Tributary to Big Creek
40,000 Road, Mile Post 3.337
Step-Pool Stream Simulation with Bed Retention Sills

Project Description

The site was chosen for culvert replacement due to the quality and quantity of upstream spawning and rearing habitat for resident Dolly Varden and the crossing’s proximity to Big Creek, a high valued salmonid stream. The stream is a high gradient (7%), incised channel with step pool morphology. Water cascaded over riprap at the outlet, creating a fish passage barrier.

Due to shallow bedrock and a narrow bankfull width, a squash pipe, embedded 30% of its height, was selected for the replacement structure. Steel bed retention sills sized to be ½ the embedded depth were welded inside the culvert every 11.5 ft (3.5 m). These sills buttress the downstream side of each step to prevent loss of bedform structure during high flows. The culvert was backfilled with a mixture of 60% class 4 riprap, 40% native streambed material. Material was placed in the culvert using a steel I-beam as a track for a 3-sided box, which was filled with rock and then pushed by an excavator into the culvert with a logging truck driveline.

Stream channel immediately above and below the crossing was graded to conform to adjacent stream conditions and improve hydraulic alignment. The culvert alignment was adjusted in the field to protect a large spruce tree not shown on the design plans.

The design and construction of this crossing is documented in more detail in an online presentation.

Material Sizing:
The channel slope provides a guide for the design of the step pools. With 11.5 ft (3.5 m) between steps, each step maintains a drop measured from crest to crest. Headwater at high flows was calculated assuming a Manning’s roughness coefficient (n) of 0.090 to account for bed roughness. This roughness corresponds with guidance provided by Oregon State University Forest Engineering Department for mountain stream containing similar sized material. Using the velocity versus stone weight hydraulic design chart in EM 1110-2-1601 yielded a stable rock size for the 50 year peak flow, which matched well with the rock size forming the steps in the reference reaches.

A similar project utilized an approach based on shear stress to size the stream simulation material.

Post Project Observations

The site experienced minor headcutting upstream that was stopped within 100 ft (30 m) of the culvert inlet by rock and log steps. Rock and sediment was deposited just downstream of the culvert behind a fallen log that causes backwater conditions. Initially left for habitat, this log has led to 0.5 - 1 ft (0.15 - 0.3 m) of aggradation below the outlet.

In the years following installation, the channel appears stable and anadromous adult Dolly Varden have been observed upstream of the culvert.

Lessons Learned

• Construction of the coffer dam to bypass the streamflow around the project site unexpectedly destabilized existing steps and caused minor headcutting. Consider locating the flow bypass structure away from existing streambed controlling features.
• While the culvert matches bankfull width, the engineer would have added additional roughness elements along the culvert walls to mimic natural stream banks, which provide low velocity resting areas for aquatic and terrestrial organism passage.
• This site was part of a larger project that replaced 25 culverts for aquatic organism passage over several years. The main lesson learned from these projects is to require full time inspection by qualified and experienced personnel. Most mistakes and failures were caused by a lack to attention to details during installation.
• While surveying the site, it is important to note key features such as trees or large rocks near the crossing that might affect placement of the culvert.

References
1. USACE EM 1110-2-1601, July 1991, USBR Nomograph Appendix 3 Plate 29
2. Oregon State University Forest Engineering Institute Notes, Vol. III, 1991

Published 09/18/07