Measurements and Modeling of Gravel Transport under Wind Waves, Vessel-Generated Waves, and Tidal Currents

Direct measurements of coarse sediment (gravel) transport were obtained over an interval of 14 months from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. The measurements were applied to validate a system of integrated numerical models that includes: a tidal circulation model, a wind-wave growth and transformation model, a vessel wake model and a one-dimensional, profile-based model. The latter model, which provides a long-term integrated assessment of the beach response to major forcing mechanisms, was the primary tool for investigating the impacts of tides, waves and wakes on the mixed sand and gravel shores of the study area. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology was implemented in tracking studies of gravel-sized sediment particles, and complemented the beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the seasonal transport patterns, which include a range of wave and vessel wake climates. Simulations of cumulative transport rate predicted with the integrated modeling system compare well with the alongshore tracer movements and capture the dominant trends and variations during the time period of the measurements. The measurements and modeling reveal that the transport is dominated by wind waves in an alongshore uni-directional process that occurs mainly in winter. However, beach response is also controlled by site-specific exposure to prevailing winds and car ferry wakes. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents; the sub-critical car ferry wakes provide a mechanism for post-storm recovery, in this low energy restricted fetch environment.