Phase 2 – ConstructionTemporary Construction, Site Preparation: Existing levees and roads that will serve as haul roads will be upgraded as needed to withstand construction traffic. All existing tidegates will have fish mitigation devices removed to maximize interior drainage and lower water levels for construction. Fish exclusion nets will be installed on existing tidegates to prevent ingress during construction. Designated refueling and stockpile areas will be constructed.
Clearing and Grubbing: The new levee alignment, landward sides of perimeter levees, and dredge spoil piles will be cleared of vegetation and topsoils. Native shrubs and trees will be stockpiled for placement in the restoration area. Non-native species will be removed. Topsoils will be used to fill existing interior ditches in order to ensure natural tidal channels can develop without being short-circuited by the linear ditches. Topsoils may also be stockpiled for use on the levee face.
First Phase of Levee Removal and Levee Construction: Levee removal will provide the conveyance capacity increase that results in reduction of flood levels over a wide area of the lower Wilson River floodplain. In general, material will be removed to slightly below natural floodplain/marsh level. This elevation is around 9 feet at the mouth of the Wilson River, increasing to 10+ feet farther upstream. Lowering areas further than this could provide some additional flood level reduction, but the cost increase would be large and the benefits temporary. The fill to be removed will be used for the new dikes and ditch filling, with any remaining soils spread on site to speed rebuilding to natural salt marsh elevations.
Because of land subsidence from diking and draining, the existing land elevations in the southeastern project area are too low to support continued use as pasture without the dikes. Given the lack of landowner interest in selling their property and the importance of preservation of agricultural lands for Tillamook County, these dikes will be lowered to 12 feet to convey floodwaters.
New and upgraded existing tidal dikes will be constructed in three segments (north, middle, and south) in order to protect adjacent agricultural lands from tidal influence in the project area. Most of the dikes will be built to the design elevation of 12 feet, with some adjustments where they tie into existing dikes or high ground. This elevation will pass river flood flows out while preventing high tides and coastal storm surges from getting in. The downstream side of each dike will have a 5:1 slope in order to pass overtopping floodwaters with minimal damage.
Levee construction will begin with excavation of interior levees and dredge spoils. The exterior will be excavated to design grade, which is just above summer high tides. If necessary, a small 1-2 foot berm will be left on the riverward side of the exterior levees to prevent tidal overtopping. Material will be trucked to the new levee alignments, laid down in lifts and compacted. There is not enough material from the existing levees to be removed on the south side of Hoquarten Slough to construct the new South Dike and therefore, material from the northern area will need to be transported over. The material will be hauled via truck or a temporary bridge may be used to move material across Hoquarten Slough. Organic soils will be used to cap the levee faces to promote vegetation. A crushed rock driving surface will top the new levee.
Drainage Structures: A new high capacity floodgate structure will be incorporated in the middle dike to replace the existing gates, provide additional conveyance capacity, and allow rapid post flood drainage. The four 5×12 foot side hinge gates on the existing flood gate at the western end of the project area will be reused on the new floodgate, and an additional four gates added. The structure is anticipated to be a cast in place concrete structure with a sheet pile seepage cut off wall. The gates are designed to function only during floods and so will be set around floodplain elevation rather than in a channel. The upper end of the relict Nolan Slough channel will be excavated to the outlet of the new floodgates to serve as the exit channel from the gates. Flood flows will pass through the gates every second or third year, a sufficient frequency which will keep the channel open and able to convey flood flows out to the main river channels and bay. Seven tidegates will be installed in the new dikes to provide equal or better drainage from adjacent pasture lands. Existing 5 and 6 foot diameter round tidegates currently installed on the western end of the site will be reused on these replacement pipes if their condition allows.
Road Decommissioning and Channel Excavation: The few roads on site, including one accessing a residence to be demolished, will have gravel surfaces removed and the roadbed de-compacted. Existing relict tidal channels will have plugs and culverts removed to allow full tidal access. In the north dike, the outlet channels will use existing or constructed sinuous tidal channels to provide connections to the main river. Excavation of a Hall Slough-Blind Slough channel will further increase connectivity. Improvements to the existing drainage ditches inside the new dike will be made as necessary to connect them to the new tidegates and ensure that equal or better drainage is maintained once the project is implemented. Note that while 1 mile of tidal channels will be excavated during construction to provide drainage from diked lands and/or improve habitat connectivity, 14 miles of tidal channels will ultimately be restored throughout the site.
Large Wood Placement: Large spruce and other trees exist along the levees and in other construction areas. Trees removed in order to implement project elements will be placed opportunistically in the wetland and channel habitats to offer predation cover for fish. Wood placement will complete the interior area work. At this point the new flood and drainage gates must be functional and the new levees built up with a minimum crest elevation of 10 feet.
Levee Removal: Once all interior work is completed the site will be ready to receive tidal waters. The levees will be breached and access to the interior floodplain will no longer be feasible. Exterior tidegates will be removed and relict channels connected to the river. Final excavation will require working within tide cycles, working back out of the project site without the benefit of loop haul roads, and more difficult sediment control measures.
Construction Completion: Final grading of new levees to design height, installation of permanent erosion control measures, hydroseeding the new levees, and repair of any damage to County or City roads used for hauling will complete construction of the project.
Re-vegetation: Native vegetation re-establishment in saline environments often occurs naturally, given the appropriate hydrological conditions. With the difficulty in predicting salinity gradients, which guides the planting strategy, a passive re-vegetation approach may be employed. Following construction and post-project monitoring, the need for a re-vegetation plan, likely focusing on higher elevation areas, will be evaluated, and developed and implemented as needed.
