The Role of Culverts in Our Watersheds
Written by Kevin Brewster, Restoration Manager
Most of the time we don’t think about road stream crossing culverts unless they are washed out from a flood and we are forced to take a detour. But how do culverts affect the day-to-day dynamics and health of our watersheds? In many important ways!
Hydraulics: How Water Moves and Affects the Landscape
The entire area upstream of a culvert, and all of the water that drains from that area and ultimately flows through the culvert is called a catchment. Catchments can range in size from a few hundred square feet (or whatever area is
enough to induce a flow of water over its surface) to many square miles. Very large catchments are more familiarly referred to as watersheds, and encompass all of the land draining to a specific stream or river. The final culvert and the point where all of the catchment’s combined waters leave the catchment is known as the pour point.
As you can see, that often unnoticed place on a highway where a stream passes under the road can have a very large impact on the upstream landscape, especially if it is the pour point of the catchment. If it becomes blocked or is washed out in a flood, the upstream and downstream effects can be widespread and sometimes devastating. Because there may be many culverts upstream of a pour point culvert, flood events can set off a cascade of culvert failures as water builds behind blocked or too small culverts until the road washes out , releasing a pulse of floodwater downstream to the next culvert and so on. This was an all too familiar phenomenon during the disastrous 2016 flood event, when over 100 culverts failed in Ashland and Bayfield Counties, contributing to the over $30M property damage cost of the event.
There are also day-to-day impacts that culverts have on our watersheds that are far less visible and dramatic than flood events, but still very important in their long-term effect on the wellbeing of our streams and ultimately Lake Superior. Our region has been prone to soil erosion since widespread deforestation took place in the early 20th Century, followed by its conversion to various types of agricultural landscapes. Water could now move much more rapidly off the land over the newly cleared areas and through agricultural and roadway drainage systems. Much of the Lake Superior shore region lies on what’s known as a clay plain formed from an ancient lake bed. Overlying the clay are extensive, highly erodible sand deposits left behind after the glaciers thawed. The combination of man-made change and local geological factors resulted in one of our biggest local environmental challenges: collapsing river banks, fallen trees across streams, rivers that run dark with sediment, and the chocolate-brown waters of Chequamegon Bay seen after heavy rains or on windy days. The Bad River watershed alone contributes over 385,000 tons of sediment to the Lake annually.
Because water can flow so rapidly through area streams and rivers, many culverts are too small to handle large flow surges. This causes two things to happen which can worsen erosion. First, the high velocity of the water passing through a too-restrictive culvert results in scouring of the stream bottom, as if a giant high-pressure fire hose were turned on. This in turn stirs up huge amounts of sediment that pass downstream. If the stream banks are largely made up of sand, as mentioned above, they too are cut away by the force of the water, collapsing into the channel and adding more sediment. Rapid flows also encourage cutting of the stream channel upstream of the culvert, further adding sediment to the stream. Another impact of high flows passing through small culverts is erosion of road bed material as water builds up behind the culvert and eventually flows over the top of the road. If the water is backed up high enough, it can force its way through the road fill and find a flow path along the sides of the culvert pipe, leading to the washing out of the entire culvert. In this way road fill material is added to the sediment load that ends up in Lake Superior during heavy rain events or spring thaw.
Impacts on Fish and Other Aquatic Life
Undersized culverts force water to travel at higher speeds, creating strong hydraulic forces that act on the stream channel bottom and banks. Like the high pressure water hose analogy above, the fast moving water digs into the channel directly below the culvert, creating what is known as a scour pool or plunge pool. Over time, this lowers the stream surface below the culvert, creating a gap between the end of the culvert and the water’s surface. Once this gap exceeds six inches, many fish, especially younger individuals, have difficulty jumping into the culvert to swim upstream. When the gap exceeds 12”, adult trout can have difficulty jumping into culverts. This type of physical barrier, known as a “perched” culvert, can cut off access to extensive river-miles of vital upstream spawning habitat for brook trout and many other species of fish. Water passing too rapidly through a culvert can also create a passage barrier even if the end of the pipe is partially submerged. This is known as a velocity barrier. Engineering studies have shown that adult trout, for example, struggle to swim upstream inside culverts when average water speed exceeds two to six feet per second, depending on culvert length. The longer a culvert is, the lower the tolerable water velocity becomes. For example, a trout-friendly sized 300’ culvert would pass water no faster than two feet per second. Juvenile fish, being less strong swimmers than adults, can be negatively affected by even lower water velocities.
A practice known as Stream Simulation further creates conditions that encourage movement of aquatic organisms by providing more natural stream conditions. This practice employs detailed survey techniques that assess a nearby stretch of stream to inform channel dimensions, placement of rocks, gravel, and other stream features that mimic the natural attributes of the stream. These projects, while often quite expensive, provide a stream crossing that provides uninterrupted natural stream habitat, and is much more resilient to repeated expensive repair of high flow and flooding damage.
Unfortunately, changing weather patterns in our region are not working to reduce problems of rapid runoff and sedimentation. Culvert washouts have been an all too common occurrence here going back decades, but recent local catastrophic flooding in 2012, 2016, and 2018 has demonstrated a troubling trend of heavy localized rainfall events. A number of widely-accepted predictive climate models have forecast continued or increasing slow moving, heavy rainfall storm systems affecting the Chequamegon Bay region. Responding to these challenges can be complicated by cost and site restrictions. For example, bedrock stream beds in some areas make it extremely difficult to install culverts large enough to handle 500 year or larger flood flows. In this case, the most economical solution is to design stream crossings that are able to withstand flooding (known as overtopping) with minimal damage to the roadbed and culvert. This can be accomplished by contouring the roadbed embankment slope to actually encourage overtopping to prevent buildup of water pressure that eventually causes washouts. The area around the culvert is heavily armored with rock, which resists the erosive action of floodwater flow, and also slows the flow with its many rough surfaces. In SRWA’s 2018 study of culvert survival after the disastrous 2016 flood event, a number of factors emerged as contributing causes to culvert washouts. One of the most important of these was road surface height above streams. Roads that were over seven feet above streams were much more likely to fail do to the larger mass of accumulating water pushing against the roadbed. A design solution for this is similar to that described above: Increasing roadway slope in and out of stream valleys to minimize road height and encourage controlled floodwater overtopping over rock armored culverts. The landscape of some locations simply won’t allow this strategy though, forcing designers to install very large culverts and/or “overflow” culverts set higher in the roadbed that start passing excess water before it reaches a level and pressure that damages the road. This approach has been used locally in a few locations, although none were designed for what are now increasingly frequent 100 year and larger flood events.
Another climate change prediction–warming stream temperatures–is also driving culvert restoration strategies. The US Geological Survey (USGS) and US Fish and Wildlife Service (USFWS) recently collaborated on development of a predictive computer model known as FishVis, which projects effects of climate change on surface water temperatures. Using stream temperature monitoring data from USFWS and SRWA, the model indicates that a worst-case warming scenario will render many of the lower reaches of area cold water streams too warm to support brook trout and other species in less than 50 years. This means that cold water fish species will only survive in our region if they have full access to cold, spring-fed headwaters where they can persist for years to come. With this as a strategic goal, USFWS fisheries biologists developed a list of streams with brook trout populations most likely to survive over decades, known as self-sustaining stream segments. The agency has prioritized grant monies from its National Fish Passage program for removal of barrier culverts that currently interrupt the connection between Lake Superior and the headwaters of these critical streams. Almost all of SRWA’s culvert restoration projects in our local watersheds have been funded by this and similar grant programs. This strategy preserves cold water fishery resources for an extended period, and buys time for larger scale, more permanent solutions to climate change to hopefully happen.
Far from being just a pipe that gets water from one side of a road to another, a culvert can have far reaching, multifaceted impacts on both a stream’s hydraulic behavior and its ecology. Historically, consideration of cost, impact on transportation, flooding, and commercially valued fisheries have been the practical drivers of culvert design and installation. Now the emerging impacts of climate change demand a more comprehensive, ecosystem-based perspective in the funding, design, and installation of fish-friendly and flood resilient culverts.