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Coastal management and sea-level rise
Catena 42 Ž2001. 307–322 www.elsevier.comrlocatercatena
Coastal management and sea-level rise John Pethick Department of Marine Sciences and Coastal Management, UniÕersity of Newcastle, Ridley Building, Newcastle upon Tyne, NE1 7RU, UK
Abstract The predicted rise in sea level due to global warming has given rise to much speculation as to the impact on erosion and accretion rates at the coast as well as increases in hazards to coastal users. This paper focuses on the spatial adjustments that coastal landforms will exhibit in response to changing energy gradients both normal to and parallel to the shore. These adjustments, in many cases, will take the form of the migration of landforms in order that they maintain their position within the coastal energy gradient. Prediction of the rates of such migration will be fundamental to the future management of the changing coastal environment. The paper discusses the impact of sea-level rise on the two basic coastal landform assemblages: those in estuaries and those on the open coast, and then goes on to examine the effect on ebb-tidal deltas that are located at the critical junction between estuaries and open coasts. In each case, the rates of landform migration under an accelerated sea-level rise are predicted and compared with existing rates using examples from the east coast of Britain. Assuming a sea-level rise of 6 mmryear, the paper predicts that estuaries will migrate landwards at rates of around 10 mryear, open-coast landforms can exhibit long-shore migration rates of 50 mryear, while ebb-tidal deltas may extend laterally along the shore at rates of 300 mryear. The implication for the management of such dynamic coastal systems, including such issues as coastal defence and conservation, are discussed. q 2001 Published by Elsevier Science B.V. Keywords: Geomorphology; Sea-level rise; Estuaries; Tidal deltas; Coastal management; Coastal defence
1. Introduction The predicted changes in the rate of sea-level rise, as a result of global warming, will have important impacts on the coastal zone, displacing ecosystems, altering geomorphological configurations and their associated sediment dynamics, and increasing the vulnerability of social infrastructure ŽChappell, 1990; IPCC, 1996; Crooks and Turner, 1999.. Taking a macroscale perspective: coastal landform morphology responds to 0341-8162r01r$ - see front matter q 2001 Published by Elsevier Science B.V. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 0 . 0 0 1 4 3 - 0
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extrinsic forces, such as those imposed by climate change, mean sea-level rise, subsidence and tidal range evolution. ŽBird, 1993; Bijlsma, 1997.. High-energy and high-impact events, from wave, tide and wind forces, which reshape coastal areas, are characterised by large spatial and temporal variability. As a result, coastal landforms themselves can display considerable morphological variation over short time periods and, consequently, can give the impression of robustness rather than sensitivity to their forcing factors. In reality, the short-term changes and spatial heterogeneity of coastal features represent a dynamic response to energy inputs that is delicately poised over medium to long term. Long-term changes to the energy environment can, therefore, result in adjustments to coastal landforms that are not anticipated by coastal users even though they may accept short-term variations as the norm. Coastal management systems tend to adopt a short-term view of the coast that recognises changes in, for example, beach profile morphology in response to storms, but are quite unprepared for mediumto long-term changes that may result in the replacement of a beach with a salt marsh. While dynamic forcing factors, such as sediment yield and catchment changes, are important controls on coastal interactions, these topics have been discussed elsewhere Žsee, for instance, Bird, 1993.. Instead, this paper will focus on the spatial adjustments that coastal landforms will exhibit in response to changing energy gradients both normal to and parallel to the shore. These adjustments, in many cases, will take the form of the migration of landforms in order that they maintain their position within the coastal energy gradient. Prediction of the rates of such migration will be fundamental to the future management of the changing coastal environment. When confronted with actual or predicted sea-level rise, most coastal managers understand that this will involve changes in the temporal response of coastal systems. For example, the frequency of the 100-year flood event on the east coast of England is predicted to increase to a 10-year interval, given current government advice of a 6-mm annual sea-level rise over the next 50 years ŽMAFF, 1993.. The implications for flood defence of such a temporal adjustment have been carefully considered by the relevant agencies, and the standards of defence are planned to increase accordingly. Such changes in the frequency of storm events will also have implications for erosion rates; and, while these are less well understood, most coastal managers understand that there will be increased erosion of, for example, soft cliffs or salt marshes. What is not so well appreciated by coastal agencies is that changes in sea level will result in a spatial response of coastal geomorphology, a concept first introduced by Bruun Ž1962.. Based on his work on beaches, Bruun predicted that in response to sea-level rise, shore profiles would respond by acting to maintain their morphology relative to still water levels. This would be achieved by translation of the landform landwards and upwards, with erosion at the landward end of the profile supplying material to raise the lower portion of the profile Žassuming a net sediment balance.. This basic idea appears to apply to all coastal landforms. Coastal morphology evolves in response to applied energy, and each landform: beach, dune, marsh, etc., develops within a specific energy level located somewhere along an energy gradient running either parallel to or normal to the shore. These energy gradients are themselves the result of changes in water depths: for example, as waves and tides enter shallow water in estuaries, or as waves are refracted along open shorelines. Increased sea level results in
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change in water depth and these energy gradients adjust accordingly. In response, coastal landforms are forced to change their location so as to maintain their relative position with regard to energy levels. Thus, as energy levels increase at a given location in an estuary, mudflats will migrate landwards to a lower energy level and could be replaced by sand beaches, which have similarly migrated landwards from more exposed situations. This Eulerian approach to landform migration along an energy gradient can, of course, be interpreted, in a Lagrangian manner, as the process by which increased energy at a given location results in changes in, for example, rates of erosion. The concept of landform migration does, however, provide a more tangible, and pragmatic, method of interpreting the degree of adjustment to sea-level rise, as Bruun appreciated in his early work. It allows calculation of the rates of migration of landforms, a geomorphological approach that may be of more use to coastal managers requiring to know how their coastline will look in 50 years time, than the more conventional process approach, which presents rates of erosion or accretion. This larger geomorphological approach will be explored here using three landform assemblages as examples: estuaries, tidal deltas and open coast beaches. In each case, the migratory response of the landform to sea-level rise is calibrated and the implications for coastal managers are considered. These implications include not only changes in risks levels for urban, industrial and agricultural uses, but also changes in conservation status and in recreational values.
2. Estuaries 2.1. Physical response The landward ‘roll-over’ transgression of estuaries as a response to sea-level rise was first suggested by Allen Ž1990. for the Severn Estuary and has, subsequently, been the subject of research in the Blackwater Estuary, Essex, ŽPethick, 1997. and the Humber Estuary ŽEnvironment Agency, 1999. ŽFig. 1.. As in the case of the 2-dimensional Bruun rule for beach profiles, the 3-dimensional estuarine transgression appears to enable an estuary to maintain its relative position within the tidal and wave energy frame. It entails a vertical upward movement, keeping pace with sea-level rise, as well as a landward movement that maintains its position in the longitudinal energy frame. The process of transgression is mainly the result of a redistribution of sediment within the estuary system itself although, in the case of the Blackwater estuary, it was found that additional sediment from marine sources provides the vertical response ŽTable 1.. In the outer estuary, the deeper water leads to an increase in waves propagating in from the open sea, and these erode the upper intertidal sediments, causing retreat of the salt marshrmudflat boundary. The eroded sediments are moved landwards to the inner estuary, where they are redeposited on the upper intertidal zone, raising the mudflat and salt marsh surfaces and resulting in a potential transgression of the landward edge of the salt marsh. The salt marshrmudflat boundary of these inner estuarine areas continues to erode as the fetch length of locally generated waves
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Fig. 1. Location of study sites: the Humber Estuary, the north Norfolk coast and the Blackwater Estuary.
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Table 1 Blackwater Estuary sediment budget 1978 to 1995 Area of estuary Žha. Area of outer estuary Žha. Area of inner estuary Žha. Mean annual volume loss outer estuary Žm3 . Mean annual volume gain inner estuary Žm3 . Net volume gain Žm3 . Estuary-wide accretion rate Žmryear. Outer estuary erosion rate Žmryear. Inner estuary accretion rate Žmryear.
5180 2196 2984 548 900 746 000 197 000 0.004 0.025 0.025
Note that figures are based on mean annual volumetric change derived from bathymetric surveys of the estuary undertaken in 1978 and 1995. Vertical accretion and erosion rates are based on volumerarea ratios. The boundary between the outer and inner estuary was set at 9 km from the mouth.
increases due to the rise in sea level. This erosion, combined with transgression of the landward salt marsh boundary, means that the estuary channel moves landward as a unit, at a rate that allows the increased tidal energy entering the estuary to be accommodated. The calculation of transgression rates of an estuary in response to sea-level rise has been approached using a regime model, in which tidal discharge at a given estuarine cross-section is related to the erosional threshold of the sediment forming the bed and banks of that section. Increased sea level in a typical trapezoidal cross-section leads to increased tidal volumes and, thus, to increased bed shear stress. In response, the cross-section widens, reducing velocities and shear stress to below the critical threshold levels and this process is repeated along the length of the estuary although as tidal volumes decrease landwards, so the cross-sectional response also decreases. A Eulerian interpretation of this process would indicate that all cross-sections have migrated landwards and the migration distance will be governed by the initial increase in tidal discharge. Prediction of this migration distance, using a regime model, was performed as part of the Environment Agency research project on the Humber Estuary ŽEnvironment Agency, 1999.. Results describe that the migration distance decreases landwards, indicating that the landward transgression is accompanied by a change in planimetric shape, the estuary becoming more expansive at the mouth as well as migrating landward. The mean migration rate of the estuary as a whole was predicted to be 1.3 m per 1 mm rise in sea level or 8 mryear assuming a 6 mmryear rise in sea level. The predicted migration rate is similar to the observed rate of horizontal erosion of the salt marshrmudflat boundaries in the outer Humber estuary that have averaged 1 to 2 mryear over the past 20 years, during which time sea-level rise has averaged 1.4 mmryear ŽShennan and Woodworth, 1992.. Migration of the landward margin of salt marshes is dependent on the slope of the hinterland. Accretion rates of 1 mmryear, keeping pace with sea-level rise, on salt marshes whose surface gradients are 1:1000 or lower, would result in a potential landward transgression of the landward margins in the order of 1 mryear, as predicted for the Humber, only if hinterland slopes were of a similar gradient. However,
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measurement of salt marsh accretion rates in the Blackwater Estuary ŽPethick, 1997. has demonstrated that vertical accretion rates on salt marsh surfaces, they are between 8 and 10 mmryear, far in excess of the current rate of mean sea level of 1.4 mmryear. If these accretion rates are indicative of a general salt marsh response to sea-level rise, then landward transgression rates similar to those predicted for the Humber could be attained on hinterland slopes as steep as 1:100. Slopes steeper than this would limit the transgression rates of the landward margins of salt marshes and set up a form of natural ‘coastal squeeze’ ŽEnglish Nature, 1992. as the salt marshrmudflat boundary continued to erode. 2.2. Management response The research on the Humber and Blackwater Estuaries, outlined above, was concerned with the theoretical development of the estuary in response to sea-level rise. In reality, reclamation in both these estuaries, and indeed in the majority of British estuaries, has effectively prevented any natural morphological response to sea-level changes. The erosional response noted above, in which the salt marshrmudflat boundary exhibits a landward transgression, is generally observed, but migration of the landward boundary of estuaries is prevented by flood embankments that restrict landward salt marsh transgression and lead to coastal squeeze. Since reclamation has removed the greater part of the intertidal depositional surface in these estuaries, sediment eroded from the outer estuary and transferred to the inner estuaries, as described above, is deposited in the subtidal channels rather than on the salt marshes. The impact of this interruption to the natural process is already causing estuarine users considerable difficulties, as navigation channels and marinas experience increased siltation, requiring expensive dredging programmes. In the Blackwater, for example, sedimentation rates in the subtidal channels of the inner estuary are now 0.06 mryear, an order of magnitude greater than the rate of sea-level rise, and reflecting the disparity between erosional areas and depositional areas in the estuary. The implications of this disparity extend to other estuarine uses besides navigation. The erosion of the upper intertidal areas means that important habitats, especially upper intertidal mudflats and salt marshes, are being lost. Many of these areas have international designations under the Habitats Regulations wCouncil Directive 92r43rEEC on the conservation of natural habitat and wild flora and fauna, adopted in May 1992x or the Ramsar Convention and, consequently, such losses due to human intervention, in the form of flood embankments, may be seen as in contravention of these designations. The loss of upper intertidal areas without corresponding transgression poses flood defence problems. Wave dissipation on the intertidal area is an important consideration in the design of flood embankments. Increased gradients of these estuarine beaches due to upper intertidal erosion leads to increased wave propagation towards the toe of flood defences, demanding expensive maintenance programmes. More serious still may be the impact on the estuarine tidal systems of the lack of overall estuarine transgression. The intervention of flood embankments in the inner estuary means that the estuary cannot move landwards in response to increased energy levels so that, as a result, tidal range
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and storm surges both increase. Management response to these problems must focus on the removal, wherever possible, of flood embankments to facilitate landward transgression of the estuarine morphology. Managed retreat is, however, not a simple palliative. The research outlined above indicates that managed retreat will be most effective in the inner estuary in order that salt marshes can transgress landward, yet this transgression is only possible if sediment is released from outer estuary intertidal areas to be redeposited in the inner estuary. This means that managed retreat of outer estuary flood embankments is necessary in order to provide sacrificial intertidal areas, a concept that may be difficult to explain to the local landowner. In addition to these difficulties affecting the estuary morphology, maintenance of the balance of ecological habitats within the estuary, especially those with international designations, demands that they are allowed to migrate at the same rate as the estuarine morphology. Thus, the erosion of outer estuary salt marshes should be balanced by their migration landwards replacing slightly more brackish marshes that should themselves migrate into areas previously occupied at the head of an estuary by brackishrfreshwater habitats. Unless this complete sequence is facilitated, by provision of areas into which such habitat migration can occur, an imbalance between habitat types will develop. Finally, research undertaken as part of the Humber research programme ŽEnvironment Agency, 1999. indicates that the location, elevation and, to some extent, size of a managed retreat area can have an impact on tidal levels in an estuary. The research indicates that restoration of a reclaimed marsh in the outer estuary has little impact on water levels, but the relationship becomes increasingly sensitive as the retreat site is moved landwards. For example, reductions in water level of 0.1 m over a 20-km stretch of estuary can result from a single marsh restoration of 300 ha at Trent Falls, some 60 km from the mouth of the Humber estuary. In contrast, a similar area of restoration located at Sunk Island, 20 km from the estuary mouth, results in no measurable change in water levels.
3. Open coasts The location of landform units along an open coast is often determined by relatively robust cell systems determining for example, pocket beaches between rock headlands. In some areas, however, where such geological controls are not present and where shallow near-shore conditions reduce wave energy sufficiently, landform units along an open coast may be both diverse and sensitively adjusted to wave energy gradients. Wave energy gradients are themselves the result of wave refraction patterns in near shore that can result in wave foci along the shore where wave height and energy are accentuated. The North Norfolk shoreline is characterised by particularly complex wave refraction patterns, the result of near-shore bathymetric variations over the tidal deltaic deposits of the Wash. Here, the shoreline lies at an oblique angle to the near-shore contours and the resultant wave-refraction pattern shows a series of wave foci spaced at approximately 10-km intervals. Fig. 2 shows the pattern for a north-easterly wave with an 8-s period. The wave foci are areas where wave rays are compressed, causing increased wave
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Fig. 2. Wave refraction patterns on the north Norfolk coast, for waves approaching from the northeast with an 8-s period. Note: focusing of wave energy due to nearshore bathymetry.
heights and energy levels, while the areas between foci experience diminished wave height and energy levels as the wave rays are spread out over wider areas of shoreline. The landform response to this sequence of energy gradients may be described with the aid of Fig. 2. Areas of high wave energy are characterised by coarse sediment beaches, such as those at Brancaster, Scolt Head Wells and Blakeney. Between these wave foci, areas of low energy are occupied by salt marshes, such as those at Thornham and Stiffkey.
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These landform sequences merge into one another and their boundaries are blurred by the variations in location of wave foci as wave approach angle changes during successive storm events. Nevertheless, the overall sequence is extremely sensitive to the imposed wave energy gradients and, therefore, to changes in sea level. The impact of sea-level rise is demonstrated in Fig. 3 in which sea level is assumed to have increased by 1 m Ži.e. taking place over 166 years at an average rate of 6 mmryear.. The wave
Fig. 3. Wave refraction patterns on the north Norfolk coast. Wave approach and period similar to those in Fig. 3 but with a 1 m increase in sea level. The increased water depths have displaced wave energy foci, relative to current baseline conditions, which in turn will cause the coastal landform units Že.g. salt marsh, beaches, sand dunes. to migrate.
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foci are shown to have migrated along the shore by 8 km, at a rate of 53 mryear, so that locations that previously experienced minimum now experience maximum wave energy, in effect the energy gradients have been reversed. The response of the landform sequences along this coast is predicted to be equally dramatic. Sand beaches, such as those at Wells, are predicted to be replaced by mudflats and salt marshes, while sand beaches will replace the designated salt marsh and sand dune areas of Thornham, Scolt and Stiffkey. These predicted changes are, of course, exaggerated here by the use of a 150-year time interval. Nevertheless, locally sea levels are rising by 1.5 mm at the present time and this appears to be resulting in the inception of landform migration, such as those predicted above. The sand beach at Holkham has been replaced by salt marsh during the last decade, while increased siltation of Wells beach is currently causing problems for local coastal managers. 3.1. Management response The alternations of wave energy and landform sequences along the north Norfolk coast are themselves responsible for the pattern of coastal management here. The tourist industries located at Hunstanton and Wells are dependent on wide sandy beaches that are a response to high-energy waves experienced here. The internationally designated nature conservation areas of Thornham, Scolt and Stiffkey are located in low wave energy areas resulting in salt marshes, sand dunes and mudflats where biological activity is at a maximum. As discussed above, landform migration may already be occurring along this coast with increased silt content of previously clean sand beaches causing major problems for the tourist industry at Wells. The economies of the towns of Hunstanton and Wells depend to a large extent on their recreational beaches. However, it seems unlikely that over the long term, migration of these towns will take place to parallel the migration of their beaches; but equally, it seems unlikely that tourists will continue to use these towns as centres if a 10-km journey to the nearest beach is demanded. In this case, the disparity between the inertia of the existing urban infrastructure and the migration of the resource could lead to the development of alternative resorts, at first as ‘mobile’ caravan parks, that could threaten the internationally important ecological habitats on this coast. The designation under the Habitats Regulations of most of this coastline poses serious management problems. Under these regulations, the competent authority must notify deterioration of designated habitats; changes in habitat due to natural processes, however, may be accepted. The key issue on the Norfolk coast is whether the natural habitat migration due to sea-level rise can take place unimpeded by human interference. In this case, existing flood defences both for agricultural land and also for the urban development along the coast may prevent landform migration and, consequently, the relative extent of different types of habitat may alter. If this is interpreted as contrary to the regulations, then it will be necessary, under their provisions, to provide replacement habitat elsewhere. Replacement of habitat that is part of a functional coastal system in an alternative location is difficult if not impossible and it appears that a major impasse is inevitable.
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4. Tidal deltas The tidal discharge from an estuary into the open sea presents a potential barrier to long-shore sediment movement along the open coast; a barrier that, in most cases, is bridged by a complex intertidal or subtidal deltaic landform. These deltas not only allow sediment transport to bypass the estuaries, they also provide significant protection to adjacent coastlines ŽDyer and Huntley, 1999.. Tidal deltas consist of two main components, a flood-tide delta located within the estuary, and an ebb-tide delta usually located seaward of the estuary and extending over lengths of the adjacent shorelines. Ebb-tide delta systems are the subject of the present discussion. Ebb flows out of an estuary force long-shore sediment-transport pathways seaward, where they commonly form a roughly triangular-shaped rampart — the ebb delta. This rampart is fed by long-shore sediment movement usually in the form of sand waves that migrate from the shoreline out to the delta. From here, they subsequently become detached and move downdrift to complete the long-shore sediment transfer across the estuary mouth ŽFitzgerald and Penland, 1987.. The sand waves that characterise these delta systems act as wave breaks and, thus, reduce wave energy along adjacent shorelines. The size and extent of ebb-tidal deltas depends on the tidal prism of the estuary and the strength of the long-shore sediment transport discharge, itself dependent on wave energy ŽWalton and Goodall, 1972.. In order to estimate the impact of sea-level rise on ebb-tidal delta dimensions, a predictive semiempirical model of delta growth has been developed for the Humber estuary. Prediction of delta volume using the relationship with tidal prism presented by Walton and Goodall Ž1972. is combined with an empirically derived relationship between delta seaward extent and tidal prism ŽFig. 4a.. Sonar imagery of sediment depth in the ebb-delta is then used to derive a maximum lateral dimension of the delta, that is, its basal width measured along the shoreline ŽFig. 4b.. A good agreement between the predicted and the observed dimensions for the present-day Humber delta was obtained ŽFig. 4c.. The delta area is predicted to be 117 km2 and its basal width to be 23 km. Observations show that the Humber delta extends south from the estuary mouth for 23 km along the Lincolnshire shore, but with almost no extension north of the mouth, an asymmetry probably due to the net southerly wave-induced current here. It is significant to note that in the protected area landward of the delta front, the shoreline is currently experiencing accretion of sand dunes and salt marshes, while on the exposed shore, south of the delta front, erosion of the sand beaches has been so severe to require ongoing annual beach sediment renourishment by the Environment Agency. The impact of sea-level rise on the lateral extent of the delta and, thus, on the protection afforded to the shoreline is predicted by the model described above. As sea level rises in the trapezoidal cross-section estuarine channel, so the volume of tidal water entering the estuary will increase, resulting in an extension of the volume and area of the tidal delta. The long-shore sediment transport rate feeding into the delta will also increase as wave refraction decreases in the deeper water. The overall effect will be for the delta and its ancillary sand waves to extend both seaward and long-shore as sea level rises, a process that will offer increased protection to adjacent shorelines.
318 J. Pethickr Catena 42 (2001) 307–322 Fig. 4. Empirical data from 9 UK estuaries ŽA. was used to derive an expression relating tidal prism and the seaward projection of their ebb-tidal deltas. This was used with the relationship derived by Walton and Goodall Ž1972. to relate variations in the length of the tidal delta baseline Ži.e. along the shore. to changes in tidal prism ŽB.. Results indicate that a sea level rise of 6 mmryear would result in an extension of the tidal delta of the Humber estuary ŽC. by 300 mryear.
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Predictions from the model, assuming a steady increase of 6 mmryear in sea level, indicate an initial increase in the ebb-tide delta area of 16 km2 , rising to 21 km2 after 100 years. The southernmost extent of the delta would, as a result of these changes, migrate south at an initial rate of 300 mryear, rising to 386 mryear after 100 years. This southerly extension to the delta may lead to a commensurate migration of the accretionrerosion boundary on this shore. The 1.5 km southerly extension of the coastal salt marsh immediately north of Mablethorpe ŽFig. 4c. on the Lincolnshire coast, over the past 40 years as evidenced from air photograph records, provides some evidence for the existence of such a migration as a result of the present-day rise in sea level Ž1.4 mmryear.. 4.1. Management responses If the example presented above of the Humber Estuary is assumed to be typical of the general response of ebb-tidal deltas to sea-level rise, then an increasing length of the coast of Britain may be expected to be protected from wave erosion as sea levels increase due to global warming. The importance of this natural form of coastal defence or the extension to this length, which will occur as a result of sea-level rise, has not been appreciated by coastal managers. The lateral extent of the Humber ebb-tide delta is predicted above to increase by between 300 m and 386 mryear, assuming an average rate of sea-level rise of 6 mmryear. This extension to the length of protected shoreline could remove the necessity for coastal defences in the shadow of the delta with significant saving in expenditure. Since conventional hard defences may cost in the region of £3000rm, this could involve savings of £1 000 000ryear on the Lincolnshire coast. Extrapolating such figures to the coast of Britain as a whole is difficult, given the range of estuarine forms that exist; but since there are 158 estuaries on the British coast ŽJNCC, 1993., it may be assumed that the reduction in the necessity for conventional defences provided by the extended tidal deltas may involve savings of many millions of pounds. This increased size of ebb-tide deltas as a result of sea-level rise appears, from the examples given above, to offer a natural mechanism for coastal defence that needs little or no management interference or financial input. However, if the sediment budget of the shoreline rather than the financial budget is considered, then the sediment-benefits of such a natural process must be set against the sediment-costs. The increase in sediment stored in an estuarine delta because of sea-level rise may be sourced from adjacent shorelines, so resulting in down-drift sediment starvation. The management fear would be that the net effect of these two processes might be to cause more erosion down-drift due to sediment loss than is reduced by the extension of the delta front. The volumes of sediment needed to supply the increased deltaic morphology vary, of course, with the size of the estuary involved. The increased sediment volumes stored in the Humber Estuary, for example, is predicted using the model described above to equal 300 000 m3ryear for a 6-mm rise in sea level. This is approximately equal to the entire sand output from the erosion of the Holderness coast to the north and represents the annual net sediment transport rate along this shoreline. If this annual volume were stored in the Humber delta, significant depletion of sand inputs along the southern Lincolnshire
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shoreline could result, an area where sand nourishment is already necessary to protect the eroding coast. The provision of self-sustaining natural defences along significant proportions of the coast due to deltaic extension offers an important coastal management tool. With the potential problems of sediment budgets in mind, it may be useful to consider the possibility of extending or even creating ebb-tide deltas along the coast as a form of coastal defence. One example is briefly described here. This involves the Royal Norfolk Golf Clubhouse, located on the shoreline at Brancaster Norfolk. Here, erosion over the past decade has resulted in the clubhouse standing proud of the shore and demands increasingly expensive hard defences to protect the building. To landwards of the clubhouse lies an area of reclaimed salt marsh and it has been proposed that a breach in the embankment surrounding this area would allow the development of a small tidal estuary with a tidal prism of 0.5 = 10 6 m3. This estuary would develop a small tidal delta of 0.01 km2 overlapping approximately 200 m of the adjacent shoreline and providing protection to the clubhouse. Moreover, this protection would extend as sea level rises, providing a self-sustaining defence that would not require further maintenance. The implementation of such a radical approach to coastal defence was prevented due to the fact that the reclaimed marsh involved is designated as an SPA under the EU Birds Directive. The proposed change in habitat type from fresh water marsh to salt marsh would contravene the Habitats Regulations, another example of the inertia inherent in our coastal management systems that prevents any long-term strategic approach to the problems posed by coastal landform migration under sea-level rise. Nevertheless, the proposal does demonstrate the potential of the process for coastal protection and may offer considerable scope in the future for accommodation to the predicted rise in sea level.
5. Conclusions Coastal landforms are extremely sensitive to medium- to long-term changes in energy inputs brought about by sea-level rise. These changes in energy gradients along the shore result in landform migration, which allow each landform to remain in its equilibrium energy niche. The prediction of the migration rate for individual landforms offers an important coastal management tool, allowing management decisions to be made to accommodate and, in some cases, to facilitate such migration. The three examples examined here demonstrate the approach and provide preliminary estimates of migration rates. Estuaries are predicted to migrate landwards and upwards in response to sea-level rise and, for the Humber Estuary, the migration rate is predicted to be 8 mryear, assuming a sea-level rise of 6 mmryear. Managed retreat of existing flood defences will be necessary to facilitate such migration. On the open coast, an example from North Norfolk demonstrates that wave refraction patterns will shift in a long-shore sense as sea level rises. Here, rates of migration of wave foci averaging 9 m per 1 mm sea-level rise per year are predicted, leading to major changes in habitat mosaics along this internationally designated coast. Positive manage-
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ment responses are difficult to predict and it is assumed that accommodation to such changes will be necessary despite the legal obstacles provided by the EU Habitats Regulations. Finally, tidal deltas are shown to protect significant length of the shoreline, but have not been appreciated as defence mechanisms. Extensions of these deltaic systems as sea-level rise is predicted here using a simple model and the possibility of initiating deltas as a form of self-sustaining defence is outlined. Here again, however, existing legislation is shown to prevent any such radical innovation in our coastal management systems. It appears that if viewed from the static position of our present approach to coastal management, sea-level rise will result in radical change to our coastal landforms. If we adopt a more dynamic viewpoint, however, then it appears that these same coastal landforms remain intact; it is only their location that will change. If we persist in applying our static coastal management systems as sea levels rise, then an increasing disparity will arise between our needs and the coastal resource. Instead, we must begin to manage change at the coast in a more positive manner.
Acknowledgements The author is indebted to Dr. S. Crooks for advice and editorial assistance and Ms. T. Skrzypcsak for the cartography.
References Allen, J.R.L., 1990. The Severn Estuary in southwest Britain: its retreat under marine transgression and fine sediment regime. Sediment. Geol. 66 Ž1–2., 13–28. Bruun, P., 1962. Sea level rise as a cause of shore erosion. J. Waterw., Port Coastal Ocean Eng. Div., Am. Soc. Civ. Eng. 88, 117–130. Bijlsma, L., 1997. Climate change and the management of coastal resources. Climate Res. 8, 47–56. Bird, E.C.F., 1993. Submerging Coasts: The Effect of Rising Sea-Level on Coastal Environments. Wiley, Chichester, UK. Chappell, J., 1990. Some effects of sea level rise in marine and coastal. Geol. Soc. Aust., Spec. Publ. 1, 37–45. Crooks, S., Turner, R.K., 1999. Integrated coastal management: sustaining estuarine natural resources. Adv. Ecol. Res. 29, 241–289. Dyer, K.R., Huntley, D.A., 1999. The origin, classification and modelling of sand banks and ridges. Cont. Shelf Res. 19, 1285–1330. English Nature, 1992. Coastal Zone Conservation: English Nature’s Rationale, Objectives and Practical Recommendations. English Nature, Peterborough, UK. Environment Agency, 1999. Humber Estuary: Geomorphological Studies. Internal Report. Environment Agency, Peterborough, UK. IPCC ŽIntergovernmental Panel on Climate Change., 1996. Second Assessment Report: Climate Change: The Science of Climate Change. Cambridge Univ. Press, Cambridge, UK. Fitzgerald, D.M., Penland, S., 1987. Backbarrier dynamics of the East Friesian Islands. J. Sediment. Petrol. 57 Ž4., 746–754. JNCC, 1993. An Inventory of UK Estuaries. Joint Nature Conservation Committee, Peterborough, UK.
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MAFF, 1993. National Flood and Coastal Defence Strategy for England and Wales. Ministry of Agriculture, Fisheries and Food, London, UK. Pethick, J. 1997. The Blackwater Estuary: Geomorphological Trends 1978 to 1994. Report to the Environment Agency, Peterborough, UK. Shennan, I., Woodworth, P., 1992. A comparison of late Holocene and twentieth-century sea level trends from the UK and North Sea region. Geophys. J. Int. 109 Ž1., 96–105. Walton, F.D., Goodall, H.G., 1972. Sedimentary dynamics under tidal influences, Big Grass Island, Taylor County, Florida. Mar. Geol. 13, 1–29.
Coastal management and sea-level rise John Pethick Department of Marine Sciences and Coastal Management, UniÕersity of Newcastle, Ridley Building, Newcastle upon Tyne, NE1 7RU, UK
Abstract The predicted rise in sea level due to global warming has given rise to much speculation as to the impact on erosion and accretion rates at the coast as well as increases in hazards to coastal users. This paper focuses on the spatial adjustments that coastal landforms will exhibit in response to changing energy gradients both normal to and parallel to the shore. These adjustments, in many cases, will take the form of the migration of landforms in order that they maintain their position within the coastal energy gradient. Prediction of the rates of such migration will be fundamental to the future management of the changing coastal environment. The paper discusses the impact of sea-level rise on the two basic coastal landform assemblages: those in estuaries and those on the open coast, and then goes on to examine the effect on ebb-tidal deltas that are located at the critical junction between estuaries and open coasts. In each case, the rates of landform migration under an accelerated sea-level rise are predicted and compared with existing rates using examples from the east coast of Britain. Assuming a sea-level rise of 6 mmryear, the paper predicts that estuaries will migrate landwards at rates of around 10 mryear, open-coast landforms can exhibit long-shore migration rates of 50 mryear, while ebb-tidal deltas may extend laterally along the shore at rates of 300 mryear. The implication for the management of such dynamic coastal systems, including such issues as coastal defence and conservation, are discussed. q 2001 Published by Elsevier Science B.V. Keywords: Geomorphology; Sea-level rise; Estuaries; Tidal deltas; Coastal management; Coastal defence
1. Introduction The predicted changes in the rate of sea-level rise, as a result of global warming, will have important impacts on the coastal zone, displacing ecosystems, altering geomorphological configurations and their associated sediment dynamics, and increasing the vulnerability of social infrastructure ŽChappell, 1990; IPCC, 1996; Crooks and Turner, 1999.. Taking a macroscale perspective: coastal landform morphology responds to 0341-8162r01r$ - see front matter q 2001 Published by Elsevier Science B.V. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 0 . 0 0 1 4 3 - 0
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extrinsic forces, such as those imposed by climate change, mean sea-level rise, subsidence and tidal range evolution. ŽBird, 1993; Bijlsma, 1997.. High-energy and high-impact events, from wave, tide and wind forces, which reshape coastal areas, are characterised by large spatial and temporal variability. As a result, coastal landforms themselves can display considerable morphological variation over short time periods and, consequently, can give the impression of robustness rather than sensitivity to their forcing factors. In reality, the short-term changes and spatial heterogeneity of coastal features represent a dynamic response to energy inputs that is delicately poised over medium to long term. Long-term changes to the energy environment can, therefore, result in adjustments to coastal landforms that are not anticipated by coastal users even though they may accept short-term variations as the norm. Coastal management systems tend to adopt a short-term view of the coast that recognises changes in, for example, beach profile morphology in response to storms, but are quite unprepared for mediumto long-term changes that may result in the replacement of a beach with a salt marsh. While dynamic forcing factors, such as sediment yield and catchment changes, are important controls on coastal interactions, these topics have been discussed elsewhere Žsee, for instance, Bird, 1993.. Instead, this paper will focus on the spatial adjustments that coastal landforms will exhibit in response to changing energy gradients both normal to and parallel to the shore. These adjustments, in many cases, will take the form of the migration of landforms in order that they maintain their position within the coastal energy gradient. Prediction of the rates of such migration will be fundamental to the future management of the changing coastal environment. When confronted with actual or predicted sea-level rise, most coastal managers understand that this will involve changes in the temporal response of coastal systems. For example, the frequency of the 100-year flood event on the east coast of England is predicted to increase to a 10-year interval, given current government advice of a 6-mm annual sea-level rise over the next 50 years ŽMAFF, 1993.. The implications for flood defence of such a temporal adjustment have been carefully considered by the relevant agencies, and the standards of defence are planned to increase accordingly. Such changes in the frequency of storm events will also have implications for erosion rates; and, while these are less well understood, most coastal managers understand that there will be increased erosion of, for example, soft cliffs or salt marshes. What is not so well appreciated by coastal agencies is that changes in sea level will result in a spatial response of coastal geomorphology, a concept first introduced by Bruun Ž1962.. Based on his work on beaches, Bruun predicted that in response to sea-level rise, shore profiles would respond by acting to maintain their morphology relative to still water levels. This would be achieved by translation of the landform landwards and upwards, with erosion at the landward end of the profile supplying material to raise the lower portion of the profile Žassuming a net sediment balance.. This basic idea appears to apply to all coastal landforms. Coastal morphology evolves in response to applied energy, and each landform: beach, dune, marsh, etc., develops within a specific energy level located somewhere along an energy gradient running either parallel to or normal to the shore. These energy gradients are themselves the result of changes in water depths: for example, as waves and tides enter shallow water in estuaries, or as waves are refracted along open shorelines. Increased sea level results in
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change in water depth and these energy gradients adjust accordingly. In response, coastal landforms are forced to change their location so as to maintain their relative position with regard to energy levels. Thus, as energy levels increase at a given location in an estuary, mudflats will migrate landwards to a lower energy level and could be replaced by sand beaches, which have similarly migrated landwards from more exposed situations. This Eulerian approach to landform migration along an energy gradient can, of course, be interpreted, in a Lagrangian manner, as the process by which increased energy at a given location results in changes in, for example, rates of erosion. The concept of landform migration does, however, provide a more tangible, and pragmatic, method of interpreting the degree of adjustment to sea-level rise, as Bruun appreciated in his early work. It allows calculation of the rates of migration of landforms, a geomorphological approach that may be of more use to coastal managers requiring to know how their coastline will look in 50 years time, than the more conventional process approach, which presents rates of erosion or accretion. This larger geomorphological approach will be explored here using three landform assemblages as examples: estuaries, tidal deltas and open coast beaches. In each case, the migratory response of the landform to sea-level rise is calibrated and the implications for coastal managers are considered. These implications include not only changes in risks levels for urban, industrial and agricultural uses, but also changes in conservation status and in recreational values.
2. Estuaries 2.1. Physical response The landward ‘roll-over’ transgression of estuaries as a response to sea-level rise was first suggested by Allen Ž1990. for the Severn Estuary and has, subsequently, been the subject of research in the Blackwater Estuary, Essex, ŽPethick, 1997. and the Humber Estuary ŽEnvironment Agency, 1999. ŽFig. 1.. As in the case of the 2-dimensional Bruun rule for beach profiles, the 3-dimensional estuarine transgression appears to enable an estuary to maintain its relative position within the tidal and wave energy frame. It entails a vertical upward movement, keeping pace with sea-level rise, as well as a landward movement that maintains its position in the longitudinal energy frame. The process of transgression is mainly the result of a redistribution of sediment within the estuary system itself although, in the case of the Blackwater estuary, it was found that additional sediment from marine sources provides the vertical response ŽTable 1.. In the outer estuary, the deeper water leads to an increase in waves propagating in from the open sea, and these erode the upper intertidal sediments, causing retreat of the salt marshrmudflat boundary. The eroded sediments are moved landwards to the inner estuary, where they are redeposited on the upper intertidal zone, raising the mudflat and salt marsh surfaces and resulting in a potential transgression of the landward edge of the salt marsh. The salt marshrmudflat boundary of these inner estuarine areas continues to erode as the fetch length of locally generated waves
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Fig. 1. Location of study sites: the Humber Estuary, the north Norfolk coast and the Blackwater Estuary.
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Table 1 Blackwater Estuary sediment budget 1978 to 1995 Area of estuary Žha. Area of outer estuary Žha. Area of inner estuary Žha. Mean annual volume loss outer estuary Žm3 . Mean annual volume gain inner estuary Žm3 . Net volume gain Žm3 . Estuary-wide accretion rate Žmryear. Outer estuary erosion rate Žmryear. Inner estuary accretion rate Žmryear.
5180 2196 2984 548 900 746 000 197 000 0.004 0.025 0.025
Note that figures are based on mean annual volumetric change derived from bathymetric surveys of the estuary undertaken in 1978 and 1995. Vertical accretion and erosion rates are based on volumerarea ratios. The boundary between the outer and inner estuary was set at 9 km from the mouth.
increases due to the rise in sea level. This erosion, combined with transgression of the landward salt marsh boundary, means that the estuary channel moves landward as a unit, at a rate that allows the increased tidal energy entering the estuary to be accommodated. The calculation of transgression rates of an estuary in response to sea-level rise has been approached using a regime model, in which tidal discharge at a given estuarine cross-section is related to the erosional threshold of the sediment forming the bed and banks of that section. Increased sea level in a typical trapezoidal cross-section leads to increased tidal volumes and, thus, to increased bed shear stress. In response, the cross-section widens, reducing velocities and shear stress to below the critical threshold levels and this process is repeated along the length of the estuary although as tidal volumes decrease landwards, so the cross-sectional response also decreases. A Eulerian interpretation of this process would indicate that all cross-sections have migrated landwards and the migration distance will be governed by the initial increase in tidal discharge. Prediction of this migration distance, using a regime model, was performed as part of the Environment Agency research project on the Humber Estuary ŽEnvironment Agency, 1999.. Results describe that the migration distance decreases landwards, indicating that the landward transgression is accompanied by a change in planimetric shape, the estuary becoming more expansive at the mouth as well as migrating landward. The mean migration rate of the estuary as a whole was predicted to be 1.3 m per 1 mm rise in sea level or 8 mryear assuming a 6 mmryear rise in sea level. The predicted migration rate is similar to the observed rate of horizontal erosion of the salt marshrmudflat boundaries in the outer Humber estuary that have averaged 1 to 2 mryear over the past 20 years, during which time sea-level rise has averaged 1.4 mmryear ŽShennan and Woodworth, 1992.. Migration of the landward margin of salt marshes is dependent on the slope of the hinterland. Accretion rates of 1 mmryear, keeping pace with sea-level rise, on salt marshes whose surface gradients are 1:1000 or lower, would result in a potential landward transgression of the landward margins in the order of 1 mryear, as predicted for the Humber, only if hinterland slopes were of a similar gradient. However,
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measurement of salt marsh accretion rates in the Blackwater Estuary ŽPethick, 1997. has demonstrated that vertical accretion rates on salt marsh surfaces, they are between 8 and 10 mmryear, far in excess of the current rate of mean sea level of 1.4 mmryear. If these accretion rates are indicative of a general salt marsh response to sea-level rise, then landward transgression rates similar to those predicted for the Humber could be attained on hinterland slopes as steep as 1:100. Slopes steeper than this would limit the transgression rates of the landward margins of salt marshes and set up a form of natural ‘coastal squeeze’ ŽEnglish Nature, 1992. as the salt marshrmudflat boundary continued to erode. 2.2. Management response The research on the Humber and Blackwater Estuaries, outlined above, was concerned with the theoretical development of the estuary in response to sea-level rise. In reality, reclamation in both these estuaries, and indeed in the majority of British estuaries, has effectively prevented any natural morphological response to sea-level changes. The erosional response noted above, in which the salt marshrmudflat boundary exhibits a landward transgression, is generally observed, but migration of the landward boundary of estuaries is prevented by flood embankments that restrict landward salt marsh transgression and lead to coastal squeeze. Since reclamation has removed the greater part of the intertidal depositional surface in these estuaries, sediment eroded from the outer estuary and transferred to the inner estuaries, as described above, is deposited in the subtidal channels rather than on the salt marshes. The impact of this interruption to the natural process is already causing estuarine users considerable difficulties, as navigation channels and marinas experience increased siltation, requiring expensive dredging programmes. In the Blackwater, for example, sedimentation rates in the subtidal channels of the inner estuary are now 0.06 mryear, an order of magnitude greater than the rate of sea-level rise, and reflecting the disparity between erosional areas and depositional areas in the estuary. The implications of this disparity extend to other estuarine uses besides navigation. The erosion of the upper intertidal areas means that important habitats, especially upper intertidal mudflats and salt marshes, are being lost. Many of these areas have international designations under the Habitats Regulations wCouncil Directive 92r43rEEC on the conservation of natural habitat and wild flora and fauna, adopted in May 1992x or the Ramsar Convention and, consequently, such losses due to human intervention, in the form of flood embankments, may be seen as in contravention of these designations. The loss of upper intertidal areas without corresponding transgression poses flood defence problems. Wave dissipation on the intertidal area is an important consideration in the design of flood embankments. Increased gradients of these estuarine beaches due to upper intertidal erosion leads to increased wave propagation towards the toe of flood defences, demanding expensive maintenance programmes. More serious still may be the impact on the estuarine tidal systems of the lack of overall estuarine transgression. The intervention of flood embankments in the inner estuary means that the estuary cannot move landwards in response to increased energy levels so that, as a result, tidal range
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and storm surges both increase. Management response to these problems must focus on the removal, wherever possible, of flood embankments to facilitate landward transgression of the estuarine morphology. Managed retreat is, however, not a simple palliative. The research outlined above indicates that managed retreat will be most effective in the inner estuary in order that salt marshes can transgress landward, yet this transgression is only possible if sediment is released from outer estuary intertidal areas to be redeposited in the inner estuary. This means that managed retreat of outer estuary flood embankments is necessary in order to provide sacrificial intertidal areas, a concept that may be difficult to explain to the local landowner. In addition to these difficulties affecting the estuary morphology, maintenance of the balance of ecological habitats within the estuary, especially those with international designations, demands that they are allowed to migrate at the same rate as the estuarine morphology. Thus, the erosion of outer estuary salt marshes should be balanced by their migration landwards replacing slightly more brackish marshes that should themselves migrate into areas previously occupied at the head of an estuary by brackishrfreshwater habitats. Unless this complete sequence is facilitated, by provision of areas into which such habitat migration can occur, an imbalance between habitat types will develop. Finally, research undertaken as part of the Humber research programme ŽEnvironment Agency, 1999. indicates that the location, elevation and, to some extent, size of a managed retreat area can have an impact on tidal levels in an estuary. The research indicates that restoration of a reclaimed marsh in the outer estuary has little impact on water levels, but the relationship becomes increasingly sensitive as the retreat site is moved landwards. For example, reductions in water level of 0.1 m over a 20-km stretch of estuary can result from a single marsh restoration of 300 ha at Trent Falls, some 60 km from the mouth of the Humber estuary. In contrast, a similar area of restoration located at Sunk Island, 20 km from the estuary mouth, results in no measurable change in water levels.
3. Open coasts The location of landform units along an open coast is often determined by relatively robust cell systems determining for example, pocket beaches between rock headlands. In some areas, however, where such geological controls are not present and where shallow near-shore conditions reduce wave energy sufficiently, landform units along an open coast may be both diverse and sensitively adjusted to wave energy gradients. Wave energy gradients are themselves the result of wave refraction patterns in near shore that can result in wave foci along the shore where wave height and energy are accentuated. The North Norfolk shoreline is characterised by particularly complex wave refraction patterns, the result of near-shore bathymetric variations over the tidal deltaic deposits of the Wash. Here, the shoreline lies at an oblique angle to the near-shore contours and the resultant wave-refraction pattern shows a series of wave foci spaced at approximately 10-km intervals. Fig. 2 shows the pattern for a north-easterly wave with an 8-s period. The wave foci are areas where wave rays are compressed, causing increased wave
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Fig. 2. Wave refraction patterns on the north Norfolk coast, for waves approaching from the northeast with an 8-s period. Note: focusing of wave energy due to nearshore bathymetry.
heights and energy levels, while the areas between foci experience diminished wave height and energy levels as the wave rays are spread out over wider areas of shoreline. The landform response to this sequence of energy gradients may be described with the aid of Fig. 2. Areas of high wave energy are characterised by coarse sediment beaches, such as those at Brancaster, Scolt Head Wells and Blakeney. Between these wave foci, areas of low energy are occupied by salt marshes, such as those at Thornham and Stiffkey.
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These landform sequences merge into one another and their boundaries are blurred by the variations in location of wave foci as wave approach angle changes during successive storm events. Nevertheless, the overall sequence is extremely sensitive to the imposed wave energy gradients and, therefore, to changes in sea level. The impact of sea-level rise is demonstrated in Fig. 3 in which sea level is assumed to have increased by 1 m Ži.e. taking place over 166 years at an average rate of 6 mmryear.. The wave
Fig. 3. Wave refraction patterns on the north Norfolk coast. Wave approach and period similar to those in Fig. 3 but with a 1 m increase in sea level. The increased water depths have displaced wave energy foci, relative to current baseline conditions, which in turn will cause the coastal landform units Že.g. salt marsh, beaches, sand dunes. to migrate.
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foci are shown to have migrated along the shore by 8 km, at a rate of 53 mryear, so that locations that previously experienced minimum now experience maximum wave energy, in effect the energy gradients have been reversed. The response of the landform sequences along this coast is predicted to be equally dramatic. Sand beaches, such as those at Wells, are predicted to be replaced by mudflats and salt marshes, while sand beaches will replace the designated salt marsh and sand dune areas of Thornham, Scolt and Stiffkey. These predicted changes are, of course, exaggerated here by the use of a 150-year time interval. Nevertheless, locally sea levels are rising by 1.5 mm at the present time and this appears to be resulting in the inception of landform migration, such as those predicted above. The sand beach at Holkham has been replaced by salt marsh during the last decade, while increased siltation of Wells beach is currently causing problems for local coastal managers. 3.1. Management response The alternations of wave energy and landform sequences along the north Norfolk coast are themselves responsible for the pattern of coastal management here. The tourist industries located at Hunstanton and Wells are dependent on wide sandy beaches that are a response to high-energy waves experienced here. The internationally designated nature conservation areas of Thornham, Scolt and Stiffkey are located in low wave energy areas resulting in salt marshes, sand dunes and mudflats where biological activity is at a maximum. As discussed above, landform migration may already be occurring along this coast with increased silt content of previously clean sand beaches causing major problems for the tourist industry at Wells. The economies of the towns of Hunstanton and Wells depend to a large extent on their recreational beaches. However, it seems unlikely that over the long term, migration of these towns will take place to parallel the migration of their beaches; but equally, it seems unlikely that tourists will continue to use these towns as centres if a 10-km journey to the nearest beach is demanded. In this case, the disparity between the inertia of the existing urban infrastructure and the migration of the resource could lead to the development of alternative resorts, at first as ‘mobile’ caravan parks, that could threaten the internationally important ecological habitats on this coast. The designation under the Habitats Regulations of most of this coastline poses serious management problems. Under these regulations, the competent authority must notify deterioration of designated habitats; changes in habitat due to natural processes, however, may be accepted. The key issue on the Norfolk coast is whether the natural habitat migration due to sea-level rise can take place unimpeded by human interference. In this case, existing flood defences both for agricultural land and also for the urban development along the coast may prevent landform migration and, consequently, the relative extent of different types of habitat may alter. If this is interpreted as contrary to the regulations, then it will be necessary, under their provisions, to provide replacement habitat elsewhere. Replacement of habitat that is part of a functional coastal system in an alternative location is difficult if not impossible and it appears that a major impasse is inevitable.
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4. Tidal deltas The tidal discharge from an estuary into the open sea presents a potential barrier to long-shore sediment movement along the open coast; a barrier that, in most cases, is bridged by a complex intertidal or subtidal deltaic landform. These deltas not only allow sediment transport to bypass the estuaries, they also provide significant protection to adjacent coastlines ŽDyer and Huntley, 1999.. Tidal deltas consist of two main components, a flood-tide delta located within the estuary, and an ebb-tide delta usually located seaward of the estuary and extending over lengths of the adjacent shorelines. Ebb-tide delta systems are the subject of the present discussion. Ebb flows out of an estuary force long-shore sediment-transport pathways seaward, where they commonly form a roughly triangular-shaped rampart — the ebb delta. This rampart is fed by long-shore sediment movement usually in the form of sand waves that migrate from the shoreline out to the delta. From here, they subsequently become detached and move downdrift to complete the long-shore sediment transfer across the estuary mouth ŽFitzgerald and Penland, 1987.. The sand waves that characterise these delta systems act as wave breaks and, thus, reduce wave energy along adjacent shorelines. The size and extent of ebb-tidal deltas depends on the tidal prism of the estuary and the strength of the long-shore sediment transport discharge, itself dependent on wave energy ŽWalton and Goodall, 1972.. In order to estimate the impact of sea-level rise on ebb-tidal delta dimensions, a predictive semiempirical model of delta growth has been developed for the Humber estuary. Prediction of delta volume using the relationship with tidal prism presented by Walton and Goodall Ž1972. is combined with an empirically derived relationship between delta seaward extent and tidal prism ŽFig. 4a.. Sonar imagery of sediment depth in the ebb-delta is then used to derive a maximum lateral dimension of the delta, that is, its basal width measured along the shoreline ŽFig. 4b.. A good agreement between the predicted and the observed dimensions for the present-day Humber delta was obtained ŽFig. 4c.. The delta area is predicted to be 117 km2 and its basal width to be 23 km. Observations show that the Humber delta extends south from the estuary mouth for 23 km along the Lincolnshire shore, but with almost no extension north of the mouth, an asymmetry probably due to the net southerly wave-induced current here. It is significant to note that in the protected area landward of the delta front, the shoreline is currently experiencing accretion of sand dunes and salt marshes, while on the exposed shore, south of the delta front, erosion of the sand beaches has been so severe to require ongoing annual beach sediment renourishment by the Environment Agency. The impact of sea-level rise on the lateral extent of the delta and, thus, on the protection afforded to the shoreline is predicted by the model described above. As sea level rises in the trapezoidal cross-section estuarine channel, so the volume of tidal water entering the estuary will increase, resulting in an extension of the volume and area of the tidal delta. The long-shore sediment transport rate feeding into the delta will also increase as wave refraction decreases in the deeper water. The overall effect will be for the delta and its ancillary sand waves to extend both seaward and long-shore as sea level rises, a process that will offer increased protection to adjacent shorelines.
318 J. Pethickr Catena 42 (2001) 307–322 Fig. 4. Empirical data from 9 UK estuaries ŽA. was used to derive an expression relating tidal prism and the seaward projection of their ebb-tidal deltas. This was used with the relationship derived by Walton and Goodall Ž1972. to relate variations in the length of the tidal delta baseline Ži.e. along the shore. to changes in tidal prism ŽB.. Results indicate that a sea level rise of 6 mmryear would result in an extension of the tidal delta of the Humber estuary ŽC. by 300 mryear.
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Predictions from the model, assuming a steady increase of 6 mmryear in sea level, indicate an initial increase in the ebb-tide delta area of 16 km2 , rising to 21 km2 after 100 years. The southernmost extent of the delta would, as a result of these changes, migrate south at an initial rate of 300 mryear, rising to 386 mryear after 100 years. This southerly extension to the delta may lead to a commensurate migration of the accretionrerosion boundary on this shore. The 1.5 km southerly extension of the coastal salt marsh immediately north of Mablethorpe ŽFig. 4c. on the Lincolnshire coast, over the past 40 years as evidenced from air photograph records, provides some evidence for the existence of such a migration as a result of the present-day rise in sea level Ž1.4 mmryear.. 4.1. Management responses If the example presented above of the Humber Estuary is assumed to be typical of the general response of ebb-tidal deltas to sea-level rise, then an increasing length of the coast of Britain may be expected to be protected from wave erosion as sea levels increase due to global warming. The importance of this natural form of coastal defence or the extension to this length, which will occur as a result of sea-level rise, has not been appreciated by coastal managers. The lateral extent of the Humber ebb-tide delta is predicted above to increase by between 300 m and 386 mryear, assuming an average rate of sea-level rise of 6 mmryear. This extension to the length of protected shoreline could remove the necessity for coastal defences in the shadow of the delta with significant saving in expenditure. Since conventional hard defences may cost in the region of £3000rm, this could involve savings of £1 000 000ryear on the Lincolnshire coast. Extrapolating such figures to the coast of Britain as a whole is difficult, given the range of estuarine forms that exist; but since there are 158 estuaries on the British coast ŽJNCC, 1993., it may be assumed that the reduction in the necessity for conventional defences provided by the extended tidal deltas may involve savings of many millions of pounds. This increased size of ebb-tide deltas as a result of sea-level rise appears, from the examples given above, to offer a natural mechanism for coastal defence that needs little or no management interference or financial input. However, if the sediment budget of the shoreline rather than the financial budget is considered, then the sediment-benefits of such a natural process must be set against the sediment-costs. The increase in sediment stored in an estuarine delta because of sea-level rise may be sourced from adjacent shorelines, so resulting in down-drift sediment starvation. The management fear would be that the net effect of these two processes might be to cause more erosion down-drift due to sediment loss than is reduced by the extension of the delta front. The volumes of sediment needed to supply the increased deltaic morphology vary, of course, with the size of the estuary involved. The increased sediment volumes stored in the Humber Estuary, for example, is predicted using the model described above to equal 300 000 m3ryear for a 6-mm rise in sea level. This is approximately equal to the entire sand output from the erosion of the Holderness coast to the north and represents the annual net sediment transport rate along this shoreline. If this annual volume were stored in the Humber delta, significant depletion of sand inputs along the southern Lincolnshire
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shoreline could result, an area where sand nourishment is already necessary to protect the eroding coast. The provision of self-sustaining natural defences along significant proportions of the coast due to deltaic extension offers an important coastal management tool. With the potential problems of sediment budgets in mind, it may be useful to consider the possibility of extending or even creating ebb-tide deltas along the coast as a form of coastal defence. One example is briefly described here. This involves the Royal Norfolk Golf Clubhouse, located on the shoreline at Brancaster Norfolk. Here, erosion over the past decade has resulted in the clubhouse standing proud of the shore and demands increasingly expensive hard defences to protect the building. To landwards of the clubhouse lies an area of reclaimed salt marsh and it has been proposed that a breach in the embankment surrounding this area would allow the development of a small tidal estuary with a tidal prism of 0.5 = 10 6 m3. This estuary would develop a small tidal delta of 0.01 km2 overlapping approximately 200 m of the adjacent shoreline and providing protection to the clubhouse. Moreover, this protection would extend as sea level rises, providing a self-sustaining defence that would not require further maintenance. The implementation of such a radical approach to coastal defence was prevented due to the fact that the reclaimed marsh involved is designated as an SPA under the EU Birds Directive. The proposed change in habitat type from fresh water marsh to salt marsh would contravene the Habitats Regulations, another example of the inertia inherent in our coastal management systems that prevents any long-term strategic approach to the problems posed by coastal landform migration under sea-level rise. Nevertheless, the proposal does demonstrate the potential of the process for coastal protection and may offer considerable scope in the future for accommodation to the predicted rise in sea level.
5. Conclusions Coastal landforms are extremely sensitive to medium- to long-term changes in energy inputs brought about by sea-level rise. These changes in energy gradients along the shore result in landform migration, which allow each landform to remain in its equilibrium energy niche. The prediction of the migration rate for individual landforms offers an important coastal management tool, allowing management decisions to be made to accommodate and, in some cases, to facilitate such migration. The three examples examined here demonstrate the approach and provide preliminary estimates of migration rates. Estuaries are predicted to migrate landwards and upwards in response to sea-level rise and, for the Humber Estuary, the migration rate is predicted to be 8 mryear, assuming a sea-level rise of 6 mmryear. Managed retreat of existing flood defences will be necessary to facilitate such migration. On the open coast, an example from North Norfolk demonstrates that wave refraction patterns will shift in a long-shore sense as sea level rises. Here, rates of migration of wave foci averaging 9 m per 1 mm sea-level rise per year are predicted, leading to major changes in habitat mosaics along this internationally designated coast. Positive manage-
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ment responses are difficult to predict and it is assumed that accommodation to such changes will be necessary despite the legal obstacles provided by the EU Habitats Regulations. Finally, tidal deltas are shown to protect significant length of the shoreline, but have not been appreciated as defence mechanisms. Extensions of these deltaic systems as sea-level rise is predicted here using a simple model and the possibility of initiating deltas as a form of self-sustaining defence is outlined. Here again, however, existing legislation is shown to prevent any such radical innovation in our coastal management systems. It appears that if viewed from the static position of our present approach to coastal management, sea-level rise will result in radical change to our coastal landforms. If we adopt a more dynamic viewpoint, however, then it appears that these same coastal landforms remain intact; it is only their location that will change. If we persist in applying our static coastal management systems as sea levels rise, then an increasing disparity will arise between our needs and the coastal resource. Instead, we must begin to manage change at the coast in a more positive manner.
Acknowledgements The author is indebted to Dr. S. Crooks for advice and editorial assistance and Ms. T. Skrzypcsak for the cartography.
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