Bioengineered design for residential storm drainage illustrates new vision, restores once-pristine creek and showcases public involvement
Mike Novak, P.E., City Engineer, Lenexa, Kansas
William J. Cunningham, P.E., Principal, The Larkin Group, Kansas City, Missouri
In Lenexa, Kansas, planting, pruning, and weeding are replacing traditional methods of maintaining stormwater channels, such as concrete replacement and sediment removal. This bioengineering trend is reflected in the mission of the City's three-year-old Watershed Management Division:
Mission accomplished: Crews recently completed restoration of the channel using a bioengineered approach. The project satisfies the City's vision for watershed management.
Restoring the creek to its natural state
Traditional engineered solutions use armored channels to convey stormwater runoff. Such solutions also may incorporate a network of underground piping, essentially burying the streambed. In contrast, bioengineered designs use natural materials with select man-made structures and materials to armor banks and stabilize slopes. This integrated approach complements a creek's natural environment and aesthetics, often delivering a solution that restores a creek to near-natural appearance.
"Whether traditional or bioengineered, mathematical computer models help us understand a creek's hydraulics and the impact of possible drainage solutions," says Larkin Project Engineer Holli Wilson, P.E., who coordinated the 83rd Terrace project. Bioengineering principles consider a streambed's shape and function, historical and current stream configurations, soil conditions, and vegetation, backed by the pure physics of runoff.
"Drainage designs to handle projected flows also needed to satisfy the City's desire to minimize potential negative impacts and restore streamway and riparian habitat," says Wilson.
Achieving that ambitious objective with a bioengineered design required a detention basin to reduce flood peaks through the channel. Expanding an existing farm pond, located nearby in a privately-owned cemetery, offered the best means to minimize disruptions to the streamway below. The City negotiated the purchase of the upstream pond plus additional land.
However, rebuilding spillways and reconfiguring the pond's waterline threatened nearby wetlands. Larkin engineers, working with Adaptive Ecosystems President Steve Parker, developed an extensive plan for upland (areas just above the waterline) and forested riparian corridor enhancements. The Corps of Engineers approved the plan, thus achieving no net loss of wetlands.
The final design expanded the pond and increased the dam height to hold more water. The dam automatically releases water from the 2.6-acre pond, controlling the flow through a spillway and stilling basin.
"Eventually the pond will not only help avoid flooding in downstream neighborhoods, it will feature amenities like fishing and a walking path," says Jim Melvin, P.E., R.L.S., Lenexa project manager.
Methods and materials
Larkin worked with bioengineering consultant David Flick of Terra Technologies to recommend methods and materials consistent with bioengineering techniques for erosion control and slope stabilization. Guidelines called for removal of non-native plants, such as shrub honeysuckle and garlic mustard, and installation of native trees, shrubs, prairie grasses and wildflowers. In addition, the entire area was inoculated with beneficial soil microorganisms to enable sustained propagation of the native plant community.
The drainage plan also incorporated some structural elements, such as a box culvert on the upper reach of the stream channel and storm drainage inlets, plus underground piping to efficiently collect, then convey, runoff through homes' side yards and into the main stream channel. This helps minimize surface erosion of adjacent properties.
Along the creek, natural materials and vegetation prevent erosion and stabilize slopes:
"This project set a high standard," notes Melvin. "There were times we weren't sure the project would happen because of potential impacts on the neighborhood. Eventually Larkin designed solutions to those concerns."
Lenexa resident Betty Keener knew the problems all too well. "Over a period of about 15 years, we lost about 10 feet of our backyard to erosion from a combination of flooding through the creek and runoff drains emptying nearby," she says.
Today, she's a vocal supporter of the restored corridor and the public involvement process that contributed to it. Noting that project construction meant losing her 60-foot pin oak, cottonwoods, and numerous landscaping projects, she says, "The rock wall opposite our yard is gorgeous, and I cannot say enough about the people with the City who worked with us. They kept us informed and they have been very accommodating."
But how does the new creek perform? During heavy spring rains this year, Keener says the creek never got high. "I'm anxious to see what happens, but I sure think it's going to work."
The project succeeded, say those closest to it, because stakeholders received a clear representation of how the final product would look and function.
Lenexa staff worked closely with homeowners, scheduling public involvement meetings and Q&A sessions as well as meeting one-on-one in backyards. "And Steve Warner on the homeowners' association was very sensitive and represented residents well," Keener emphasizes.
The communication paid off:
"Hard engineered solutions like concrete are not nearly as attractive, and will require more maintenance over the long haul," says Melvin. "As time goes on, the bioengineered materials and design we've built here will improve, not degrade."
Bill Cunningham, P.E., can be reached at (816) 361-0440 or (800) 488-5275 or via e-mail at BCunningham@larkin-grp.com. Mike Novak, P.E., can be reached at (913) 477-7680 or via e-mail at email@example.com.