Flood Control and Recreation Site
THe problem
The Wilson, Trask, and Tillamook Rivers merge to form an alluvial plain at the head of Tillamook Bay on which the City of Tillamook is located. The Wilson River flows within natural levees at a higher elevation than its floodplain due to thousands of years of sediment accumulation caused by floods. Thus, when the Wilson River floods over its banks, water cannot return to the river channel. Instead, water flows south and west across the floodplain to meet the Trask and Tillamook Rivers, where it again backs up against levees. This results in recurrent, damaging floods within Tillamook’s Highway 101 business corridor. The creation of the Southern Flow Corridor Land Owner Preferred Alternative Project (Southern Flow Corridor) joined Tillamook County and Port of Tillamook Bay in contracting with Northwest Hydraulic Consultants, along with HBH Consulting Engineers and Shannon & Wilson as subconsultants, to develop and implement a plan to reduce damaging floods affecting Tillamook County and the City of Tillamook. The project includes the restoration of 521 acres within the floodplain to their natural salt marsh condition.
THE CHALLENGES
During the Southern Flow Corridor Project, the project team encountered:
Soft and compressible native soils that presented numerous soil stability and settlement issues that threatened to delay the schedule;
A former mill site cleanup involving contaminated soils;
A 612-acre project site requiring coordination between contractors and designers; and
The potential for high tidewaters during construction that could flood the area.
THE SOLUTIONS
The project team developed and implemented the following plan:
1. Construct approximately 1.6 miles of new setback levees and upgrade .4 miles of existing levees to protect adjacent agricultural lands from tidal influence.
2. Construct new culverts, a new floodgate structure, and a 4 million-gallon reservoir; connect and construct new tidal channels to discharge flood waters into nearby sloughs and Tillamook Bay.
3. Fill existing agricultural ditches with native materials, eliminate plugs and culverts from relic tidal channels, remove rock and gravel surfaces from roads, and decompact roads to ensure tidal channels can form that will naturally restore the salt marsh.
4. Remove approximately 5 miles of man-made levees built up around the Wilson River to provide the conveyance capacity to lower flood levels.
The Southern Flow Corridor Project has already resulted in decreased duration of impact of floodwaters on Highway 101 during the earliest Wilson River flood event on record.