Digger Creek

Concrete Bottomless Arch Culvert

Case Study Contributors

  • Howard Dashiell and Alex Straessle, Mendocino County Dept. of Public Works
  • Ross Taylor, fisheries biologist with Five Counties Salmonid Conservation Program

Location
Tributary to Pacific Ocean, Northern California, USA. MAP

Project Type

  • Bottomless Arch Culvert
  • Roughened Channel

Pre-Project Conditions

  • Steel Pipe Arch: 6 ft (1.8 m) wide, 4 ft (1.2 m) high, and 60 ft (18.3 m) long, 1.2% slope
  • Undersized culvert, overtopped road at 7 year return flow

Pre-Project Barrier

  • Leap barrier
  • Complete barrier for all age classes of salmonids

Hydrologic Characteristics

  • Drainage Area: 1.23 mi2 (3.2 km2)
  • Average Channel Width: 8 ft (2.4 m)
  • Average Channel Slope: 2.5%
  • 2-year Peak Flow: 110 cfs (3.1 cms)
  • 100-year Peak Flow: 387 cfs (9.4 cms)

Ecological Value

  • 2.3 miles (6.0 km) of habitat, once fish passage is restored at highway culvert located 500 ft upstream
  • Historically present: coho salmon, steelhead, and coastal rainbow trout

Project Characteristics

  • Precast Concrete Arch: 12 ft (3.6 m) wide x 4 ft (1.2 m) high x 60 ft (18.3 m) long
  • Constructed roughened channel in culvert using angular cobbles and small boulders mixed with fine sands and silts
  • Slope of constructed streambed in culvert:
    • Upstream 40 ft = 1.2%
    • Downstream  20 ft = 8.8%
  • Culvert walls lined with angular rock to prevent scour of footings

Challenges and Lessons Learned

  • Temporary construction easement conditions
  • Landowner specific aesthetic and revegetation requirements
  • Using a continuous slope through the culvert rather than a short oversteepened section would improve fish passage conditions

Project Lead
Mendocino County Department of Transportation

Project Funding

  • California Dept. of Water Resources
  • California Coastal Conservancy
  • Mendocino County Dept. of Transportation

Total Project Cost

Construction $ 93,204
Materials $ 91,535
Engineering $ 44,218
Easements $ 5,489
Planning $ 2,981
Total $ 237,427

Completion Date
October 2003


Project Description

The existing CSP pipe arch was undersized and in poor condition.  The 3 ft (0.9 m) perched outlet, along with an inadequate jump pool, created a barrier to all age classes of coho, steelhead, and resident trout.  Currently, the State Highway culvert 500 ft (152 m) upstream is blocking passage to 2.3 miles (6 km) of upstream habitat.

The replacement structure is a roughened channel inside a pre-fabricated concrete arch culvert set on cast-in-place footings.  The footings were installed level, 4 ft (1.2 m) below grade with angular rock lining the footings to prevent scour. 

The streambed through the culvert was constructed from a mix of angular cobble to small boulders to provide sufficient roughness and maintain the relatively steep channel grade.  To prevent subsurface flow through the larger material, fine material (sand and silt) was mixed and jetted into the bed. The upper 40 ft (12 m) of the channel bed inside the culvert was constructed at a 1.2% slope, while the lower 20 ft (6 m) is oversteepened at 8.8%.  The upstream channel was expected to incise 2 ft (0.6 m) and release fine sediments, exposing spawning gravels.

The aesthetic treatment required by adjacent land owner included the use of stamped concrete wing walls resembling cobblestone and replanting the site with non-native vegetation.

Post Project Observations and Lessons Learned

Monitoring in 2004 and 2005 found the new crossing to be a significant improvement in both fish passage and storm flow conveyance. Since construction, headcutting exposed bedrock 30 ft (9 m) upstream of the crossing, preventing additional upstream incision.  Longitudinal profiles of the culvert streambed showed the slope through the steeper section had decreased and the slope break had moved upstream.  While it is much steeper than the adjacent channel, the roughness of the cobbles and small boulders may provide pathways of reduced velocities for fish to move through.  Future adjustments to the channel bed will likely further decrease the slope.

Published 02/23/07