Gower SOS

[ In: M.R. Bennett & P. Doyle (eds) Issues in Environmental Geology: a British Perspective, p. 326-373. ]

Off-shore sand banks and their role in coastal sedimentary processes:
The Welsh coast of the outer Severn Estuary, SW Britain
Dr Siegbert Otto

Holistic & Integrated Services, The Granary, Teifi Wharf, Cardigan SA43 3AA
Telephone: 01239 614564, Fax: 01239 621987, E-mail: ziggy-otto@supanet.com

Introduction
Sedimentary processes in the coastal zone are the result of complex interactions between tides and tidal currents, and waves and wave-induced currents. When waves (or swells) propagate into shallow water they are modified by refraction, shoaling, and energy losses. Refraction of waves entering shallow coastal waters is often the overriding factor in governing sediment entrainment and deposition, sediment transport paths and ultimately landform evolution. The waxing and waning fortunes of beaches and spits can also often be explained by changing patterns of wave refraction and associated long shore currents. Natural and man-made obstructions result in gradients of incident wave energy along the coast, thus giving rise to areas of sediment deposition and / or erosion. Long-term changes in relative sea level and short-term variations, brought about by fluctuating tidal levels, not only determine the fetch over which wind can act on the water surface and consequently potential maximum wave heights, but also the extent to which subaqueous topography interferes with the incident waves. Offshore, subaqueous highs, such as sea mount ridges, dumped spoil heaps and sandbanks, are the most common cause for the generation of strong wave energy foci along the coastline. It is therefore by no means surprising that the most severe coastal erosion is occasionally found concentrated in the more sheltered parts of bays (Caston, 1967) and estuaries (Otto, 1996c).
The purpose of this publication is: (a) to present the results of wave refraction modelling from two areas along the Welsh coast of the outer Severn Estuary in south-west Britain, (b) to illustrate the importance of off-shore sand banks on controlling in-shore wave climate, (c) to relate the modelled wave climate to coastal morphodynamics and sedimentary processes, (d) to consider the wider implications of this investigative approach, and (e) to recommend it particularly for coastal and estuarine environmental assessment studies. Prerequisite for the holistic appreciation of the results of the hydrodynamic modelling and their wider implications is a thorough understanding of the estuarine morphodynamics and sedimentary processes - present as well as past - and it is for this reason that a detailed review of the study area is presented.
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Dredging
The inner Bristol Channel and outer Severn Estuary supply an estimated 15 % of United Kingdom sea-dredged aggregates (Figure 5). In 1995 1.52 x 106 tonnes of sand and occasional gravel were landed at local ports of the region (Table 1). This means that the approximate equivalent of one year's fluvial sediment supply entering the system (i.e. 1.98 - 2.25 x 106 tonnes) is at present yearly being dredged. It is impossible to establish how much sediment has been redistributed through over-spilling and screening, or how much has been introduced by severe wave action into the active sedimentary environments of the high intertidal zone.

Table 1 Summary of Port Statistics for Marine Dredged Aggregates 1995 in the outer
Severn Estuary and inner Bristol Channel (data supplied by The Crown Estate)

Landing Ports 1995 Tonnages
Avonmouth 501,229
Barry 84,175
Bridgwater 52,928
Cardiff 447,814
Newport 436.331
Total 1,522,477

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Conclusions
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Although the sediment transport paths suggest a contemporary constructive process with sand banks and flats either in equilibrium or in a state of net-accretion, intense dredging activity over several decades has probably removed more sediment than has entered the system. There is no modern source of sand, and if we accept the concept of bedload parting to be active (somewhere) in the Bristol Channel, the sand resources up-estuary from this zone must be considered relatively finite. Sediment is undoubtedly being redistributed by the vigorous hydrodynamic regime and pushed further into the estuary by a continuous rise in sea level, but a net gain of sand is not possible by these means. Parker and Kirby's (1982) suggestion that the volume of sandbanks has increased significantly over the last 100 years has - as yet - not been investigated or quantified, and is indeed less likely. Intense dredging in the inner Bristol Channel and outer Severn Estuary must therefore be very carefully considered, as sand banks and flats may become sediment starved by dredging of the sources which would otherwise feed them.
It is noteworthy that the dredging areas - as part of The Crown Estate - fall under the jurisdiction of the Department of the Environment, and thus require an environmental assessment prior to dredging approval in order to establish "whether the dredging will cause, or modify, refraction of waves and thus lead to significant changes in the wave pattern" (Department of the Environment, 1989). The author is not aware of any wave refraction studies undertaken to date to assess the impact of aggregate dredging on coastal sedimentary processes in the outer Severn Estuary (or for South Gower for that matter!).
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Any proposed aggregate extraction and spoil dumping operations, or building of coastal protection measures should be scrutinised in the light of comprehensive environmental impact studies. Ideally this should include the type of wave climate modelling approach adopted here for a relatively small area of an otherwise large estuary. It explains successfully the distribution of coastal morphologies and the causes for their erosional or accretional behaviour, and will also allow predictions to be made regarding bathymetric changes, for instance owing to dredging, spoil dumping and / or sea level rise, and changes in the local wind regime. However, prerequisite for a full appreciation of the results of any type of coastal hydrodynamic modelling is a thorough understanding of the temporal and spatial variations of coastal change, whether naturally caused or by anthropogenic interference.
Concluding on a more general note, erosional problems in the coastal zone cannot be viewed without consideration of shallow off-shore belts which are still affected by wave action (in particular during storms and / or low tide conditions), and which in turn modify heights and directions of waves subsequently incident along the coast. Poor understanding of the causes and effects of sedimentary processes in the wider coastal zone may result in inadequate planning to counteract saltmarsh and beach erosion and flooding of the hinterland. Within the context of coastal zone management and flood defence issues, off-shore sand banks, intertidal mud and sand flats, saltmarshes and beaches, must all be appreciated as a tiered defence system. Any large-scale modifications in the off-shore zone by natural events (e.g. extreme storms) or anthropogenic interference (e.g. dredging or dumping of spoil) will inevitably have a significant impact on the shoreline. It is in this light that dredging and sand / gravel extraction policies should be reviewed, and options considered to modify, and possibly incorporate, these sand banks into a system of submerged shoals, as large scale hydraulic modifications of the system are most likely to yield best results for the retardation of coastal erosion.
THUS FAR BITS AND PIECES FROM A RATHER LONG PAPER I PUBLISHED IN 1998.

RE Start Bay (South Devon):
650, 000 tonnes of shingle were dredged offshore from Hallsands (Start Bay - South) between 1897 and 1902. The village of Hallsands lost subsequently 12 houses in 1903 due to coastal erosion, as higher waves were now (after dredging) able to come closer inshore. The village had to be abandoned in 1917. There can be but little doubt that dredging and coastal erosion are linked in this case. Removal of sand banks does modify wave refraction and wave energy along a coastline that might otherwise be protected by offshore shoals. On the other hand, sandbanks can also modify wave patterns to an extent of creating wave foci (erosional hotspots) along a coastal stretch. Each case has to be looked at individually! In case of the South Gower beaches, Gower SOS should demand for a wave refraction study to be conducted, modelling scenarios such as before dredging, present state and future anticipated state, with sandbanks lowered or even totally removed. I have extensive experience in wave climate modelling, and would encourage Gower SOS to go down that road.

About myself:
I am an environmental and geoscientific consultant (T/A Holistic & Integrated Services, The Granary, Cardigan), currently specialising in environmental management strategies and risk appraisal, including Coastal Zone Management and Environmental Impact Assessments. In August 2000, I completed an Estuary Management Plan (EMP) for the Teifi Estuary, West Wales, the first EMP - and to date the only one - for an all-Welsh river.