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Denver Ultra-Urban Green Infrastructure Guide
A guide for Denver for the design, installation and maintenance of "ultra-urban" water quality treatment BMPs for low-impact development



This design rendering by Stream Landscape Architecture show its vision for 4-way bumpouts in Denver. Bumpouts are becoming more prevalent as a means of calming traffic and expanding vegetation within the road right-of-way. A bumpout stormwater planter 40' long can provide almost twice the water quality capture volume (WQCV) as a streetside stormwater planter the same length. A streetside planter typically has an inside width of 5'; the bumpout stormwater planter can be up to 12' in width. Areas with a minimum width of 5' feet can incorporate trees. One corner bumpout planter and one midblock bumpout planter can satisfy Denver's water quality requirements for a city block from crown to ROW line for Denver's local and collector street classifications. Denver's four and six lane arterial classifications require additional planters, such as two corner and two mid-block bumpouts.

Stream Landscape Architecture of Denver, a firm that specializes in landscape architecture design for water resources and recreation related work, teamed with Muller Engineering to co-develop a guidance manual for the city and county of Denver (CCD) and its Urban Drainage and Flood Control District (UDFCD) for the design, installation and maintenance of "ultra-urban" water quality treatment best management practices (BMPs) for low-impact development in the city.

The EPA, which is committed to providing technical assistance to communities working to incorporate green infrastructure, provided contractor support for the development of the checklists in the manual. Tetra Tech prepared plan review, construction inspection and maintenance checklists, as well a number of renderings.


Stream Landscape Architecture teamed with Muller Engineering to co-develop a guidance manual for the city and county of Denver and its Urban Drainage and Flood Control District. The manual provides detailed BMPs for streetside stormwater planters, bumpout stormwater planters, green gutters and alleys, and tree trenches.

"These BMPs are regionally appropriate landscape elements that meet EPA guidelines for removing pollutants from urban stormwater, and also reduce stormwater runoff volumes (and downstream flooding) though short-term storage, infiltration, and uptake of stormwater by plants," explains Stream Landscape Architecture.

Developing these multifunctional, green infrastructure elements required the landscape architects and engineers to coordinate intensively with numerous city and regional agencies, including those focused on stormwater management, transportation/roadways, parks and forestry and urban design/planning.




A streetside stormwater planter is intended to provide water quality treatment of runoff. Stormwater runoff enters the streetside planter through a curb opening and chase type inlet, spreads over the planting media, infiltrates and exits through an underdrain. Inlet width is based on upstream impervious area and street slope, with a minimum width of 2' feet and a maximum width of 3'. The inlet cover specified has to meet ADA accessibility requirements and be of the Neenah R-4999 heavy-duty bolted trench grate type.

The purpose of the manual was to provide the city guidance in integrating large-scale green infrastructure through best management practices (BMPs) for water quality treatment and stormwater detention. The BMPs are designed to aid engineers, designers, planners, developers and municipalities to implement attractive, maintainable and sustainable stormwater elements that enhance the streetscape and pedestrian environments.

The team developed guidelines for BMPs to capture and treat runoff from public roadways and sidewalks, as well as from more typical urban site-generated runoff sources such as rooftops and parking lots. The majority of BMPs include features which collect trash and sediment, provide infiltration, promote biological uptake and support a variety of feasible vegetation alternatives which provide water quality benefits, but also improve aesthetics through natural elements and reduce the heat island effect.


The Western Prairie typical planting plan for a stormwater planter is primarily native grasses and herbaceous perennials arranged in a natural way and amended based on which species thrive. Stormwater planters require a significant amount of maintenance to keep them weed free and healthy. Prairie plantings can be more forgiving and require less maintenance than the more formal "modern matrix," which uses drought tolerant, hardy natives and groups them in regular clusters.
Images: Stream Landscape Architecture

The manual provides detailed BMPs for streetside stormwater planters, bumpout stormwater planters, green gutters and alleys and tree trenches.

A special focus of the manual was the planning and design of treatment facilities for runoff generated by public streets and roadways.


A green alley handles stormwater by sloping toward a central permeable pavement section, which typically runs within the central third of the alley for its full length. PICPs can greatly increase the attractiveness of alleys. A green alley, however, should not be installed where hazardous materials are loaded, unloaded or stored. If contamination exists an impermeable liner is necessary. In all cases, the outlet from the underdrain (4" PVC pipe with two 45? bends and a threaded cap set 2 inches below the top of the pavement) must be accessible via an inlet or manhole.


The upstream end of a block is generally not an effective place to install a tree trench; the middle or downstream end of a block is usually conducive. If serving just the public ROW (street and pedestrian zone), three tree trenches with three trees each can satisfy the water quality requirements for an impervious area measuring 400 feet (a typical city block) by 30 to 34 feet (street crown to ROW) for Denver's local and collector street classifications. Trees are typically located 4-6' from the back of curb. The preferred treatment surrounding the tree is a 4" curb with no tree grate. This allows the soil surrounding the tree to be visible while protecting the soil from compaction. When tree grates are used, a minimum separation of 4 inches between the grate and the tree trunk should be maintained. Grates shall be easily maintainable and need to be inspected on an annual basis. Another option is to use PICPs on top of the tree trench and continuing this to within 6-12" of the tree trunk.

As seen in LASN magazine, January 2018.

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December 8, 2019, 8:29 am PDT

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