Mouth of the Columbia River North Jetty Lagoon Fill

Jetty1The mouth of the Columbia River (MCR) is known as one of the most treacherous coastal inlets in the world. Consistent navigation through MCR bar was finally secured by construction of three rubble-mound jetties, which included the North Jetty. Since initial construction, approximately 2,100 feet of the North Jetty head has deteriorated and is no longer functional. Structural degradation of the North Jetty has accelerated in recent years because of storm activity, loss of sand shoal material at its foundation, and continued recession of the jetty head. Along with the degradation of the jetty itself, the landward side of the North Jetty between stations is eroding and has created a depression, or tidal lagoon, that fills with water on the incoming tide. This lagoon appears to be formed primarily by the erosion and piping of accreted sand with tidal exchange and wave surge through the deteriorated jetty structure. Surface water flow and groundwater flow from McKenzie Head Lagoon on the north side of Jetty Road also likely contributes to the loss of material through the jetty.

HENDERSON was selected to support Catworks Construction as the environmental habitat and native plantings specialist in completing construction that included filling the lagoon with sand and rock fill up to an elevation of 15 feet NAVD88. The lagoon fill work included construction of a filter zone (consisting of riprap, shot rock, and gravel) adjacent to the jetty stone to prevent sand migration through the jetty, followed by upland bulk filling with imported and on-site borrow sand. An Erosion Protection zone was also part of the lagoon fill at its western-most end. On-site borrow sand was excavated from the eastern part of the site, and the resulting borrow pit was reclaimed as an intertidal wetland. In addition, a 48-inch diameter corrugated culvert that extends under Jetty Road, and associated with the McKenzie Head Lagoon system, was replaced, and the road surface repaired in the culvert area. The construction sequence was designed to accommodate high tides, storms, and groundwater levels.

HENDERSON’s role included redevelopment of the Corps’ planting plan to include appropriate available materials and, ultimately, installation of 500 feet of sand fence, removal of 2.25 acres of invasive Scotchbroom and Himalayan Blackberry, and 153,000 trees, shrubs, and herbaceous plantings.

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