Case Study Contributor
Location
Tributary to Meadow Creek
South Fork Clearwater River Watershed, Idaho, USA MAP
Project Type
Pre-Project Conditions and Barriers
Watershed and Channel Characteristics
Ecological Value
Project Characteristics
Challenges
Project Contributors
Project Funding
Completion Date
September 2006
Project Cost (2006)
| Construction | $ | 192,309 |
| Engineering & Design | $ | 12,000 |
| Total | $ | 204,309 |
Project Summary
The Nez Perce Tribe and the Nez Perce National Forest have been working together to fund replacement of barrier culverts since 2003. This watershed restoration effort includes road decommissioning, soil restoration, and barrier removals.
This project replaced an existing 7 ft (2.1m) x 140 ft (43 m) CMP with a 15 ft (4.5 m) span x 108 ft (33 m) open bottom structural steel plated arch pipe on concrete stem wall footings. A natural stream channel with a slope of 6.6% was constructed through the new crossing using native streambed material from the site mixed with larger rock. The channel inside the culvert included a well defined active channel and streambanks. The bed mixture included fine material to minimize porosity and maintain surface flow. The banks also provided a pathway for smaller terrestrial animals that travel along the riparian corridor; allowing them to migrate through the culvert rather than crossing over the large fill and roadway. A rock vortex weir was constructed upstream of the culvert inlet for grade control to minimize headcutting.
The crossing is located along a sharp switchback in the road within a narrow valley and has a 26 ft (8 m) high fill above the culvert. Excavation of this extremely large amount of fill required working with steep embankments that made equipment access difficult. A concrete pump truck and a crane positioned on the road were used to pour footings and lower materials.
This new crossing allows passage for all aquatic organisms. Native spring/summer Chinook salmon, steelhead and cutthroat trout are now able to access upstream spawning and rearing habitat that was blocked by the old culvert. The new culvert also allows the crossing to pass flood flows and larger debris, reducing the risk of road failure and sediment delivery to important steelhead and salmon spawning habitat downstream.
Post Project Observations and Lessons Learned
The extremely large fill volume took longer to excavate than was estimated. Space for storing the fill material was limited and resulted in steep side slopes creating access problems to the streambed.
A substantial bedrock shelf was found at the footing depth. The contract called for cast-in-place footings and stem walls which were modified in the field to reduce the amount of bedrock to be removed using a rock hammer. Construction using precast footings would have been much more costly at this site, requiring the removal of several feet of bedrock to reach subgrade design depths. A geotechnical investigation that included borings may have alerted designers to the subsurface conditions.
Post construction monitoring shows little change in the site profile or cross sections, suggesting the new channel is stable and functioning as intended. The upstream weir and several boulders placed in the culvert have created resting area and pools throughout the length of the new culvert for use by migrating fish. Post construction planting included native grass seed mix and native shrubs and trees on the new cut slopes which have helped control erosion and will provide shade and slope stability.
Published 05/27/08