Culvert Replacements
A Well Designed Crossing from Vermont Stream Crossing Handbook, Vermont Fish & Wildlife
Why replace culverts?
Replacing culverts has many benefits for people and the ecosystem!
Many culverts in the state are too small for the stream they are in- they were made to pass through a certain amount of water, but not to account for the complex hydrology and ecosystem. Many culverts are also just old and not functioning well, especially in the big storms we are getting more and more often.
- Reduces flood risk: Appropriately-sized culverts allow more water and debris to pass through, reducing the risk of streams overtopping and flooding roads. Additionally, when water passes through a too-small culvert it is constricted (like the traffic jams when two lanes suddenly become one). This suddenly slows the water upstream, which can cause scourpools and bank cuts above the culvert. On the other side, the water is faster and stronger as it is released at the outlet (like how when you put a thumb over a garden hose- the water comes out of the smaller opening with more pressure and speed), causing greater erosion and damage downstream too. This means more land and infrastructure getting washed out in large rain events and storms.
- Improves water quality: All of that erosion means sediment entering our waterways, reducing water quality in the stream and in the rivers and lakes that it feeds. Reduced flooding also reduces the pollutants from roadways and infrastructure, such as road salt, fertilizers and pesticides, and household chemicals being carried into waterways.
- Improves aquatic organism passage: Undersized culverts prevent the passage of many aquatic organisms including important fish such as trout, and invertebrates essential for healthy functioning ecosystems. Aquatic organism passage is usually impeded by a combination of factors. The increased velocity in an undersized culvert causes scouring downstream that over time can result in the culvert becoming "perched", or raised above the water's surface, forcing organisms to jump to reach the culvert- which can be done by some organisms at some life stages, but not all. The high velocity of water and lack of resting areas within the culvert also makes passage difficult or impossible for many organisms. Good culverts for aquatic organism passage have open bottoms or riffles to capture sediment and recreate the natural streambed.
The culvert below is undersized and perched. Aquatic organisms would be unable to pass through this culvert and the constriction of waterflow would increase the flooding risk and increase velocity, causing more erosion of the streambed and banks both upstream and downstream. The culvert upstream is bank full-width and incorporates the natural streambed and slope, making is ideal for aquatic organism passage and reducing flood and erosion hazards.
For more information, check out Vermont Fish and Wildlife's Stream Crossing Handbook:
AOP HANDBOOK.pdf