Channel Armoring
Protecting the waterways
No single erosion control product could prevent Minnesota’s Elm Creek from seeping over its eroding banks and onto a busy township road. So officials with the state office of the US Department of Agriculture’s Natural Resources Conservation Service (NRCS) turned instead to a menu of solutions designed to stop erosion along the creek’s banks.
The stream, which winds through Minnesota’s Martin County, had encroached to within 40 feet of a road that local schools used as a bus route. Keeping this road usable was key: If floods had forced the buses to drive along any other roads, it would have taken them significantly longer to get students to their classrooms.
To stop this erosion, the NRCS relied on a combination of solutions that included planting willow trees, installing brush mattresses, and creating a new rock-walled streambank for the creek.
How has this menu of services performed? Crews installed the erosion control measures way back in 1998. They are still protecting Elm Creek today and even survived a 2001 flood, during which more than 5 inches of rain fell in Martin County in April. That’s twice the amount of rain that typically falls in the month.
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Photo: CalTrans |
| Demolition on the West Approach in San Francisco |
For Sonia Maassel Jacobsen, a hydraulic engineer in the St. Paul, MN, office of the NRCS, the project’s success proves the wisdom of relying on a variety of techniques to protect the waterways that run through and alongside the agency’s construction projects.
“For us, using a variety of methods is always the preference,” Maassel Jacobsen says. “When you use only one material, there is a greater chance of something going wrong. Whenever you can, it’s better to use a variety of methods. Your chances of success improve when you use a little variety.”
Maassel Jacobsen isn’t the only government or private-sector engineer who relies on this “many methods” approach when it comes to stabilizing the banks of drainage channels and other waterways and planning construction projects adjacent to them. By relying on several different types of erosion control methods, these engineers say, they are able to build more powerful defenses against erosion.
The good news is that engineers can choose from more methods than ever to protect channels and other waterways. These methods include both bioengineering techniques and manmade products.
For example, in drainage channels in which water flows only periodically—such as channels designed specifically for stormwater runoff—engineers may be able to simply line the channels with healthy vegetation. The plants will slow the flow of water, preventing soil from washing away.
Higher flow rates and a greater volume of water requires more creative methods of protection. Engineers may line waterways with geosynthetic fabrics such as turf reinforcement mats and, again, plant new vegetation. This combination provides increased protection from erosion.
In other cases, when the flow is even more powerful, engineers may add concrete protection to their arsenal of erosion control measures. Because simply lining a stream with concrete comes with several drawbacks—it doesn’t allow water, for instance, to seep into the soil—engineers may opt for a product such as articulated concrete blocks or gabions.
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