Green Valley Ranch East, a 600-acre master planned community situated approximately seven miles south of Denver International Airport in Aurora, Colorado, boasts a unique natural feature running through its landscape. During the community's initial planning stages, the design team, led by landscape architecture firm Terracina Design, recognized the importance of integrating Tributary T, a tributary of the Platte River, into the parks and open space network and implementing effective stormwater management solutions.
To comply with Colorado Water Law, all new construction must collect, detain, filter, and release all surface runoff to the local storm drain system. Green Valley Ranch East's regional detention and drainage facilities meet these requirements, while creating an aesthetically pleasing open space network that provides abundant habitat for local wildlife.

Project Goals
The design team set several goals in conjunction with the drainage system including developing a functional and visually appealing drainage concept, blending seamlessly with the outdoor recreation design, establishing connections between community amenities and the open space and drainage corridors, implementing innovative engineering solutions for flood protection, and restoring and enhancing natural ecosystems.

Terracina Design embraced the opportunity to celebrate Tributary T by strategically locating parks, open spaces, trails, and community amenities along the corridor to create a comprehensive open space network. Prior to development, Tributary T was a shallow, mostly dry, depression. The increased stormwater flows, resulting from the community's impervious areas and roof tops, necessitated the redesign and construction of the corridor. To accommodate these flows, the design team collaborated with the local flood control district, Mile High Flood District, and the City of Aurora's Public Works and Water departments to create an engineered geomorphic channel. The concept behind this geomorphic design is to mimic natural water systems, ensuring resilient flood protection, and restoring and enhancing the area's natural ecology. To make Tributary T accessible and enjoyable for all residents, the team designed the maintenance road to serve as a regional trail, providing access from adjacent neighborhoods, improved by pedestrian bridges and low water crossings.

Drainage as an Entry Feature

 
Tributary T flows from south to north and eventually joins First Creek at the community's entry point, providing an opening for the team to enhance the required detention ponds with permanent water features. This unique design opportunity arose due to the site's natural drainage, necessitating the placement of detention facilities at the primary entry to the community. Terracina Design envisioned permanent ponds that double as water quality and detention facilities during storm events, while also providing a picturesque view across the water to a bridge and waterfall element. These ponds, along with the detention basins located to the northeast and directly south of the tributary channel corridor, serve as detention and water quality basins for approximately one third of the community.

With Colorado's arid climate and stringent water rights regulations, it is uncommon to find retention basins (wet ponds) integrated into new developments. The retention basins were constructed using an impervious natural clay lining. To ensure that these ponds effectively fulfill their primary function of stormwater collection, it was necessary to maintain a permanent water surface slightly below the top of the bank. A boulder edge was utilized along the pond edge to maintain the required freeboard, allowing the water surface to fluctuate and occasionally run overtop the boulder edge, while also preserving a visually pleasing appearance. To maintain the specified water surface elevation, the two wet ponds rely on special water rights to an aquifer, automatically pumping water when the surface falls below the specified elevation, typically due to evaporation. During storm events, excess water from the eastern pond flows west through surface overflow swales and an equilibrium pipe into the western wet pond. The excess water from the western pond eventually flows into an outlet structure, releasing into Tributary T, which then joins First Creek, ultimately leading to the Platte River.

Engineered Stilling Basins
In addition to creating permanent water ponds at the entry, Terracina Design explored designs to improve the aesthetic of the engineered stilling basins (forebay structures) and drop structures. The eastern wet pond captures stormwater flows from the community through a series of trickle channels to the bridge, under which it cascades down an aesthetically engineered drop structure into the pond. The drop structure, constructed from grouted boulders and custom concrete textures, serves dual purposes: controlling and directing stormwater during significant storm events and acting as an aesthetic waterfall, providing aeration and circulation to the pond through a circulation pump located in the eastern wet pond. Both ponds benefit from mechanical aeration for additional water quality improvement. The wet ponds boast multiple aesthetically enhanced and engineered stilling basins (forebay structures). Terracina Design worked closely with hydraulic and structural engineers, Aurora Public Works, and the Mile High Flood Control District to design aesthetically pleasing forebay structures. These structures incorporate boulder baffle blocks, enclosed stilling basins with a combination of concrete and grouted boulders, and stepped concrete retaining headwalls connected to boulder retaining walls with curvilinear geometry.

Plant Selection
A critical aspect of the project was the careful selection of plant species to cater to multiple ecosystems present on the site. The Tributary T corridor to the south of the wet ponds features a small wetland. Terracina Design collaborated with ERO Resources Corporation, an environmental consultant, to establish a revegetation plan for the disturbed areas. The wet ponds accommodate riparian plant communities from the water surface elevation to the ten-year flood elevation, upland plant communities from the ten-year flood elevation to the 100-year flood elevation, and high-altitude prairie plant communities above the 100-year flood elevation. The site's plant palette and seed mix selections successfully promote a healthy ecosystem and attract various wildlife, including waterfowl, wetland and prairie birds, hummingbirds, birds of prey, mule deer, coyotes, frogs, various pollinators, and occasional pronghorn antelope.

Outcome
Green Valley Ranch East stands as a model of successful community planning, incorporating effective stormwater management, aesthetically pleasing design, and the restoration of natural ecosystems. The careful integration of Tributary T and associated drainage facilities into the community, along with innovative engineering, showcases the harmonious coexistence of a functional drainage system and a flourishing ecosystem within the community.

Team List:
Terracina Design (Landscape Architect and Planner)
Oakwood Homes (Land Development)
Calibre Engineering (Civil, Hydraulic, and Structural Engineers)
EV Studio (Structural Engineers)
Wright Water Engineers (Water Resources)
ERO Resources (Wetland Mitigation)
Aurora Water
Mile High Flood District