Bridges and Culverts

Caledonia County Conservation District supports the inventory, design and replacement/retrofit of undersized or inadequate stream crossing structures. Working closely with the VT Agency of Natural Resources, over 500 bridges and culverts have been inventoried in Caledonia County to better understand their impacts on habitat, fish passage and stream geomorphic conditions.

What’s wrong with an undersized culvert?

PerchedCulvert_1Undersized culverts create plugged inlets and perched outlets. When a culvert is undersized, too much water and debris attempts to pass through the structure. Water and debris will back up, create pressure and/or plug the inlet of the culvert which creates erosion and washout. Also, an undersized culvert creates a powerful flow of water from the outlet of the structure, much like putting your thumb over a garden hose. This powerful flow of water creates erosion at the outlet, typically scouring a pool and eventually causing a “perched” culvert that is sitting above the stream bed. Fish are no longer able to pass through these structures, and the structures become susceptible to washout.

What’s wrong with an undersized bridge?

On a larger scale, an undersized bridge has a similar impact to rivers as culverts do to streams.  When a bridge is undersized, it creates a channel constriction where too much water, debris or ice attempts to pass through the structure.  This can cause ice and debris jams leading to a channel avulsion around the structure.  Downstream of an undersized bridge, you’ll often see a scoured channel.  Similar to the effect of putting your thumb over a garden hose, channel constrictions cause rivers to increase in power leading to pools and channel bed erosion downstream.

Culvert Replacement in Walden, VT: September 2014

The town of Walden replaced two culverts with support from the Caledonia County NRCD, US Fish & Wildllife and Vermont Agency of Transportation. Both culverts were undersized, and were not able to pash fish. The culverts were in close proximity to each other on the same stream, so both were replaced to restore fish passage and improve flood resiliency. Photo left: Culvert Replacement Mountain View Road, Walden VT