55th Avenue South Culvert Replacement Project
The 55th Avenue South culvert is located approximately 1,000 feet south of South 277th Street in unincorporated King County between the cities of Kent, Federal Way and Auburn. The culvert carries Bingamon Creek under 55th Avenue South. The existing culvert is unable to carry stream flows during storm events, which leads to flooding, and its small size and depth makes fish passage difficult.
The 55th Avenue South Culvert Replacement Project will replace the existing 3.3'x2.4'x55' metal corrugated culvert with a 16'x8.5'x41' concrete box culvert. The project will:
- Help reduce roadway flooding
- Provide roadway safety improvements
- Improve fish passage and habitat
- Raise and widen the roadway
- Realign and widen the stream
- Widen the distance between the guardrails by two feet
- Place retaining walls on both sides of the existing road embankment
This work is part of Sound Transit's Sounder South Expanded Service project. King County is administering construction on behalf of Sound Transit.
Frequently asked questions
The U.S. Army Corps of Engineers (Corps) issued a permit to Burlington Northern Santa Fe (BNSF) railroad in 2015 related to Sound Transit's Sounder Easement 4 project. The permit conditions required that fish passage improvements compensate for project impacts at the Hill Creek Tributary South culvert under the BNSF tracks and the Hill Creek Tributary South culvert under 29th Street Northwest in the City of Auburn. The 55th Avenue South culvert conveys Bingamon Creek, which is in the same Green River watershed as the Hill Creek tributaries. Sound Transit’s project mitigation to provide a fish passable culvert was to be implemented off-site by 2018, which was subsequently extended to 2019.
Presently, the 55th Avenue South culvert consists of a 55-foot-long elliptical corrugated metal culvert that is 3.3-feet wide and 2.4-feet high. In addition to presenting a fish passage barrier, the culvert is unable to carry stream flows during two-year storm events, which leads to water overtopping the road. The project proposes to replace the existing culvert with a 16-foot-wide by 8.5-foot-tall by 41-foot-long box culvert to alleviate flooding and improve fish passage at the site. The base of the culvert will be three feet below the existing stream bed and will contain fish-friendly streambed gravel and cobbles.
As part of the project, the roadway will be raised to accommodate the taller culvert. Over the culvert, adequate depth of cover will be provided so that Puget Sound Energy (PSE) can relocate its gas line here instead of below the new culvert. King County standards require that distance between guard rails, which are severely sub-standard, be widened by two feet to improve traffic and pedestrian safety. Raising and widening the road will require fill and retaining walls to be placed on either side of the existing road embankment. To accommodate these walls, a small tributary to Bingamon Creek will be realigned toward the edge of the roadway right-of-way. All permanent work will be in the existing King County right-of-way.
Post-project stream channel conditions will be improved with the realigned stream being somewhat wider and having a gravellier substrate. In Bingamon Creek, the undersized pipe will be replaced with a stream simulation culvert with fish-friendly substrate throughout, including a low-flow channel. Native plantings will be adding during the fall/winter after construction is complete.