Utilizing Sediment Tracer Studies to Evalaute Transport Pathways at the Mouth of the Columbia River, Usa

During 2006 to 2009, two sediment tracer studies were implemented at the mouth of the Columbia River (MCR) to improve our understanding of littoral sediment transport pathways along the ocean margins of the inlet. Science-based data from these tracer studies is being integrated with other analysis methods to evaluate if dredged material can be preferentially placed within the coastal near shore to augment the sediment budget of the inlet and adjacent coastal margin. This paper summarizes the MCR tracer study methodology and results, with specific emphasis of how the general limitations for tracer studies were addressed and overcome. Results from these studies conclusively show that tracer particles released along the coastal margin of the MCR (at water depth of 13-14 meters) were transported into the active littoral zone of Oregon and Washington, with tracer deposition occurring on beaches. The results indicate that the sand-sized sediment at MCR is dispersed differently along the northern side of the inlet as compared to the southern side. Introduction and Study Motivation The Mouth of the Columbia River (MCR) is located on the Pacific Northwest Coast of the USA, at Long-Lat 124W-45 N, between the states of Oregon and Washington (figure 1). Vigorous wave-current interaction at the MCR inlet is motivated by the confluence of large ocean waves interacting with a prominent & spatially variable ebb-tidal delta, a 2.5 meter estuarine tidal range, and the riverine flow regime of the Columbia River; making the MCR inlet one of the most energetic and dangerous coastal inlets in the world. Littoral sediment transport rates near the MCR are expressed in terms of millions of cubic meters per year and can be highly variable in space and time. The adjacent coastal margins north and south of the MCR have been accreting in a spatially-variable manner for many decades, in response to jetty construction during 1885-1939. However, the post jetty construction accretion rate is slowing, and has reversed in several areas close to the MCR inlet leading to recession of the coastal zone over the foreseeable future. The morphology recession rate near the inlet appears to be increasing and the areal extent of shoreface recession is expanding. In additional to compromising the stability of the jetties and the MCR Report Documentation Page Form Approved