Half Moon Bay, Grays Harbor, Washington: Movable-Bed Physical Model Study

Abstract

Abstract : A 1-to-50 scale physical model of Half Moon Bay, Grays Harbor, WA, was constructed at the U.S. Army Engineer Research and Development Center's Coastal and Hydraulics Laboratory in Vicksburg, MS. The purpose of the physical model was to support studies being conducted by the U.S. Army Engineer District, Seattle. Specifically, the model results will be used to assess the potential long-term response of the Half Moon Bay shoreline to expected storm waves and surge levels, provided the breach fill between South Beach and the bay remains intact. The physical model eroded the June 2003 shoreline until a near equilibrium was achieved in the model with the dune recession line closely matching the existing vegetation line. This result indicated Half Moon Bay is approaching an equilibrium shoreline planform shape as it adjusts from an influx of sediment resulting from the 1993 breach. However, scale effects in the physical model related to sand transport mean that the dune recession reached in the model for a constant water level and wave energy is less than what would occur in Half Moon Bay under the same constant conditions. Therefore, some additional erosion of the dune should be expected in the coming years. The physical model did not include the benefits of placing dredged material in Half Moon Bay. The model demonstrated the gravel/cobble transition material is mobile with substantial erosion and transport of the gravel from the area of original placement toward the eastern portion of the bay.