Project Summary

The two 4 ft (1.2 m) diameter culverts were located above a sewer line on a service road. Both culverts were: a depth and velocity barrier for adult and juvenile salmon and trout, undersized for high flows, and prone to sediment and debris plugging. At fish migration flows water was concentrated into the left barrel, which had a 4.6% slope. Additionally, the right bank at the culvert outlet had been scoured, causing a large spruce tree to fall across the stream blocking the outlet and impeding fish passage.

A corrugated steel pipe-arch culvert (squash pipe) with a width 130% wider than the active channel was selected for the project. The oval shape of the squash pipe allowed adequate width while minimizing the height to fit between the sewer line and the roadbed. However, the new culvert has inadequate hydraulic capacity for the 100-year flow. A rocked “critical dip” was constructed in the roadbed to allow flood flows to overtop the road and flow back into the channel.

The new culvert was placed flat at a minimum of 1 ft (0.3 m) below the natural streambed to create a simulated stream bottom. It could not be placed lower due to the elevation of the sewer line crossing. After installation, a series of six 1.5 ft (0.46 m) diameter rock were placed in the culvert, spaced approximately 2 ft (0.61 m) apart to create flow velocity areas for juvenile fish to rest. The culvert and existing outlet pool were backfilled with a mixture of river-run gravel, ranging in size from fine sands and silts to 2 inch (14 mm) gravels. The inlet and outlet fill slopes were armored with ¼ ton riprap to stabilize the fill and prevent erosion. The rootwad of the spruce tree blocking the outlet was utilized to armor the bank and provide shelter for fish.

A boulder grade control structure was constructed 30 ft (9.1 m) downstream of the pipe. The structure was positioned to stabilize the channel gradient throughout the new crossing and help retain streambed materials within the culvert. The weir was constructed of two rows of rock, with the top of the structure placed 0.5 ft (15 cm) above the existing channel bed. Footer rocks were placed below the predicted scour depth, which was assumed to be the thalweg elevation of Widow White Creek at the confluence with the North Fork, 80 ft (15.2 m) downstream of the crossing.

Post Project Observations

Post project spawner surveys recorded adult coho and two possible redds upstream of the culvert. The vegetation survey recorded approximately 40% survival on willow cuttings and vigorous growth within the first year. A visual survey in 2006, 5 years after completion, found no noticeable change in the shape, depth, or composition of the streambed material in the culvert. The downstream grade control weir continued to function as designed.

Challenges and Lessons Learned

Open bottom structures, including bridges, were considered for the site but determined to be problematic due to the likelihood of scour, exposing the sewer line.

This project was proposed following changes in the Federal and State guidelines for fish passage improvement projects, which lead to unanticipated costs. Lesson learned included clear communication with permitting agencies about the requirements for survey, fish passage and hydraulic design, dewatering, and erosion control. Developing realistic cost estimates and early submittal of permit applications would have helped the project.

The contractor used an undersized piece of equipment for lifting and placing the pipe-arch culvert. This led to difficulties in placing it at the correct location and placing it flat.

Banks were not created inside the culvert. Consequently, even at low-flow the culvert is wetted from wall to wall, failing to provide a pathway for terrestrial species. Banks also create variation in depth and velocity, with slower water along the edges of the channel that smaller fish swim through.

Construction did not begin until mid-October, when base-flow began to increase as evapotranspiration decreased. This caused base-flow to nearly double during construction, creating significant challenges in dewatering the construction area and maintaining water quality standards (less than 5% increase in turbidity from upstream to downstream of site). Pumps had insufficient capacity and there were no backup pumps. Additionally, there was inadequate energy dissipation provided at the outlet of the bypass hose, causing some scour of the channel bed and increases in turbidity. To avoid this, consider base-flow may change with season, provide sufficient pumping capacity, use gravity bypass systems whenever possible, and use approved energy dissipation techniques at the outlet of all bypass lines.

Published 04/19/07