Enselman Road Culvert Replacement
The Stillaguamish Tribe of Indians, with sponsorship from the Washington Department of Fish and Wildlife, replaced a 48-inch corrugated metal pipe culvert and a series of failing log weirs that have become a complete fish passage barrier in Kackman Creek at 55th Avenue NE in Snohomish County, Washington. The existing culvert and road embankment are also failing due to a variety of geomorphic, geotechnical, and structural issues because of a naturally steep channel gradient, highly erodible soils, and stream channel incision 20 feet below the roadway grade. Herrera is leading a team of biologists, geomorphologists, geotechnical, structural, and hydraulic engineers to replace the culvert with a new 110-foot, single-span bridge and accompanying soldier pile retaining walls. The site posed significant logistical challenges for construction: 55th Ave NE is a dead-end road and will require continuous traffic access during construction, the roadway must be widened to meet County standards and yet the Right of Way (ROW) boundary lies on the existing western edge of asphalt requiring an overall roadway shift to the east, construction access to the stream is challenging given the steep slopes and the fact that vegetative disturbance must be minimized given its existing role in providing both quality buffer habitat and soil stability. To address these challenges, Herrera led the interdisciplinary design team through a comprehensive value engineering effort that resulted in a staged construction approach that optimized construction feasibility, low-cost, and reduced risk. Herrera’s technical contributions to the project included evaluating fish passage at the culvert, conducting a geomorphic reach assessment, determining the new structure span, and designing the new channel geometry, alignment, and roughened channel streambed consistent with WDFW guidance. Herrera also designed a self-mitigating revegetation and mitigation plan for temporary and permanent impacts to stream and wetland buffers. Herrera led the preparation of the final bid documents for the improved roadway, new bridge and retaining walls, a realigned channel, instream log structures, bank restoration elements, and the channel, wetland and buffer mitigation areas and supported the client during project construction. Construction was completed in 2020.