Landscape Performance Benefits
- Prevents 76,000 gallons of annual stormwater runoff from entering the city’s aging combined sewer system by collecting and reusing runoff from a 40,000 sf area of the courtyard and office building roof and reusing it in the courtyard for irrigation and water feature replenishment.
- Eliminates the use of potable water in the landscape, saving 76,000 gallons of water and approximately $730 per year. The use of native and drought-tolerant plants, as well as high-efficiency irrigation systems, helps by reducing the amount of water needed for irrigation by 62%.
- Reduces summer rooftop temperatures by using a combination of vegetated and light-colored roofing materials. Air temperatures above the green roof were an average of 3.5°F cooler than above the light-colored roof areas and overall peak roof surface temperature is estimated to be approximately 40°F cooler than a conventional black roof.
- Will sequester over 12,000 lbs of carbon annually and supply an estimated 24,000 sf of shade, when the 34 street trees reach their mature size, as projected by the 900 cu ft of structural soil available per tree.
- Provides a space of respite and relaxation for visitors, office workers and residents, with users observed to spend an average of 21 minutes in the courtyard.
- Creates an active streetscape with a 58-ft wide landscaped sidewalk where an average of 90 individuals were observed using the space for outdoor dining during the summer months.
- Contributes to the estimated $11.5 million in annual city tax revenue generated by the full transit-oriented development (TOD) project, which includes commercial office space, retail and residential units.
At a Glance
Sasaki Associates, Inc.
Former Land Use
2221 I Street NW
Washington, District Of Columbia 20052
Located in downtown Washington, DC, The Avenue is a mixed-use transit oriented development (TOD) of office space and residential units over street level retail. Formerly referred to as Square 54, the 3.5-acre site is bordered by George Washington University (GWU) to the south, the Foggy Bottom-GWU Metro Station and GWU Hospital to the east and the busy Pennsylvania Avenue corridor to the north. The development is enhanced by a collection of public and private green spaces including a central courtyard with water feature, a retail-lined sidewalk promenade and rooftop patios. Through the implementation of innovative stormwater management strategies, these spaces collect and reuse rainwater falling on the site while also offering visitors, office building employees and residents a pleasurable outdoor experience.
- Almost 60,000 sf of public space and over 21,000 sf of private space is provided for visitors, employees and residents by the courtyard, streetscape and rooftop patios of the site.
- A specially designed structural system supports the water feature and vegetation of the courtyard, which is located directly above the five level underground parking garage.
- A 7,500 gallon cistern, located below the central courtyard in the underground parking garage stores stormwater collected from a 40,000-sf area of the office roof and courtyard.
- Two sand filters, an ultraviolet (UV) sterilizer and an ionizer clean collected stormwater, which is then reused in the courtyard for irrigating the landscape and replenishing the water feature. While a standard infiltration system uses chemical applications of chlorine or bromine to kill bacteria, the ionizer releases copper and silver ions into the water, killing algae, bacteria and viruses. This allows plants to grow directly in the water feature and provides a chemical-free water source for birds and insects. The filtration system only requires cleaning weekly, as opposed to daily, water quality monitoring, and long-term maintenance is reduced because equipment and pool surfaces will not need to be replaced due to chemical damage over time.
- Aquatic vegetation is placed within the water feature in perforated planters, allowing the roots to provide supplemental filtration to the water.
- The landscape of the central courtyard provides green views for interior-facing offices and 34% of residences, while a combination of 17 benches and 175 linear feet of seat wall provides places for visitors to spend time in the park spaces.
- An ADA compliant ramp provides public access to the interior courtyard via 23rd street, drawing users into and through the site. Directly across the street from this entrance is the GWU Hospital with over 1,600 employees.
- Two 4,000 sf green roofs are located on the office building and the larger of the two residential buildings. The green roofs are laid out in a series of custom designed trays made up of a water retention layer, a drainage layer, filter fabric, 5 in of lightweight engineered soil and a variety of succulent sedum plantings.
- 80% of plant species utilized in the landscape are native or drought-tolerant to reduce irrigation needs.
- A 58 ft wide retail-lined sidewalk promenade runs along I street and aligns directly with the entrance to the Metro station across 23rd street. The promenade is 56% wider than the minimum city standards for a street, and is designed to provide gathering places while also allowing efficient movement to the station. The adjacent Metro station serves over 42,000 riders on an average weekday and the expanded promenade accommodates the high volume of pedestrian traffic while also providing 188 seats for outdoor dining.
- 35 street trees provide pedestrians with shade and protection from traffic while large planting beds filled with mixed perennials, low shrubs and flowering trees, provide colorful seasonal interest.
The design of the central landscaped courtyard was difficult due to a combination of factors. First, the courtyard is located directly above the five levels of underground parking for the development, potentially limiting what could be built. Second, the public entrance to the space at 23rd street is approximately 15 ft (one story) lower in elevation than the courtyard. Third, the city required a little more than half of the courtyard to be designated as public space, and to encourage public use, this space had to be as open and accessible as possible. On the other hand, the courtyard space next to the residential buildings was designated as private, so access to this space needed to be restricted and residents needed to feel secure at night. Finally, surrounded by twelve story buildings, the courtyard would receive very limited sunlight, potentially restricting the vegetation that could be utilized in the design.
A series of design solutions were employed to meet the challenges of constructing the central courtyard. First, the landscape architect coordinated with the project engineers to design the structural system over the parking garage to allow for the planting of large trees, lawn and grasses. Second, to provide public access to the site via 23rd street, a long ramp was designed to accommodate the fifteen foot elevation change from the street to the courtyard. Third, for the public courtyard area to be open and accessible, the entrance ramp is ADA compliant and low profile grasses were planted along it to allow for unobstructed views into the courtyard. Alternatively, the private courtyard space is divided from the public space by a low fence of metal cables, restricting physical access while still allowing visual access across the landscape. At the public entrance, gates designed to be “invisible” to users when open are secured at night and a smaller gate allows residents to gain access to the private space using a key. This design attempts to provide a balance between the city’s desire for public access and the residents’ need to feel secure. Finally, to allow more sunlight in the courtyard, the residential property was split into two buildings connected by a two story lobby. This south-facing lobby is approximately 120 ft shorter than the residential buildings, allowing light into the courtyard when the sun is lower in the sky.
- Stormwater on the site is collected, filtered and stored in a cistern under the central courtyard. This water is utilized in lieu of potable water for the irrigation of plants and to replenish water lost to evaporation in the water feature. The storage tank and pumping and filtration systems cost approximately $42,000 to install and have an initial maintenance cost of $500 per year. With 76,000 gallons of stormwater collected and reused annually, the cost of the system could be offset with savings in potable water in approximately 33 years.
- In order to achieve a certain landscape aesthetic soon after completion, large caliper trees were specified for the interior courtyard design. The larger trees required a crane for installation and the crane was only on site for a limited time during construction, forcing the trees to be planted earlier in the construction process than would normally occur. Although an attempt was made to protect the trees, they were covered by construction dust and many suffered severe negative effects. All of the large trees in the courtyard area eventually had to be replaced with smaller caliper trees towards the end of the construction process. In hindsight, once it became clear that the only time the larger trees could be installed was early in the construction process, the best strategy may have been to compromise on the design intent and use a smaller caliper tree that could be installed later, increasing the chance of survival. Alternatively, a comprehensive construction plan could have been developed before the larger trees were installed, detailing how they would be protected during the construction process to maintain their health.
- The courtyard water feature presented one of the main design challenges, due to the client’s interests in both sustainability, and achieving a particular aesthetic. The landscape architect wanted the design to collect and clean stormwater without chemicals, but neither the designers nor the client wanted the more natural-looking aesthetic of a typical planted stormwater feature, such as a bioretention area or constructed wetland. An innovative solution was developed utilizing a combination of sand filters, an ultraviolet (UV) sterilizer, an ionizer and a small number of aquatic plants to filter the water instead of plants and soil. The client was pleased with the sustainable aspects and clean aesthetic of the design, and therefore willing to pay for the higher initial cost. Because the designers carefully considered what would be appropriate for the site and worked closely with the client throughout the process, they were able to implement a design that met their goals of collecting and treating stormwater, satisfied their client and fit well with the character of the urban environment.
- In order to reduce the urban heat island effect of roofs, LEED recommends utilizing light-colored, highly reflective materials or green roofs or a combination of the two. This project incorporated an extensive green roof system for some areas and light-colored pavers for the resident and employee rooftop patios. While the designers had intended to use the lightest colored pavers to maximize reflectivity and environmental benefits, they soon realized these pavers would create a surface that was not comfortable for people due to its brightness. Creating rooftop spaces for residents and employees was a goal established early on in the project, so the designer was willing to use slightly darker pavers, with less reflectivity, in order to provide a space where people would enjoy spending time. Prioritizing their objectives and remaining flexible with their plan allowed the designers to adjust when they realized that their goal of reducing urban heat island effect conflicted with creating a comfortable space for people on the roof.
Developer: Boston Properties, Inc.
Executive Architect: Hickok Warner Cole Architects
Design Architect: Pelli Clarke Pelli Architects/ Landscape
Architect: Sasaki Associates, Inc.
Structural Engineer: Thornton-Tomasetti Group
MEP Engineer-Office component: TOLK
MEP Engineer-Residential component: KTA Group
Civil Engineer: Wiles Mensch Corporation
Geotechnical Consultant: ECS
Preconstruction Services Consultant & General Contractor: Clark Construction
Role of the Landscape Architect
The landscape architect collaborated with the design architect on the overall site masterplan early in the design process, deciding on the massing of buildings and the shape and access plans for outdoor spaces. As the masterplan advanced, the landscape architect worked closely with the owner and other team members to design green spaces and the stormwater management system and to emphasize the importance of landscape in the final site plan. Working with city officials to rezone the site and negotiate the incororation of public space and sustainable features was another key aspect of the landscape architect’s work.
Case Study Prepared By
Research Fellow: Victoria Chanse, PhD, Assistant Professor, University of Maryland
Research Assistant: Jen Salazar, PhD candidate, University of Maryland
Firm Liaison: Mark Delaney, Senior Associate, Sasaki Associates, Inc.