August 2003

Watershed management approach enhances wooded suburban stream corridor

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:

• Protect water quality and reduce the impact of new development on local streams.
• Enhance opportunities to create greenways and trails while protecting the environment and preserving wildlife habitat.
• Build community support through environmental education.
• Work with residents to enhance both natural and built environments, ultimately conserving, even restoring, natural resources.

As its progressive approach to watershed management took shape, an unusual October storm inundated Kansas City, including a pristine, wooded channel that runs through residential Lenexa in eastern Kansas. Water engulfed streets, flooded homes, and caused slope failure. Once floodwaters receded and damages were tallied, the City commissioned The Larkin Group to study the corridor for potential restoration. Along with flood prevention, the City wanted the area to look as if the channel had not been disturbed.

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:

• Custom seed mixes and plants — Terra Technologies developed seed mixes and specified plantings appropriate for eastern Kansas ecotypes. Primarily native grasses, vegetation closest to the creek bed will develop a complex network of fibrous root systems, strengthening stream banks and protecting even relatively steep slopes. In the riparian buffer, a mix of grasses, trees, shrubs, and native wildflowers will vegetate this transitional zone.

• Stacked stone walls — Along the channel's steepest section and curves where water velocity would be the fastest, stacked stone walls evoke natural stone ledges and increase both strength and aesthetic appeal.

• Coir logs — Logs made of fibrous materials from coconut trees stabilize the toe of slopes along channel edges.

• Stone gabions — Where a slope required anchoring to protect against undercutting and widening and potential slope failure, designers selected stone gabions that will be hidden as vegetation matures and cascades over the bank.

• Other protection — Erosion control blankets (ECBs), TRM (turf reinforcement matrix), and wire TRM provide additional slope protection while plantings fill in. ECBs actually decompose as plants mature and assume the role of erosion control.

Active public involvement process gets results
Public participation has become critical to the success of all projects that involve stakeholders. The Lenexa community not only expects active participation, it demands it.

"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:

• By responding to complaints regarding the corridor, the project team learned valuable information about erosion, water levels during rainstorms, flooded houses, and other concerns. The City developed options, reporting back to neighbors via a series of small neighborhood meetings.

• Neighborhood input highlighted the desirability of a visual transition along the corridor from residents' manicured lawns and gardens to the rustic creek ecosystem.

• No properties were condemned while securing easements; all easements were negotiated.

• Public works representatives coordinated each resident's plantings. Each homeowner could choose native trees listed in the guidelines. Owners also could specify placement on their property, as long as the location did not damage the channel design or interfere with flow. In some instances, landscape materials were stockpiled for installation.

• The City received no complaints during construction, thanks in large part to daily interaction between construction inspectors and residents.

The good news doesn't end with completion of construction.

"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 mnovak@ci.lenexa.ks.us.