Written by: Joe Norton, project manager of TU’s Upper Connecticut Home Rivers Initiative
Additional comments by Daryl Kenny
If you’ve ever wondered about the “how” of a culvert replacement project, TU’s Joe Norton invites us along on this beautifully documented project at Wood’s Brook, on the Indian Stream watershed in New Hampshire…no heavy lifting required!
From the historic log drives of the late 1860s, right up to the 1980s, land use practices have severely impaired the Indian Stream watershed. However, there are a few bright spots. Indian Stream’s numerous small tributaries have largely recovered with pools, riffles, ample instream woody habitat and very good populations of young brook trout. These tributaries act as brook trout nursery streams that provide larger mainstem trout for both Indian Stream and the Upper Connecticut River. However, many of these small nursery streams still have culverts that are barriers to brook trout migration. Adult brook trout need access to these small coldwater streams for spawning and to escape the warm summer water temperatures in Indian Stream’s mainstem.
One of those culvert replacement projects was on Woods Brook, a small tributary in Indian Stream’s East Branch watershed. Woods Brook has abundant young native brook trout, but its only stream crossing – a 4-foot corrugated aluminum pipe – was a migration barrier at most flows. In addition, the culvert was severely undersized, and non-aligned with the stream, making it prone to washing out and causing stream sedimentation during high water events.
The project’s funders were Orvis, Basil Woods TU Chapter, Eastern Brook Trout Joint Venture, and the Upper Connecticut Mitigation and Enhancement Fund.
Below is a journal documenting the project to replace the Woods Brook culvert with a 14×40-foot bottomless arched culvert that will allow access for trout and other native species, including amphibians.
The first order of business was to remove the old 4-foot culvert. Then the ground was leveled and sloped in order to lay the base plates for the multi-plate arch. By the end of the day, the eight-piece base plate was laid in place.
Following the installation of the base plates, TU and the contractor crews began assembling the multi-plate. Several sections were left open in order to install the boulder grade controls and the final coarse streambed material.
With the culvert partially assembled, crews begin to install the three grade control weirs. These grade controls will aid in keeping the stream centered within the culvert, allow the creation of pools below the weirs, and will eliminate scouring that could cause a “head cut” up the stream. Head cuts can be caused by disturbing the streambed, which in turn can cause a cascading effect of gravel, cobble and boulders upstream. This causes the stream to destabilize above the crossing. Two of the three grade controls were installed. The third, at the outlet of the crossing, will be installed last.
After finishing the grade controls, ledge material (angular crushed granite) was wrapped in textured fabric and placed inside on the edges of the new crossing in order to stop the stream from moving inside the crossing and eroding the base plates. Following this, simulated stream bed material was added. This streambed material is mixed at the pit with the proper amount of fines, gravel, cobble and boulder based on our “pebble counts” conducted during the design phase. This gives the new streambed the closest approximation to the natural streambed mixture above and below the crossing. With the finishing of the multi-plate, the critical work is complete. Next, material was placed in stages outside the culvert in order to rebuild the road crossing, and a packer was used to strengthen the material and avoid leaving spaces that could erode.
As luck would have it, at the end of Day 5 the heavens opened up. While rain would have been unfortunate during construction, this was good news as it raised the stream’s water level and allowed the streambed material in the culvert to settle in, and the new natural streambed began the “pool carving” process below the control weirs. The road approaches were finished, the boulder headwalls on each end of the culvert were completed, and finally, native grass seed was planted and covered with straw mulch.
Now that you’re an expert on culvert replacement (don’t try this at home!), learn more about the Orvis/TU 1,000 Miles Campaign, and how you can help reconnect 1,000 miles of fishable streams.