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By Emily Clegg, The Nature Conservancy
In stewarding our natural resources, we can’t be too cautious about protecting the aquatic life in our rivers and streams. Indeed, thousands of manmade dams and stream diversions have seriously degraded and obstructed the natural habitat of our native fisheries. But that’s precisely where precast concrete can help.
A case in point is the Great Lakes Basin. This vast area is home to more than 30 million people in the United States and Canada, covers about 295,000 square miles, and contains about 354,800 miles of rivers and streams. The five Great Lakes hold one-fifth of the world’s freshwater surface, making this area one of the most valuable natural resources in North America. The Great Lakes commercial and recreational fishery alone is worth more than $4 billion per year and is home to more than 150 species of fish.
Many of these species of fish and other organisms rely upon moving freely throughout the Great Lakes and their tributaries to access habitats for spawning and rearing. Therefore, ensuring the connectivity of these systems is vital. Recent estimates indicate that more than 275,000 manmade dams and stream crossings exist in the Great Lakes Basin. That equates to a manmade structure approximately every 1.25 river miles in every river and stream flowing into the Great Lakes(1)!
Obstacles to natural flow
Several environmental agencies and organizations have identified aquatic connectivity, or lack thereof, as one of the top threats to the Great Lakes. Connectivity is fragmented or interrupted when barriers obstruct rivers and streams, altering ecological processes such as the transportation of nutrients and natural water flows, and restricting migratory aquatic organisms like fish, insects, mussels, amphibians, reptiles and some mammals, often causing population fragmentation or isolation.
There are three significant ways that manmade structures can become obstacles to the natural flow of rivers and streams:
Perched culverts. Dams and perched culverts remain the most obvious barriers to the upstream movement of organisms. At the downstream end, perched culverts often create a waterfall with a large pool that forms a physical barrier to organisms. Upstream, perched culverts often increase water levels and temperatures by confining the water flow, making a wider channel and slowing the flow, which deposits sediments.
Undersized structures. Other less common characteristics of barriers include undersized structures, which cause an increase in the velocity of the river or stream that is too swift for some animals. When structures are too long, animals may feel deterred to enter them. If there is a debris blockage, too much sediment, or not enough water in the culvert during low-flow times, organisms cannot pass through at all.
Other characteristics of manmade barriers. Several key characteristics influence the passage of fish, insects and other animals, and require consideration when designing structures. These include water temperature, swiftness or velocity, turbulence, structure sizing, slope, angle to the road, length and localized amounts of light, and debris and sediment.
Three-sided precast concrete culverts are rapidly becoming part of the solution to this looming issue in the Great Lakes.
Precast concrete culverts ensure natural connectivity
Many types of structures can address connectivity issues, but one of the easiest and most effective ways to ensure connectivity is to use three-sided precast concrete box culverts or arches. They allow water and debris to pass freely, saving on maintenance costs, and they are less likely to fail or wash out, substantially extending service life.
Many state, provincial and federal infrastructure agencies and local organizations that have specified three-sided precast concrete box culverts or arches to achieve connectivity goals, such as fish passage, have also started seeing other benefits during extreme weather events like Hurricane Irene (August 2011) and Hurricane Sandy (October 2012).
For example, during Hurricane Irene, Green Mountain National Forest received up to 12 in. of rain in less than a day(2). The Forest Service found that recently replaced stream crossings with a natural bottom designed and built as wide or wider than the river’s bank incurred virtually no damage to the structure. In comparison, crossings with undersized, poorly aligned culverts experienced severe damage(3).
Lack of connectivity is one of the greatest identified threats to the Great Lakes, yet may also be the easiest to fix. Rather than replacing stream crossings with the lowest cost option or the same type of structure that previously existed, many government agencies are now considering connectivity and aquatic organism passage when designing new structures.
Natural-bottomed precast concrete structures have proven to be an extremely important and effective tool in the restoration of aquatic connectivity in the Great Lakes Basin by allowing for natural flow regimes, transporting nutrients and restoring access to key aquatic animal habitats.
Repairing a ford with precast concrete landscaping blocks for easy movement of a portable bridge in the Two Hearted River Watershed, Mich.
Example A – Before
Example A – After
1. A ford located in the Lake Superior Basin across the West Branch of the Two Hearted River (a renowned trout stream made famous by Ernest Hemingway) near Newberry, Mich. The ford was a barrier to fish and other aquatic organisms during low flow periods, confining water upstream and creating steep rapids downstream with a large sediment plume from too much sediment running down the road into the river.
2. In 2010, with funding from the Great Lakes Restoration Initiative, The Nature Conservancy removed the ford across the West Branch of the Two Hearted River down to the original stream channel, stabilized the banks with field stone, and used 2-ft by 2-ft by 6-ft precast concrete landscape blocks to create staging pads for the later use of a portable bridge. The bridge can be moved between two other similar sites in the Conservancy’s Two Hearted River Forest Reserve to accommodate timber harvesting. The use of portable bridges in forest operations has resulted in 98% less sediment entering the stream than if a culvert had been installed.4
3. A portable bridge used for timber harvesting operations over Chapel Creek near Munising, Mich. (Lake Superior Basin). Portable bridges reduce impact to streambed disturbance, accommodate large water volumes, reduce the amount of sediment into the stream and allow for full aquatic organism passage.
Repairing an undersized and damaged four-sided box culvert with a three-sided precast concrete box culvert in the Pine River Watershed, Wis.
Example B – Before
Example B – After
4. A stream crossing of Forest Road 2168 over the Long Lake outlet, a tributary to the Pine River, in the Chequamegon-Nicolet National Forest near Florence, Wis., was a four-sided concrete box culvert. The crumbling culvert proved too narrow for the outlet, so as it came time for replacement, the Forest Service designed a new wider structure with a natural bottom.
5. The Forest Service replaced the Long Lake outlet and Forest Road 2168 crossing in 2011 with a 20-ft span and 4-ft rise, open-bottom precast concrete box with wing and headwalls. This wider span allows for a natural flow regime out of the lake.
6. The Long Lake outlet three-sided concrete box culvert has a natural bottom with designed rock veins to reduce stream swiftness, create more natural pool and riffle formations, enhance in-stream habitat for aquatic organisms, and prevent erosion. During low and normal flow, dry banks under the structure allow for small terrestrial animal passage as well as aquatic passage.
Replacing three undersized perched culverts with a precast concrete arch with a natural bottom in the Popple River Watershed, Wis.
Example C – Before
Example C – After
7. The Little Popple River and Popple Road crossing located in the Chequamegon-Nicolet National Forest near Florence, Wis., consisted of four 48-in.-diameter, undersized, perched culvert pipes.
8. The Forest Service removed the four culvert pipes in 2010 and replaced them with an attractive precast concrete arch having a 24-ft span, 7-ft rise and 10-ft wing walls. A natural bottom was designed with streambed material and shaped to mimic a natural stream channel and banks, with rock veins at the inlet and outlet to create pool and riffle formations.
Replacing a perched and undersized culvert that was a complete barrier with a three-sided precast concrete box culvert in the Pine River Watershed, Mich.
Example D – Before
Example D – After
9. The Silver Creek and State Road crossing located near Cadillac, Mich., consisted of a perched 6-ft by 8-ft elliptical culvert. This inadequate crossing blocked all fish passage from the Pine River into Silver Creek, a cold-water tributary and nursery stream for rainbow, brown and brook trout.
10. The undersized, perched culvert was replaced in 2009 with a 12-ft by 10-ft, three-sided precast concrete structure.
11. A new three-sided precast concrete structure also allowed for approximately 10 ft of elevation change in 400 ft within the channel to accommodate stream stability and aquatic organism passage.
Replacing two unsightly perched-barrier culverts with a precast concrete box culvert in the Tawas River Watershed, Mich.
Example E – Before
Example E – After
12. The Silver Creek and Monument Road crossing near Tawas City, Mich., with two 48-in.-diameter by 53-ft-long culverts perched at the outlet, was a barrier to aquatic organism passage and caused a large scour pool just downstream of the road/stream crossing.
13. As a joint project between the Huron – Manistee National Forest and Iosco County Road Commission, the two perched culverts were removed and replaced with a precast natural-bottom culvert that allows for aquatic organism passage.
Emily Clegg has spent the past 10 years studying and working in freshwater environments in the Great Lakes and Columbia River Basins. She is currently the manager of the Two Hearted River Connectivity Project and a member of Great Lakes Project’s Watershed Connectivity Strategy Team. Contact her at firstname.lastname@example.org.