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Stormwater Capture on K Street11-29-10 | News

Stormwater Capture on K Street

Timmons Group, Richmond, Va. ?EUR??,,????'?????<

The west sidewalk is pressed concrete block pavers (downtown D.C. standard) with 8x12-in. granite curb and brick gutter. The exterior bioretention planter seatwalls are constructed of Jerusalem gold honed stone with Rugo granite caps. The planter at the northwest corner of the building incorporates the water feature, which spills into an interior walled infiltration basin, then to the underground cistern. Filter fabric lies between the soil and walls. The side slopes are planted with 3-in. pine mulch. Washed river stone (6-8-in. dia.) sits atop a geotextile fabric layer. A perforated hope underdrain with sock leads to the cistern.

During the recent ASLA annual convention of landscape architects in Washington, D.C., one of the field sessions was ?EUR??,,????'?????<

This was not the only group to tour the site. Staff members from the EPA?EUR??,,????'?????<

Let?EUR??,,????'?????<

Stormwater infiltration is via two levels of green roofs and three bioretention planters on the ground. Excess water from the green roof tray systems, plus water from the building?EUR??,,????'?????<

Rooftop Stormwater Capture
Stormwater on the roof is infiltrated via two tiers of green roofs. The lower tier green roof (an 8-inch soil tray system) is an outdoor terrace for people to enjoy. The upper tier green roof (with the 4-inch trays) is only accessed by maintenance personnel.

Excess water is slowly released through small openings in the Hydro-tech green roof trays and go to the 12,000-gallon underground cistern on site. Voids within the rigid trays are greater than the volume of a one-year storm event. If there were a greater storm, the overflow would go to the cistern.

This plan view shows the three primary planting areas, including two tiers of green roofs, one with 4-in. of soil (in blue), and one with 8-in. of soil (in green) for the terraces and then the streetscape bioretention plantings (in brown).

Engineered Soil
For the upper and lower tier green roofs, the American Hydrotech ?EUR??,,????'?????<

?EUR??,,????'?????<

1/16?EUR??,,????'?????<

Course to Medium Sand: 0-30%

Perlite, Sphagnum (soil additive): 0-30%

Composte: 5-30%

Water from the cistern passes through a filter, then flows over a two-foot weir to feed three interconnected bioretention areas framed by low walls. The bioretention areas at street level are not large enough to handle all plaza hardscape runoff alone, however, the combination of the bioretention and the cistern do the job, plus accommodate any overflow from the green roofs.

The Upper-Tier Green Roof
The upper level green roof comprises 3,800 sq. ft. Green roof design involved selection of durable plants with potential for four-season coverage and flowering color. After consulting with Emory Knoll Farms of Maryland, the top five growing sedums for the D.C. area (USDA hardiness zone 7a) were selected for this green roof, four evergreens (sexangulare, album, floriferum ?EUR??,,????'?????<

The bioretention planters on the north side of the building are thickly planted.

iris germanica

begonia ?EUR??,,????'?????<

Salvia nemorosa ?EUR??,,????'?????<

Coreopsis verticillata ?EUR??,,????'?????<

liriope muscari

uniperus horizontalis ?EUR??,,????'?????<

calamagrostis x acutiflora ?EUR??,,????'?????<

ilex glabra (?EUR??,,????'?????<

Cornus sericea ?EUR??,,????'?????<

ilex crenata (?EUR??,,????'?????<

How the species were laid out was critical. The kamtschaticum was spread sporadically across the upper-tiered green roof, however, the other plant species were organized in bands. Where two varieties came together, plantings were staggered across the line to avoid the look of an abrupt shift. Species were alternated across the gradient, separating the varieties of least moisture tolerance, and keeping the most durable along the edges and anchoring the center areas.

The green roof, of course, experiences weather extremes. The selected sedums are not only extremely drought tolerant, but because D.C. gets periods of prolonged rain, they are ?EUR??,,????'?????<

The upper tier green roofs (4-inch soil trays) cover the central portion of the rooftop (pictured) and a strip down the east side. These are accessed only by maintenance crews. Five sedum varieties (four evergreen and one deciduous) were organized in bands, but alternated across the gradient, separating the varieties of least moisture tolerance and placing the most durable plants in the center and along the edges. As this layout matures, the hardiest species are positioned intermittently in all areas to hold soils during a possible hard kill of all other plant species.

The Lower Tier?EUR??,,????'?????<
The 8-inch soil depth tray system on the building?EUR??,,????'?????<

The detail of the bioretention planters at street level shows the placement of two planters on the north side of the building (picture), and the one on the northwest wing that incorporates the water feature. There are six 'Hightower?EUR??,,????'?????<

Terrace Plants
The interactive nature of the terrace required thoughtful plant selection. Species need to be lightweight (nixing the temptation to use cacti), produce minimal leaf litter and survive under winter and summer climate extremes. Maintenance issues were, of course, an important consideration, as were the aesthetics of beautiful, large flowers and intense colors.

While emphasis on this ?EUR??,,????'?????<

Ornamental grasses were also included: Schizachyrium ?EUR??,,????'?????<

Clumpers were used to avoid migration of the grasses. Seed production was a concern, especially for Stipa.

Consistent maintenance will be required, but was judged worthwhile for the textural and movement afforded bygrasses.

The building owner desired bamboo. The general consensus was that bamboo would not do well on a green roof, given the climate effects. Also, bamboo is not particularly drought tolerant. Containing the bamboo, i.e., keeping bamboo roots from migrating into the green roof plantings, was another issue. Some of these initial concerns were solved by placing the bamboos (Fargesia robusta) in raised planters and protecting them will a glass-framed open?EUR??,,????'?????< Top Photo: Pau Hana Productions

Terrace Planters
The building owner desired bamboo. The general consensus was that bamboo would not do well on a green roof, given the climate effects. Also, bamboo is not particularly drought tolerant. Containing the bamboo, i.e., keeping bamboo roots from migrating into the green roof plantings, was another issue. Some of these initial concerns were solved by deciding to create raised planters for the bamboo and protecting them will a glass-framed open?EUR??,,????'?????<

Selecting the appropriate bamboo specie was the next challenge, not an easy matter considering there are 10,000 species! With the bamboo in planters, it was a given these plants would need more attention and require high fertilizer levels. Temperate varieties are typically deciduous and are of the ?EUR??,,????'?????<

The lower tier green roof terrace on the west side of the building employs 8-inch soil trays. The terrace view with the Washington Monument is looking south. The other view is looking north. Grades mimic the bioretention beds at the street level. Long sweeps of paver bands complement this green roof. Sedums predominate, but perennials include Achillea, Allium (bulb), Coreopsis, Delosperma, Dianthus Helianthemum, Rudbeckia, Talinum and Thymus (herb). There?EUR??,,????'?????<

On the Ground
The visual connection between the green roofs and the ground plane are three heavily vegitated bioretention planters framed by low seat walls (16-inches tall) and six ?EUR??,,????'?????<

The planters do not receive direct plaza drainage, nor, even if so designed, would they be of sufficient size to handle all plaza hardscape runoff. The combination of the bioretention areas and the cistern do handle the plaza?EUR??,,????'?????<

About 66 percent of the cistern?EUR??,,????'?????<

LEED Gold Certification
The 1050 K. Street Building received LEED Gold Certification. The water quality and quantity treatments strategies significantly improved the sites stormwater runoff volume, without compromising the attractiveness of the streetscape and landscape.

The Lenkin Co. and Tower Companies call the 1050 K Street NW building in Washington, D.C. the ?EUR??,,????'?????<

1050 K. Street Washington, D.C. Team

LEED???????(R)???? Gold Certified/Streetscape Bio-retention/2-Tiered Green Roof

Developer
The Tower Companies, The Lenkin Company Management Inc. Bethesda, Md.

Landscape Architect/Horticulture
Timmons Group, Richmond, Va.
Lu Gay Lanier, LA, FASLA, LEED AP: Landscape Architect/Environmental Scientist
Neal Beasley, VCH: Horticulturalist/Landscape Designer
Charlene Harper, PE, LEED AP: Civil Engineer/Stormwater Specialist

Architect
Hickok Cole Architects
Washington, D.C.
Jason Wright, AIA, LEED AP

Civil Engineer
Timmons Group
Richmond, Va.

General Contractor
Fairmont Builders
Bethesda, Md.

Green Roof System Manufacturer
American Hydrotech Inc., Chicago

Irrigation Specialist
Tommy Edwards, CLIA

Landscape Contractor
Ruppert Nurseries
Gainesville, Va.