HIGH VOLUME SEDIMENT TRANSPORT AND ITS IMPLICATIONS FOR RECREATIONAL BEACH RISK

摘要

In a coastal region where there are large pressures on the beach resources through recreational usage, understanding the levels and characteristics of risk to the beach user is paramount. A morphodynamic evaluation of beaches in the high-energy, macro-tidal southwest of England was made between July 2006 and February 2008. Levels of physical beach hazards presented to the beach user, both spatially and temporally, by waves, tides and surf-zone currents were assessed and calibrated against lifeguard rescue and usage data. Large seasonal variations in wave energy lead to significant annual morphodynamic transition of the popular west coast beaches from a erosive planar beach face with linear shore parallel bars in winter to a highly three dimensional accretionary system in spring/summer with pronounced low-tide bar/rip systems and enhanced mid-tide bar morphologies. In many locations this general transition is modified through sediment supply, geological constriction and freshwater drainage. This annual transition drives temporal variations in beach hazard through (1) the temporal variability morphology (especially rip currents, the cause of 68% of all incidents during 2005-200'7); and (2) large tidal excursion during spring tide periods exposing low tide rip systems increasing the rate of change of the temporal hazard signature. Periods of high morphologically driven beach hazard coincide with seasonal peaks in beach user numbers, increasing recreational beach risk.