The Winkelman Revetment on the Snoqualmie River near Duvall protects the Tolt pipeline, a major water supply serving the Seattle metropolitan area. Channel migration, exacerbated by a 50-foot deep scour hole at the apex of a sharp meander, has caused bank failure and could threaten the pipeline if not addressed. Deer Creek, a small fish-bearing tributary, flows over a small dam and through an undersized culvert into the river at the revetment, severely limiting fish passage opportunities to upstream wetlands and streams.
NHC was retained to perform technical analyses, evaluate alternatives, and engineer a design to stabilize the eroding bank. Improving fish habitat and protecting adjacent agricultural lands were additional project goals.
NHC constructed one- and two-dimensional hydraulic models of the project reach, led a detailed alternatives analysis, and identified four likely alternatives. The alternatives analysis utilized findings from hydraulic modeling, geomorphology, habitat conditions, and geotechnical analysis. Design challenges included the bend scour hole, which results in risk of deep seated bank failure risk, and construction challenges of working in up to 25 feet of water even during summer low flow.
NHC developed final plans and specifications for the preferred design to address toe erosion, bank protection, and reconnection of an off-channel wetland for habitat improvement. The project consists of limited rock placement for improved geotechnical bank stability, innovative modular log jacks that allowed for deep water log placement, a log revetment placed on a re-shaped bank, a habitat alcove that recreates the original Deer Creek outlet and will provide excellent fish habitat, and a new flapgated box culvert that will vastly improve fish passage and benefit agricultural drainage. NHC also provided support to King County during the construction phase.