Hammered by a Major Flood Earlier in the Decade, Denver Decided to do Some Digging.
By Sam Stevens
Severe flooding in Colorado caused the loss of lives, destruction of property and the evacuation of more than 11,000 people in 2013. Following this and numerous other instances of flooding, the City and County of Denver committed to an ambitious, $298 million stormwater infrastructure initiative – Platte to Park Hill: Stormwater Systems Project. The coordinated and community-led approach consists of a series of engineering-focused projects that will address flood protection in northeast Denver neighborhoods.
Denver’s Montclair Basin, which encompasses the area affected, experiences a high flood risk given it lacked an adequate drainage system and is the City’s largest watershed basin. It is also challenged from a water treatment perspective, with no open waterway to allow natural or man-made facilities to address water quality. Stormwater modeling and evidence from previous storms shows that during moderate to large storm events, the existing pipe systems reached capacity quickly and the excess runoff is carried into neighborhoods at depths of three feet or more – causing a many issues for these at-risk communities.
In order to address these flooding and environmental issues, as well as improve community amenities, multilayered goals for the project were established early, including providing critical flood protection, improving water quality, increasing neighborhood connectivity, enhancing public spaces, and upgrading infrastructure with green elements.
The three projects are currently underway with expected completion of the water treatment elements by Fall 2019 and the entire project by the end of 2020. The projects include:
- City Park Golf Course Redesign
- 39th Avenue Greenway and Open Channel
- Globeville Landing Outfall and Park
Integrating a 200-acre-foot detention basin into an urban golf course
To begin, the City knew it needed to uncover existing storm canals and drainageways that had been buried under 150 years of urban development, forcing stormwater into pipes. The “daylighting” of buried channels started at the golf course — cutting through pavement and re-engineering old streams and canals to ultimately create over a mile and a half of naturalistic riparian corridors across the three projects – and allowing the team to create natural bioswales throughout.
The first part of this takes place at City Park Golf Course, where 800 linear feet of brick pipe installed in the early 1900s was removed, opening up the system to a 1,200-linear foot meandering stream. The stream was then seamlessly integrated into the golf course – controlling stormwater and enhancing playability of the course. A trash vault was also installed offline of the storm system, intended to collect debris picked up by stormwater during rainfall events, and up to 2,200 cubic feet of trash prior to emptying.
Another essential part of the course redesign was the inclusion of a 200-acre-foot detention basin. Golf courses have a proven track record of providing neighboring communities with effective water quality and flood control protection and integrating stormwater detention basins into golf courses allows the course to capture and temporarily hold floodwater, slowly releasing it over time.
The new regional trash vault, forebay and wetland channel form the first treatment train in the re-engineered Montclair Basin, enhancing the quality of the water as its conveyed to the South Platte River.
While the treatment train is essential to the Platte to Park Hill project, the City was challenged with smartly integrating these elements into a beloved City course. Through a collaborative design process, the drainage improvements, including re-grading of 200 acre-feet of detention, were integrated in such a way as to become part of the course itself. Playability is now enhanced and challenged by the drainage and stream features.
Using green infrastructure in an urbanized area to capture a 100-year storm
Even with the City Park Golf Course redesign to integrate these stormwater features, the Montclair Basin still lacked the infrastructure for handling the water produced during a 100-year flood event. Therefore, the City opted to combine a greenway and stormwater pipe system to create the backbone of the needed drainage system.
At a location lower in the basin where stormwater concentrates, an East-West corridor extending from 39th Avenue was selected for the greenway. This greenway will both intercept overland flows in larger storms and help naturally clean stormwater in smaller events as it flows and conveys to a river. A key challenge with the project was completing the project in a tight urban corridor with businesses and residents surrounding the two-mile-long project site. Therefore, a robust community outreach process was established, enabling the City to uncover design aspects most important to the community which included gathering and recreational spaces, incorporation of native landscapes and materials, and increased connectivity for pedestrians, drivers, and bikers.
The City reconstructed the urban landscape where possible, slowing down and filtering the water through vegetation to remove contaminants, all while controlling storm runoff and nourishing greenery to help residents endure the climate shift toward droughts and rising temperatures. Greenways help remove waterborne contaminants by exposing them to sunlight and slow and control stormwater across wider channels and gentler banks. To further the treatment process, over 50 street side stormwater quality planters were installed to collect and filter water through various layers of vegetation and soils. Additionally, two new trash vaults add 1,000 cubic feet of trash collection in the basin
Precast, reinforced concrete boxes were installed underground on both sides of the greenway, connecting the overall system and allowing for future extension further into the basin. Storm flows will be intercepted and directed from newly constructed boxes on one end through the greenway and back through additional new boxes and pipes leading to the Globeville Landing Outfall – which ultimately empties into the South Platte River. Another key component was careful planning and designing around an existing 100-year-old, 120-inch brick pipe. The existing pipe was inspected and found to be in good condition, allowing for reuse. The reuse of this pipe helps convey a significant portion of the stormwater to the South Platte River and reduces the need for another bored tunnel under the Union Pacific Railroad.
The new greenway project will replace the City’s aging infrastructure with nearly 12 acres of new recreational open space and increased connectivity. Three vehicular and two pedestrian bridges will connect people across the channel, and the site will feature the first shared street in Denver. The new green infrastructure will better control stormwater, improve water quality and public safety, increase plant diversity, and improve natural ecosystems – all of which were top priorities for the City and community. Once complete, the new space will include more pathways and bridges, 5,000 square feet of community gardens and an outdoor amphitheater.
Engineered outfall within Superfund site and railyard
Globeville Landing Park is the final stop for stormwater traveling from northeast Denver to the South Platte River. Five hundred linear feet of large diameter pipes were installed to carry stormwater under 17 freight train tracks and two heavy commuter rails, and through an existing Superfund site, before it rises to the surface at a natural open channel with a capacity of 2,300 cubic feet per second.
The City opted to design an open, natural stormwater drainage system, a nationally recognized best practice for moving stormwater, and also a way to provide habitat for wildlife and ecosystem restoration while improving overall water quality. The outfall is in a key location, where much of the water from the Montclair Basin naturally enters the South Platte River. It is designed to dissipate the high-energy stream of water and features a separate and stable, impermeable lining installed three feet below the finished open channel, protecting groundwater at the outfall. It also contains a UV vault that treats low flow stormwater prior to discharging it into the South Platte River.
Next to the outfall, the new community park will feature active play areas, grassy areas for sports and recreation, multiple bike and pedestrian paths, outdoor seating and shaded gathering places for families, access to the river, and improved safety and visibility of the park. Upon completion, Globeville Landing Outfall and Park will not only move stormwater, it will become a desired and safe place for families to gather within the community.
All aspects of Platte to Park Hill: Stormwater Systems Project are currently underway with drainage functions operational and expected completion of each project throughout the next year and a half. Beyond providing flood protection for more than 5,000 residences and establishing the first inhabitable stormwater outlet structure, the project will create 12 acres of new parkland in communities, add three miles of pedestrian facilities, increase open space, and breathe new life into one of Denver’s beloved golf courses.
Sam Stevens, engineering supervisor, infrastructure project management, City and County of Denver. Stevens is an Engineering Supervisor with the Infrastructure Project Management team at the City and County of Denver. He has over 17 years of experience as a Civil Engineer working for the Los Angeles County Department of Public Works, several consulting firms in the Denver Metro area and in various divisions within the City and County of Denver.
*This article was originally published in Civil + Structural Engineer in June 2019