BEACH-DUNE DECOUPLING DURING AN OFFSHORE WIND EVENT: PRINCE EDWARD ISLAND, CANADA

The role of offshore wind events in the maintenance and evolution of coastal foredunes is gradually being recognized and understood. It was commonly assumed that sediment was unlikely to be mobilized across the beach during offshore flow conditions because of fetch restrictions. If sediment transport was initiated under powerful winds, the flux rates would be small and directed toward the sea, yielding a net loss to the beach-dune system. A small number of field studies have successfully documented flow and sediment transport pathways during offshore wind events, and the data indicate that large recirculating eddies can be established in the lee of large foredunes (above the beach), yielding obliquely onshore transport pathways across the beach that may be critical to initiating embryo foredunes and repairing wave-cut scarps.

An experiment on the north-east shore of PEI, using sonic anemometers and laser particle sensors during offshore winds showed that there can be active transport on the crest and beach with essentially no (or minimal) transport across the seaward facing slope of this 8-m high foredune. The regional wind at a height of approximately 4 m above the dune crest was essentially offshore (20^o-25^o) with speeds from 7.5 to 8.5 ms^-1. At 0.2 m above the dune crest, the direction was within 5^o of offshore and the 15-minute mean speed was about 6-8 ms^-1, whereas sand transport was intermittent varying between 4-15 counts s^-1. At a distance of only 2-4 m downwind from the crest (and extending to the base of the dune), wind speed decreased to less than 2 ms^-1 and transport rate was negligible. Topographic steering of the flow vectors down the face of the dune was evident, to the extent that there was alongshore and slightly onshore mean wind directions (100^o-130^o) across the beach. Despite relatively small mean wind speeds of about 4 ms^-1, there was significant sand transport (up to 12-13 counts s^-1) in the alongshore direction because of unrestricted fetch. Given that the seaward-facing, vegetated slope of this large foredune experienced virtually no transport during this event, the active transport zones on the dune crest and on the beach can be considered to be effectively 'decoupled' in regard to continuity of sediment pathways. Offshore winds are therefore believed to contribute to dune building and stabilization.