Purpose and Approach:

In recent years – like other cities across the country – the City and County of Denver has amplified its use of “green infrastructure” to manage stormwater, cool city streets, create more equitable neighborhoods, and provide safer connections for bicyclists and pedestrians. By adopting the best practices outlined in Denver’s earlier “Ultra-Urban Green Infrastructure Guidelines (UUGIG)”, capital projects are increasingly weaving valuable natural processes back into the urban fabric by implementing green infrastructure. Many municipalities that have embraced the use of green infrastructure to address stormwater quality and volume challenges have done so to meet regulatory MS4 requirements. Often this approach also occurs in cities with “combined sewer overflows” (CSOs) where storm sewer capacity can be hamstrung by increased demand from sanitary capacity as density increases. Denver does not have CSOs, however current MS4 requirements do limit the occurrences of required stormwater control measures (SCMs) to relatively larger consolidated development sites, and even though many transportation projects do not trigger a regulatory post-construction SCMs requirement, there is a tremendous opportunity to include green infrastructure into more frequent (often smaller) applications to add pervious, vegetated areas to the urban landscape for filtration and infiltration of stormwater runoff – and to provide additional life-quality benefits to people in the community.

While Denver’s UUGIG were specifically developed for site-scale ultra-urban green infrastructure SCMs on projects that do trigger compliance requirements, it has been realized that UUGIG- type SCM’s are often over-engineered for the project context, and too expensive for projects that would still benefit from the use of green infrastructure strategies. In addition, the Continuum philosophy that “a lot of a little can be far more effective than a little of a lot” has gained significant traction in the municipal stormwater infrastructure realm, thus the Denver Green Continuum: Streets Guidelines (Continuum) was born. While the Continuum is intended to be a companion document to the recently adopted “Denver Complete Streets Design Guidelines” (which focuses on public street configurations that accommodate the pedestrian realm, safety and accessibility, and utilities) – the practices detailed within the Continuum are very appropriate to be put into practice in both public and private spaces. In short, the Continuum is a guide for designing novel and scalable right-of-way green infrastructure that targets both stormwater management and urban heat effects at the interface of the right-of-way and public/ private realms.

The Continuum categorizes green infrastructure into five “Levels of Green” (LoG) where higher levels (i.e. LoG’s 4 and 5) address greater stormwater volume control on the order of 80%-100% of MS4 required capture volumes – but require much higher degrees of engineering and design, are expensive to build, have more invasive construction impacts, and often limit the urban tree canopy because of the amount of structure required. Lower and mid-levels (i.e. LoGs 1,2, and 3) provide a healthier tree canopy and increased shade, with simpler design, for a much lower cost, with less impact – and can still provide significant stormwater runoff mitigation when implemented more frequently across urban areas. The Continuum is a progressive planning guide that documents the policy and climate drivers that led to the development of the Continuum philosophy; outlines the criteria for each LoG; illustrates the Components and Control Measures that may make up each LoG; and summarizes the analysis and engineering principles behind the criteria and performance metrics for each LoG. The document also includes examples of how the Continuum can be applied to urban street projects and how “mixing and matching” different LoG’s can help address stormwater and heat island more effectively while also allowing for flexibility in urban design and integration into the context.

Scope and Role of the Landscape Architect:

The landscape architect worked in tight collaboration with the client in thought leadership and leadership of the consultant team (which included civil engineers and other technical experts); research on relevant green infrastructure practices and compatible urban design approach; comprehensive stakeholder and staff workshops; advancement of the Continuum philosophy; development of LoG’s and Components and Control Measures; and overall graphic and document production.

Context:

The Continuum is meant to provide context, guidance, and inspiration for implementing a variety of green infrastructure strategies designed to reduce stormwater runoff, improve water quality and/or mitigate urban heat along streets in Denver, however pertains to – and can be adapted to – a variety of conditions across the arid west region or in municipalities that could benefit from innovative stormwater quality, runoff reduction, and urban heat management approaches. The Continuum aims to provide a more robust, versatile, and practical set of green infrastructure tools and solutions that are applicable to all types of city streets and conditions regardless of MS4 requirements.

Special Factors, Environmental Sensitivity, and Project Significance:

In Colorado, research indicates that climate change is responsible for more variable precipitation patterns, warmer ambient temperatures, more intense wildfires, and poorer air and water quality. Climate models project that the average temperature will increase by 2.5°F to 6.5°F by 2050 – which will continue to exacerbate environmental and public health problems. To mitigate these impacts and protect Denver’s high quality of life, green infrastructure has been identified as a critical tool to be implemented on a greater citywide scale. The Continuum builds upon other City policies and guidance documents with intent to expand the extent and rate of implementation of green infrastructure, in large part, to mitigate and adapt to increasing climate-change impacts.

The Continuum defines five levels of green that range from simply adding vegetation with proper growing conditions to streetscapes, to planting trees in depressed landscapes, to fully engineered stormwater quality detention planters that store regulatory water quality volumes. By offering a greater suite of green infrastructure resources, Denver can more rapidly implement green infrastructure solutions. The Continuum contains information for project planners, designers, and managers who are implementing green infrastructure along with transportation or other physical infrastructure improvements. The document is meant to provide context, guidance, and inspiration for implementing a variety of green infrastructure strategies designed to reduce stormwater water runoff, improve water quality, and mitigate urban heat in Denver – and ultimately expands the toolbox for stormwater control measures (SCMs). Each chapter builds upon the premise of the Continuum philosophy and vision of utilizing green infrastructure as a natural climate solution as outlined in Chapter 1. Chapter 2 describes the design principles of SCM design to meet stormwater and heat objectives. Chapter 3 provides context by introducing and defining the five Levels of Green (LoG) – which are groups of SCMs distinguished by performance when reducing runoff or temperature. It also contains performance and design criteria summaries for each LoG. While a certain LoG may be better performing for one objective, different LoG’s can be combined on a project to provide multiple outcomes and benefits. Chapter 4 is designed to inspire decision makers, planners, and designers by showing graphic examples of SCMs and the components that make them up. Examples are paired with narrative text explaining why each SCM is appropriate in context and how it’s components can be designed to fit into a specific LoG. Components are pieces that, when assembled, form an entire SCM. Chapter 4 also provides options for key components and guidance for selecting from those options. Chapter 4 intentionally shows graphic SCM examples as well as component options to provide some technical guidance while encouraging designers to be creative. Chapter 5 offers guidance for project managers to understand the heat and stormwater needs of their project, which can be used to identify the appropriate level or levels of green. Chapter 5 lists the considerations important for green infrastructure projects specifically during the planning and design process. Finally, the Appendices contain additional useful data and information including plant lists for the SCMs and a summary of the technical analysis performed to establish the performance and design criteria for each LoG.