Hell Roaring Creek
Culvert Replacement and Channel Re-grade with Step-Pool Morphology

Project Summary

The original project was to remove a concrete weir with a 7 ft (2.1 m) drop that blocked resident Bull Trout from reaching upstream habitat. In November 2006, during the project planning phase, the weir failed during high flows. This resulted in rapid headward incision that progressed about 100 ft (30 m) upstream until it reached the culvert outlet. The newly perched culvert became a migration barrier, changing the focus of the project.

The stream crossing is at the apex of an alluvial/glacial outwash fan that slopes at about 3.4%. The channel has a recent history of avulsion upstream of the crossing, and in several instances had migrated around previous crossings. There was considerable discussion of  rerouting the stream to one edge of the fan, but property ownership and County road concerns precluded this option. Fixing the stream in its current position on the fan, as designed, will require recurring maintenance.

A bottomless arch placed on concrete footings located below the anticipated scour level replaced the original culvert. The new culvert was designed to span the bankfull channel and have a mobile bed consisting of natural streambed substrate. To facilitate passage of debris, the culvert was sized to avoid overtopping the inlet (HW/D < 1) at the 50-year design flow of 1,400 cfs (40 cms). The channel was regraded 250 ft (76 m) downstream and upstream of the culvert. Portions of the regraded channel profile were controlled with boulder weirs designed to form step-pool morphology.

At this site, channel stability appears to be correlated with coarseness of the bed material and sediment transport capacity rather than channel geometry. The new channel design was based on several upstream and downstream reference reaches, stable stream patterns reported in published literature, and application of analytic methods. The proposed channel contains a combined step-pool and plane-bed morphology (Montgomery and Buffington, 1993). Design bankfull width and depth are 18 ft (5.5 m) and 1.35 ft (0.4 m), for a width to depth ratio of 13. Entrenchment ratio at the 25 year flow is 1.5 to 1.7. Using a Rosgen typology, this is an A3 channel.

The channel design consists of three step-pool reaches and three natural plane-bed reaches. The two step-pool reaches downstream of the culvert are 72 ft (22 m) and 37 ft (11 m) in length. The step-pool reach upstream of the culvert is 54 ft (16 m) long. Each step-pool reach maintains a gradient of about 6.5%.

Step-Pool Channel Design

The designed step-pool configuration uses a primary step followed by a constructed scour pool and a secondary step placed 12 ft (3.7 m) downstream. Primary steps are spaced 18 ft (5.5 m) apart and the step height (primary step to primary step) is 1.2 ft (37 cm). The drop between the secondary step and primary step is 0.5 ft (15 cm). Residual pool depth between weirs is 1 ft (30 cm).

The hydraulic environment of the design channel was modeled using HEC-RAS. Tractive Force method for incipient motion of the D84 particle was used to size the bed material for the new channel. The design storm for stability of the steps is a 25 year peak flow. The Froude number associated with the hydraulic jump generated by the step geometry is 2.4. This is a weak submerged jump bordering on an oscillating jump that is expected to provide good energy dissipation and recirculation.

Lessons Learned

Coordinating the timing of the contracting, the construction, and manufacturing and delivery of the culvert required detailed project management. Having the Soil and Water Conservation District purchase the culvert early and provide it to the contractor helped avoid construction delays.

Published 03/24/08