Widow White Creek

Box Culvert Retrofit
and Rock Weirs

Case Study Contributors

  • Don Allan, Redwood Community Action Agency
  • Michael Love, Michael Love & Associates

Location
McKinleyville, Northern California, USA. MAP

Project Type

  • Hydraulic Design
  • Angled Baffles in Box Culvert
  • Downstream Boulder Weirs

Pre-Project Conditions

  • Concrete box culvert: 8 ft (2.4 m) wide x 6 ft (1.8 m) tall x 72 ft
    (21.9 m) long
  • Culvert Slope = 0.4%
  • Outlet perched 3 ft (0.9 m) above downstream channel bed

Pre-Project Barrier

  • Wide, flat culvert bottom created depth barrier for salmonids
  • Excessive leap at outlet
  • Complete barrier to all fish

Watershed Characteristics

  • Drainage Area: 1.97 mi2 (5.1 km2)
  • Average Channel Width: 7-9 ft (2.1-2.7 m)
  • 100-year Peak Flow: 700 cfs (20 cms)
  • High Fish Passage Design Flow: 50 cfs (1.4 cms)
  • Low Fish Passage Design Flow:
    0.35 cfs (0.01 cms)

Ecological Value
Provide access to 1.5 miles (2.4 km) of upstream spawning and rearing habitat for coho salmon, steelhead trout and coastal cutthroat trout

Project Characteristics

  • Install 4 wooden baffles in culvert,
    • 8 ft (2.4 m) long placed perpendicular to flow
    • Spaced 20 ft (6 m) apart
    • Side-sloped from 1 ft (30 cm) on left to 0.5 ft (15 cm) on right
    • Outlet baffle is V-shaped
  • Construct 3 boulder weirs downstream of culvert, 1 ft (0.3 m) drops between weirs

Challenges and Lessons Learned

  • Work space and access limited by adjacent landowner
  • Finding suitable discharge location for streamflow bypass
  • Permitting and cost overruns
  • Baffles should be skewed instead of perpendicular to flow to further concentrate flow and simplify installation
  • Originally designed and constructed with only 2 weirs, when 3 weirs were needed
  • Lower weir partially failed and downstream banks scoured prior to addition of 3rd weir

Project Contributors

  • Redwood Community Action Agency
  • Michael Love & Associates
  • Graham Mathews & Associates
  • Environmental Restoration Services

Project Funding

  • California Dept. of Water Resources

Total Project Cost

Engineering/Planning $ 9,991
Materials $ 7,960
Permit Fees $ 8,269
Construction $ 35,792
Total $ 62,012

Completion Date
September 2003


Project Summary
The existing stream crossing is an 8 ft (2.4 m) wide x 6 ft (1.8 m) tall x 72 ft (21.9 m) long concrete box culvert with a 3 ft (0.9 m) perched outlet. Although the culvert has a slope of only 0.4%, the flat bottom results in insufficient water depths at most fish passage flows. Retrofitting the culvert to provide fish passage for adult and juvenile salmonids involved installing baffles inside the culvert to increase water depths and constructing rock weirs below the culvert outlet to decrease the drop height.

Three redwood baffles were installed in the culvert and a redwood weir was installed across the culvert outlet. The main purpose of the baffles was to increase water depth. The baffles were placed perpendicular to flow and spaced 20 ft (6 m) apart. They slope from a height of 1 ft (0.3 m) at the left wall to 0.5 ft (0.15 m) at the right wall. The side-slope concentrates the majority of flow along the right side while providing slower water along the left. The weir at the culvert outlet has a 4 inch (0.1 m) deep by 12 inch (0.3 m) wide notch cut in it to concentrate plunging flow and provide good leaping conditions.

The project funding request was developed prior to adoption of new Federal guidelines for salmonid passage at stream crossings. Project costs were developed assuming two weirs would be acceptable, with 1.5 ft (0.46 m) drops over each weir. However the new guidelines, which were published after design completion and just prior to project implementation, required maximum drop heights of 1 ft (0.3 m). Permitting agencies agreed to allow construction on the condition that a 3rd weir would be added once funding was secured. To allow for subsequent installation of the 3rd weir, the first two weirs were constructed higher than originally designed, resulting in a 2 ft (0.6 m) drop over the lower weir. During the first winter, a large scour hole formed below the lower weir, causing it to partially collapse into the downstream pool.

With additional funding secured in 2003, a 3rd boulder weir was installed.  Additional rock was placed on the bank to protect the weirs from scour and flanking.

The streambed has been stable since construction of the 3rd weir. Willows cut to access the site were replanted after the project was completed and re-growth has been excellent.

Lessons Learned
The redwood baffles were made from timbers cut to fit the culvert width exactly. However, the logistics of placing them at a right angle to the culvert walls and forming a water tight seal to prevent leaking on the sides was extremely difficult. The preferable approach is to skew the baffle relative to the culvert walls to make installation easier. Skewing the baffle with the low side pointing upstream also concentrates flow towards one side and improves both fish and debris passage.

During the design phase, it appeared that 2 weirs would be adequate to maintain the desired grade and water surface drops. However, the increased water surface from the outlet weir and the increased drop due to the formation of the downstream scour hole were not anticipated. A 3rd weir should have been added to the original plans but construction cost, limited topographic survey, and a need to minimize channel disturbance resulted in design of only 2 weirs. The addition of the 3rd weir stabilized the streambed and water surface elevation.

Due to difficulties with permitting, construction began in mid-October, just before the onset of winter rains in Northern California. During construction a large rain event caused flows to rise substantially and resulted in failure of the sandbag cofferdam and streamflow bypass system. Continued rain and high flow saturated soil conditions, and the potential for additional rain rushed the completion of the project and prevented construction of a 3rd weir. Starting well before the onset of the rainy season would have provided a larger window to address unanticipated challenges that arose during construction.

References

Washington Department of Fish and Wildlife Environmental Engineering Division. 2003. Fish passage design at road culverts: a design manual for fish passage at road crossings. May 2003. http://www.wdfw.wa.gov/hab/engineer/cm/.

 

Published 04/20/07