Westerly Creek at Stapleton

Challenge

To make way for the restored stream channel, nearly 4,000 linear feet of culvert pipe and over 860,000 cubic feet of contaminated soil had to be excavated and removed. The park needed to not only convey stormwater discharge, remove overbank areas from the regulatory floodplain, and sustain water quality, but also be a safe and maintainable continuous greenway corridor. Passive recreation amenities and significant habitat areas needed to be integrated to minimize disruptions to wildlife. Aesthetic appeal, maintenance, habitat value, and resiliency to both flash floods and periods of drought were other essential considerations.

Solution

To protect surrounding development from flooding, the restored stream channel was set approximately 25 feet below adjacent urban grade in an 800-ft wide, 0.75 mile long basin. Excavated soil was relocated to Stapleton’s Central Park, capped, and covered. Designed for aesthetic appeal and habitat value, the 18-ft wide, 2-3 ft deep channel has gentle meanders and 4:1 sloped vegetated banks. Stormwater outfall structures, forebays, and an in-stream drop-structure (added to control grade change) were designed to be visually appealing, easily maintainable, and fully accessible. The planting palette includes species that provide year-round visual appeal, have good habitat value, and are genetically suited to Denver’s semi-arid climate.

• Buried riprap was installed along the channel at a 2:1 slope, approximately 3 feet below the invert of the channel, and varying in width, from 75 to 150 feet. This bioengineering technique fortifies the sinuous meanders against powerful storm events, yet enables typical erosion and deposition to occur. Additionally, it enables vegetation to grow overtop, down to the water’s edge, which is important for supporting habitat.
• Vegetated coir logs, termed “biologs”, and vegetated coir mats, termed “wetland-sod”, were installed during the early stages of restoration to provide immediate protection against stream bank erosion. Designed with a pre-vegetated mix of contract-grown woody riparian species and herbaceous wetland species, this technique enabled the riparian vegetation to establish more quickly, providing almost immediate habitat and visual appeal.
• As the first step of the park’s stormwater treatment train, local runoff enters the site through crescent shaped concrete outfall structures. Complete with “Staplestone” forebay basins, water is both slowed and filtered through these structures before flowing into the vegetated water quality ponds.
• The six water quality ponds cover approximately 10 acres and further reduce suspended sediment as well as filter out nutrients and bacterial components before waters spill into Westerly Creek.
• The plant palette includes 85% native and naturalized species for wetland, riparian, and upland landscape types. Aesthetics, habitat value, natural associations, low maintenance, and drought tolerance strongly influenced species selection.
• Native material was grown locally or provided by growers along the western flyway to ensure western genotypes, particularly for wetland species. Local cottonwoods were dug within a 30-mile radius of the project site.
• Pollination is facilitated by forbs, such as liatris, gaillardia and prairie coneflower, and woody species such as honey locust and serviceberry, which provide habitat and nectar.
• To create significant habitat, adequate space, food, and shelter were all considered. An abundance of wildlife inhabit the park, including small fish, toads, frogs, crayfish, bats, butterflies, American avocets, snowy egrets, cormorant, killdeer, golden eagles, red-tailed hawks, whitetail deer, muskrat, coyote, and beaver.
• To help minimize wildlife disruptions, the park’s primary trail maintains a 30-foot distance from the stream edge, except for at crossings.
• Concrete from the former airport runways, termed “Staplestone”, was recycled locally and reused throughout the site in various applications, including the outfall forebay basins, dry stack seating walls, and pathway edging. Pieces vary from 12 to 16 inches thick and have no steel reinforcement. Remnant white or yellow reflective markings and/or grooved surfaces can still be seen on several pieces, an indication of their former use.

• By planting 50 acres of native prairie grasses instead of bluegrass sod, the annual water budget was reduced by approximately 27.9 million gallons per year, saving an estimated $72,000 in annual irrigation costs. In 2008, the park’s irrigation system was switched over to supply non-potable water, creating an additional annual cost savings of approximately $2.23 per 1,000 gallons as compared to cost of potable water.
• By using a Natural Areas adaptive maintenance and management regime for the park’s native prairie vegetation, the city saves on average $2,240 per acre per year over the cost of maintaining a typical Denver city park. The savings result from an annual decrease per acre of 155 labor hours, 130 gallons of fuel, 131 lbs of fertilizer, and 24 lbs of herbicides.
• The cost of using the “Staplestone” (recycled runway concrete) was approximately 75% of the cost of using cast‐in‐place concrete for short retaining walls and the floors of the forebays. Savings resulted from onsite recycling of an onsite concrete product, as compared to crushing or hauling offsite for different recycling purposes.

• Several areas of vegetation experienced difficulties in establishment. After evaluation, it was determined that there was a mismatch between soil conditions and vegetation type. To help improve plant establishment rates, a soils analysis should be performed prior to, or early in, the design process, so that the planting palette can be designed to match specific soil conditions. Particular consideration should be paid to soil composition and salts.
• Original prairie seed mixes contained only 1-2 varieties of forbs, comprising less than 1% PLS/acre. However, to emulate a native prairie ecosystem more closely, prairie seed mixes should contain upwards of 20% PLS/acre of forbs and legumes. To enhance aesthetic appeal for blooming in the spring, summer, and fall seasons, at least three species of forbs should be included, which will also help in pollination. The inclusion of legumes will enhance soil nutrient levels, particularly the macronutrient nitrogen.
• The variety and abundance wildlife found within the park has far exceeded expectations, and at a surprisingly rapid rate. To help educate residents about cohabiting with wildlife and minimize potential conflicts, several informational signs have been placed throughout the park, discussing nesting birds, black-tailed prairie dogs burrows, and the presence of coyote.
• This landscape presents a tremendous opportunity for learning about urban prairie/ stream restoration. Ideally, a long-term monitoring plan should have been established alongside the construction of the park so that performance data could be collected and analyzed on a regular basis.

Project Team

Landscape Design Team: EDAW|AECOM, Forest City Stapleton, Stapleton Development Corporation, Park Creek Metro District, Denver Parks and Recreation
Civil Engineer: Matrix Design Group
Structural Engineer: Brown and Caldwell
Irrigation Designer: EDAW|AECOM
Ecologist: ERO
General Contractor: MA Mortenson
Landscape Contractor: ValleyCrest Development
Wetland Construction Consultant: Sue Scherner
Native Grass Seeding and Hydromulching: Peak n’ Prairie
Geotechnical: Kumar and Associates
“Staplestone” Contractor: Territory Unlimited
Concrete Contractor: Castle Rock
Specialty Concrete: Technology Constructors, Inc.

Role of the Landscape Architect

Landscape architects were an integral part of this multi-disciplinary effort, along with hydraulic engineers, structural engineers, irrigation engineers, and ecologists. In advocating for the use of bioengineering strategies, the landscape architects led the design and grading of the stream channel, as well as design of plantings, irrigation system, trails, bridges, and architectural features.