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Regional Analysis of Sediment Transport and Dredged Material Dispersal Patterns, Columbia River Mouth

The Columbia River entrance and adjacent nearshore shelf environments have been studied extensively over the past 50 years in response to engineering activities associated with navigation at the entrance, the interest of the U.S. Atomic Energy Commission, and the development of black-sand placers as economically-viable deposits. Although significant knowledge has been gained on the dispersal and deposition of river-derived sediment along the shoreline and onto the continental shelf, little effort has been made to quantify historical sediment dynamics near the entrance using existing, long-term, regional-scale data sets. Material placement in authorized disposal sites seaward of the Columbia River entrance has led to unacceptable mounding. Two problems have been created by the mounds: 1) dredged material extends beyond the designated disposal site limits, and 2) ships are reporting adverse sea conditions believed to be created by shoaling over the mounds. As such, the focus of this study was on quantifying regional sediment transport dynamics at and adjacent to the mouth of the Columbia River using historical shoreline and bathymetry change data sets as a tool for siting and managing nearshore dredged material disposal sites.

Three bathymetry surfaces were compiled for quantifying nearshore geomorphic change. Volume change estimates were established for specific polygons to relate grouped cut and fill relationships with natural and human processes. For the overall area, four distinct depositional trends were identified. One, the modern ebb-tidal delta developed as a result of jetty construction. Currently, it resides about 3-km seaward of the original feature in about 30- to 40-m water depth. The deposit contains about 276 Mcm of sediment, approximately half of which comes from the old ebb shoal. Two, the depocenter for sedimentation on the ebb shoal is to the north of center, and it migrates to the north with time. Three, northward-directed sediment transport from the entrance has resulted in net accretion along the shoreline and on the continental shelf seaward of Long Beach Peninsula. Four, erosion south of the south jetty is the result of sediment blocking by the jetty and subsequent transport towards the ebb shoal and onto the continental shelf.

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