Application of a Bottom Boundary Layer Model in Contrasting Wave and Current Environments: Grays Harbor, Washington

In the fall of 1999, stations for measuring waves, currents, and sea level were established near the entrance of and within Grays Harbor, Washington. The purpose of this study was to assist the U.S. Army Corps of Engineers (USACE), Seattle District (NWS) in the design and maintenance of the Grays Harbor federal navigation project. Benthic tripods were deployed inside, within, and outside the harbor entrance for a period of $2\text{months}$ beginning in September. The contrasting wave and current environment between the open coast and the protected bay provide an opportunity to apply a bottom boundary layer model designed for arbitrary wave and current conditions including the limiting case of pure currents. Results show that, on the inner shelf, the total bottom shear stress is dominated by the wave-induced orbital motions. In the bay, waves are weak and the total bottom stress is dominated by the tidal current. The highest sediment transport is seen on the shelf and inlet throat, and it is correlated with periods of elevated wave activity. Within the sheltered regions of the harbor, sediment resuspension is reduced several orders of magnitude, but in general it is not correlated with the timing of the high waves. Rather, stronger currents and associated stresses during the spring phase of the tidal cycle tend to produce the greatest sediment transport. Closer to the inlet throat, waves and spring tidal currents together play a role in sediment mobilization. Although wave heights exceed $0.5\mathrm{m}$ in the bay only 12% of the time, failure to include combined wave and current algorithms leads to a 20–70% underestimation of net sediment transport. Analysis of the modeled sediment transport magnitude at sites with weak waves and strong tidal currents indicate that wave-current interaction can be neglected if the ratio of the current magnitude to the wave orbital velocity is greater than about 40. For smaller values of this ratio, wave-current interaction should be included.