Puyallup River 128th Street Bridge and Road Design CLOMR

128th Street in Pierce County will be widened from two to four lanes to accommodate a new arterial that will serve a large masterplan community near Orting, WA. A new, two lane bridge will be constructed parallel to and downstream from the existing 128th Street bridge across the Puyallup River. WSE was retained to complete the hydraulic design investigation for the new bridge and associated road approaches, and to prepare a FEMA CLOMR submittal.

Role/Services:

• Hydrology – Computing peak design discharges for the Puyallup River.
• Developing a HEC-RAS 2D hydraulic model to evaluate performance of the existing and proposed crossings and to develop a preferred solution that satisfies zero-rise floodplain development requirements. The analysis revealed that in addition to the bridge, two large culverts (10ft x 60ft and 10ft x 100ft) will need to be added through the east approach fill to prevent rises in 100-year flood levels upstream.   
• Working with designers to accommodate the new road alignment and associated project grading elements.
• Providing Pierce County right-of-way staff data needed to negotiate right-of-way and flood easements.
• Developing updated floodplain maps and associated work products for the County. A revised floodway analysis was also developed using the 2D model which accommodates updated peak discharges and the road project.
• Preparing and supporting review of a FEMA CLOMR submittal to update the existing floodplain maps and permit the project.

Challenges/Outcome: Achieving zero-rise was extremely difficult and required significant creativity and coordination with the project design team, Pierce County, and Newland Communities. The project is on-going with permitting review underway.