Collaborative Research: CROSSTEX - An Experimental Study of Onshore Bar Movement

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ABSTRACTOCE-0351297Cross-shore sediment transport processes play a crucial role in the annual episodic cycles of erosion and accretion on most beaches. Great strides have been made in our understanding of erosional processes during intense storm events, and models have shown reasonable skill in predicting storm bar formation and offshore migration. A parallel understanding of accretional processes, particularly the landward migration of bars, has not yet been obtained. Consequently, predictive models of cross-shore transport processes exhibit little skill in predicting morphology change for any extended sequence of erosional and accretional events. Part of the lack of present understanding stems from a paucity of available data sets. In the field, measurement of profile evolution during an accretional event requires having both a detailed surveying program and sufficient hydrodynamic instrumentation in place at the right time in order to allow testing of process based models. The Duck '94 experiment has provided one such data set. However, cross-shore processes in the field can be masked or altered by the lateral diffusion of the large longshore transport volume, and can be further complicated by the evolution of two dimensional bathymetric variations, leading to longshore variability of the cross-shore transport pattern. Large scale laboratory experiments have been conducted at near-to-field scale, eliminating most of the concerns over scaling issues and providing an isolated look at purely cross-shore processes. However, the existing data sets do not provide a long enough look at the shoreward movement of bars during accretional events, and do not provide measurements of the multiple events that are needed to provide both calibration and a blind test of predictive models. The PI's will conduct an extended cross-shore morphology experiment to examine the shoreward migration of shore-parallel sand bars under accretional wave conditions. The experiments will be conducted in the large wave flume facility at Oregon State University. The experimental program will primarily examine the relative roles of suspended load versus bedload modes of transport. Also included will be whether "plug-flow" (mass failure of the upper layer of the sand bed) plays a significant role during shoreward bar movement events. Extensive use will be made of hydrodynamic models (based on Boussinesq or nonlinear mild-slope formulations) to design experimental conditions and assist in choice of instrument placement. The acquired data set will be used to test the predictions of both the hydrodynamic models and the sediment transport models, which are driven for extended periods by the hydrodynamic models. This data set which will adequately describe the hydrodynamics and sediment transportmechanisms associated with a significant period of shoreward bar movement, will provide a test base for evaluating modern sediment transport models which is presently lacking. The improvement of process-based predictive models which could result from access to such a data base could markedly increase the coastal engineering and nearshore oceanography communities' ability to predict the seasonal variability of shoreline erosion and accretion in the U.S.'s heavily populated coastal zones. The results of this project will be disseminated broadly through publications in peer-reviewed journals, presentations at international and national conferences, workshops for scientists interested in the data set and for industry members and coastal managers, inclusion of the results in classroom material, and participation by undergraduate students.