Case Study Contributors
Location
Armuchee Creek Watershed, Chattahoochee-Oconee National Forest, Georgia, USA
Project Type
Pre-Project Conditions
Pre-Project Barriers
None
Watershed Characteristics
Ecological Value
Project Characteristics
Challenges
Project Contributor
US Forest Service
Project Funding
US Forest Service
Completion Date
1990
Total Project Cost
| Materials | $ | 8,000 |
| Construction | $ | 2,000 |
| Total | $ | 10,000 |
Project Summary
This low-water crossing was experiencing chronic erosion as drivers widened the approaches due to the deepening of the stream at the crossing location. To stabilize the crossing and improve safety the low-water crossing was reconstructed using geocells; an expandable three-dimensional polyethylene, honeycomb-like structure that is placed on top of a geotextile fabric laid across the stream channel. The cells are 4 in (10 cm) deep on the approaches and 8 in (20 cm) deep across the channel. They are filled with gravel to provide a stable driving surface that retains a roughened bottom to the stream channel. The use of the geo-cells for low-water crossing applications were first tried on this Forest in 1986, and summarized in the January-February 1987 edition of Engineering Field Notes, published by the US Forest Service.
This project was the first in a series of low water crossings within the watershed. The second utilized concrete planks and the newest utilized linked concrete blocks.
Post Project Observations
Crossing has been in use continually for over 17 years and has survived numerous large storm events. The downstream edge of the crossing remains at grade, and only a small riffle has formed. The roughness of the stone filled Geocells may help prevent water from accelerating and causing downstream scour, which often creates a drop along the downstream edge of the crossing. Fish likely have little to no difficulty swimming onto and across this low water crossing.
Along the upstream edge, portions of the geotextile fabric have come loose and need to be re-bedded. The top edge of cells have been worn down and raveled. Some additional stone needs to be added to fill in rock that has been scoured out. The crossing width needs to be restricted from further widening and approaches could use additional stone.
Lessons Learned
Construction and installation in the wet was difficult due to flowing water. Additional personnel and construction equipment, including a backhoe was needed. The cells should have been placed lower across the channel and the upstream edge of geo-textile should have been embedded into streambed to prevent it from becoming loose.
This type of low-water crossing appears well suited for low-volume roads crossing stable low-gradient channels.
Published 03/25/08