Ensuring Culverts Help Not Hinder Michigan's Streams
April 12, 2012
Providing for one of modern society's most basic needs,roads, can make it difficult for the Department of Natural Resources to fulfill one of its key responsibilities: protecting Michigan's natural streams from degradation.
Whenever a road crosses a stream, interference with the stream course is a potential side effect. Altering the course or flow of a stream can cause disruption in fish passage, siltation and other undesirable consequences. The trick is crossing the stream in a way that allows the stream to continue to exist in its natural state. If it were possible to build a suspension bridge at every stream crossing, that would be relatively simple to accomplish.
Truth is, funding for roads is not unlimited. Compromises must be made. Often, the simplest, least expensive way to cross a stream involves placing a culvert (a covered channel that allows water to pass beneath a road, railway or embankment) in the stream and building the road over it. However, unless they're properly sized and installed, culverts can bring their own baggage. Fortunately, new Department of Environmental Quality permit categories for stream crossings, for which the DNR's Fisheries Division lobbied heavily, should, in the years ahead, minimize the impact of culverts on streams.
According to the DNR, the main issue with culverts is that they have, historically, been designed only for hydraulic purposes, to move water, but concentrating only on hydraulics ignores other potential outcomes. In recent years, DNR officials have found that culverts not only affect the streams themselves, but can also have negative effects on fish and wildlife populations, too.
"We began looking at more stream-friendly designs, looking not only at hydraulics, but at environmental concerns, impacts on fish and wildlife, and public safety concerns," explained Chris Freiburger, a DNR fisheries biologist who works on stream protection and restoration. "What we're trying to design are culverts that simulate how the stream should function naturally. That not only benefits the stream, but also fish and wildlife, public safety and long term maintenance and replacement costs.
"If we can meet the needs of the stream, we're not only able to pass water, but we can pass sediment and provide for fish and wildlife, too."
Undersized culverts, anything less than bank-full width (the maximum width a stream attains, can cause flooded roads and impoundment of water upstream. The stream may get wider and collect organic material above the crossing, resulting in higher water temperatures.
These improperly sized culverts cause problems with fish passage in two ways:
- During high flows, fish cannot overcome the high water velocities.
- During low flows, there may not be sufficient water depth for the fish to go through the culverts.
If that erosion continues, Freiburger said it can lead to a perched culvert.
Perched culverts, culverts that are set at a higher elevation than the stream's water level, are an obvious problem. When confronted with a perched culvert, fish attempting to migrate upstream simply hit a dead end. There is nowhere for them to go.
"We usually think of fish movement occurring during spring and fall migrations," Freiburger said. "But one of the things we've learned over the years is that fish are moving daily, as well as seasonally, to meet life history requirements,for feeding, juvenile migrations or water temperature reasons, not just for spawning."
Oversized culverts cause other problems. Because culverts block out the sun, there's no terrestrial vegetation on the banks to prevent erosion or the banks are removed during construction. The stream channel will eventually widen to the width of the culvert. That reduces the velocity of the water moving through there, which affects the stream's ability to move sediment.
"During high flows, it passes fish," Freiburger said. "But during low flow periods, the water is spread out across a wider channel and often it's too shallow for fish to pass. Fish are unable to find cooler water upstream during low-flow periods.
"We used to think that if you were using a culvert, use as big a culvert as you can," he explained. "We've found out that isn't so."
Oversized culverts or crossings can be improved by building a flood plain within them, to prevent the stream from widening. This can be accomplished by setting rocks along the sides, allowing sediment to fill and create a bank, allowing for animal passage.
In many places, stream crossings involve multiple culverts, usually installed because the original culvert is inadequate to handle high flows.
"What happens with multiple culverts is, while one culvert remains open, the other culverts tend to fill up with debris or sediment," Freiburger explained. "So when you need the capacity during high flows, they are plugged up and unable to move water."
Last fall, after years of reviewing existing culverts, state officials concluded that a majority of them were affecting the streams and fish passage. The DEQ, in partnership with the regulated community, developed and approved new permit categories for culverts.
Now, culverts must be bank full width and buried to allow for natural stream bottom within the culvert and they must be set on the same slope as the stream bed. Engineers must measure the stream at upstream and downstream reaches to determine what the natural bank full width of the stream is in order to make sure they use the properly sized culvert.
An evolving issue with stream crossings is amphibian and reptile passage, Freiburger said. Most species migrate along the bank, not in the water. Some states mandate flood plains be constructed through stream crossings to allow for amphibian, reptile and mammal passage.
"We're not there yet," Freiburger said. "But others states, primarily in the Northeast and West, are already mandating them."
Many Michigan agencies are working together and with federal partners to better stabilize and protect the state's natural streams. Learn more about how they're sharing resources and developing common methods for collecting stream data at www.michigan.gov/streamteam.