Alternative 3: Raise Roadway

Alternative 3 will elevate the roadway above the Base Flood Elevation of 258.7 feet with a minimum freeboard of 3 feet. The three existing fish barrier culvert crossings will be replaced with fish-passable structures, and 10’x10’ precast concrete culverts are proposed. The dimensions of the culvert are based on the width of the existing stream and anticipated depths of flow to allow for a 100-year flood event.

Three options for implementing this alternative have been evaluated:

• Pin Pile-supported Slab: This option consists of constructing a concrete slab supported by piles driven into existing ground, eliminating any future settlement.
• Lightweight Fill: This option would excavate some of the existing roadway and place low density material to raise the roadway, which would result in a net reduction in weight of the roadway, thereby stopping or reducing settlement.
• Overbuild and Long-term Settlement: This option would raise the road with excess material, allow time to settle, and then open to traffic.
• Ground Improvements: This option would install concrete walls or columns in existing ground to reduce settlement.

Construction impacts

Raising the roadway would have the most substantial up-front construction cost and traffic impacts. Northbound and southbound construction would happen sequentially, with traffic reduced to a single lane in each direction during construction.

Potential results

Raising the roadway profile will offer the best long-term solution relative to the other alternatives. Raising the roadway above the existing water surface elevation and supporting the extra weight with one of the options listed above will reduce or eliminate settlement of the roadway. In addition, three fish-passable culverts will be installed, which will benefit fish and allow flood waters to pass under the roadway.