Case Study Contributors
Location
Armuchee Creek Watershed, Chattahoochee-Oconee National Forest, Georgia, USA
Project Type
Low-water crossing retrofitted with precast concrete boat-ramp planks
Pre-Project Conditions
Pre-Project Barriers
None
Watershed Characteristics
Ecological Value
Reduction of sediment inputs and while maintaining access to upstream habitat for many fish species, including redeye bass, Coosa darter, Alabama rainbow, which carry freshwater mussels.
Project Characteristics
Challenges
Project Contributor
US Forest Service
Project Funding
US Forest Service
Completion Date
1994
Total Project Cost
| Materials | $ | 12,000 |
| Construction | $ | 3,000 |
| Total | $ | 15,000 |
Project Summary
Erosion from the approaches of the low-water crossing was a source of chronic sediment delivery to the stream. Drivers were widening the approaches as the roadbed in the stream became deeper. To stabilize the crossing and reduce erosion, a concrete plank low-water crossing with rocked approaches was constructed.
The crossing design was based on an article published in US Forest Service Engineering Field Notes and local experience. Six inch (15 cm) thick precast concrete planks, designed for use with boat ramps, were placed across the channel to provide a stable driving surface. Planks were placed on geotextile fabric and additional planks were placed on their side to form a curb and keep traffic on the roadbed. Concrete median barriers laid on their sides were used as a buttress along the downstream edge of the crossing.
This project was the second in a series of low-water crossings within the watershed. The first utilized a plastic geo-cell membrane filled with gravel and the newest utilized linked concrete blocks.
Post Project Performance
The crossing, with some modifications, has performed fairly well for over a decade. After construction downstream channel scour and down-cutting created a 10 in (25 cm) drop onto the face of the concrete median barrier. Also, stream velocity between adjacent planks may have created a velocity barrier to upstream fish movement during low-flow conditions. A downstream rock weir was constructed to protect the crossing from scour, which backwaters the crossing and improves passage conditions over the planks. However, the hydraulic conditions created by the rock weir may hinder low-flow passage of weaker swimming fish species.
Portions of the geotextile fabric have come loose and needs to be rebedded and the approaches could use additional stone.
Lessons Learned
Construction, including placement of the geo-fabric and planks, is difficult in a live stream due to water velocity. Concrete planks and median barriers were very heavy and difficult to handle and transport. Elevation of concrete planks should have been slightly lower and the upstream edge of geotextile should have been embedded lower into the streambed.
Higher water velocities over the planks tend to scour the channel along the downstream edge of the crossing, forming a small drop and exposing the face of the concrete median barriers. Use of this low-water crossing design may need to include addition of a low-profile downstream rock weir to prevent fish passage and structural issues caused by scour.
Published 03/25/08