Interannual Sandbar Variability within the Columbia River Littoral Cell

Abstract:

Examining nearshore sandbar behavior is highly relevant to understanding past and predicting future coastal change because of the role that these morphologic features play in regional sand budgets. However, because of the difficulty and expense of collecting nearshore bathymetric data, relatively few long-term data sets of sandbar morphology exist. At sites where sufficient data has been collected, a common phenomenon observed is a cyclic, multi-year offshore migration of sandbars. This cycle typically follows a general three-stage conceptual model of net offshore migration (NOM) originally proposed by Ruessink and Kroon (1994) whereby each individual bar is: (1) generated close to shore, (2) migrates seaward, and (3) decays at the outer margin of the nearshore zone. One such location where a clear NOM cycle has been observed is in the Columbia River Littoral Cell (CRLC). The CRLC is a high energy, dissipative coast located in northwest Oregon and southwest Washington which is characterized by fine sand and an intense wave climate. A beach morphology monitoring program that includes annual nearshore bathymetry surveys and quarterly subaerial beach topography measurements was initiated in 1997 (Ruggiero et al., 2005). Data from this long-term field effort indicates that there are large spatial and temporal differences in bar behavior throughout the littoral cell, but that in general there is a net offshore movement of the bars on interannual time scales. In this study, the high-resolution coastal profile data from the beach monitoring program are used to specifically evaluate bar dynamics along the Long Beach subcell of the CRLC. Parameters such as bar crest position from the shoreline, bar crest depth, and bar height have been extracted from the dataset and are being used to evaluate both spatial and temporal trends in bar morphology. Initial results indicate that NOM cycles have approximately three-year return periods in Long Beach. Further analysis will focus on the alongshore variability of these cycles within the subcell. The results from this project will improve our understanding of bar behavior on interannual to decadal timescales which, as previously described, is integral for improving our predictive capabilities of shoreline change.