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


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  • Using wave and wind data from nearby buoys and gauges, real time kinematic global positioning system (RTK-GPS) and light detection and ranging (lidar) topographic survey data, and a robust video record, we have quantified the Large Scale Coastal Behavior (LSCB) of a dissipative end member beach in the Pacific Northwest. This study of Agate Beach from 1992 - 2001 reveals important observations of beach behavior on temporal and spatial scales that have received little attention in recent nearshore research. Similarly, the high-energy conditions characteristic of the Agate Beach study site define it as an dissipative end member that is not well understood. In order to describe the variability of the system at spatial scales of hundreds of meters to kilometers and time scales of months to years, regression models for wave parameters and the beach sediment response were developed consisting of 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 sec ± 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/yr ± 2.6, βTp = 0.15 sec/yr ± 0.04, βθ = 3° S/yr ± 1.0). Agreement between the long-term trends in wave statistics and morphology suggest a directly forced beach response. Assuming alongshore transport of sediment at Agate Beach is wave-driven, the long-term increase in significant wave heights (βHs) and change to a more southerly approach in wave direction (βθ), coincident with the 1997-98 El Niflo/ 1998-99 La Nina sequence, correlate with the increase in sediments along the beach (ΔVb = l.84x10⁵ m³). Predictions of wavedriven alongshore transport estimate a net accretion at Agate Beach (ψ net = 2.73x10⁸ m³) over the 9 year record length. In addition to the long-term increasing trend in sediment volume, a seasonally based fluctuation in sediments is observed (Avb = 7.85x10⁴ m³ ± 2.13x10⁴). Video image analysis shows this increase in subaerial beach sediment volume at the northern end of the Newport littoral cell also coincides with the long-term offshore migration of the outer sand bar (βoBx = 11.0 m/yr ± 0.8). This result also suggests accretion of sediments in a wider crossshore region than observed in the survey record. Similar to the signature of beach volume variations, the cross-shore position of the outer sand bar also varies with season (AQBX = 114.9 m ± 4.2). The seasonal migrations in the outer sand bar position displays much larger variations than the long-term behavior described by βOBx. Analysis of 27 topographic surveys resolves the cross-shore structure of the time varying beach surface. Using empirical orthogonal functions (EOF), 2 distinct eigen-modes of variance describe the seasonal patterns of sediment behavior at Agate Beach. The first mode describes 34% of the variance and 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% of the variance) 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.
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