The sediment response of a dissipative beach to variations in wave climate

Using wave and wind data, topographic survey data, and a robust video record, we have quantified the Large Scale Coastal Behavior (LSCB) of Agate Beach, a dissipative end member beach in the Pacific Northwest USA. Nine years of study (1992–2001) contain important observations of the intra-and interannual sediment response under variations in wave climate. er to describe the variability of the nearshore system, wave forcing and beach response were modeled as annually periodic functions superimposed upon long-term trends. The amplitudes of the seasonal periodicity in significant wave heights (AHs=0.94 m±0.06), dominant wave period (ATp=2.1 s±0.1), and mean wave direction (Aθ=12.3°±2.0) exhibit larger variability than the long-term trends observed within a year (βHs=6.7 cm/year±2.6, βTp=0.15 s/year±0.04, βθ=3°S/year±1.0). Assuming alongshore transport of sediment at Agate Beach is mostly wave-driven, and that the northward flux of sediment goes to zero at the northern end of the beach, the observed long-term increase in significant wave heights (βHs) and change to a more southerly approach in wave direction (βθ) (coincident with the 1997–1998 El Niño/1998–1999 La Niña sequence) result in net accretion along Agate Beach. Beach survey data collected by RTK-GPS and lidar quantify the increase in subaerial sediment volume, ΔVb=1.84×105 m3. This change in volume corresponds to a mean bed elevation increase of 0.21 m over the 8.7×105 m2 area of study. The amplitude of annual variation in sediment volume was AVb=7.85×104 m3±2.13×104 m3. image analysis (1992–2001) also quantifies a long-term offshore migration of the outer sand bar (βOBx=11.0 m/year±0.8) coincident with the increase in subaerial beach sediment volume at the northern end of the Newport littoral cell. This result suggests accretion of sediments in a much wider cross-shore region than observed within the beach survey record. Similar to the signature of beach volume variations, the cross-shore position of the outer sand bar also varies with season (AOβx=114.9 m±4.2). The agreement between long-term trends in wave statistics and morphology suggests a directly forced beach response. irical orthogonal function (EOF) analysis of the topographic survey record at Agate Beach resolves two distinct eigen-modes of variance associated with seasonal patterns of sediment flux. The first mode (34%) is related to the summer growth of a dune field that is limited to elevations above MHW, z=1.076 m. Analysis of concurrent wind field measurements shows this mode of variance is well correlated with aeolian processes. The second mode (21%) is wave-driven, and corresponds to the seasonal behavior of the beach surface below MHW. Observations show the MHW elevation serves as a transitional zone between dune related and wave-driven processes that affect the seasonal evolution of Agate Beach.