Project Summary

The existing culvert was constructed in 1939 and estimated to pass the 50-year peak flow. The culvert was in poor condition and had a steep concrete floor that created a depth and velocity barrier for adult and juvenile coho salmon and steelhead trout. Water velocities of 10-12 feet per second (3.1-3.7 m/s) were measured during low to moderate winter migration flows, twice the maximum water velocity recommended by the CA Dept. of Fish and Game for salmonid fish passage. The stream is relatively steep and consists of gravel with large cobbles up to 12 inch (30 cm) in diameter.

The replacement structure was a steel plated arch culvert mounted on spread footings designed as concrete grade beams. The top of the new spread footings were installed 2 ft (0.6 m) below the existing culvert floor and were 3 ft (0.9 m) deep. The bed inside the arch culvert was constructed at a 3% slope, which was the estimated stable elevation and slope of the stream. The stable slope was based on a channel (longitudinal) profile over 900 ft (274 m) long. The channel bed between the footings was constructed of imported river-run gravel, which was smaller than the bed material in the adjacent channel. Revegetation of the site was conducted by local community groups.

Post Project Observations and Retrofit

During the first winter after construction, in November 2005, high streamflows scoured a deep trench along one of the footings, undermining it for approximately 80 ft (24.4 m) and causing the footing to crack and rotate. The footings were retrofitted in December 2005 to repair the damage and stabilize the channel grade in the culvert. Initial efforts focused on pumping concrete into the void below the beam to support the foundation and prevent further settling. The next step entailed excavation, forming, and pouring of two weirs spanning the width of the culvert spaced approximately 50 ft (15.2 m) apart. The weirs strut the footings and are tapered to direct flow toward the center of the culvert. Finally, ½-ton rock was placed along the entire length of the footings using a front-end loader to armor against scour.

Following the retrofit, the site was monitored during the next series of storms and has been inspected several times since. The culvert has not shown any further signs of damage.

Lessons Learned

Equipment problems in this remote location lead to an inconsistent concrete pour and a cold joint along the footing. The storm damage caused cracking along the cold joint, which may have been avoided with more quality control.

Aspects of the design, such as footing depth, may need to be revised based on field observations during construction. In some cases, the original streambed can be identified in the excavation, but requires an experienced inspection team. Additionally, it may be necessary to excavate deeper than indicated in the design to find competent material to place the footing onto.

The streambed material placed in the new culvert was much smaller and more mobile than the large cobble dominated substrate found in the natural channel. This made the new bed in the culvert highly susceptible to scour during initial high flows, likely contributing to the undermining of the footing. This can be avoided by placing streambed material that more closely matches the size of the natural channel substrate.

Scour along the smooth concrete footings, referred to as trenching, is commonplace. Adding rock banklines within the culvert, which was done as part of the retrofit, can prevent trenching and produce slower velocities along the edges suitable for smaller fish.

Published 04/24/07