Cherry Creek Greenway

Purpose and Approach:

In 2014, the client group set out to mitigate detrimental impacts to Cherry Creek from decades of urbanization and flooding in an area rich with Denver’s heritage, and which comprises one of the most heavily used trail segments in the City. They aspired to expand flood capacity, bolster channel stability, enhance natural function, and better the experiential qualities of the 40-acre and 1-mile-long reach of severely degraded creek corridor just upstream of its confluence with the S. Platte River in downtown Denver. Prior to the project, severe degradation and erosion resulted in a 10- to 20-foot-deep incised channel that disconnected the floodplain from natural hydrology – ultimately exacerbating the loss of healthy vegetation and habitat, and threatening urban infrastructure including major utilities, trails, and roads.  Since its completion in early 2022, the project has surpassed expectations for restored stream-corridor function while improving flood conveyance, integrating over 2 miles of new trails, crossings, and community connections, enhancing habitat and water quality, and creating better open-space recreation.

Scope and Role of the Landscape Architect:

Through a collaborative teaming approach with engineers, ecologists, and other experts, the landscape architects co-led (with civil) all design aspects including the channel, crossings, trail configurations, grading and landforms, recreation facilities, soil health, and vegetation. The following elements were included as the landscape architect’s scope:

• Comprehensive, multi-year stakeholder engagement process and workshops with community leaders and focus-groups, jurisdictional partners, planners, and operations staff
• Corridor Vision Plan and Construction Documents for channel and creek features, natural areas, multimodal trails, recreation facilities, and community connections
• Bidding and construction implementation services including advisory role in vegetation monitoring and adaptive management.

Context:

The project site is in a rapidly evolving area about 6 miles upstream of downtown Denver and 4 miles upstream of the popular Cherry Creek district. There are hundreds of professional, industrial, and commercial businesses, 8 parks, and thousands of households within only a few miles of this regional recreation destination. The area’s demographic foundation ranges widely in socioeconomical, cultural, and ethnic character – and several jurisdictions of municipal/county, and quasi-governmental organizations overlap to manage infrastructure in the area. The completed project gracefully complements both current and future context by accommodating and enhancing natural systems function and by integrating characteristics that support necessary urban infrastructure.

Special Factors, Environmental Sensitivity, and Project Significance:

Prior to the project, this stretch of Cherry Creek had never received channel stabilization improvements, thus the project was one of the most significant and holistic channel corridor restorations in the Denver area since early 1900’s configurations (i.e., channelization) of many waterways that accommodated much of Denver’s urbanization. Decades of increased runoff caused by urbanization in the watershed, coupled with erodible sandy soils, resulted in massive lateral migration and up to 20 feet of vertical channel downcutting throughout the project reach. Additionally, the rate of erosion was exacerbated by the upstream Cherry Creek Dam which dramatically reduces sediment loads in daily flows (creating more erosive water characteristics), and annual dam “sediment flush” releases which mimic flow events equivalent to 2-year and 10-year flood events in alternating years.

The Cherry Creek Restoration Project is extraordinary in that it leveraged a variety of innovative strategies with a goal to exceed the performance of a traditional stream stabilization project and create solutions that address community values and experiential qualities with equal importance as connecting hydrology to the adjacent landscape, healthy vegetation, geomorphological improvements, and satisfying complicated hydraulic requirements for flood flows and yearly “sediment flush” releases. To achieve this goal with high-performance, the project team used collaborative multi-jurisdictional partnerships, and a natural systems design approach that intentionally blended functional drainage infrastructure, greatly improved recreational amenities, enhanced ecological functionality and habitat, and increased community connectivity and mobility solutions. Additionally, the integration of an underutilized adjacent parcel, and an innovative strategy to raise the channel thalweg by up to 8’ allowed the design team to develop a channel configuration that lengthens the stream significantly and mimics a naturally stable channel system. This approach also engaged the natural hydrology to shallower soils helping to establish and sustain a denser and broader variety of vegetation typologies, provide additional flood conveyance, and to stabilize the side slopes with far less visible riprap ‘hard armoring’. The project improved trail-user experience and community connections by integrating the primary trail into the functional channel cross-section, which increased flood capacity, and enhanced experiential qualities by moving the trail away from the busy roadway and closer to the creek itself.

Successful completion of the project was founded on a more than 2-year effort to engage community members and multiple associated stakeholders with use of a carefully developed vision plan that looked holistically at the corridor. The plan envisioned progressive channel stabilization measures, environmental and water quality benefits, transportation improvements, equitable access to nature, open space and recreation opportunities, and prospects to increase quality of life characteristics for the surrounding public. The plan provided an invaluable tool to gain stakeholder participation and for fundraising. From 2014 to 2017, the partner group contributors/dollars expanded from 2/$2m to 6/$14m.

Channel and grading design

Nature-based channel features including wide floodplain benches, a sinuous active channel with riffle-pool sequences, and riparian vegetation were all implemented to improve floodplain connection, raise the groundwater table, lower flow velocities, and create a high functioning, lower maintenance stream system. “Riffle” drop structures integrate repurposed semi-rounded rock acquired from gold mining spoils to create less abrupt – and more natural looking stream grade-control features. Additionally, one of the areas with significant grading was in a known area of hazardous materials landfill. The design and construction approach of the site required creative cut/fill and soils management strategies to minimize exposure to – and management of – massive amounts of asbestos and other materials.

Additional channel and grading stats:

• 60,000 cubic yards of earthwork (incl. 150 CY of disposed asbestos)
• 20 drop structures – including 16 “riffles”, 2 sculpted concrete with trail crossings, and 2 modified (existing) boulder tie-ins
• 9 storm sewer outfall renovations and 2 water quality diversions

Recreation, trails, and community connections

Realigning the primary regional trail along one side of the channel and the incorporation of a secondary soft-surfaced trail along the other provides a drastically enhanced – and much safer – trail experience for both high-speed regional trail users and those looking for a more casual and peaceful experience. Additional connections to the adjacent communities help provide more equitable access to nature and recreation for thousands of underserved community-members who live in the area, along with a better commute for those who use the trail to get in and out of downtown Denver and the Cherry Creek district.

Additional recreation, trails, and community connections stats:

• 7 new/improved community connections
• 7,200 linear feet of new concrete trail
• 3,400 linear feet of new soft surface trail
• 3 trail creek crossings
• 3 rest areas with seating, trash cans, and shade

Soils and revegetation

The pre-project site had extensive weeds and little existing topsoil, and importing topsoil for the entire site was cost prohibitive. Through collaboration with ecologists and engineers, the landscape architects used strategic soil testing to develop an amendment plan which integrated compost and aged wood chips into native site soils to aid in chemistry and moisture retaining properties, used a native plant palette, and incorporated a site-wide irrigation plan to support healthy vegetation. In combination with lower flow velocities achieved in the channel design, this vegetation approach enabled the project to reduce the use of rock in drop structures and bank stabilization by as much as 30%, allowing the reallocation of budget to irrigation and soil preparation to develop sustainable – and durable- vegetation for channel stability.

Additional soils and revegetation stats:

• 40 acres of seed
• 382 Trees, 1514 shrubs, 73,699 plugs, 1,928 willow stakes
• 1,840 LF of willow fascines
• 40, 000 LF of irrigation lines