Rain, Rain, Bring It On

July 24, 2006

As a society, we have come to recognize that human development affects the environment in many forms, and that we are better off when we can minimize that effect. Described in this article are three HNTB Corp. projects-each in a different area of the country-that approach water-quality enhancement using best management practices and treatment trains:

  • Bioretention in Lenexa, Kan., and Milwaukee, Wis.; and
  • Best management treatment trains and low-impact development in Seattle, Wash.

2020 vision

As a society, we have come to recognize that human development affects the environment in many forms, and that we are better off when we can minimize that effect. Described in this article are three HNTB Corp. projects-each in a different area of the country-that approach water-quality enhancement using best management practices and treatment trains:

  • Bioretention in Lenexa, Kan., and Milwaukee, Wis.; and
  • Best management treatment trains and low-impact development in Seattle, Wash.

2020 vision

Lenexa, Kan., is a rapidly developing suburban community in the Kansas City metropolitan area. Home to nearly 45,000 residents, the city encompasses a 34-sq-mile area, approximately two-thirds of which is experiencing development pressure. Lenexa contains the upper reaches of four main watersheds that ultimately flow to the Kansas and Missouri rivers.

To accommodate its rapid growth, the city initiated Lenexa Vision 2020, a citizen-driven, long-range community plan, in 1996. To develop the plan, Lenexa surveyed its citizens and found strong interest in a storm-water management program. In fact, nearly 80% of the survey respondents expressed interest in a program that would (1) reduce flooding, (2) improve water quality, (3) preserve the environment and open space and (4) provide for new recreational opportunities in the undeveloped portions of Lenexa. These elements form the foundation for the city's storm-water management program, titled "Rain to Recreation," which views storm water as an amenity, not a liability.

As part of the program's approach to watershed management, the city constructed a 7.5-acre lake, located in the valley of a tributary of the Cedar Creek watershed, with three sediment forebays, three wetlands and two bioretention facilities. The lake and associated elements reside within nearly 60 acres of parkland, much of which is conserved riparian corridor. The dam for the lake is Mize Boulevard, a four-lane divided major arterial street that generates storm water from both directions. The two bioretention facilities of about 3,000 sq ft each were designed for entry from the south and north of the lake. They accept runoff from the street before it enters the lake and are the key to preserving the lake's water quality.

The bioretention cells were designed to address storm-water quality in the street runoff before it entered the proposed lake. This would mean treating a volume of water equivalent to 1.37 in. of rain within 24 hours, or approximately 90% of all rain events in any given year in the Kansas City area. Further, the city desired to limit each bioretention cell's tributary area to 2 acres and require a drawdown time of 24 hours.

"The project also had vertical constraints," said Bob Gilbert, project manager in HNTB's Overland Park, Kan., office. "The elevation of the street storm sewer outlet and the permanent pool elevation of the lake dictated that the bioretention cells be placed near the edges of the lake so they could function without the lake's influence during a water quality storm event. In addition, since the cells were designed for street runoff, we had to determine how to deal with energy dissipation as the fast-flowing runoff entered the cells."

To keep within the constraint of 2 acres per bioretention cell, an offsite (outside of right-of-way) tributary area of 5.25 acres was excluded and treated by the developer. Runoff from the street storm sewer system resulted in using 15-in. to 18-in. pipes with velocities of 7 ft to 10 ft per second. To address the energy at the outlet, and to encourage flow dispersion, the city utilized a riprap stilling basin that met HEC-14 design requirements. In addition, larger riprap stones were placed around the stilling basin perimeter, in combination with Broomsedge and Crane's Bill plants, to create a stiff screen for the runoff to travel through to the rest of the bioretention cell. The overall length of the flow dispersion was increased from a point source to sheet flow approximately 20 ft long.

Specific design elements are as follows:

  • Soil mixture: 2.5 ft deep consisting of the following:

Planting soil..........30%




Organic matter......10-20%

Hardwood mulch.........20%


  • Hardwood mulch layer: 3 in. across the entire cell;
  • Nonwoven geotextile fabric: bottom of the cell;
  • Underdrain system: 4-in. perforated HDPE spaced at approximately 20 ft with cleanouts at the end of each line;
  • Underdrain rock: Two gradations consisting of No. 57 and No. 7 stone, double washed, placed surrounding the underdrain pipes, trapezoidal in shape to encourage lateral flow. This choice avoided use of rock on the entire bottom of the cell, with a geotextile over the top, because others have found that such an approach tends to clog; and
  • Nonwoven geotextile fabric: This was placed only over the top of the trapezoid-shaped rock over the underdrain pipes.

A call to capture

In 1999, Milwaukee embarked on an ambitious plan to redevelop the Menomonee River Valley. Its overall goal was to create several thousand new jobs and more than a million sq ft of new light-industrial development-all in the context of sustainable development principles. The valley lacked the local streets and infrastructure to support redevelopment. In addition, the area contained hundreds of thousands of sq ft of abandoned industrial buildings, contaminated soil, lack of storm-water quality and quantity controls, significant amounts of land within the floodplain and asbestos-containing debris.

In February 2003, the city of Milwaukee initiated design of the Canal Street reconstruction and extension project in the lower Menomonee River Valley. Undertaken to promote new development and redevelopment in the Menomonee Valley-particularly in the 100-acre site of the former Milwaukee Road Railroad Shops-this project consisted of reconstructing Canal Street from 6th Street to 25th Street, a distance of 1.2 miles, and extending Canal Street 1.3 miles west from 25th Street to Miller Park, the home of the Milwaukee Brewers baseball team. The Canal Street extension included constructing a new bridge over the Menomonee River and two land bridges crossing existing railroad lines.

Within the last several years, the Milwaukee Metropolitan Sewerage District (MMSD) adopted new rules regarding the discharge rate of storm-water runoff from new development and redevelopment areas. Similarly, the Wisconsin Department of Natural Resources (WisDNR) adopted new rules regarding the quality of storm-water runoff discharged from new development and redevelopment areas. These new rules from both the MMSD and WisDNR were incorporated into the city of Milwaukee Municipal Code of Ordinances.

"For the Canal Street Extension Project, the goal of stormwater management was to capture and treat the first 1/2 in. of storm-water runoff," said Curt Hulterstrum, water resources department manager in HNTB's Milwaukee office. "Though less than the 1.34-in. capture and treatment desired by Lenexa, Kan., it has similar pollutant capture performance because of climate differences."

The plan for controlling storm water in the developing and redeveloping portion of the lower Menomonee Valley consists of three regional bioretention facilities. Storm-water runoff from the shops site, including Canal Street, will be controlled in the proposed treatment and detention facility termed "Stormwater Park." Storm-water runoff between 16th and 27th streets will be treated by the 25th Street bioretention facility. This facility includes a 38-cfs lift station to deliver storm-water runoff from the service area to the bioretention facility.

Both Stormwater Park and the 25th Street bioretention facility are under construction. Storm-water runoff from the area between 16th and 6th streets will be controlled by Emmpak Facility, a regional facility that is being designed. It is likely that a lift station also will be necessary at this location to provide treatment of storm-water runoff from the proposed service area.

Friend of the fish

In the northwest, water quality treatment for 80% removal of suspended solids has been routine since the early 1990s. Biofiltration swales (engineered ditches that filter the water through tall grass), biofilter strips (engineered grass lawn areas to treat sheet flow) and wet pools (dead storage ponds sized to hold the annual runoff volume) are apparent wherever recent highway projects have been completed. An example of this is HNTB’s work supporting Washington State Department of Transportation improvements along the State Rte. 18 (SR-18) corridor. In the very rural setting of SR-18, the corridor crosses numerous pristine streams that are critical as part of the region's declining salmon spawning habitat. Designs often dot the landscape, with as many as five storm-water treatment facilities per mile. Storm-water flow control and water quality best management practice (BMP) sites are developed to protect each tributary that crosses the corridor to assure there will be no adverse impact at any point along the natural stream system. To minimize the size of these facilities, they would be designed to collect and convey runoff from the pollution-generating surfaces (roadway) separately from the clean runoff from lawn and embankment areas.

In more recent years, since 2002, some Puget Sound-region salmon and trout species have been listed on the endangered species list. Local agencies have responded with more stringent storm-water criteria, giving special attention to any facility with highway segments with greater than 30,000 average daily traffic demand. For these facilities (including most of the freeway system in the Seattle area), there is an added focus on "enhanced treatment" to remove the metals, such as copper and zinc, generated by highway uses. Stormwater flow control and water-quality treatment sites still are developed to protect each tributary that crosses the corridor, but the level of engineering is far greater than before, requiring complete treatment trains to obtain the required water quality.

Water-quality facilities include multiprocess enhanced treatment systems. Examples of these include storm-water treatment wetlands, with a pretreatment wet pool followed by a variable-depth (1-ft to 3-ft) wet pool with a diverse emergent aquatic planting. This second wetland cell provides added filtering residence time to settle metals. Once the metals settle into the bottom sediments, the plants eventually absorb them. In highly urban areas, where water quality must be accommodated within an underground vault, treatment is accomplished in a similar dual treatment system made up of a wet pool followed by a sand filter. These facilities can be very expensive to build and operate, either due to high property-acquisition costs or construction and maintenance costs.

"These requirements have driven HNTB to look to low-impact development techniques for possible alternatives," said Bob Ivarson, practice director, water resources, in HNTB's Chicago office. "These include treatment facilities that address pollutant sources in smaller, more frequent facilities. It can be as simple as changing the erosion protection for guardrails and poles from being galvanized-a source of zinc-to paint or rusting steel."

"The biggest development in this area has been the DOT's development of a new treatment facility called the 'ecology embankment,'" said Alan Black, storm-water section leader, HNTB Corp., Bellevue, Wash. "This approach allows water-quality treatment along the full length of the highway by treating the sheet flow off the shoulder through a series of roadside features that are built into the grading."

The ecology embankment has four components: The no-vegetation zone assures that sheet flows remain dispersed and uniform over the treatment system. A biofiltration grass strip provides initial treatment by collecting trash and larger sediments. The third element, a porous "ecology mix" media (made up of mineral aggregate, perlite, dolomite and gypsum), is sized to pass 91% of the storm flows from the surface flows in the biofilter strip, through the media, to an underdrain system. Less frequent storms, with peak flows higher than the two-year recurrent storm event, would pass over the filtration media to a ditch or collection system that conveys these flows (along with the clean flows from the underdrain system) to the flow-control facility.

Make a low impact

As in the water-quality treatment techniques, flow-control BMPs for highways look to low-impact design techniques for inspiration. Infiltration is the first choice for flow control. Even in low-permeability areas, consideration of infiltration capacity can significantly reduce the detention volume requirements at a given site. Porous pavement has been approved for use on bike paths and maintenance access roads. Surface treatments on roadway embankments also can be used to convert the surface runoff response to match pasture land cover rather than the more typical grass lawn condition. HNTB applied all of these techniques along S.R. 509 in Seattle and found that the optimized storm-water management facilities would cost an average of $10 million per mile.

"This inspired a new approach, which we call 'watershed initiatives,'" Ivarson said. Looking beyond the project limits to the watershed level allows HNTB to create efficiencies of scale, thereby achieving higher levels of resource protection and cost savings.

"In the case of S.R. 509, we realized far greater flow control and a project cost savings of $16 million by investing in watershed basin plan recommendations for a regional detention facility and other improvements that reduced flow-control measures along the corridor," Ivarson said. "We are continuing to look for these 'watershed initiative' approaches as we develop the much larger 30-mile I-405 corridor east of Seattle."

Storm-water best management practices, such as bioretention, ecology embankments, storm-water wetlands and low-impact development, whether applied individually or in treatment trains, are turning storm water from a liability into an asset. The next step in storm-water management may just be the sharing of our lessons learned both regionally and nationally.

About The Author: Black is a section manager for HNTB Corp. and the storm-water design lead on the I-405 corridor program in Seattle; Gilbert is a project manager, water resources, for HNTB and the storm-water designer for the Mize Lake project in Lenexa, Kan.; Hulterstrum

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