Progress / Science

Aquatic Organism Passage through Bridges and Culverts

A goal of the Vermont Rivers Program is to properly size bridges and culverts to pass a range of higher flood flows and the sediment, tree debris, infrastructure debris, and winter’s ice that otherwise might plug the road crossing structure and cause a road washout and disrupt travel or cause property damage to a neighbor. A related goal is to provide aquatic organisms the ability to pass through bridges and culverts in normal low flows, and is required for design and permitting of road stream crossings by Vermont state laws. Aquatic Organism Passage (AOP) includes fish, aquatic reptiles and amphibians, and the insects that live in the stream bed and are the food source for fish and has a profound influence on the movement, distribution and abundance of populations of aquatic species in rivers and streams. By state law, impacts to both fish and wildlife are considerations in Stream Alteration Permits as other wildlife (deer, bear, raccoons, squirrels, ducks, turtles, etc.) may also pass through road stream crossing structures rather than climb the road embankment into traffic.

Our goal is to provide a channel inside the road crossing structure that simulates characteristics of the adjacent natural channel upstream and downstream that will present no more of a challenge to movement of organisms than the natural channel. The design team can look upstream and downstream for a naturally stable stream section (reach) that can be referenced (a reference reach) for simulating the natural conditions inside the bridge or culvert. A site review will identify the natural channel characteristics such as dimension, channel pattern and slope, and the amount, size and mobility of stream bed boulders, cobbles, gravels and sediments. All of this information is readily gathered and necessary to design a natural channel bed (gravel substrate) through the road crossing structure and achieve AOP during construction.

These pictures of the 2013 replacement culvert on Route 9 in Searsburg show a good example of an AOP box culvert. The natural channel appearance and large boulders reduce flow velocity that scours and erodes downstream channels and stream banks of neighboring landowners, as well as provide easy passage for both aquatic and terrestrial organisms. This is a win for our need to maintain Route 9 traffic, a win for the downstream neighbors in reduced erosion on their land, and a win for the environment in connecting aquatic and terrestrial wildlife populations that can now move under the road in search of food and refuge in the wilds of Searsburg – and a win for those folks who drive to Vermont to hunt and fish and hike and annually contribute to our state economy.

The state encourages early contact with a River Management Engineer (RME) to meet on-site to discuss appropriate measures to achieve AOP before a stream crossing design is finalized, so that AOP barriers are not created by design and the permit process is not delayed by design plan revisions. The RME contact information can be found on the Vermont Watershed Management Division website at http://www.watershedmanagement.vt.gov/rivers/docs/rv_contact.pdf for anyone wishing to call to schedule a site visit.

Old undersized culvert outlet – caused increased water velocity and scoured channel bed.

Old undersized culvert outlet – caused increased water velocity and scoured channel bed.

Old culvert interior – surface too smooth to slow water velocity and too shallow for AOP

Old culvert interior – surface too smooth to slow water velocity and too shallow for AOP

 

 

 

 

 

 

 

 

 

New culvert interior showing bed roughness - slows velocity and good fish habitat!

New culvert interior showing bed roughness – slows velocity and good fish habitat!