Holistic Framework for Assessing the Functional Integrity of Navigation Structures at the Mouth of the Columbia River

This paper presents a framework for reducing lifecycle costs of maintaining the engineered navigation entrance at the mouth of the Columbia River (MCR), an ocean entrance located on the Pacific Northwest coast of the United States. At MCR, three jetties with a total length of 15.7 km were constructed on massive tidal shoals during 1885–1939, and the morphology of the inlet has been changing ever since. The jetties have experienced significant deterioration since construction, mainly due foundation instability associated with the erosion of tidal shoals on which the jetties were built. A jetty breach at MCR would result in rapid in-filling of the 17 meter deep navigation channel. Considerable effort has been expended to rehabilitate the MCR jetties and more is needed to prevent a catastrophic breach. Is placing more stone on the jetties the best way to sustain navigation through the MCR? A holistic approach is used to rationally examine the functional performance of the jetties, optimal repair strategy (which structures to repair first and where); and maximize the utilization of dredged sand as a resource. Elements of this paper also include: A) Development of a digital elevation model (DEM) to analyze the MCR jetties and adjacent morphology. B) Application of a 2-D hydrodynamic model to investigate the impact of present/future jetty deterioration (and reconstruction) upon the morphology and sediment budget at MCR. This paper concludes that a targeted placement of dredged sand can reduce erosion of the sand shoals, mitigate deterioration of jetty foundation, and prevent the need for future jetty repair. The framework discussed in this paper, which deals in part with employing regional sediment management, can be readily applied to similar ocean inlets where jetties are used to control sand shoal migration and retain a viable navigation channel.