) and has shown that throughout most of the Tollesbury managed realignment site, the number of intertidal invertebrate species increased between 1995 and 1998. Over the same period, the number of species common to both the realignment site and the marsh outside the study site continued to increase, but still remained below that of the surrounding marsh. Natural colonisation of the realignment area by saltmarsh plants occurred alongside experimental introductions and has been monitored by ITE. Of all the saltmarsh species planted within the experimental site, sea aster and common saltmarshgrass were able to establish in greatest numbers and continue to survive with varying degrees of success. There are now also populations of both species established within the site, outside the experimental plots. The formation of saltmarsh along the southern edge of the new sea wall, between August 1996 and October 1997, is clearly visible and its spread has continued.
The effects of intertidal invertebrates on the colonisation of intertidal mud by saltmarsh plants are under investigation by Queen Mary and Westfield College. The main aims of this research are to test the hypothesis that there are two alternative stable states at the saltmarsh-mudflat interface: one dominated by animals (particularly ragworm), which prevents plant colonisation; and the other dominated by
plants, which prevents colonisation by burrowing animals by the presence of dense root systems. Bathymetric studies were undertaken by HR Wallingford between 1994-1999 to determine how the surrounding creek systems might be affected by the formation of the realignment area. The analysis showed that during this period the whole estuary deepened. The data indicated that the increased tidal volume moving through the estuary has modified all of the channels, but that a relatively stable situation is now emerging. Changes within and outside the area will continue to be monitored until 2002. The data is providing valuable information on managed realignment, increasingly seen as a key element to sustainable long term flood and coastal management (Ledoux et al, 2005), as a potential technique to alleviate the problems of rising sea level. It also provides an important insight into the development and exploitation of such areas by coastal wildlife, particularly under reasonably ‘natural’ conditions where no heavy engineering of new creeks and sea walls has taken place.
References
Clark, J. R. (1996). Coastal Zone Management Handbook. Boca Raton, CRC Press.
(accessed April, 2005)
Goudie, A.S. (1993) Land Transformation. In, Johnston, R.J. (Ed.) The Challenge for Geography A Changing World: A Changing Discipline. The Institute of British Geographers Special Publication Series, 28. pp 117-137.
Goudie, A.S. (1994). The geomorphology of Great Britain. BCRA Cave Studies Series, 5. pp 3-7.
Johnson, D. W., 1919, Shoreline processes and shoreline development: New York, NY, John Wiley and Sons, Inc., 584 p.
King, C.A.M. (1980). Physical Geography. Blackwell Publishers.
Ledoux, L., Cornell, S., O’Riordan, T., Harvey, R. and Banyard, L. (2005). Towards sustainable flood and coastal management: identifying drivers of, and obstacles to, managed realignment. Land Use Policy. Vol. 22, Iss. 2; 129-144.
Shepard, F.P. (1945). Revised Classification of Marine Shorelines. J. of Geology, 45:602-624.
Suess, E. (1888). “The Face of the Earth”.
Valentin, H., 1952, "Die Kusten der Erde," Petarmanns Geogr. Mitt (Erg.): 246.
Viles, H. and T. Spencer (1995). Coastal Problems: Geomorphology, Ecology and Society at the Coast. London, Edward Arnold.
