Tributary to Big Creek
40,000 Road, Mile Post 2.49
Step-Pool Stream Simulation with Bed Retention Sills

Project Description

The site was chosen for culvert replacement due to its proximity to Big Creek and the high quality spawning and rearing habitat utilized by both resident and anadromous Dolly Varden.  The stream is high gradient, with an incised channel and step-pool morphology.  Stream gradient is controlled by steps formed by bedrock, angular cobbles, and woody debris. 

The replacement culvert is a 6.5 ft (2 m) diameter CMP at 4% slope and embedded 40% of its diameter.  The culvert was backfilled with a streambed mixture composed of 60% USFS Class 4 rock mixture and 40% native streambed material.  The Class 4 rock gradation is as follows:

% of rock
by weight                      Cubic dimensions
   20                            18 - 20 in (450 - 500 mm)
   30                            14 - 18 in (350 - 450 mm)
   40                              6 - 14 in (150 - 350 mm)
   10                                0 - 6 in (0 - 150 mm)

Because the material that forms steps in steep streams in infrequently mobile, the larger rock in the mixture was sized to be stable at the 50 year flow. The stability analysis was based on several equations that predict mobility as a function of rock size and average boundary shear stress.

When constructing steep streambeds it is important to include stable grade controlling features.  The bed in the culvert was shaped to create a step-pool morphology with an average slope of 4%, similar to the adjacent channel. Steel bed retention sills 1.3 ft (0.4 m) tall were welded into the culvert every 10 ft (3 m) to buttress the steps and maintain bedform during high flows. Rocks larger than the D84 were used to construct rock steps, and placed along the culvert walls to create channel margins.

The bed material was placed in the culvert using a 3 sided box pushed along a steel I-beam by an excavator with a driveshaft from a log truck.  This provided a 20 ft (6 m) reach into the culvert, requiring filling from both ends.  About 33 ft (10 m) of channel below the crossing was regraded and two rock weirs were constructed for grade control.

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

Post Project Observations

In the years following installation, anadromous adult Dolly Varden were observed upstream of the culvert, which was a complete barrier prior to the project.

Lessons Learned

• The site experienced minor headcutting upstream, which was stopped within 100 ft (30 m) of the culvert inlet by stable rock and log steps. Headcutting was attributed to the project’s coffer dam used to dewater the site. Its placement destabilized a grade controlling bedform upstream of the inlet. Care should be given when siting the coffer dam to avoid disturbing existing grade controlling channel features.
• When sizing the culvert to the active or bankfull channel width it is important to include sufficient roughness elements along the culvert walls to mimic natural streambank roughness and provide low velocity areas for aquatic organisms. In this project the designer would have preferred larger rock was used along the margins to create distinct banks
• The bed retention sills were added as a safety factor. Currently, for slopes less than 6%,  the designer uses carefully placed large rock instead of steel sills to form bed features. The rock is sized using a stability analysis.
• Dumping material along the culvert wall scratched the galvanized coating, causing the metal to corrode more rapidly. The designer recommends carefully placing large material against the walls on a bedding of sand instead of dumping.
• This project was part of a larger effort that replaced 25 culverts for aquatic organism passage. The main lesson learned from these projects is that they require full time construction inspection by qualified and experienced personnel. Most of the mistakes and failures were caused by a lack to attention to details during installation.
  

Published 09/17/07