CHARACTERIZATION OF COARSE SEDIMENT TRANSPORT ON A MIXED SAND AND GRAVEL BEACH: CHERRY POINT AQUATIC RESERVE, BLAINE, WASHINGTON

Coarse-sediment processes are still not fully understood, especially when it comes to coarse-sediment transport in mixed sand and gravel (MSG) beach systems [Curtiss et al. 2009]. This study examines coarse-sediment (large pebbles to large cobbles) transport behavior on an MSG beach at Cherry Point Aquatic Reserve in the Georgia Strait of Northwest Washington. Radio Frequency Identifier (RFId) Passive Integrated Transponder (PIT) tags were used to trace movement of large pebbles and cobbles (51 - 129 mm in diameter) at a range of different beach elevations during several winter and spring storms of 2012. Wave heights and period were measured with a pressure sensor offshore of the study site and combined with wind, current and water level data recorded by nearby weather stations into a comprehensive data set to assess forcing of cobble transport. Tide, wind and wave parameters were then input into the nearshore numerical model XBeach [Roelvink et al. 2009] to investigate the critical relationships between oceanic and atmospheric factors and the behavior of coarse sediment at the Reserve. Measured displacement of pebbles and cobbles recorded long-shore transport in both directions, with little cross-shore movement. Transport to the southeast under infrequent long-fetch storm waves occured at velocities up to 8 times greater than transport to the northwest associated with prevailing winter SSE storms. This bi-directional transport supports observations made by Bauer [1976] and adds a new facet of beach behavior to the uni-directional net-drift cell model [Schwartz et al. 1991, Johannessen & Chase 2006]. During moderate energy wave activity, the duration of elevated wind speeds above 4 m/s controls the extent of tracer displacement. During high energy winter storms, fetch distance and significant wave height control displacement. Another important discovery is the immobility of lower beach sediments compared to those of the middle and upper beach. Computer modeling of wave forcing, roller energy and bed shear stress identified a critical water level needed to bring roller waves close enough to break over the beach profile and affect coarse-sediment transport. These findings help inform how MSG beach systems operate today and how the systems are likely to respond to sea level rise in the near future.