7.0 EROSION ISSUES

7.1 The Scottish coastline stretches over 11,800 km in length, including the shorelines of the mainland and the islands²⁵. Much of the Scottish landscape and coastline was formed initially through the processes of glacial erosion and deposition²⁶, and the sea continues to change the land through wave action, sediment movement, erosion and accretion²⁷.

7.2 Scotland's Marine Atlas identifies erosion from vessel wash as a pressure for coastal habitats. A key issue for the Draft Ferries Plan is the increase in the number of vessel movements on existing routes, and the introduction of new routes, and the effect that these may have on the coastline and/or seabed.

Current Situation

7.3 Coastal erosion and accretion are significant problems affecting many coastal communities around the world. Natural wave action, tidal currents and drainage are typically the main drivers of coastal erosion, acting to effectively "wear away" the coastal surfaces. In more recent times, human activities have played a significant role in coastal erosion, through such practices as land reclamation and the construction of infrastructure such as harbours, jetties and marinas, which can affect coastal processes by restricting the movement of coastal sediments²⁸ and increase the vulnerability of an area to erosion. It is expected that erosion processes are likely to increase in the future, with additional pressures from rising sea levels and erosion/deposition associated with climate change²⁹.

7.4 The main factors affecting susceptibility of a coastal area to erosion comprise the sediment size and size distribution, which can vary markedly even in small areas. In addition, the existing level of disturbance or erosion of a shoreline can make an area more susceptible to the effects of wave wake and coastal erosion.

7.5 A survey of Scotland's coast (SNH, 2000)²⁵ identified the following characteristics:

hard coasts, comprising rocks and cliffs (70%);
soft coasts, comprising unconsolidated gravels, sand and silts (29%); and
infrastructure, comprising harbours, sea walls, etc (less than 1%).

7.6 Of these, 75% of the coast was considered broadly stable, 8% accretional, and 12% erosional (there was insufficient data available for the remaining 5% at the time of study³⁰). The erosional portion of coastlines largely consists of beaches, sand dunes, conglomerates/soft-rock cliffs, machair and marshes with muddy sediments^31,32. This information is summarised in Figure 7.

7.7 Coastal erosion is a largely natural process affecting most coastlines in Scotland, with erosion rates generally estimated to range from <1mm/year for hard igneous cliffs to greater than 4m/year for dune coastlines in the Western Isles³³. Apart from this, there is little data currently available on estimated or actual erosion rates for most of the Scottish coastline. However, several specific coastal areas are known to be subject to active erosion and/or accretion: the east coast from Montrose to Dunbar, the Firth of Clyde, the inner Moray Firth, and the Northern and Western Isles. Coastal protection plans have been introduced in these areas²⁷.

Potential Effects

7.8 As noted in Paragraph 7.2, Scotland's Marine Atlas identifies vessel wash as a pressure on Scottish coastlines. The waves from vessel wakes effectively add energy to coastal systems, and by doing so can alter coastal processes, particularly in environments where natural wave patterns and wave energy are low³⁴, such as semi-enclosed seas and other sheltered waterways.

7.9 Wave and wake actions from vessels have been shown to contribute to changes in coastal processes, and can result in significant localised erosion and accretion of sediments³⁵, ³⁶. Studies of wave and wake patterns globally have indicated that impacts on coastal zones can occur at distances of up to 7-10 km from the sailing line, particularly in confined coastal waters with low natural wave energies³⁶.

Figure 7. Erosional Characteristics of Scotland's Shoreline

7.10 Research indicates³⁷ that the factors affecting vessel wave wake and the potential for coastal erosion include:

Vessel type, size, weight, and hull design and shape - can greatly affect wave wake generation by a vessel and the subsequent impacts of the wake on surrounding waterways.
Vessel speed - higher speeds typically increase vessel wake, and restrictions in specific waterways have been introduced in Denmark, Sweden, New Zealand (Marlborough Sounds)³⁶ and Australia (Gordon River)³⁸ amongst others. These restrictions target vessels with known wave wake characteristics, and are intended to minimise the effects of coastal erosion.
Distance of the vessel from the shoreline - wave crests diverge and there is typically dispersion of wave energy into the subsequent waves with distance, indicating that the closer a vessel moves to the shoreline, the larger the likely wave impacts.
Depth of water - as water depth decreases, waves typically become less dispersive and wave height becomes larger.
Extent of the natural currents and waves on the waterway - the effects of wash wake will typically be larger in sheltered and reasonably calm waters in comparison to waters with larger natural wind waves.

7.11 At present there is little or no data about vessels affecting coastal and/or seabed erosion, apart from work done on the fast ferry link between Scotland and Northern Ireland, at Loch Ryan.

7.12 The ports and ferry routes where proposals in the Draft Ferries Plan would lead to change were identified, and existing coastal data was reviewed to identify areas which may be susceptible to erosion in their vicinity (Eurosion 2000³⁹, SNH²⁵ and SCAPE Coastal Zone Assessment Survey⁴⁰). An assessment was undertaken to ascertain whether the proposed changes would lead to increases in erosion, taking into account ferry information, where this is known (i.e., ferry type, design speeds and need for manoeuvring). The detailed results of this assessment are presented in Appendix 3.

7.13 In summary, these results show that increases in the number of vessel movements and/or vessel turning movements could result in an increase in erosion at sensitive locations. However, in many cases the increase in vessel movements does not appear to be large enough to be likely to result in significant effects. At more sensitive locations, the significance of effects is uncertain.

7.14 In consequence, the indirect effects of erosion on biodiversity, landscape and cultural heritage are also uncertain.

Mitigation

7.15 As the significance of effect is uncertain, it is not possible to identify mitigation measures at this stage.

7.16 The Scottish Government will explore, with ferry operators, harbour authorities, statutory advisers and others if erosion is an issue at the identified locations. This will include the consideration of local-level erosion issues as part of the STAG-type appraisals to be undertaken prior to the finalisation of the Draft Ferries Plan.