Case Study Contributor
Rob Sampson, State Conservation Engineer, USDA-NRCS
Kootenai River, Idaho, USA
Provide access to 1.5 miles (2.4 km) of low gradient habitat for adult Bull Trout (Salvelinus confluentus)
The old crossing was a complete barrier to Bull Trout due to a large drop over riprap below the culvert outlet. The replacement crossing was originally designed using a concrete box culvert. A step-pool bed form was designed to fit inside a 12 to 15 ft (3.6 to 4.5 m) wide box culvert. However, fish passage constraints created by the box culvert lead to the use of a prefabricated bridge with a 40 ft (12 m) span. To control the stream grade, a step-pool channel reach containing 13 boulder weirs placed over a 180 ft (55 m) of channel was designed. The upstream end of the step-pool reach was placed immediately upstream of the bridge.
The undersized culvert had caused substantial deposition upstream of the site, resulting in an ill-defined channel and floodplain and creating a small forested wetland. Originally there were discussions of constructing a ‘desired’ stream channel upstream of the site. The final design maintained the upstream channel grade using the boulder weirs. Removing the undersized culvert is anticipated to mobilize the upstream stored sediment, allowing the channel to seek its own location.
Analytical techniques were used to calculate water depths, velocities and shear stress associated with the step-pools. Bedload transport continuity though the reach was evaluated using the Meyer-Peter Muller approach. Published studies of step-pool channel morphology were used to determine weir spacing and drop heights. Water depths and velocities of the weirs were evaluated to ensure they satisfied Federal requirements for passage of adult salmonids, such as Bull Trout.
The final design included 13 weirs, each having 0.75 ft (23 cm) drops and spaced 11 ft (3.4 m) apart. Fine material was specified to be pressure washed into the gravel and boulder matrix to ‘seal’ the weirs and maintain surface flow. The project was allowed to be constructed within the flowing stream. Although this increases downstream turbidity, it makes it easier to determine if the weirs were sealed. A similar step-pool design was utilized on the Hell Roaring Creek project in an adjacent watershed.
A significant constraint was discovered about one hour prior to the rock placement. A County Road and Bridge worker revealed that a concrete water line crossed through the stream close to where the upstream weir was to be placed. There were no as-built records to locate the line so the entire suite of rock structures was moved about 20 feet downstream. This created substantial problems with the designed grade and required shortening the spacing and lowering the elevations of the rock weirs. The drop over the constructed boulder weirs was increased to 1 ft (0.3 m) and the overall slope of the step-pool reach was increased to approximately 7%.
The 20 Mile Creek Road is the sole access for many homes, making traffic control challenging. A portable bridge was used for a temporary crossing downstream of the construction site.
The site experienced a flow between a 10 and 25 year recurrence interval about 3 weeks after completion. No damage was observed. Hydraulics and recirculation patterns appear to look good from a fish passage standpoint.