storm history

wave environment

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Perhaps the most striking examples of the effect of air-borne wave water come from Esha Ness. Images show spray reaching heights of 80 m above the Dore of Holm on Stenness. The roof of the Esha Ness lighthouse at over 55 m OD has also had to cleared of debris on occasions. In freezing temperatures the spray falls as sheets of ice!

Wave action on cliffs

Seli Geo, Bressay. Wave scour of cliffs, with the maximum height of scour decreasing in the relatively sheltered and shallow inner part of the geo.

Breaking waves require that the base of an incoming wave is in contact with the sea bed and on parts of the outer coast of Shetland deep water close inshore means that only the largest waves break at the cliff and smaller waves are reflected seawards from the cliff face. It is important however not to over-emphasise this effect for many waves are only a few metres high and many cliff bases show reefs close inshore. What is clear is that abrasion has limited effects because the boulders that are the tools of abrasion are not generally available to be thrown against the cliff base.

Waves that impact on the cliff face will generate hydrostatic pressures due to the instantaneous or momentary compression of water and air in fissures. Reported measurements show pressures of up to 690 kPa, with durations of only 10 ms (Rouville et al., 1938) and 435 kPa (Bullock et al., 1999). The effective pressures will vary according to the forward motion of the impacting mass of water. Although these forces have yet to be measured directly on Shetland cliffs, the evidence of open cavities that narrow into closed fractures requires that they are sufficient to prise open existing fractures. The jetting effect of water under pressure squirting through cavities will also tend to remove loose debris and to widen fissures.

On stepped or sloping cliff faces the upper part of the wave may be thrown or jet upwards. A substantial volume of water may move rapidly up the cliff face or, if thrown into the air, may descend on the upper part of the cliff. Both action will generate powerful sluicing actions, removing loose debris. The forward motion of the water may also be very rapid - it seems to be sufficient to detach large blocks high on cliffs. This is the origin of the scoured zone present on cliffs. The stripping of vegetation down to bedrock may reach heights of 40 m and form zones up to 100 m wide. The absence of this zone on a cliff and the extension of turf to the cliff edge implies that no storm water reaches the cliff top.

On cliffs on deep water coasts at 10-40 m asl the wave may break on the cliff top and generate a jet of water that moves at high velocity across the ramp or platform of the cliff top. These green water waves are capable of detaching very large blocks. A axis lengths of cliff top boulders frequently exceed 1 m on Shetland and reach 3 m at Scat Ness. Distances of carry are also considerable, reaching 60 m at the Grind of the Navir.

The effect of spray is usually restricted to the vegetation of the coastline but on Shetland the water thrown up by waves is itself a powerful agent of transport. This is scarcely surprising as spray reaches many tens of metres above cliff tops. Any cliff top walk in western Shetland will encounter zones where platy rock fragments from coin- to plate-size litter the turf on the cliff top. Turning over these fragments reveals green grass, so the debris moved recently. These fragments represent air throw debris, often sourced from the cliff top and from erosion of the overlying till and soil. The small fragments may be thrown far inland but the larger pieces are probably moved by a combination of very high winds during storms and splashing spray. Over time the small fragments are covered by the growth of turf to give a form of machair.