Project Summary

An abandoned concrete flashboard dam constricted streamflow through a 4 ft (1.2 m) wide notch, which was less than half the width of the upstream active channel. The upstream channel was visibly aggraded due to the constriction. The constriction created a velocity barrier at high flows for adult steelhead and resident rainbow trout. A 2 ft (0.6 m) drop across the foundation of the dam created a low-flow barrier for juvenile salmonids.

Part 1
The concrete sidewalls between the opening of the dam were removed to eliminate the channel constriction. The dam’s foundation was left in-place to maintain the upstream grade. However stored sediment in the upstream channel mobilized, causing unexpected down-cutting of the channel bed. The down-cutting extended upstream for 200 ft (31 m) until reaching the outlet of a concrete box culvert, causing it to become perched above the downstream channel by 3 ft (0.9 m). The previously at-grade culvert was constructed after the flashboard dam and subsequent channel aggradation. Prior to this project, offset baffles had been added to the culvert to increase water depths.

Part 2
Three boulder weirs were constructed to improve fish passage by eliminating the perched culvert outlet. Weirs were built below the outlet spaced 20 ft (6 m) apart using 1-ton rock positioned to create 1 ft (0.3 m) drops. The following winter, the upstream weir collapsed and the second weir was flanked, creating an 18 ft (5.5 m) long by 3 ft (0.9 m) deep scour hole under the adjacent access road. The concrete road surface began to sink, preventing vehicle traffic from using the roadway.

Part 3
The final modification was to excavate below channel grade and build a concrete wall along the road edge to prevent flanking and undercutting of the roadbed. The 3 boulder weirs were rebuilt and tied into the form wall by embedding them into the concrete. To fill the scour hole under the road, the contractor core drilled through the roadbed every few feet and pumped concrete into the void. A concrete notched weir was added to the culvert outlet to improve leaping conditions and increase water depth on the outlet apron, downstream of the first baffle.

Post Project Observations and Lessons Learned

The initial oversight was failing to fully anticipate channel grade adjustments caused by removal of the flashboard dam. Keeping the dam’s concrete foundation in-place was expected to maintain the upstream channel grade. However this did not address the impact of the constriction on upstream sedimentation and channel grade. Surveying a longitudinal channel profile combined with field observation may have indicated the extent of down-cutting to expect and grade control could have been included in the original project.

Due to the proximity of the access road, the first set of constructed boulder weirs were not sufficiently tied into the bank, resulting in flanking of the boulders and undermining of the roadway. Embedding the end of the boulder weir into a lateral concrete wall worked well to avoid further flanking and scour. Post project observations indicate the weirs appear to be stable and functioning as designed.

The pre-existing baffles did not include a downstream weir, leading to a draw-down in the water surface as it approached the plunge at the end of the apron. This increased water velocities, decreased water depth, and created undesirable conditions for leaping. For perched culverts with baffles it is usually necessary to add a weir across the outlet to avoid these conditions.

Published 05/01/07