Project Summary

The project site was a road-stream crossing on a small tributary immediately upstream of its confluence with Ti Creek. Ti Creek supports steelhead trout and the tributary potentially contains good rearing and spawning habitat.  The pre-existing CMP was undersized with a 3.4% slope and a 5 ft (1.5 m) drop at the outlet into a shallow pool.  The culvert was deteriorated and also considered a barrier to all life stages of steelhead trout.  Due to its poor condition, it was scheduled for replacement with a new crossing that provides fish passage.

The new culvert is a 12 ft (3.7 m) diameter by 70 ft (21.3 m) long corrugated steel pipe, embedded 3 ft (0.9 m) (25% of diameter) below the predicted stable channel grade.  The stable grade was based on the channel elevation of the tributary at its confluence with Ti Creek.  The new culvert bottom was placed 8 ft (2.4 m) lower than the bottom of the old culvert.  Several boulders were placed along the bottom and sides of the culvert to help anchor the bed material and prevent it from washing out. The culvert was then backfilled to a depth of 3 ft (0.9 m) with 1-inch (2.5 cm) minus clean gravel containing no fines.

Since the new streambed in the culvert was roughly 5 ft (1.5 m) lower than the upstream channel, the project involved regrading a 70 ft (21 m) section of channel above the culvert inlet at an oversteepened slope. Three boulder weirs were constructed for grade control using ½ ton to 1 ton rock.  Each weir was spaced roughly 20 ft (6.0 m) apart and constructed with two rows of boulders that spanned 20 ft (6.0 m) across the channel.

Challenges

Lowering the culvert created a 5 ft (1.5 m) difference in grade between the inlet and the upstream channel.  Approximately 100 ft (30 m) upstream the valley narrows, making heavy equipment access difficult and limiting the length of channel that could be regraded during construction. 

The crossing is 30 ft (9.1 m) from the confluence with Ti Creek and backwater effects on culvert hydraulics, capacity, and sediment transport were unknown.

Post Project Observations and Lessons Learned

The site experienced several flow events exceeding a 2-year storm during the first winter after construction. During these storms the boulder weirs failed and the upstream channel incised.  After one winter the incision headcut roughly 250 ft (76 m) upstream, exposing larger streambed material characteristic of the bed material found in the upstream channel.  The overall bed elevation inside the culvert remained as constructed and the large boulders placed in the culvert remain stable.  However, the washed gravel placed in the culvert appears to have been replaced by bed material transported from upstream.

Given the drop height and spacing between weirs, the rocks used for the boulder weirs were insufficient in size.  Spacing and drop heights were governed by length of accessible channel.  A stability analysis may have indicated use of a larger rock size to maintain the grade. 

In retrospect, it may have been preferable to allow the channel to naturally regrade rather than attempt to maintain the existing grade with boulder weirs.  There are no critical resources upstream to protect and the resulting incision helped expose a more stable channel bed.

Published 03/19/07