STORM-GENERATED CURRENTS AND OFFSHORE SEDIMENT TRANSPORT ON A SANDY SHOREFACE, TIBJAK BEACH, CANADIAN BEAUFORT SEA

Waves and currents were measured for three weeks in 3.4 and 5.0 m water depths on the sandy shoreface of Tibjak Beach, located on the eastern side of Kugmallit Bay in the Canadian Beaufort Sea. During the field study, five significant wind events resulted in strong shore-normal components of surface wind stress which induced sea level set-up at the coast. Because of the coastline configuration, storm surge conditions and horizontal pressure gradients varied along the eastern side of the bay as a function of small changes in wind direction. One of the westerly storms was responsible for raising the water level by 0.96 m above mean sea level in the nearshore zone. Northwesterly winds resulted in less significant storm surges at Tibjak Beach, but induced higher water levels to the south, near the head of the bay. During the northwesterly storms, strong seaward-directed near-bottom currents associated with downwelling conditions were observed. These currents, with mean velocity up to 0.49 m s-1, are believed to be primarily driven by wind-induced hydrostatic pressure gradients caused by the set-up of coastal waters. Once they began to develop, the seaward currents were directed offshore for periods of at least 6 to 9 consecutive hours; during one storm, near-bottom currents were flowing offshore for more than 18 hours. High wave orbital velocities were measured during these events which induced significant sediment remobilization as the threshold of sediment motion under oscillatory flow was largely exceeded. Because medium- to fine-grained sands prevail on the shoreface, a significant sediment transport load is probably moved down the shoreface by such currents during major events, thus contributing to offshore sediment dispersal.