Morphodynamics of prograding beaches: A synthesis of seasonal- to century-scale observations of the Columbia River littoral cell

Findings from nearly two decades of research focused on the Columbia River littoral cell (CRLC), a set of rapidly prograding coastal barriers and strand-plains in the U.S. Pacific Northwest, are synthesized to investigate the morphodynamics associated with prograding beaches. Due to a large sediment supply from the Columbia River, the CRLC is the only extensive stretch of shoreline on the U.S. west coast to have advanced significantly seaward during the late Holocene. Since the last Cascadia Subduction Zone (CSZ) earthquake in 1700, with associated co-seismic subsidence and tsunami, much of the CRLC has prograded hundreds of meters. However, the rates of progradation, and the processes most responsible for sediment accumulation, vary depending on time scale and the morphological unit in question. Remarkably, the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major CSZ event, most likely due to an increase in sediment supply resulting from inlet jetty construction. In some locations detailed beach morphology monitoring reveals that at interannual- to decadal-scale the upper shoreface aggraded about 2 cm/yr, subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline, the shoreline prograded about 4 m/yr, and 1 to 2 new foredune ridges were generated. A detailed meso-scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m(3)/m/yr accumulated between 12 m (seaward limit of data) and +9 m (crest of landward-most foredune). Gradients in alongshore sediment transport, net onshore-directed cross-shore sediment transport within the surf zone, and cross-shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the CRLC during the observational period. Direct observations of beach progradation at seasonal- to decadal-scale are put in context of measured or inferred changes over time scales of decades to centuries. (C) 2016 Elsevier B.V. All rights reserved.