SEATTLE DISTRICT AND PORT RELEASE DRAFT FEASIBILITY REPORT ON SEATTLE HARBOR DEEPENING PROJECT
The U.S. Army Corps of Engineers Seattle District released the Seattle Harbor Navigation Improvement Project Draft Feasibility Report and Environmental Assessment for a 30-day comment period from August 2 to 31. The Port of Seattle is also the non-federal sponsor working with the Corps to complete the study. The two held a public meeting for the proposal on August 18.
The tentatively selected plan for the Seattle Harbor Navigation Improvement Project includes deepening the East and West Waterways to 57 feet below mean lower low water (MLLW) increasing the channel width by 550 feet plus widening the approach reaches to 700 feet. Ac-cording to the study approximately 777000 cubic yards will be dredged from the West Waterway and approximately 341000 cubic yards from the East Waterway. The estimated cost (at October 2015 price levels) was $64086000.
The existing 6109-foot long West Waterway will be deepened from -34 feet MLLW to -57 feet MLLW. The 6000-foot long East Waterway will be deepened from -34 feet MLLW and -51 feet MLLW to -57 feet MLLW. The 1232 feet at the southern end of the East Waterway would maintain its authorized width of 500 feet and depth of -34 feet MLLW.
Both channels which are 51 feet deep present challenges as some shallower spots add navigational and safety challenges. The study evaluated several deepening alternatives.
The report estimates the dredging will take two years while adhering to the work windows for the protection of sensitive fish species. The work will use a clamshell dredge and a 1200-cubic-yard to 3000-cubic-yard bot-tom dump scow barge will transport and place dredged material to an open-water disposal site. Contaminated material will go to a contained flat deck material barge for transportation to a transloading facility where it would be mechanically treated for placement at an upland landfill. Dredged material in the West Waterway is classified as a sandy silt while material in the East Waterway is a slightly finer silty sand. Production rates are estimated to range between 2000 and 3000 cubic yards per day but will depend on the actual dredging equipment used the nature of the dredged material and environmental conditions. Most of the material unsuitable for open-water placement is located in the top four feet below the mudline. Thus production will likely increase as the project progresses.
The interagency Dredged Material Management Program (DMMP) works to collaboratively manage and regulate the placement of dredged material from projects in Washington between the Corps Seattle District; EPA Region 10; Washinton State Department of Ecology; Washington State Department of Natural Resources. DMMP agencies determine what dredged mate-rial can be disposed of at open-water disposal sites. A full DMMP suitability determination could not be completed in the three-year study time but a full determination will be completed during the pre-construction engineering and design (PED) phase. Initial estimates have 24 percent of the dredged material (about 183000 cubic yards) requiring upland disposal. Approximately 594000 cubic yards was estimated to meet the open-water placement requirements at the Elliott Bay DMMO open-water site.
