Seasonal patterns of coarse sediment transport on a mixed sand and gravel beach due to vessel wakes, wind waves, and tidal currents

Direct measurements of coarse sediment (gravel) transport are presented and analyzed from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA that is exposed to wind waves, vessel wakes, and tidal currents in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology is implemented in two coincident yearlong tracking studies of sediment particles, and complemented with beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the dominant seasonal transport patterns, which include a range of wave climates. During winter storms, the predominant transport is due to wind waves in an alongshore uni-directional process, whereas tides and wakes play a relatively minor role. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents. Although tidal currents are not sufficient to initiate sediment movement alone, the combination of tidal currents and vessel wakes generate significant transport and provide a mechanism for post-storm recovery, re-distributing sediment onshore. Morphologic response occurs as a seasonal fluctuation of the upper beach profile from steep to flat and in sediment composition from gravel to coarse sand between non-storm and storm conditions respectively. These results, which are unique in their duration, suggest that mixed sand and gravel beaches experience different modes of behavior over the range of forcing conditions observed during a typical year. They point to the need for including grain composition in modeling mixed sand and gravel beach response and the need for long term observations of both forcing and response.