TAXI II

Challenge

The project was well underway, with a developed architectural design for the TAXI II building, when the developer engaged Wenk Associates to rethink the landscape and develop more innovative and cost-effective solutions to the site’s infrastructure challenges. Previously a landfill, the site was extremely flat, and was to be bisected by the 550-ft-long by 60-ft-wide “bar building” of TAXI II. Although the siting and shape of the long building supported the long-term strategy for phased development of the entire 15-acre parcel, it created a “dam” between the northwest portion of the site and the Platte River, which the site ultimately drains into. In order to create positive drainage, a traditional piped stormwater system would have required importing substantial amounts of expensive fill material to raise the building finished floor elevation (FFE) by an average of 4 ft. However, the economic recession at the time made the developer want to keep site development costs to a minimum. The developer also wanted to keep rents and purchase prices affordable to encourage artists, design firms, and other creative enterprises to relocate to this emerging arts district in a declining industrial zone in central Denver, which was extremely important to spur future development of TAXI’s later phases. Moreover, TAXI II also needed to accommodate nearly 400 surface parking spaces for its projected 500 employees, leaving very limited room for open space.

Solution

A more holistic approach was developed to integrate pedestrian spaces and landscape areas with the site’s stormwater infrastructure. The entire site drains to a system of surface swales and 7 stormwater gardens that convey, infiltrate, and cleanse stormwater runoff from roofs, streets, and parking areas, eliminating the need to import expensive fill and install any curb and gutter or underground storm sewer. 2 existing flood detention ponds constructed for the first phase of TAXI were also integrated into this system, so no additional detention ponds were needed for this phase. The curbless approach allowed for more usable open space, with the “undefined” streets and parking lot serving as outdoor plaza spaces for community events such as Denver’s annual National Food Truck festival. Simple, artistic paint lines were used on the asphalt to indicate pedestrian and plaza areas, and over 130 canopy trees were integrated into the green spaces, parking, and the loop road to reduce the perceived presence of the large parking lot. In addition, the cottonwood trees, wet meadows, wildflower prairies, and natural materials such as crushed stone, bring the Platte River’s appearance and ecology into the site and blur the lines between development and nature. 

• The surface stormwater system uses 7 Porous Landscape Detention (PLD) gardens, a series of surface swales, and 2 pre-existing shallow detention ponds to convey, infiltrate, detain, and cleanse stormwater runoff from the roofs, streets, and parking areas. 9 downspouts with associated splash plates or metal grating runnels direct runoff from the 2 building rooftops into the surface stormwater system, while the evenly-distributed PLD gardens allow stormwater infiltration to occur as close to where rain falls as possible.
• 2,950 sf of native grasses were planted as plugs or seeded within the 7 PLD gardens. Major functional species include blue flax (Linum perenne var. lewisii), Rocky Mountain iris (Iris missouriensis), prairie verbena (Verbena binpinnatifida), prairie cordgrass (Spartina pectinata), tufted hairgrass (Deschampsia cespitosa), and western wheatgrass (Pascopyrum smithii).
• 53,443 sf of short-grass prairie and 7,050 sf of mixed-grass prairie were seeded with high-diversity mixes in the stormwater conveyance and other landscape areas. Major species include blue grama grass (Bouteloua gracilis), green needlegrass (Stipa viridula), upright prairie coneflower (Ratibida columnifera), prairie Junegrass (Koeleria macrantha), and sand dropseed (Sporobolus cryptandrus).
• 7,500 sf of drought-tolerant flowering perennials were seeded at building entries and in other highly visible landscape areas. Major species include California poppy (Escholzia californica), Missouri evening primrose (Oenothera macrocarpa), Siberian wallflower (Cheiranthus allionii),  red dome blanketflower (Gaillardia pinnatifida), and lance-leaved coreopsis (Coreopsis lanceolata).
• About 100 deciduous trees, including both native and adaptive species, were planted in the stormwater conveyance and other landscape areas. Narrow leaf and plains cottonwoods were positioned to shade west-facing building facades from the afternoon summer sun and reduce building cooling requirements, while alders and hawthorns well-suited to soil moisture fluctuations make up the understory.
• About 38 deciduous street trees, including Shademaster honeylocust (Gleditsia triacanthos var. inermis ‘Shademaster’), swamp white oak (Quercus bicolor), and Autumn Purple ash (Fraxinus americana ‘Autumn Purple’), line the entry and internal circulation drives. Combined, the 130 trees planted on the site provide a canopy that reduces the perceived dominance of the large parking lot.
• 230 pieces of concrete bin-blocks made from surplus “washout” concrete from a concrete plant adjacent to the TAXI parcel were used for seat walls, benches, and retaining walls for the gardens and swales, greatly reducing the cost of materials.
• Other site elements and furniture, including a 16-ft-long metal fence, 2 bicycle racks, 3 painted concrete planters (made from recycled storm pipe), and an outdoor patio enclosure (with recycled industrial conveyors) were fabricated from repurposed industrial materials that underscore the project’s industrial context and history.
• The TAXI II Plaza is a 6,000-sf outdoor patio that features 1 PLD garden and a dense tree canopy, providing much-needed outdoor eating, social, and event space.
• A 10,000-sf open space at the entrance to the TAXI II building with its 6-ft  TAXI II Hill provides outdoor space for relaxing, reading, meetings, yoga, and other activities.

• Site construction costs were reduced by around $2.55 million through various design strategies, including elimination of curb and gutter and subsurface stormwater infrastructure, application of native seeding, and use of recycled materials. 
• Groundcover planting material cost was reduced by 76%, or $90,400, by using native seeding in most landscape areas while limiting ornamental grass usage to only highly visible areas.
• The use of a surface stormwater system eliminated the need to import $40,000 of fill material to raise the building’s FFE to create positive drainage with a traditional piped stormwater system.

• Pilot projects play an important role catalyzing changes to municipal ordinances, as was the case with TAXI II’s Stormwater Controlled Landscapes (SCLs). The landscape architect met regularly with the City of Denver’s Department of Water to refine the design and seek necessary approvals for stormwater quality treatment. In the end, TAXI II became the first project in the metro-Denver area to use Porous Landscape Detention (PLD) gardens as a means to meet requirements for stormwater quality. Since TAXI II, Denver’s stormwater codes have gradually evolved to recognize the stormwater attenuation functions of SCLs, with significantly reduced detention volume requirements if SCLs are present. 
• The site’s surface stormwater system has continued to function as designed after 10 years of service with little maintenance. The direct sheet flow to the Porous Landscape Detention (PLD) gardens and the sloping transitional grass buffers at parking lot edges have effectively prevented sediment from clogging the PLD soil media, while the interconnected linear swales designed with 0-1% longitudinal slopes encouraged vertical infiltration in the highly permeable soil media.
• A more rapid natural succession has occurred in areas that receive higher levels of stormwater runoff. For example, over the life of the project in the PLD gardens or near building downspouts cottonwood plantings have thrived, growing from 1-in caliper to over 30 ft tall in 10 years. Native coyote willows have self-seeded and formed several colonies at various locations. Riparian grasses and sedges have expanded as well. On the other hand, the understory of seeded meadow has thinned due to shading by trees and has evolved into native riparian plants.
• Deciduous street trees lining the entry and interior circulation drives have failed to thrive. The 4-ft-by-6-ft planting pits excavated in the drives and parking following installation of the asphalt and base course were inadequately sized to allow the trees to thrive. 
• Seeking out a landscape maintenance company that has experience with natural landscapes is very important. Although it was difficult when TAXI II was first built, it gradually became easier to find qualified companies with the surge in popularity of xeriscapes in Colorado over the past decade. Before the companies start a maintenance regime, it is essential to conduct a site walkthrough and training to ensure proper identification of native species.

Bicycle Racks: Madrax; Model: CTL-3-IG-G, Color: Galvanized
Dryland Seeding: Designscapes, Inc.; Short Grass Prairie Seed Mix, Mixed Grass Prairie Mix, Wildflower Seed Mix, PLD Seed Mix
Mycorrhizal Seed Inoculant: Pawnee Buttes Seed, Inc.; Mycorrhizal AM120
Erosion Control Blanket: Revex, Inc.; Greenfix America: WS072B Double Organic Net Straw, Straw/Coir Erosion Control Blanket
Sod: Designscapes, Inc; Colorado-grown Kentucky Bluegrass/Fescue blend
Bin-block: Ready Mix Concrete Plant adjacent to the site
Bollards: 10” OD Diameter corrugated galvanized pipe, filled with gravel
Pedestrian Bridges: Peterson Company; traction tread grating
PLD Soil Mix: Designscapes, Inc.; 33% peat, 66% sand
Geotextile for Crusher Fines Areas: Mirafi 140 N
Geotextile for PLD Areas: Mirafi, Nonwoven Geotextile Fabric ASTM D4751- AOS U.S. STD. Sieve #40 to #60 ASTM D4632- MIN. Grab Strength 120 lbs

Project Team

Landscape Architect: Wenk Associates, Inc.
Associate Architect - Building Massing and Materials: Will Bruder & Partners Ltd.
Associate Architect - Residential Unit Design: David Baker & Partners 
Associate Architect - Owners’ Representative: Alan Eban Brown Architects 
Associate Architect: Harry Teague Architects
Civil Engineer: York Engineering Services
General Contractor: Mortenson Construction Company
Landscape Contractor: Designscapes

Role of the Landscape Architect

The landscape architect was responsible for all site planning and conceptual site engineering including overlot grading and surface stormwater system design. The firm collaborated with architects to establish indoor-outdoor relationships between landscape areas and the circulation system, prepared conceptual and final landscape design plans, and administered stormwater system and landscape construction. The landscape architect was specifically called upon to reduce costs for the landscape portion of the project.