Beach Erosion during Minor Storm

Over 12 m³ of sand per meter of beach was eroded at Debidue Island, S.C. during a minor northeast storm on September 23, 1974. Process measurements and beach surveys made before, during, and after the storm isolate the period of maximum erosion and suggest the threshold process values at the initiation of erosion. Storm processes on the 23rd included winds of 9.1 m/s from the northeast, 6.5 sec breakers 100 cm to 120 cm high, and average wave steepness (H^o/L^o) equal to 0.012. The beach was 50% steeper than normal and contained well-developed cusps before the erosion, facilitating formation of a berm scarp. The bulk of erosion resulted from the undermining and slumping of the scarp at high tide. Threshold process values for Debidue Island at the onset of erosion (for tan β = 0.055) are estimated to include: wind velocity 7 to 9 m/s; wave height > 100 cm; wave period < 7.0 sec. These process values are exceeded up to 14% of the time, but significant erosion will only occur a few days each year due to the influence of beach slope, beach morphology, and water level on the erosional efficiency of waves.