Project Summary

The site consisted of a corrugated metal pipe arch and an overflow culvert. The overflow pipe captured roughly 25% of the stream volume at low flow. The main culvert had a 0.7 ft (20 cm) outlet drop and both culverts created a severe velocity barrier for Westslope cutthroat trout at nearly all flows. Both culverts combined constricted the channel to less than 50% of stream’s bankfull width. A large depositional area had formed upstream of the undersized culverts due to frequent ponding. Nearly the entire Red Butte Creek Watershed had burned the year prior to the project. Higher storm flows and debris transport were expected following the fire, increasing the likelihood of culvert plugging and failure. For these reasons, this crossing became a high priority for replacement.

The culverts were replaced with a pre-cast concrete bridge spanning 48 ft (14.6 m). During construction, the overflow pipe was kept in place and used for water diversion.
The bridge rests on pre-stressed concrete beams supported by geocells filled with gravel. The option of using an open bottom pipe arch was explored, but the bridge was determined a more economical and practical solution due to the steep channel slope and high flows.

The stream has a step-pool morphology with a channel slope of 7 to 9%. The stream channel under the bridge was built to have a bottom width of 18 ft (5.5 m) and banks sloped back 1.5H:1V and armored with riprap. This created a channel that had similar dimensions to the adjacent natural channel. To control the grade and match the adjacent channel morphology, five rock weirs were built through the project reach to form step pools. The weirs were constructed with two rows of rock. The footer rocks were 3 ft (0.9 m) diameter and buried into the streambed. The footer rocks were topped with rocks of similar size or slightly smaller. The weirs were designed to be convex shaped with gaps of roughly ½ a rock diameter wide between the top rocks to allow passage in very low flows. The number and spacing of the weirs was determined by upstream reference reaches.

Challenges

The primary challenge was assessing the biological trade-offs of providing access to upstream habitat for native Westslope cutthroat trout and allowing non-native brook trout to colonize this newly accessible habitat. Upstream of the culvert barrier only native, genetically pure, Westslope cutthroat trout are found. Downstream of the culvert, both native cutthroat trout and non-native brook trout coexist. Keeping a barrier in place provides some management opportunity to keep the non-native population from out-competing the genetically pure stock above the crossing.

In the end, it was decided that the benefits of fish passage outweighed the risks. The implications of this decision are unknown. Monitoring the cutthroat population is possible, but without baseline data, drawing definitive conclusions is difficult.

Post Project Observations

The rock weirs were difficult to construct as shown on the design plans due to the challenges of working in the natural substrate, which is dominated by large boulders. Once the project was done, it was virtually impossible to find the weirs as they blended in with the coarse stream substrate. As a result, the constructed channel functions as a mix of step-pools and cascades.

In steep step-pool stream reaches such as this one, the large bed material is infrequently mobilized and when transported, may only move short distances. Reconstructing the channel morphology through the new crossing on steep boulder dominated streams is necessary. If the step-pools are not constructed through the crossing, the channel may take decades to form its natural morphology, possibly remaining a passage barrier to fish in the interim.

The new reach appears to function well as a mix step-pool / cascade type morphology due to the large native material used to fill around the constructed boulder steps. Designing for cascades instead of a step pool morphology may have simplified the construction process but may not have been as stable. A stability analysis could have be used to determine whether boulders placed in the stream in a cascade formation would be sufficient to maintain the grade.

The channel under the bridge creates a wide variation in water velocities and depths at both low and high flows, providing numerous suitable pathways that both small and large fish can swim through. Based on observations during Spring flows, the new channel easily passes fish during a 2-year flow event and may pass fish at even higher flows.

Published 09/26/07