New Life at an Old Airport

At the Stapleton Airport brownfield development site in Colorado, a creek once relegated to culverts now serves as a centerpiece for sustainable communities.

By Carol Carder

When the city of Denver, Colorado decided in 1989 to build the new Denver International Airport instead of expanding landlocked Stapleton Airport, city planners faced a challenge … and an opportunity. They needed a blueprint to convert the nation’s largest urban infill project into a vibrant thriving community. At 4,700 acres, or one-third the size of New York’s Manhattan Island, Stapleton is one of the largest underdeveloped parcels of land in the heart of a major U.S. city.

In 1990, a group of civic leaders created the Stapleton Redevelopment Foundation, a nonprofit group financed by private philanthropy. In 1993, Denver Mayor Wellington Webb appointed the Citizen’s Advisory Board of 42 members to oversee creation of a redevelopment plan. After two years of work and extensive community outreach, the foundation produced the 1995 Stapleton Redevelopment Plan, affectionately known as the “Green Book.” The vision embraced a new sustainable community with the soul of an old Denver neighborhood. The plan balanced home life and workplace, marketplace, and green space. In 1998, Forest City Enterprises, a developer based in Cleveland, Ohio known for its expertise in brownfield redevelopment, was selected as master developer.

Peter Calthorpe, urban planner and renowned proponent of New Urbanism, drew up concepts for the mix of open space, residential, retail, and commercial components for Forest City Enterprises. BRW, now URS Corporation, came on board as the lead consulting engineer for the redevelopment, first to dismantle and direct recycling of the old airport, then to plan and implement the infrastructure to support redevelopment.

A key civil engineer at URS responsible for the program management at Stapleton is Dennis Arbogast, P.E. He honed his airport conversion skills as project and design manager for six and a half years on a prior Denver airport conversion, the Lowry Air Force Base. His diverse college education started with a degree in forestry from Colorado State University. Then he decided to change his career track to follow his interest in construction, where he had previously worked as a carpenter and pipeline installer. He earned a B.S. in civil engineering at Colorado School of Mines, then worked for a small firm in design and surveying. Next he moved to Wright Water Engineers before focusing on civil engineering at Lowry.

Engineering New Urbanism

“Although New Urbanism has its origins in urban planning, the engineers, surveyors, and landscape architects transform the vision into reality,” says Arbogast. New Urbanism denotes a pedestrian-friendly design with contiguous open space. Houses with front porches sit close to the street and close to each other with just 10 feet between housing units. Garages are tucked in the rear facing alleys. Local streets are 30 feet wide, 2 feet narrower than the typical city standard. Traffic meanders through the neighborhoods with roundabouts at many intersections.

URS takes the concept drawings for land parcels from Calthorpe and guides a design team that populates them with roadways, utilities, and contiguous open spaces. Then, M.A. Mortenson, the construction manager, reviews the plans for cost and constructability. Next, Forest City sells the finished lots to builders. Approximately 14 different builders construct units ranging from apartments and townhouses to starter homes and estate homes at Stapleton.

This diversity in home types and the piecemeal development of individual parcels challenged the URS infrastructure designers. Plus, the narrow streets and shorter alleys made utility infrastructure more complicated and prone to conflicts, according to Arbogast. Many of these deviations from standard meant negotiating with the city, the water company, and the parks department for approvals. “Our job is to simplify the project so we have a standard approach for placement of utilities, service lines, transformers, and pedestals,” Arbogast explains. “We need to make sure the elevation and location of sewer line and other product align from one parcel to the next.”

At build-out by 2020, Stapleton will have more than 81 miles of sidewalks spanning the contiguous open spaces. Public green spaces will comprise 30 percent of the community with a park system of 1,116 acres. The centerpieces of green space are the 80-acre Central Park and 85-acre Westerly Creek corridor. Numerous pocket parks and greenways throughout Stapleton mean every resident is just steps away from shared green space. “While a lot of projects have some New Urbanism features; we are an example of putting them all together,” Arbogast concludes.

A key component of the public open space is the resurgence of the central and southern portions of Westerly Creek, an $18-million project. (The northern portion estimated at $5 million is yet to be developed.) When Stapleton Airport was developed in 1929, the natural north-south flow of Westerly Creek was encased in two parallel underground pipes 66 inches and 108 inches diameter. In the redevelopment, Westerly Creek would be converted from this utilitarian, undersized conveyance channel to a diversified waterway, parks, and open space system.

Lead designer of the rebuilt Westerly Creek is Robert Krehbiel, the director of water resources at Matrix Design Group and a licensed professional engineer in Colorado. Matrix is a planning and engineering firm based in Colorado Springs, Colorado with other offices around the country. Krehbiel has more than 20 years experience as a civil engineer and in water resources with a specialty in drainage master planning. He earned a B.S. degree from the Colorado School of Mines and a master’s in hydraulic engineering from Colorado State University. He too is familiar with airport conversion, as he worked with Dennis Arbogast on the master drainage plan for the Lowry Air Force Base redevelopment.

As director of engineering for Forest City Enterprises, Derek Brown supervises and directs all planning, engineering, and environmental activities related to the Stapleton Redevelopment. Previously, the civil engineer worked for M.A. Mortenson as construction manager for Stapleton and served as senior construction project manager overseeing the Westerly Creek project. He remarks, “The project is one of the most unique projects in the country. Building a city from an old airport and truly creating a community from that is an amazing opportunity and privilege.”

Improving Water Quality
While residents enjoy the restored Westerly Creek with bicycle and pedestrian paths and enclaves to sit in and contemplate nature, to the designers, the creek and its associated ponds represent a flood control and water quality enhancement system. The Westerly Creek Watershed consists of 18.5 square miles of mostly developed land in Denver and Aurora. The Stapleton Redevelopment site contributes 1.75 square miles of the drainage area. Due to the hard-pipe connection of storm sewers between Lowry and Stapleton, the watershed is vulnerable to rainfall events and historically has produced high flows under even typical summer storm events. “The previous airport floodplain was a thousand feet wide and one foot deep over the former runways,” Krehbiel explains. “By improving the channel to an enhanced riparian corridor, we took valuable developable land out of the regulatory floodplain.”

The base flow of Westerly Creek is 3 cubic feet per second (cfs), while the 100-year flood is about 6000 cfs. The Urban Drainage and Flood Control District required stabilizing the new channel by burying riprap in the banks. A unique feature of the Westerly Creek corridor is the construction of regional water quality ponds. In typical development, every commercial property requires a water quality control pond. At Stapleton, the water quality treatment occurs at the end of the pipe. Regional stormwater ponds within the Westerly Creek corridor provide water quality treatment at each outfall before the urban runoff can enter the stream system. For example, from the box culvert at 29th Avenue, the “first flush” of stormwater goes through three ponds (constructed wetlands) before entering Westerly Creek. “We had a 600- to 800-foot-wide corridor to work with, which is very unusual, as many developments would have given us a 200-foot-wide corridor at maximum for this stream channel,” Krehbiel observes.

Architectural features designed to accommodate drainage requirements include a stormwater outfall and a four-foot drop structure. Four bridges cross the creek and are part of the trail system for pedestrians, bicyclists, and equestrians. Ecologically, the corridor is targeted for a variety of small mammal and bird species that historically inhabit the Sand Creek corridor to the north. Habitat is provided with the planting of native and drought-tolerant trees and shrubs, wetland plants, and grasses, creating ecozones similar to eastern Colorado foothills and prairie wetland transitioning to a mid-grass prairie environment.

Michelle Leach, LEED AP (Leadership in Energy and Environmental Design accredited professional), Matrix director of sustainability, knows plant species well, as she formerly worked with EDAW, the landscape architects. She worked extensively in the field with the landscape contractors assuring the project was built to the plans. Also, she documented the work with photos shown here and many more.

Start by Demolishing and Recycling
Initial work required the demolition of approximately 10 acres of existing airport runway over the Westerly Creek corridor and excavation of approximately 754,000 cubic yards of material. Pipes were removed, creek channel cut in, and banks stabilized with buried riprap. ValleyCrest, one of the landscapers, built several boulder jetties using 128 four- to five-foot boulders to slow creek flow. ValleyCrest also built a 280-linear-foot, 2-foot high wall using Staplestone, chunks of crushed recycled runway, near a set of benches on a trail.

“At our peak three months in summer, we had 55 guys putting in 60 hours a week planting trees, native plants, wetland plugs, wetland sod, and erosion blanket,” remarks Tim Hearlihy, ValleyCrest project manager. The wetland plants and sod came in the form of biologs, two-inch thick coirs made of coconut fiber rolled into rope-tied logs. Then, plugs of wetland plants are inserted and grown in small shallow ponds until the root mass comes out the bottom. For wetland sod, grass plugs are inserted into the biologs. “Westerly Creek was a huge undertaking that worked well thanks to the great partnerships with Mortenson, Matrix, EDAW, and the construction team,” concludes Jim Ransiear, ValleyCrest site superintendant.

A major sustainable feature of the development is the removal and recycling of 6.5 million tons of runways, taxiways, and parking pavements from the airport. Recycled Materials Inc. started removal in 1999 and took six years in the recycling, as a million tons a year is as much as the market can absorb, according to Rick Givan, RMI project manager. Some went off site, such as 200,000 tons for biota barrier to cap landfills at the neighboring Rocky Mountain Arsenal. Most has been recycled on site as base for pavement and foundations and in water features and public spaces.

What does the future hold for Westerly Creek? “The goal envisioned by Denver District 5 Council member Marcia Johnson, who supported this project, is to eventually connect Westerly Creek at Stapleton with the restored Westerly Creek at Lowry when Aurora and Denver have funding,” Leach says. Currently, the creek dives underground into pipes below neighborhoods at Montview Boulevard, Stapleton’s southern boundary, and emerges at Lowry. When that happens, a major environmental problem caused by closing a major airport will have been transformed into a huge livable and sustainable community Denver can point to with pride.

Carol Carder is a freelance writer based in Denver, Colorado who specializes in construction.