Spatial and Temporal Variability of Dissipative Dry Beach Profiles in the Pacific Northwest, USA

Abstract

Dissipative beaches in the U.S. Pacific Northwest are subject to a marked seasonality in wave climate and water levels, which leads to periodic oscillations in the morphology of the typically dry part of the beach profile. The back-and-forth, seasonal sediment exchange between the emerged and submerged parts of the beach system induces two main dry-beach profile-equilibrium configurations. During approximately 70\% of the year, the dry beach adapts its configuration to a uniform positive slope from the mean high-water level to the dune toe. The remaining 30\% of the time, typically corresponding to summer, the profile adopts a berm-like profile. These changes are quantified by studying intra-annual and interannual variations of the dry-beach profile shape. For intra-annual variations, a monthly profiling campaign between July 2014 and October 2015 from South Beach State Park (Oregon) was used. For interannual variations, 17 years (1997-2015) of quarterly beach profiles at 31 transects spread along the four subcells that constitute the Columbia River littoral cell were used. Several morphological phenomena have been identified via the application of two data-mining routines: the K-means clustering technique (KMA) and empirical orthogonal functions (EOFs). KMA clustering illustrates the main equilibrium configurations that the dry-beach profile experiences over time, whereas the EOF analysis explains the variability of the data in space and time. These analyses allow for examinations of berm formation and destruction as well as the shifting of the profile between summer/winter configurations-among other changes induced by the cross-shore sediment exchange, such as beachface and dune toe erosion and recovery.