Project Summary
The existing 10 ft (3 m) diameter steel plate culvert crossed under US Highway 101. It was 300 ft (91.5 m) long with an 80 ft (24.4 m) long concrete trapezoidal outlet apron, and was considered a depth and velocity barrier to all life stages of steelhead trout. Also, the winter before construction a drop 2 ft (0.6 m) drop formed below the outlet apron, creating a leap barrier.

The culvert inlet was extended 31.5 ft (9.6 m) upstream in the summer of 2000 as part of a highway widening project. The project included the installation of 35 notched concrete weirs inside the culvert, and 8 on the outlet apron to improve fish passage conditions. Weirs in the culvert are 1.3 ft (0.4 m) high, span the width of the culvert, and are spaced 10 ft (3.0 m) apart. They were built with low-flow notches that are offset to create sinuosity. The apron weirs are 2.3 ft (0.7 m) high by 14.5 ft (4.6 m) wide (spanning the concrete channel), with a low-flow notch located in the center. Due to the anticipated loss of hydraulic capacity associated with the weirs, an overflow culvert was added to the crossing. Placed above the original culvert, it also serves as a crossing for cyclist, pedestrians, and terrestrial animals.

Culvert Inlet
Extending the culvert changed the approach angle between the upstream channel and the inlet, creating a poor alignment. The following winter large storm flows scoured behind the headwall and eroded the adjacent stream bank. In 2003 a stream bank stabilization project was implemented using bioengineering techniques, including native fill wrapped in coir fabric, willow cuttings and a “brush box”.

Performance of Weirs
The weirs in the culvert and on the outlet apron were intended to increase water depths and decrease velocities to improve fish passage. They were expected to produce plunging flow as a pool and weir fishway during low flows (5 to 10 cfs) and transition to streaming flow during higher flows. At high flows the weirs would act as baffles, or large-scale roughness elements. However, only limited hydraulic analysis was conducted to predict their performance and the weirs completely filled with sediment during the first winter. The sedimentation created a smooth flat bed throughout the structure, likely producing insufficient depths and excessive velocities for fish passage at most flows.

Sedimentation of the weirs may be attributed to a combination of upstream conditions and design characteristics:

1. Recent wildland fire in upper watershed increased sediment delivery.
2. The project rerouted the highway drainage upstream of the culvert resulting in new bank failures and sediment input into the stream system.
3. The horizontal weir shape causes flow to spread out across the weir crest rather than concentrating the flow and focusing scour forces.
4. Turbulence produced by the weirs during streaming flow was insufficient to keep pools scoured.

Downstream Channel Adjustments
During project design the drop at the outlet apron was less than 6 inches (0.15 m). The winter prior to construction the channel downcut an additional 2 ft (0.61 m). Then, addition of the weir at the end of the apron added another 1 ft (0.30 m) of drop. The channel continued to downcut another 10 inches (0.25 m), creating a 4 foot (1.2 m) leap barrier for steelhead trout. The degradation can be attributed to high velocities exiting the apron and scouring the downstream channel. Adding the weirs may have contributed to additional downcutting due (1) the increased drop created by the outlet weir and (2) starving the downstream channel of sediment as the pools filled-in between the weirs.

Roughened Channel
To address the drop at the apron, a roughened channel of grouted rock was constructed in 2003. Construction access was limited, requiring a crane to lower materials and equipment into the site. The roughened channel was confined to 50 ft (15 m) in length to fit within the right-of-way.

The roughened channel has a 1.5 ft (0.46 m) drop at the apron and two boulder weirs with drops of 0.7 ft (0.20 m) and 1.8 ft (0.56 ft). One weir was constructed of new rock and the other built with boulders located at the site. Between the apron and boulder weirs are two 23 ft (7 m) long rock roughened chutes at 6% and 8.5% slopes. Confinement in this reach of the creek made it difficult to embed rock in to the channel bottom and banks. Concrete was used to fill the voids and secure the rocks to resist scour. The top one-half of the rock thickness is free of concrete to provide roughness for reducing velocities and increasing depth.

Due to the limited access, the roughened channel does not have pools below each drop, making it difficult for fish to successfully leap over them. Pools also dissipated energy and further reduce water velocities. Following completion, the natural channel downstream of the project downcut, creating a large drop that blocks fish passage.

At an overall slope of 11%, the grouted channel was extremely steep. The downcutting is likely due to channel scour caused by high velocities exiting the roughened channel. This might have been avoided by constructing a longer roughened channel to lower its slope, reduce drop heights, and include pools. Efforts are currently underway to purchase downstream right-of-way and stabilize the downstream grade with a series of rock weirs.

Lessons Learned
Consider the geomorphic and hydraulic impacts beyond the project area. For this project, including a interdisciplinary team of engineers, hydrologists, fisheries biologists, and geomorphologists may have identified and addressed potential problems during the preliminary design phase.

Poor culvert alignment can increase the risk of debris plugging, scour adjacent banks, and reduce capacity. When extending or installing a culvert, consider the impacts on alignment between the culvert inlet and approaching channel.

When installing weirs or baffles it is essential to perform a thorough hydraulic analysis that considers flow regimes (plunging and streaming), weir or baffle shape, drop heights, and sedimentation. For case studies that examined weir and baffle hydraulics, see Jon Hatt Creek, Upper Salsipuedes Creek, and Peacock Creek.

The natural streambed below a lined or hardened channel is typically susceptible to scour and downcutting. It is a good idea to include a transition area that dissipates energy and reduces velocity before flow enters the natural channel. Addressing this in the initial design phase may have prevented the need for subsequent retrofits.

Limiting the project length to the right-of-way can make it extremely difficult to satisfy fish passage objectives while maintaining a stable channel. To achieve the project’s objectives it may be necessary to extend the project reach.

Published 05/01/07