Coast erosion at a nuclear waste shaft, Dounreay, Scotland

The risk from coast erosion to a shaft containing nuclear waste at Dounreay , Caithness is assessed. The physical setting of the shaft, data on the rat es of erosion in the Devonian Flagstones and the drift capping of the local cliffs and future influences on erosion rates and flooding are summarized. A two-layer model of superficials and flagstones is used to address the imm ediate concern of lateral erosion towards the shaft. Superficials' erosion is assessed from a comparison of successive cliff surveys. Flagstones' eros ion proceeds chiefly by the deepening of slots in the weaker interbeds, wit h associated cantilever failures and block removal through wave action. The rate of slot development is estimated from the growth rate of tafoni withi n them. Whether erosion proceeds by superficials' or flagstones' erosion al one or in combination depends on the level of rockhead in the cliffs in rel ation to tide and wave levels. These threshold rockhead levels vary with th e morphology and degree of exposure of the cliffs. They will also rise with time relative to Ordnance Datum in response to the general rise in sea lev el. Estimates of the periods remaining before the shaft is exposed by coast ero sion are made through the above model, both planimetrically and sectionally . They range from about 160 to over 240 years. Flooding by the sea is likel y to commence about 400 years hence. The UKAEA plans to remove the nuclear waste from the shaft within the next two decades, well before the above fac tors would have an impact.