Clear Creek

Bottomless Arch Culvert

Location
Mattole River Watershed, NW California, USA. MAP

Project Type

  • Bottomless Arch Culvert w/Concrete Footings
  • Stream Simulation Design

Pre-Project Barrier

  • 9 ft (2.74 m) diameter CMP, 85 ft (25.9 m) long at 8.5% slope, culvert bottom lined with smooth channel iron
  • Steep culvert slope presented difficulty to all ages of migrating salmonids. High velocities and shallow depths in culvert blocked all juvenile and most adult salmonids from moving upstream, except possibly when backwatered by the Mattole River

Channel Characteristics

  • 100-year Design Flow: 425 cfs (12.0 cms)
  • Drainage Area: 1.24 mi2 (3.21 km2)
  • Active Channel Width: 15 - 24 ft (4.5 - 7.3 m)

Ecological Value

  • Provide access to 3,550 ft (1,082 m) of potential spawning and rearing habitat for all life stages of coho salmon and steelhead
  • Provide cold water refugia from the warm waters of the Mattole River

Project Characteristics

  • Single Radius Structural Steel Plate Arch on concrete footings
  • Culvert Dimensions: Span = 18.0 ft (5.5 m), Rise = 9.25 ft (2.8 m), Length = 110 ft (33.5 m)
  • Concrete Spread Footings: Base Width = 3 ft (0.9 m), Height = 3 ft (0.9 m)
  • Design Bed Slope: 5.4%

Challenges and Lessons Learned

  • Excavating through bedrock to place footings to achieve desired slope
  • Traffic bypass bridge over the Mattole River
  • Shifting alignment after construction
  • Backwatering at river confluence can cause deposition and compromise conveyance capacity

Project Contributors
Humboldt County Dept. of Public Works

Project Funding
California Dept. Fish and Game
Humboldt County
California Coastal Conservancy Grant

Completion Date
November 11, 2001

Total Project Cost
Approximately $264,641

 


Project Description

The existing 9 ft (2.74 m) diameter corrugated metal culvert, 85 ft (25.9 m) in length, was identified as a total barrier to all juvenile salmonids. The steepness (8.5%) of the culvert created a velocity barrier at all migration flows to all but the strongest adult salmon and steelhead. Additionally, the culvert constricted the stream channel, slowing water during storm flows and causing bed material to deposit upstream of the inlet. A steep transition formed between the aggraded upstream channel and the culvert inlet, which further hindered fish passage. This site received a priority ranking of 7th in the Humboldt County Inventory and Fish Passage Evaluation Report.

A bottomless arch culvert with concrete footings was selected as a replacement structure for the site. Dimensions of the arch were determined by using streambed simulation methodology and field observation of the stream channel width and slope. The bottom of the new culvert was placed 1.5 to 2 ft (0.46 to 0.61 m) below the elevation of the replaced culvert, and was installed at a slope of 5.4%. This matched the slope of the upstream channel beyond the influence of the culvert, as identified in a sufficiently long profile of the channel. This design provides a natural stream bottom with sufficient width to avoid constricting the active channel. In addition, the new crossing was designed to pass a 100-year flood event, reducing the potential for culvert failure. The objective was to construct a streambed within the culvert that resembled a natural channel, thus providing passage conditions similar to those found in the adjacent stream channel.

 

Post Project Observations and Lessons Learned

Observations of the site in November 2001 and February 2002 recorded headcutting of the upstream channel through accumulated sediment stored at the culvert inlet. The head cutting was relatively minor and did not progress very far upstream. In some cases, the stored material near the replaced culvert is excavated to prevent it from being mobilized.

Upstream headcutting has caused the channel to migrate so now the stream enters the culvert at an angle, resulting in undermining of the footings near the inlet. Poor channel alignment with the culvert inlet also increases the risk of plugging and decreased hydraulic efficiency. In hindsight, anticipating channel adjustment and improved crossing alignment could have avoided undermining of the footings.

Located near the confluence of the Mattole River, the culvert experiences extreme backwater conditions when the main stem of the River rises. This occurred during a 2002 storm event resulting in over 6 ft of deposition within the culvert. The stream has since downcut through the deposited material. The accumulation of bedload at stream confluences is a well known morphological trait, but the extent to which it can compromise the conveyance capacity of a small tributary culvert is an important lesson. Increasing the height of the culvert to provide additional head room can help prevent culvert plugging and potential culvert failure.