Case Study Contributor
Mike Derrig, Hydrologist, Lassen National Forest
Location
Antelope Creek Watershed, Lassen National Forest, Northern California, USA. MAP
Project Type
Pre-Project Conditions
Pre-Project Barrier
Watershed Characteristics
Ecological Value
Project Characteristics
Challenges
Project Contributors
Project Funding
Completion Date
October 2005
Total Project Cost
| Engineering/Design | $ | 4,500 |
| Materials | $ | 15,050 |
| Planning/Permitting | $ | 4,500 |
| Construction | $ | 159,370 |
| Total | $ | 183,420 |
Project Description
The pre-existing 6 ft (1.8 m) diameter CMP was undersized and prone to overtopping. The pipe restricted sediment transport, resulting in aggradation in the upstream channel. The 7% culvert slope and drop at the outlet were barriers to all age class of rainbow trout and other native aquatic organisms.
A multi-plate bottomless arch culvert with concrete footings and inlet headwall was selected as the replacement. The culvert was sized wider than the active channel width and provided sufficient capacity to convey the 100-year flow and associated debris with a headwater depth less then 80% of the culvert height. Located at a bend in the river, the inlet headwall was added to improve inlet hydraulics and minimize flow contraction, which can cause scour of the streambed inside the culvert.
A mix of cobble to 1-ton boulders was used to form a concave channel in the culvert with the expectation that high flows will adjust the channel and form step-pools. The larger boulders were placed along the footings to prevent scour and form a low-flow stream bank. The bed material was placed by an excavator after construction of the footings and before the installation of the culvert. Streambed material was sized to match the downstream bed material. Slope of the downstream channel, characterized as a stable boulder step-pool morphology, was used to determine the 7% design grade for the crossing. The scour pool at the outlet was filled to form a continuous gradient and bed material through the project reach.
During excavation bedrock was found 2 ft (0.6 m) above the design depth of the footings. The contractor was able to excavate through the bedrock for one of the footings. On the other side, the bedrock was too hard for excavation and the footing was cast on the bedrock above the design depth.
A small amount of gravel and fines had aggraded immediately upstream of the old culvert. Given the stable nature of the downstream channel and the small quantity of aggraded material, it was left in place and allowed to flush downstream. Entering the upstream channel to excavate the stored material would have further disturbed the channel bed and banks. During the first winter, the material moved through the project site and may have helped to seal the new bed in the culvert.
Post-Project Observations and Lessons Learned
Visual post-project inspection following large flow events (5 to 10-year return period) found the boulders in the culvert had rearranged, forming the desired step-pool geometry. In conclusion, the design and construction techniques proved adequate to meet the project objectives.
Published 06/08/07