A New Strategy For Stormwater Management

Coralville’s ambitious stormwater program uses green infrastructure to filter pollutants and reduce runoff and protect waterways.
A New Strategy For Stormwater Management
The Coralville Flood Protection System along the Iowa River was completed in 2014. The former power plant in the background is now the Iowa River Power Restaurant.

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Heavy rainfall in 2008 inundated the Coralville, Iowa, area. Six inches of rain fell on already saturated ground over a 48-hour period in June. It wasn’t the wettest year on record, but 2008 brought too much rain over too short a time. Damages and property loss were widespread and unprecedented.

“We’re downstream of a U.S. Army Corps of Engineers flood control reservoir on a large watershed and adjacent to the Iowa River,” says Dan Holderness, city engineer. “In most years the Coralville Dam does its job, but we had water coming over the spillways and the river crested 5 feet above that, so a lot of bad things happened.

“Same with the ‘93 floods: We had a lot of soil erosion, damage to buildings and infrastructure, closures of many public facilities, including University of Iowa Hospitals and Clinics, and the VA medical complex. Access from I-80 was cut off, and patients and staff couldn’t get in. The UIHC had to be closed for nearly a week.”

The 1993 event marked the genesis of their flood control program, but the 2008 event is what really triggered Coralville’s efforts to mitigate stormwater runoff. Over 200 businesses and 400 residential units were damaged by that flood. The total commercial damage was $21 million, with another $4.4 million for residential and $14.4 million for public facilities and cleanup costs. Flood control measures are now nearing completion. The stormwater program is a work in progress but is already yielding impressive results.

Green infrastructure

The Coralville area watershed suffered from both urban and agricultural runoff issues, with the usual mix of fertilizers, pesticides, petroleum products, animal waste and trash. With many riparian habitats under their watch and recreation and fishing a popular attraction for residents and visitors, these runoff issues had to be addressed.

Coralville’s stormwater program requires infiltration practices on all new or redesigned development, including municipal projects. Engineered practices include:

  • Addition of permeable paver systems
  • Tree filter boxes and rock chambers
  • Bioretention cells and bioswales along roadsides
  • Detention basins in strategic locations, such as sandy areas where the design requires a smaller footprint or under parking lots
  • Green roofs on municipal and commercial buildings
  • Road deicing with a mix of salt and beet juice to improve adhesion and delay the need for resalting
  • Detection and elimination of illicit discharges, including concrete slurry
  • Concrete washout facilities on construction sites

There are two green roofs in the city. The larger one is at North Ridge Pavilion, a recreation facility. It was Iowa’s first municipal green roof, designed by Conservation Design Forum in Elmhurst, Ill. It’s a high-profile feature, visible from the interstate, with 1,700 square feet of attractive green cover. “When you drive by on I-80, you can’t miss it,” says Amy Foster, stormwater coordinator.

Private construction of green roofs is virtually nonexistent. Retrofits on existing framing is constrained by load limits and funding, and the temperate climate eliminates much of the HVAC energy savings that you’d see in hotter climates.

Keeping the system split

Coralville has a single sanitary treatment plant handling about 2.2 mgd on an average dry day, with a total capacity of 6.6 mgd. “We’d like to keep it that way,” Holderness says. “We don’t want to add a stormwater treatment plant.

“Our goal is to put as much of that rainwater as possible back into the ground so it can feed our local water resources. Infiltration practices clean and cool the water while also recharging the aquifers. If we can do that, we can avoid the cost and complexity of mechanical treatment.”

There are many lakes and streams in the area, including the Iowa River and its tributaries, so minimizing stormwater runoff and making sure it isn’t harmful to the ecosystem is imperative.

One notable example is the system of bioretention structures along Coral Ridge Avenue, a high-traffic thoroughfare. “That was actually the result of an expansion from two lanes to four,” explains Foster. “We received grants totaling $263,000 from the Watershed Improvement Review Board and from Rockwell Collins [a local aerospace contractor] to do that project.

“We used a combination of bioretention cells and swales on the sides of the road. But we needed more capacity in the middle since there are steep drop-offs along the outside shoulders. So we engineered the center median swales with a good soil base and installed overflow to the sides to keep water off the road.”

The University of Iowa and the Johnson County Soil and Water Conservation District are working as research partners with Coralville on this project. They installed flowmeters to monitor overflow vs. infiltration rates. It’s an atypical design, but one that was dictated by existing topography.

“We got lucky on this project,” Foster says. “We ran into some natural pockets of sand. On top of that we used a combination of washed rock, tiles and engineered soil with mulch and native plants. We’re trying different combinations of engineered soil in the bioretention cells and swales because we really don’t know yet which mix works best in roadway projects like this.”

Residential and commercial efforts

At the residential level participation is voluntary, but there are incentives. Rebates are available at 50 percent, up to a maximum of $2,000, to do any or all of the following:

  • Gutter output diversion into porous paving systems, bioretention cells and swales and rain gardens
  • Installation of cisterns to sequester on-site runoff
  • Soil quality restoration (decompaction and addition of compost to the soil)

“We’ve got $25,000 each year authorized by our City Council to make this program work and a formal application process for these rebates,” Foster says. “It’s been pretty popular with the public and a lot of our citizens are already on board.”

Builders and developers are encouraged to reduce roof footprints and road widths where feasible. To date, compliance has been good. Reconstruction of parking lots to include permeable paving and/or retention basins has been done on a limited basis. “I can’t say yet if we’re staying ahead of runoff, but I feel like we’re getting it done overall,” Foster says. “We do what we can with what we have to work with.”

Within the City of Coralville, the majority of runoff can be characterized as urban. But on the periphery of the city, especially along the 25 miles of Clear Creek (a tributary of the Iowa River), there’s the usual agricultural runoff. Ongoing communication with adjacent municipalities and farmers via the Clear Creek Watershed Board has helped reduce runoff of pesticides, fertilizer, animal waste and topsoil.

The stormwater ordinance

Coralville enacted a revised stormwater ordinance on Earth Day in 2014. All new development must follow city guidelines for stormwater runoff, including infiltrating 1.25-inch rainfall events using proven stormwater practices. Those rainfall events represent 90 percent of the storms that cross Iowa.

Guidelines require use of bioretention cells and swales, detention basins, permeable paving and soil quality restoration to meet ordinance goals. They also require site plan design to take advantage of existing pockets of sandy soil where available and engineering for effective drainage and infiltration.

At the citizen level, Coralville promotes washing cars on lawns instead of driveways, or at a commercial carwash where effluent is fed into the sanitary system. People are using the commercial carwashes more, but not many use their lawn. Still, the promotion raises public consciousness of the issue. “At least it gets people thinking about how, otherwise, all that soap goes into the river,” Foster says. “I always wash my car on my lawn.”

Education, examples, encouragement

Coralville Mayor John Lundell put in a porous pavement driveway at his home to serve as an example of what the average citizen can do. “Mayor Lundell, along with the rest of our City Council, has been extremely supportive,” Foster says.

Lundell won an award in 2013 from the Iowa Stormwater Partnership (ISP) for the work he did at his home and the leadership he provided to the City of Coralville over the past decade. A video of Lundell’s driveway repaving project is available to contractors and residents and is a popular download from the city website.

The ISP provides training and certification for professionals as well as resources for the public. “This is a wide-ranging program, and you don’t have to be a professional to participate,” Foster says. “They’ve provided training for students, homeowners, landscapers and contractors. Everyone’s welcome. It’s like a club where everyone can learn from each other.”

Recruitment programs are also in force to offset the industry-wide problem of “graying personnel.” That happens at both university and high school levels through education and internship programs. Coralville currently hosts two interns from the university who are studying for careers in the field of water management.

The EPA provides additional training through its Brownfields Assessment Grants program, in which many local students have participated. EPA grant funds pay for a part-time Brownfields coordinator position that is filled through recruitment from the UI Urban and Regional Planning Master’s Program. The coordinators perform day-to-day management of the Brownfields program.

“We were awarded our first Brownfields Grant in 1999,” Holderness says. “To date, we’ve been awarded 14 grants totaling over $3.2 million and brought in 13 or 14 different coordinators from the UI Master’s Program.

“One of the things we do for outreach is, every spring, we bring in four classes of junior high students to do things like wetland creation, restoration and planting. We try to get them involved in the environment. They get some ownership and pride, and we like to think we’re grooming the environmental stewards of the future.”

Keys to success

Foster proudly notes, “I really think Coralville’s stormwater program is one of the best and most advanced in Iowa. One reason for this is that we talk with other stormwater departments throughout the state. We learn what they’re doing, what works and what doesn’t work.”

Training is also an important component. Foster and Holderness attend seminars and conferences whenever they can, seeking to fill any gaps in their knowledge and looking for new state-of-the-art solutions to stormwater problems.

Support from the mayor, administration and City Council has also been crucial to their success. “We have a very supportive city administration and City Council,” Foster says. “They understand the importance of protecting water quality in our community.”

An additional tactic is to seek out all possible sources of funding at the state and federal levels, as well as through NGOs. “We’ve got some great people writing grant applications,” Holderness says. “Since the 2008 floods, we’ve been awarded over $66 million in funding for flood mitigation projects, and we couldn’t have accomplished what we have without that. We’ve been very aggressive about going out and finding funding.”

Coralville expects to complete its flood mitigation efforts by 2017 with close to $80 million in total investment. When that critical work is done, more resources will be diverted to green infrastructure, always a work in progress. The future of Coralville looks both dry and green.


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