Regional scale sandbar variability: Observations from the U.S. Pacific Northwest

Abstract Understanding sandbar dynamics and variability is integral to developing a predictive capacity for nearshore flows, sediment transport, morphological change, and ultimately for determining coastline exposure to damaging storm waves. Here we report on a nearshore bathymetric data set from the U.S. Pacific Northwest (PNW) that stretches from Point Grenville, Washington to Cascade Head, Oregon, over approximately 260 km in the alongshore and includes 8 distinct littoral cells. We describe and quantify the morphological variability of sandbars on a regional scale, using 560 individual cross-shore transects, as well as attempt to explain the inter-littoral cell variability via relationships to various environmental parameters. The cross-shore extent of the bar zone extends over 1 km from the shoreline in the northern part of the study area, but only to about 600 m from the shoreline in the southern part. Maximum bar crest depths are typically 7 m below MLLW. Bar heights range from a step in the cross-shore profile to over 3 m from crest to trough. The northernmost littoral cells typically have two or more subtidal sandbars per cross-shore profile whereas the littoral cells in the southern part of our study area have only one bar. The mean depths of the bars, however, are much more consistent across littoral cells even while the upper shoreface slope significantly increases from north to south, requiring that the maximum bar distance from the shoreline decreases from north to south. Results from a limited study of the temporal variability suggest that while data collected over large spatial scales captures significant amounts of overall sandbar variability, it does not completely characterize the variability over the entirety of the net offshore migration cycle.

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