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Stormwater Capture on K Street

Timmons Group, Richmond, Va. o Editor, Stephen Kelly


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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 "Streetscape Innovations in D.C." One of the stops on that tour was at 1050 K. NW Street, an example of landscape architects leading a team in developing and implementing a stormwater system whose collection apparatus is part of the streetscape.

This was not the only group to tour the site. Staff members from the EPA's Office of Wetlands, Oceans and Watersheds, the Office of Wastewater Management and the Office of Science and Technology also wanted to take a look at how the building at 1050 K. Street was handling urban stormwater. After their visit, Lisa Hair, PE, EPA Office of Water, commented: "Your building design has stormwater management features that will help achieve improved water quality in our streams, bays and other surface waters. A full suite of tools employed at the building provide the 'treatment train' approach of incremental stormwater capture and on-site use. It is great to see how the green roofs, sequential bioretention and cistern are integrated to provide the most efficient use of the stormwater that falls on the site. The green roof and bioretention plantings are highly aesthetic, attractive features, even in the middle of December.

Let's tour 1050 K. Street ourselves. First, this is an 11-story building in the central downtown, just a short walk west from the convention center. Before the stormwater improvements, the building site was typical of D.C. streetscapes and urban environments in general--97 percent impervious. The project design reduced that number to 67 percent. That's a 30 percent improvement, which means only a major storm will allow runoff from the property. LEED stormwater credit 6.1 requires a 25 percent reduction in peak flow rate and volume of stormwater leaving the site.




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's air conditioning, collects in a 12,000-gallon underground cistern on site. Cistern water is filtered and pumped through the plaza water feature. The water, via drip lines, is distributed to the three bio-retention basins and to the green roofs. About 66 percent of the cistern's volume is allocated for irrigation using ET referenced programming via satellite. In the event of a major rainstorm, the cistern has drains to release overflow into the storm sewer.

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 "Garden Root" layered system was chosen. The company's record of success (more than 500 projects and years of technical data to support its use), plus a single source warranty that included the plants were deciding factors.

"Lite Top," a lightweight engineered soil was specified. It consists of:

  • 1/16"-3/8" Lite Top Lightweight Aggregate: 45-70%
  • 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 'Weihenstephaner gold,' spurium 'John Creech') and one deciduous variety, Sedum kamtschaticum. Why a deciduous plant for the green roof? Kamtschaticum grows fleshy and tall (6 in.), is remarkably tough and drought tolerant and offers blooming color (including orange fall colors). This species is a mounding type and there's little concern of it taking over other sedum varieties.





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

  1. iris germanica
  2. begonia 'Dragon Wings'
  3. Salvia nemorosa 'Viola Klose'
  4. Coreopsis verticillata 'Moonbeam'
  5. liriope muscari
  6. uniperus horizontalis 'Blue Chip'
  7. calamagrostis x acutiflora 'Karl Foerster'
  8. ilex glabra ('Compacta' inkberry )
  9. Cornus sericea 'Silver and Gold'
  10. ilex crenata ('Compacta' Japanese holly)


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 "mostly moisture tolerant." In very wet conditions, kamtschaticuma and evergreen 'John Creech' will prevail, avoiding a "complete kill."




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--The Terrace Green Roof
The 8-inch soil depth tray system on the building's terrace (the lower green roof) offers a significantly more diverse plant palate, with greater textures and colors than the upper green roof. The terrace green roof offers sweeping displays of sedum/perennials and some ornamental grasses. Grades mimic the bioretention beds at the street level. Long sweeps of paver bands complement this green roof.




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' willow oaks with amended soils and tree wells. Beneath the plaza are four levels of parking.

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 "semi-intensive" green roof (the lower level greenroof with the terrace) was placed on sedums, perennials are represented, the most prominent being Allium schoenoprasum (chives), Coreopsis verticillata 'Moonbeam' (tickseed), Delosperma nubigenum 'Basutoland' (ice plant), Dianthus deltoids 'Mountain Mist' (Maiden Pink), Dianthus gratianopolitanus 'Firewitch' (Cheddar Pink), Sisyrinchium angustifolium 'Lucerne' (blue-eyed grass) and Thymus (thyme).

Ornamental grasses were also included: Schizachyrium 'The Blues' (little blue stem grass), Sporobolus heterolepis (prairie dropseed) and Stipa tenuissima (feather grass).

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-aired structure on the north side of the terrace green roof.
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-aired structure on the north side of the terrace green roof.

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 "runner" type. The idea was to have tall plants visible from the street and adjacent buildings. For such visibility, the bamboo would have to grow to 10-12 feet tall and preferably be an evergreen. Given these needs, the landscape architects were limited to few varieties of the clumping type, and these are not a cold tolerant. After consulting with Kurt Bluemel Nurseries of Baldwin, Md., Fargesia robusta was selected. These bamboos will need strict monitoring.




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's limited use of ornamental grasses: Festuca (blue fescue), an eastern native, Schizachyrium scoparium (little blue stem), Sporobolus, a drop seed prairie grass and Stipa (Korean feather grass).


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 'Hightower' willow oaks with amended soils and tree wells. There are two planters on the north side of the building and one on the northwest wing, which incorporates the water feature. Water from the underground, onsite cistern passes through a filter, then is pumped over a two-foot weir to feed the bioretention planters.

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's stormwater runoff, any overflow from the green roofs and the chiller water from all 11 stories. In the case of a major storm, the cistern has drains to allow runoff released into the storm sewer.

About 66 percent of the cistern's volume is allocated for irrigation. Roof and planter irrigation is via drip lines, which is of course very efficient use of water, limiting water lost to overspray, wind and evaporation. The irrigation control system uses ET referenced programming via satellite that downloads local NOAA weather station information. Beneath the plaza are four levels of parking.

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 "most sophisticated building they've ever designed. From the outside, the soaring angles of the rising roofline make the building appear taller than its neighbors. The glass facade and outdoor landscaping blur the line from exterior to interior." This LEED Gold certified building was the winner of the 2010 NAIOP (Commercial Real Estate Development Association) Award of Excellence.

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.


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November 20, 2019, 2:03 pm PDT

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