4.1 Processes

Every site is influenced by physical processes that cause erosion and recovery of the dunes. These include winds, waves, water levels and currents that drive cross-shore and longshore sediment transport. In turn these processes are influenced by topography, bathymetry, geomorphology, river flows, coastal structures and dredging operations. Dunes are also influenced by rainfall, temperature, sediment chemistry and natural grazing as these factors affect the type and health of the vegetation. There are numerous publications that discuss these processes in general terms (for example, Ranwell and Boar, 1986), but a shoreline manager should also be aware of how these processes work at the site of interest.

A good starting point for developing an understanding of the shoreline is the geomorphological and recent history. Information on the coastal and nearshore surface and drift geology is available from British Geological Survey publications and maps. Recent evolution can be derived from analysis of historic map series, charts, aerial photographs, surveys and engineering records. Important sources of regional information are provided by the Coastal Cells series (HR Wallingford, 2000) and the Beaches of Scotland series (Countryside Commission for Scotland, 1969-1981), both of which are fully referenced in Section 6.1 and Figures 6.1 and 6.2. The former series of 11 reports was commissioned in 1996 by SNH, SOAEFD (now SERAD) and Historic Scotland to provide a broad understanding of the character and processes of the coastal regime around the Scottish coast. The latter series of 19 reports provide a detailed description of every sandy beach and dune system in Scotland including morphology, land use, management pressures and the nature and extent of coastal erosion; although some information is now dated, this series is of considerable value to coastal managers. Recent information is also available for some areas where Shoreline Management Plans and analogous studies have been completed (Section 6.1, Figure 6.2) (Plate 20).

Plate 20

Plate 20 A series of erosion concerns, such as here at Dysart, has prompted Fife Council to prepare a Shoreline Management Plan for its entire coastline.

Useful information can also be derived from discussions with local residents and beach users, and from more formal consultation with other interest groups (Section 4.4). The resulting picture may be one of long term erosion, accretion or stability, combined with short term cycles of rapid erosion and recovery. Care should be taken if the data set is skewed to the very recent past and is dominated by records of erosion; such information may be atypical of the long term situation, and may lead to a hasty decision to undertake unnecessary works.

Reasons for the evolution may be an obvious change in local shoreline processes caused by specific human activities, such as the construction of a harbour, the dredging of a channel or an increase in recreational use of the dunes. Less obvious may be the effects of medium term (1-20 years) changes in weather patterns, tidal cycles (up to 19 years) or a gradual reduction in sediment supply from a natural source.

Met Office data may reveal reasons for changes to dune stability. Unusually severe storms over Spring tide periods may have caused rapid erosion, while a sustained shift in the dominant wind directions may have caused change to the longshore drift direction, causing erosion in one area and accretion elsewhere. Alternatively, a drop in average wind speeds may have reduced the aeolian transport of sand that feeds the dune face, or a prolonged dry spell may have reduced the vigour of foredune vegetation and restricted dune recovery.

The causes of erosion within an estuary may be difficult to understand. Shoreline evolution is often linked to the ever changing distribution of channels and banks. These features are influenced by local wind waves, waves entering through the estuary mouth, ebb and flood tide currents and fresh water flows, as well as human activities such as dredging, reclamation and the construction of training walls or navigation facilities. Prediction of evolution will require the services of a specialist estuary consultant.

Having established the background to the site evolution, and prior to committing any significant effort to erosion control, it is advisable to establish a monitoring programme. The objective of monitoring would be to expand the desk based knowledge of the processes active at the site and to develop a first hand understanding of the extent of the erosion problem. Minor shoreline works might be undertaken during the monitoring period, but the implementation of any major schemes should be delayed until the managers have confidence in their understanding of the site and the potential consequences of their actions. Monitoring approaches are outlined in Appendix 2.