By Kim Hartley HawkinsOVERVIEW:
Who: Metro Nashville Public Works
What: First Tennessee street to incorporate low-impact design into the public right of way
When: October 2008 to October 2009
General contractor: Roy T. Goodwin Contractors, Nashville, Tenn.
Landscape architect: Hawkins Partners Inc., Nashville, Tenn.
Cost: $3.1 million in capital improvement funds
Deaderick Street was part of Nashville's history long before “Music City” was incorporated in 1806. The capital of Tennessee is on the Cumberland River, so the street provides ready access to and from one of the South's busiest shipping ports. But even with the state Legislative Plaza at one end and the city's Public Square on the other, the three-block stretch of central business district looked old and worn out.
Today, though, anyone can see how the street links the two levels of government. In addition to providing a civic axis, the street plays a major role in the city's economic life. Downtown workers, residents, and tourists pass beautiful plantings as they stroll along, and people who ride to the area have a safe place to stow their bicycles.
Recently accepted as a national pilot project for SITES, the Sustainable Sites Initiative established by American Society of Landscape Architects with the Lady Bird Johnson Wildflower Research Center, the transformation gave Metro Nashville Public Works the perfect opportunity to use low-impact design to manage stormwater.Alleviate flooding, promote commerce
In the 1970s Deaderick Street was reconfigured from two lanes surrounded by three- and four-story buildings to four lanes with traffic buzzing past buildings five times as tall.
It was also the site of the Metro Transit Authority's transit mall. When the authority announced plans to move the mall, public works saw an opportunity to turn the street into a visually striking and pedestrian-oriented byway that contributes to long-term sustainability goals.
The street is located within the Kerrigan Basin, one of the city's 10 combined storm sewer basins, which occasionally overflowed to the Cumberland River. Only 2,000 square feet of surface — primarily in the form of 4x4-foot street tree grates — was pervious. Any stormwater mitigation would be seen as an asset, especially if it also promotes pedestrian activity through additional retail, commercial, and residential development.
Following six months of meetings with other city departments and the public, public works established the project's goals:
- Re-establish the connection between the city's Public Square and the state's Legislative Plaza
- Implement as many sustainable/green initiatives as possible
- Establish multimodal opportunities
- Build in a way that supports short-term, mid-term, and long-term needs as well as promotes residential, commercial, and retail development
- Create a public and private initiative
- Provide a project on time and within budget.
The long-term master plan identified additional development potential of 40,000 square feet of retail and restaurant uses, 11,000 square feet of office use, and 94 residential housing units. Potential retail uses included small-scale sidewalk kiosks that might function as a newsstand, flower shop, or sandwich shop. The absence of the transit mall greatly reduced average daily traffic volume, making the street more welcoming to pedestrians and providing on-street parking. The planning also considered special events and festivals with provisions for water and electrical needs.
The master plan included extensive analysis of all current codes, license requirements, and regulations that might be required for vendors and potential kiosk tenants. A land use/special events matrix was created to address existing codes and laws and suggestions for amendments to allow for activities like outdoor dining and restrictions on serving alcohol within the right of way.
Deaderick would become the first street in Nashville and the state to incorporate low-impact design features within a public right of way. With a primary goal of sustainability, education was needed at many levels to explain the approach.
From the outset, public works wanted to explore the sustainability and aesthetic potential of low-impact design while also evaluating the engineering aspects to provide a balanced solution. During the planning process, the department studied the successes of other cities — such as Portland, Ore., and Seattle — and research from low-impact implementations within private development sites in the region.
Learning about the implementation of green streets extended beyond public works to the many city and state departments involved in the project. Ultimately, eight wayfinding kiosks were designed to include graphic panels that interpret the benefits of street trees on the environment and the community as a whole and explain how infiltration works.
Storm drains and natural solutions
The plan incorporates three primary low-impact design elements: continuous street trees and planting zones on both sides of the street, bulbouts, and the addition of a central median.
Street trees are planted within 8-foot-wide rain gardens or 8x8-foot tree grates.
Sidewalks have a cross slope of 1% to 2% to drain into the gardens' 6-inch depressed soil areas. Bioswales have filter fabric, a 1-foot aggregate drainage layer, and an underdrain system to tie into the storm sewer, all of which are under 4 feet of engineered soil. Infiltration rates early in the demolition/excavation process far exceeded the required ½-inch/hour rate, so these measures were eliminated from all design elements on the street.
High-traffic pedestrian areas that needed a higher percentage of paved surface received the grated-tree solution. The planting areas are connected to each other with a 3-foot trench of structural soil. The soil is capped with 4 inches of porous concrete pavement to allow root zones to meet the minimum 100-square-foot root zone requirement.
Bulbouts at each corner provide safer crossings for pedestrians, the ability to accommodate future retail kiosks, and infiltration bioswales to be incorporated within downhill slopes.
These bioswales are planted with grasses, perennials, and sedges in 2 feet of engineered soil. Existing conditions to be accommodated included storm drains. To allow for infiltration, the bioswales provide a 12-inch depression with an overflow built up with concrete pavers around the existing storm drain that allows for drainage during major storms.
The north end of the street had an existing slope of 7% to 8% and velocity of water flow presented a challenge. Flows are dissipated by using a modified curb grate and concrete pavers mortared and set at varying levels instead of loose gravel or stone as originally proposed.
The 8-foot median reinforces the visual axis between the civic termini at each end of the street while adding pervious surface area.
An inventory conducted to prepare for the project recorded 97 street trees representing eight species ranging in size from 3- to 24-inch-caliper, all planted in 4x4-foot tree grates. More than 60% were in poor to hazardous condition.
The street's east/west orientation presented some vegetation challenges. The south side is heavily shaded by office buildings, while the north side is in direct sunlight. To address those variables, species selection included native and adapted species with ferns, hosta, hellebores, itea, oakleaf hydrangea, carex, and juncus for shady areas; and callicarpa, daylilies, catmint, and cotoneaster among others for the sunnier areas.
Species were carefully chosen in cooperation with public works, which maintains the plantings. Native plants make up 53% of the species; 102 4-inch-caliper shade trees were specified using overcup oak and Princeton elm; and more than 4,249 shrubs, perennials, sedges, and groundcover plants.
The project was implemented block by block to allow as much of the street as possible to remain open during construction.
In addition to weekly team meetings on site, public works offered weekly information meetings and e-mails to local businesses and stakeholders and provided an onsite construction trailer with full-time staff to address construction conflicts or emergencies with affected property owners. The Tennessee Performing Arts Center in particular required careful coordination with scheduled set ups and break downs of shows and patron access.
The project incorporates additional sustainability measures beyond storm-water management, including:
- Low-flow irrigation nozzles and soil moisture probes that use at least 33% less water
- LED traffic signals, pedestrian lights, and guidance lighting
- Solar-powered parking meters
- Recycled crushed concrete for sidewalk subbase
- Recycled coal fly ash with concrete
- A high percentage of recycled steel in streetlights, tree grates, site furniture, and wayfinding kiosks
- Bicycle storage facilities
- Educational kiosks that explain the benefits of urban trees, rain gardens, and sustainable infrastructure.
Public works is using lessons learned from the project as the model for new roadway projects. The incorporation of sustainability measures at every level makes Deaderick Street one of the Southeast's first green streets.
— Kim Hartley Hawkins (firstname.lastname@example.org) is the founding principal of design firm Hawkins Partners Inc., Nashville, Tenn.