Project Summary

The width of the existing culvert is 64% of the stream’s bankfull width. The culvert is in good condition and the future of this road segment is uncertain. Since the road may be closed later, replacement was not considered an option. The upstream channel was in good shape with pool-riffle morphology at a 2% slope. Below the culvert outlet the channel was scoured to bedrock to the confluence with Canal Creek, 140 ft (43 m). There was a drop of 1 ft (0.3 m) at the culvert outlet, which was believed to be a partial barrier to juvenile salmonids.

The culvert was retrofit in 2003. The streambed was raised over a 100 ft (30 m) reach downstream of the culvert outlet using 20 randomly distributed boulders and 3-6 in (8-15 cm) pit-run angular basalt rock. The boulders were put in the channel to create roughness and hold the basalt rock in place. The result was a streambed that looked like a lag deposit from a debris torrent. No material was added to the streambed inside the culvert.

Challenges and Lessons Learned

This project demonstrates that for culverts with only minor downstream channel degradation, it may be possible to raise the streambed and add needed roughness using randomly placed imported material. This approach is particularly applicable to low gradient channels and culverts. The angular 2-3 inch (5-8 cm) basalt fragments may have locked together better than stream gravels, helping to hold the constructed streambed in place. The channel roughness added by the boulders is aiding in natural streambed substrate deposition.

There was uncertainty whether the raised downstream channel will remain stable given that the existing culvert constricts the bankfull channel width, causing higher outlet velocities and scouring forces. The physical limitations in applying this approach are unknown.

Post Project Observations

After one winter (2003-2004), the streambed downstream of the culvert appeared stable, maintaining its slope of 1.2%. Additional sediment was being deposited downstream of the boulders. No erosion from winter flows was noticed and there was no change in the streambed upstream from the culvert. Within the culvert the streambed aggraded 0.5 ft (0.15 m) with sands and gravels.

In January 2006, storm flows in the Oregon Coast Range reached the 5 to 7 year recurrence interval. An additional 0.75 ft (0.23 m) of sediment was deposited inside the culvert for a total depth of 1.25 ft (0.38 m) at the culvert outlet. The sediment layer in the culvert now covers the entire culvert floor. Downstream of the culvert the channel bed has remained unchanged since 2004, and no changes were observed in the streambed upstream. In January 2007, chinook salmon were observed spawning in the raised streambed.

Published 09/26/07