100-YEARS OF SHOAL EVOLUTION AT THE MOUTH OF THE COLUMBIA RIVER:IMPACTS ON CHANNEL, STRUCTURES, AND SHORELINES

摘要

The deep draft navigation project at the Mouth of the Columbia River, USA (MCR)rnconsists of a dredged navigation channel 8 km (5mi) long that extends through a jettied entrancernbetween the Columbia River and the Pacific Ocean. During 1885-1939, three rubble mound jettiesrnand a series of pile dikes were constructed at MCR to stabilize the inlet, confine flow within the 2rnmile-wide entrance of MCR, and prevent encroachment of shoals into the navigation channel. ThernMCR rubblemound jetties (north, south, “A”) were built on flood/ebb tidal shoals. The shoalsrnprotected the jetties from excessive waves and currents. The re-direction of currents through thernjettied entrance of MCR resulted in the discharge of 375-600 million cubic meters (500-800 mcy) ofrnsand from the estuary to the ocean and re-orientation of the tidal inlet. Much of the present-dayrnPeacock Spit and Clatsop Spit were formed by sand discharged from MCR during/after jettyrnconstruction, however, these spits and tidal shoals have been eroding since jetty construction. Thernerosion trend of sand spits/shoals at MCR is problematic for both the longterm stability of the jettiesrnand the cost-effective sustainability of the MCR project. The key to preventing additional jettyrndeterioration will be to prevent scour of the jetty toe; I.e. Shore up the sand shoals upon which thernjetties were built. Using up to 3 million cubic meters (4 mcy) per year of dredged sand to shore-uprnthe tidal shoals and forego jetty “re-construction” appears to offer a superior method (economically)rnfor maintaining MCR navigation.