Coastal Processes

Winds

The predominant wind directions are from the south-west and south. High wind speeds (in excess of 70 mph) occur on a regular basis within the more exposed St Kilda archipelago and wind erosion of the vegetated cliff slopes is prevalent, especially where this occurs in association with salt spray erosion/weathering.

Waves

The isolation of St Kilda along the north-western edge of the continental shelf also means that the islands are constantly subject to severe high energy swell waves which are generated across the open expanse of the North Atlantic Ocean.

Significant wave heights exceed 5m for over 10% of the year, and 2m for over 75% of the year. The predicted 50-year wave height immediately to the west of the Outer Hebrides is 35m, which is significantly greater than for other parts of the UK coast and the 50-year wave height further to the west, around Hirta will almost certainly be above that value.

The dominant southerly and south-westerly wind directions mean that the most frequent high-energy storm waves are also generated from these directions. However, even Village Bay on Hirta which is considered one of the safest anchorages on the islands (the only other is Glen Bay), is notoriously treacherous during high-energy south-easterly storms. Open fetch conditions occur in all directions, and the coastline is prone to attack by storm waves generated from every sector.

Bathymetry

The cliffed coastline of the islands and stacks that comprise the archipelago plunges to depths of –40m. Water depths shallow abruptly in two steps from –120m to –80m and then to –40m at the foot of the clifflines around the islands, thereby facilitating large, high energy Atlantic swell waves to run and break relatively unimpeded onto the exposed coastline.

Figure 3.7: Perspective view of the St Kilda Platform from the north-west. This was produced by BGS using ArcScene (ArcGIS 8.2 Desktop) software to manipulate a 3D model of the St Kilda region, based on digital elevation data supplied by Scottish Natural Heritage. Separate colour ramps are applied to elevations above and below sea level. Sea level is further accentuated by including a transparent layer to represent the sea surface. A ‘hillshade’ is applied to highlight topographic variation (Azimuth 315 degrees, Altitude 30).

Tides

The tidal range is oceanic and thus comparatively low at 2.9m (Spring Tides). Storm surges can result in an elevation in tidal height, and calculations on the heights of the 50-year storm surge around the UK along the West Highland coast including St Kilda indicate that for a limited time water levels could be up to 1m higher than predicted values.