A LABORATORY INVESTIGATION OF WAVE-DRIVEN DUNE AND BEACH MORPHOLOGY CHANGE UNDER HIGH STORM SURGES

Coastal morphological changes occur at different scales and at different rates that depend on the surface wave forcing conditions and mean water level elevations. Storms deliver both energetic waves and elevated mean water levels, and can cause large volumes of sediment to erode and induce major changes over a short time. The present study uses a sand beach-dune system in the laboratory to understand wave-driven morphological evolution at high water levels. In a 35.5 m long, 0.9 m wide and 1.2 m deep wave flume with regular (monochromatic) waves that have a significant wave height of 0.15 m and propagate toward a 0.65 m high sand dune, erosion under two different water level regimes that represent high storm surges is investigated. The water levels correspond to the overwash and inundation regimes on the Sallenger (2000) storm impact scale for barrier islands. The work is intended as an extension of previous work focusing primarily on the swash, collision and overwash regimes. Waves are measured using six capacitance wave probes located along the flume, and morphological evolution of the beach-dune system is monitored throughout the experiments using a laser scanner. The final beach profiles are very different for the overwash and inundation experiments, with morphological equilibrium occuring after 1110 and 362 minutes of testing respectively. Landward recession of the dune crest is balanced in both cases by seaward advance of the beach toe and is variable in time due to the development and migration of sand bars. The observations indicate that severe erosion occurs in the inundation regime over a time scale that is much shorter than in the overwash regime, suggesting catastrophic destruction of barrier islands under extreme storm conditions.