Dr Adrian Hall, University of St Andrews

The record of storm hazards

The Shetland Islands lie along the storm tracks of mid-latitude depressions. Intense depressions generate high winds and agitated sea states, resulting in hazards that include wind damage, extreme waves, inundation, ship wreck and coastal erosion. The extremes of wind, with maximum recorded gusts in excess of 150 mph, mean that the storm hazard on Shetland is perhaps greater than anywhere else in the British Isles.

At least six major storms are recognised in the 20th century in which wind speeds exceeded 60 knots. Each storm occurred in the winter months of December through to February. Each caused the impact of large waves at the coast, with associated erosion and local inundation. Documentary evidence of earlier storms is less reliable but major storms are identified in 1634, 1669, 1792 and 1862. An unexplored archive of information about past storms is provided by sediments in bogs close to current sea level and by boulder deposits on cliffs on the exposed outer coast of Shetland. Preliminary dating of sands from boulder deposits at Villians of Hamnavoe indicates that at least one major storm occurred in the early 17th century.

An association exists between the highest wind speed in gusts recorded at Lerwick and reports of damage from storms. Observations after the 1992 and 1993 storms indicate also that wave water reached higher on cliffs in these storms than in any more recent storm up to February 2005. The storm hazard therefore is generated by the most intense storms, although local impacts can be severe from events of lower magnitude where an unfortunate coincidence between tide, wind direction and coastal configuration funnels water to create surges at the heads of voes.

Intense storms are generated at the Polar Front. Storm intensity reflects in part the pressure gradient between the Icelandic Low and the Azores High. The most intense storms deepen rapidly, within 12 hours, and the storm system may also be fast moving. The time window for forecast and warning is correspondingly small. The cyclone centre may be no more than 50 km across and impacts along the 120 km N-S length of the Shetland Islands can therefore vary greatly. The maximum winds and associated wave heights occur in the SE quadrant of the depression system. Maximum impact can be predicted where this zone crosses the coast.

The track of the storm determines the locus of impact on Shetland. The 1992 and 1993 storms tracked NE, causing maximum damage on North Mainland and Unst. The 1953 storm tracked SE, with damage concentrated in South Mainland. As the storms cross Shetland the wind may swing to the SE, giving high seas on coasts including Out Skerries and Mousa and may be accompanied by tidal storm sure in Lerwick. This pattern appears to have been followed by the 1900 storms which inundated Clickhimmin and led to widespread damage up to 5 m OD around the harbour at Lerwick.

Storm hazards in the future: possible impacts of climate change

Estimated sea level rise on Shetland over the next century is around 1 m. Any future change in storminess will be superimposed on this, extending inland the limit of storm driven wave water. The period since 1955-1994 saw a steady rise in storm intensity and an associated increase in average and extreme wave heights in the North Atlantic (WASA). Most models indicate further increases in storminess as a result of any increase in mean annual air temperatures in the NE Atlantic.

In the 21st century storm intensity is predicted to increase, with the focus of intensity migrating eastwards across the Atlantic, so becoming greater in the Shetland-Norway area.

The latest STOWASUS models indicate significant local and seasonal variation in sea states in the eastern North Atlantic. An increase in significant wave heights is predicted in Norwegian waters in winter but at 60° N in North Sea there may be little apparent change even for the largest waves with a 100-year recurrence interval.