Churchill Barrier No. 4

The Churchill Barriers were constructed between 1940 and 1945 to protect the British Naval Fleet, based in Scapa Flow (Cormack and Cormack, 1992). The barriers comprise 4 solid-fill causeways which link Mainland Orkney to four smaller islands. The barriers have radically altered the sediment movement, affected the current and wave regime of Scapa Flow (Murray et al., 1999) and cut off tidal flow with the North Sea.

Churchill Barrier No 4 links the former islands of Burray and South Ronaldsay. Sand which accumulates on the narrow strip west of the barrier is blown over the causeway under the prevailing winds to add to an expanding dune system. During easterly gales waves will refract around either Burray Ness or Grim Ness and then approach the barrier from the east as a curved series of wave crests.

Detailed stratigraphical, geomorphological and surveying work was undertaken in 2007 to investigate the geomorphological and coastal processes that have led to the development of a large sand dune system at Churchill Barrier 4. Photographs show sand has been accumulating along the eastern side of Barrier 4 since approximately 1961 (Figure 2) and a dune-field now occupying 1km² has developed.

The closures of the Sounds between Scapa Flow and the North Sea have provided a sheltered environment allowing northward longshore drift to transport sediment along the eastern side of Churchill Barrier 4. Water Sound Bay is situated between two rocky headlands and has a shallow gradient beach which is topped by an exposed dry sandy beach approximately 15m wide. Extensive sand bars situated offshore of the barrier are a possible source of sediment for dune formation (Glennie, 1990). In addition, sand accumulates on a narrow strip to the west of the Barrier 4 which is blown over the causeway under the prevailing winds, adding to the expanding dune system.

The main region of accumulation is in the backshore where the sand has reached road level (5.6m OD). This area comprises of a series of vegetated dunes with an average altitude of 7.3m OD. Large parts of the dunefield have been partly or fully stabilised by vegetation, primarily consisting of marram grass (Ammophila arenaria) and lyme grass (Leymus arenarius) with patches of oyster plant (Mertensia maritima) and frosted orache (Atriplex sabulosa). The degree of vegetation cover and thus stabilisation decreases towards the water line. In the south eastern corner of the dunefield, sand has been mined since 1997, with a total of 55,000 tonnes removed (Aurora Environmental Ltd. 2003). Although the mined area is replanted to aid sand stabilisation, mining practices have created a man made blowout in the dunefield, which resembles a trough blowout, with an elongated shape, steep slopes and a deep depression (Gutiérrez-Elorza et al., 2005). The upper slopes of the blowout are less vegetated which has resulted in sediment being removed and the erosional walls becoming over-steepened, leading to some slumping within the deflation basin. The airflow within the basin then re-mobilises the sediment in a downwind direction (Hesp, 2002). This dunefield has now cut off Churchill Barrier 4 from direct marine influence. The northward longshore drift is also clearly evident on the eastern side of Churchill Barrier 3, where a small vegetated sand dune system is developing. During low tide the offshore sand banks at Barrier 3 extend northward to Barrier 2, where a small area of beach has established.