Unified purpose ensures success of planned community in Colorado

Building in the city limits doesn’t have to be "limiting," even though principles of sustainability may be unfamiliar terrain for developers, planners, and city staff. If key decision-makers are presented with the facts and brought into collaborate, greater sustainability can be a reality in new communities being built. With a great design and a city staff that was open to new ideas, residential sections of The Village at Centennial, in Centennial, Colo., have become an excellent example of the developer, planner and city staff coming together.

Grassed stormwater conveyance reduces runoff and improves stormwater quality.	Design and construction team Project name: The Village at Centennial Owner: KB Home, Denver Developer: Castle Prim, LLC, and Stone Ridge Capital, LLC, both in Millville, Calif. Civil engineer: Nolte Associates, Inc., Denver Architect: KB Home Planner: Michael Ohara, KTGY Group, Denver

Currently under construction, The Village at Centennial is a mixed-use development of single-family residential, multi-family residential, apartment, commercial/retail, and municipal areas. When complete, the 138-acre community, centrally located in the city, could have up to 600 multi- and single-family residential units and potentially up to 1 million square feet of commercial and retail space. Two green elements of the multi-family portion of the project illustrate how collaborative spirit contributes to effective sustainable planning on the municipal level. The first design element relates to pedestrian and vehicle traffic circulation, and the second concerns an alternative treatment for conveying stormwater.

Pedestrian and vehicular movement
Narrower streets will reduce the overall imperviousness, the heat island effect, and pollution. The Nolte Associates, Inc. (Nolte) team, with input from the developer’s planner, negotiated with city officials to allow the 1-mile-long on-site public road narrower than the standard road width. A standard city street requires street radius and width that would have dramatically increased impervious acreage and reduced the efficient use of the land within this infill site. Narrower streets also minimize the heat island effect and pollution.

Based on an internal study by Nolte, narrowing a typical 1-mile stretch of a local asphalt concrete (AC) road saves approximately $30,000 in construction cost and $150,000 in long-term maintenance. The move could also save 125 tons, or about five truckloads, of asphalt concrete and 265 tons, or almost 11 truckloads, of aggregate base. Eliminating 1 foot of pavement saves money, uses fewer non-renewable resources, and reduces impact on the environment. These savings are simply for initial local road construction. The savings increase exponentially when analyzing larger roads and accounting for future maintenance costs.

In the City of Centennial, the minimum width of city street right-of-way (ROW) is 50 feet, with a street dimension of 36 feet curb-to-curb and attached walks. The city agreed to allow a 32-foot ROW for the Village at Centennial, measured curb to curb, and placing attached and detached sidewalks within maintenance easements.

To encourage the residents to travel more by foot, a series of pedestrian trails are planned to connect the overall development’s residential sections, office buildings, and a 38-acre retail center to the adjacent Cherry Creek State Park. This trail connection will also link to a public transportation node adjacent to the retail center that will provide light rail service through the Denver metroplex system.

Also considered was the environmental impact of the actual road work. Negative impact occurs during several phases of road construction: production of the materials, material haul to the site, roadway installation, and post-installation maintenance.

During the production, haul and installation of AC roads, a significant amount of diesel and gasoline is consumed. Eliminating 1 foot width per mile of roadway, would save 200 gallons of gasoline and 500 gallons of diesel. Additionally, the reduction in fuel use would prevent 15,000 pounds of CO2 from being released into the atmosphere, which is about two times what the average car produces in an entire year.

During the heating and installation of pavement, a significant amount of volatile organic compounds (VOCs) are released into the atmosphere. Narrowing the roadway would keep almost 400 pounds of VOCs ― equivalent to 15 times what the average car produces a year ― from polluting the air.

Along with narrower streets, the city agreed to reduce the design speed limit from the standard 30 mph to 25 mph and to implement guest parking on only one side of the street. One-side, on-street guest parking on narrower-than-normal streets with close building setbacks produces a traffic-calming effect and decreases driving speeds. The on-street parking plan also reduces the need for a series of guest parking lots.

With building setbacks measured from ROW, this new road section allows greater density and more efficient use of space, providing more useable public open space than would be possible with a more dispersed site.

Storm drain conveyance
Pocket parks and trails connect activity areas throughout the development, encouraging residents to walk to nearby stores. Nolte designed more than 3,760 linear feet (LF) of pedestrian trails, excluding attached sidewalks, on this project. Nolte then placed an 8-foot-wide meandering concrete walkway, running between the southern and northern ends of the project. Situated within a 50-foot-wide grassed stormwater swale that enhances water quality, the walkway serves a dual purpose by also accommodating on-site maintenance by the water/sanitary district.

Multiple conventional designs, presented in early phases of design development by other land planners, would have placed a major on-site storm drainage conveyance system entirely within underground pipes with guest parking lots above. These designs would also have increased stormwater runoff and negatively affected water quality downstream.

Instead, at Nolte’s suggestion, the developer opted for wide, grassy roadside swales. The adjacent roadway slopes very gradually from south to north, allowing a design that produces non-turbulent flows into the adjacent swale. The design naturally filters the stormwater and results in cleaner water as it flows to the adjoining regional detention pond.

The pedestrian trail runs along, around, and across the swale. The drainageway has become a major landscape feature, enhancing the development entrance with trailside benches, downcast and shielded pedestrian lighting, and trash receptacles strategically located by the landscape architect.

The trail connects to the paralleling public roadway using two 30-foot pedestrian bridges, required by the city, crossing the riparian-planted drainageway. Public plazas feature sand-bedded concrete pavers, shaded seating, and bicycle racks. They are connected by the trail system within public open space, which occupies 32 percent of the project site.

Overlot grading activities were substantially reduced by producing two distinct row townhouse styles that provided for an efficient use of land: a tuck-under design and a detached garage unit, which allowed the alley loaded layout to conform to the gently sloping existing landform. Floor area ratios for the development ranged from approximately 7.2 d.u. per acre in the lower density paired model portion of the development to more than 17 d.u. per acre in the townhome section, including the 32 percent open space and common area.

Throughout the entitlement process, Nolte presented its sustainable vision to city staff, in some cases supporting a design that was contrary to approved development standards. Without the support of city staff, this project would not have been able to move forward in such a successful and sustainable way.


Mark Bellinghausen, P.E., a senior engineer at Nolte Associates, Inc., has been providing site engineering design services since 1996. He can be contacted at 303-220-6400 or via e-mail at mark.bellinghausen@nolte.com.