For years, the Wilson, Trask, and Tillamook Rivers have caused recurrent, significant floods that resulted in damage to the nearby agricultural lands, the City of Tillamook, and Highway 101. This created economic hardship and emotional stress for residents and landowners. In addition, the County experienced multiple national emergency declarations due to flooding. Shannon & Wilson joined a design team tasked with implementing solutions to the flooding, mitigating contaminated soils on the project site, and restoring part of the project area to its natural salt marsh state.

The team's proposed improvements included removing approximately five miles of existing levees, filling 3.3 miles of existing ditches, constructing 1.6 miles of new levees and upgrading of 0.4 mile of existing levees, constructing new culverts and a new floodgate, constructing a four million-gallon reservoir, and creating new tidal channels in the formerly contaminated area. For the construction of the levees, Shannon & Wilson contended with soft and compressible native soils that presented slope stability and settlement issues that threatened to delay the schedule. Using Settle 3D and SLOPE/W, Shannon & Wilson performed advanced numerical soil settlement calculations along with global stability analyses. After analysis, the project team evaluated alternatives to mitigate the settlement and stability issues, along with their associated cost and schedule impacts. The team decided on a combination of staged construction and preloading to meet timeline goals and increase the likelihood of final stable structures.

The total estimated economic benefits accrued from avoided flood damages over a 50-year project life of the Southern Flow Corridor Project are $9.2 million. Direct economic benefits from increased sport fishing opportunities due to increased Coho and Chinook salmon production are estimated at $4.6 million to $7.7 million over a 50-year project life. The Southern Flow Corridor Project has already resulted in decreased duration of impact of floodwaters on Highway 101 in 2017, during the earliest Wilson River flood event on record.