Duke University Water Reclamation Pond

Challenge

• Provide water to support University cooling needs and thereby reduce University dependence on municipal potable water.
• Collect runoff from the Duke campus and increase drought resilience in response to extreme drought in 2008 while establishing a model for stormwater infrastructure on university campuses.
• Improve hydrological and ecological quality of the pre-existing stream corridor.
• Preserve as many existing trees as possible and use site-harvested timber for site structures.
• Create habitat for native wildlife.
• Provide a living laboratory for environmental sciences and general ecological education of students while educating the public about stormwater management. 
• Create a beautiful destination and broadly accessible landscape for relaxation and recreation for students and community members. 

• The 5.5-acre pond collects rainfall and surface runoff from 265 acres of Duke University’s West Campus, 135 acres or 51% of which is covered by impervious surfaces. Most of this water arrives through a storm culvert that passes under Circuit Drive and empties into the pond forebay.
• The pond holds 16.4 million gallons of water at normal pool capacity and has an operating capacity of 6.7 million gallons within a 4-ft fluctuation. The operating capacity can provide water for a minimum of two weeks during a water shortage.
• Duke Pond provides 85-90 million gallons of water annually to the Duke University chiller plant, which draws an average of 250,000 gallons per day and a maximum of about 700,000 gallons in summer when cooling needs are greatest.
• Nearly 30,000 individual plants representing 95 species are found on site, 97% of which are native to the southeast region including bald cypress (Taxodium distichum), important perennial pollinators such as milkweed (Asclepias sp.) and goldenrod (Solidago sp.), and several woodland shrubs such as buttonbush (Cephalanthus occidentalis) and fothergilla (Fothergilla sp.). The 95 unique plant species include 36 trees, 21 shrubs, and 38 herbaceous species. The 27,567 new individual plants consist of 111 grasses, whips, and seeds; 25,725 herbaceous; 1,259 shrubs; and 472 trees. Adding 16 species of canopy, subcanopy, and understory trees to the existing matrix of 20 canopy tree species increased overall tree diversity on the site by 80%. 
• Wetland plantings added 54 species found in submergent, emergent, and wet mesic zones. Wetland species surveys taken as part of ongoing monitoring counted 65, 27, 40, and 35 unique species in the fall of 2015, 2016, 2017, and 2018, respectively. The declines in 2016 (attributed to drought) were mostly recovered in 2017 and effectively stable in 2018, but reduced by nearly half (54%) compared to the designed wetland matrix. The changing water levels and natural emergence, migration, and succession patterns encourage adaptation and allow resilient species to flourish.
• 1,596 coniferous and hardwood trees (including 657 pine and 119 oak) that were removed were repurposed into the wood used for decks and handrails. The remaining wood was mulched or milled and stored for reuse on campus, and some lumber was sold to local suppliers to support Duke Forest programming.
• Almost one mile of crushed stone paths (4,706 lf) and a wood bridge (100 lf) provide access around the entire pond and forebay. A 560-sf covered pavilion with a 1,100-sf boardwalk and a 4,560-sf amphitheater provide additional space for recreation and education.

The project has a rigorous water quality monitoring regime that addresses the regulatory, infrastructural, and ecological imperatives connected to the site and construction of the project. Because the pond was built over an existing streambed, Duke Facilities Management (FMD) is required by state regulation to perform annual water quality monitoring for five years after construction of the project. In addition to this requirement, FMD is coordinating with the Duke University Wetland Center (DUWC) and Nichols School of the Environment to measure, analyze, and evaluate the impact of variable water levels at the pond on water temperature and quality, plant attrition, and other metrics.

Baseflow nutrient and sediment data from 2015 to 2018 has led the DUWC to conclude that Duke Pond retains a majority of the nitrogen, phosphorus and sediment that enters and so successfully reduces downstream nutrient loads (Richardson, Flanagan, Ho. August 2019). Further, the pond traps over 90% of total suspended solids during typical storms and most sediment during large storms. Generally, Duke Pond meets the primary required water quality standards and has little impact on temperature or nutrients in downstream aquatic ecosystems.

DUWC monitoring reports available to the CSI research team were completed in 2016, 2017, and 2018. By the time of the first pond vegetation survey in July 2015, the four most ‘visible’ taxa in the three primary littoral zones met DUWC expectations based on transplantation, project location and character as a land-water interface in the Piedmont region: common spikerush (Eleocharis palustris), blunt spikerush (Eleocharis obtusa), common rush (Juncus effusus), and rice cutgrass (Leersia oryzoides) (Richardson, Flanagan, Giuliano, Ho. May 2016). The 2018 survey builds on this assessment and notes “good evidence that species richness was dynamic not only through time of year but year to year as well as locally within a meter elevation change caused by random hydroperiod fluctuations. It is clear that the fluctuation of water levels drive water front vegetation distributions and succession,” (Richardson, Flanagan, Ho. August 2019).

The changing water levels have also clearly impacted plant succession. The wetland plant community around Duke Pond appears to be expanding. After an initial decline, species counts have recovered and community composition has shifted. Several minor species have also expanded. As of the 2018 plant survey, spike rush (Eleocharis palustris), needle spike rush (Eleocharis acicularis), flatsedge (Cyperus reflexus), valley redstem (Ammannia coccinea), and fall panicgrass (Panicum dichotomiflorum) are the only originally planted species that remain as sizable colonies (Richardson, Flanagan, Ho. August 2019).

Natural patterns of succession and more relaxed management in wetland planting areas seem to have combined to encourage adaptation and allow resilient species to flourish.

Construction of Duke Pond cost approximately $11.5 million. This cost, although 28% higher than the estimated $9 million initial budget for a conventional stormwater retention pond, enhanced ecological and cultural value by creating a high-performance landscape and hydrological park accessible to the university community and the broader public. Projections suggest that savings for water costs alone will cover the cost of the project by 2025.

• The wetland plants, many of which were selected for their suitability in submerged conditions, were delivered to the site before grading was completed. These plants were stored for two early summer months out of water, and therefore required additional irrigation until sitework and grading was complete and the pond’s water level could be increased to its designed elevation. The first pond vegetation survey by the Duke University Wetland Center (DUWC) in July 2015 noted a setback in establishment of the original planned species matrix, but species distribution largely met expectations based on transplantation and subsequent emergence, migration, and succession.
• Goose pressure on the almost 92,000 newly-installed wetland plugs required installation of nearly 6,000 linear ft of protective fencing. While wetland plants would typically create a vertical barrier to discourage geese, the birds were able to move easily between land and water where new plants had not yet established. As a temporary measure, nylon twine fencing was installed at 6 and 18 in above normal pond level on 2x2 stakes about 25 ft on-center. As of late 2019, the twine had not been removed and so has become somewhat unsightly over time: oak stakes have deteriorated and wetland plants have become entangled in the twine. During construction, contractors used dogs to deter geese, but long-term alternatives (for example, audio deterrents) were limited by the University’s urban location.
• Maintenance of meadow and wetland plantings as well as the pond water body itself has presented many opportunities and challenges based on operational and performance constraints. Due to insurance and liability issues, the facilities management department cannot legally enter the water to clean and maintain the pond. Maintenance of submerged areas has been contracted to pond management companies who are accustomed to maintaining golf course ponds, which have very different aesthetic goals. These companies typically provide more conventional maintenance programs that include mowing turf grasses and leaf-blowing. Some perennials and other herbaceous plants have been suppressed; anecdotally, this suppression may be attributed to over-mowing of the hillsides. More recently, maintenance teams have become better acquainted with alternative management regimes recommended to promote a somewhat wilder meadow aesthetic.
• Various invasive species have infiltrated the meadow areas, and Facilities Management may not have been adequately trained or prepared to manage them effectively. Efforts to manage aggressive invasive species are ongoing, including the sustained removal of five species: Lespedeza, floating primrose willow (Ludwigia peploides), black willow (Salix nigris), creeping smartweed (Persicaria longiseta), and lady’s thumb (Persicaria maculosa). Lespedeza seems to be the greatest offender. According to the design team, concerns around design fees meant that detailed specifications for maintenance were not included in the project documents, though abbreviated instructions were included in construction drawings. Ideally, a meadow-specific contractor would have been enlisted to install meadow plants and establish a maintenance contract for 3 or 4 years. Duke Facilities Management is currently developing more meadow-specific maintenance regimes.
• White oak handrails from site-harvested timbers have started to rot. As designed, handrails were lightly sanded to not only prevent splintering but also leave saw marks visible as clues about the milling process. Without preservatives, they started to rot from the lower corners of the exposed end-grain. Deck boards have also experienced some rot, likely due to lack of air space between boards (installed at ADA-standard 1/4-in gap) where boards swelled more than anticipated. These installations may have benefitted from larger gaps for better air and water movement. A few boards have been replaced. Such maintenance by replacement is not unusual for decking boards with prolonged exposure to moisture. The pumphouse rain-screen has performed without such issues. 
• An early (but not primary) focus of the project was the expanded walkability of the University campus and introduction of new paths. The primary trail around the pond shows signs of frequent use, and informal paths or cut-throughs encroach from the project edges. In particular, two desire-lines from Erwin Road suggest the project’s importance in the campus circulation system and its value to students and visitors as a threshold into campus. A bus stop along Erwin Road to the west was installed after the pond’s completion but not directly as part of the pond’s construction. The design process did not include specific coordination to link pond paths with adjacent transportation nodes, or any outreach to the broader campus and adjacent community to understand desire or benefit for such connections. However, integration or formalization of this emergent network could further amplify pedestrian routes on campus.

Stone (vertical): Duke stone from Duke University stone quarry (Hillsborough, NC)
Aggregate path material: Luck Stone granite, permeable aggregate (Pittsboro, NC)
Wood preservative: Eco Wood Treatment, low-maintenance wood sealer (Bridgewater, NS CAN)

Project Team

Landscape Architect: Nelson Byrd Woltz Landscape Architects
Client: Duke University; University Landscape Architect; Duke Facilities Management
Civil Engineer:  McAdams Civil Engineers
Geotechnical Engineer: Geotechnologies
Mechanical Engineer: Affiliated Engineers
Structural Engineer: LHC
Architects: MEP
General Contractor: LeChase Construction
Grading Contractor: Mid-Atlantic Infrastructure Systems
Utility Contractor: Mid-Atlantic Infrastructure Systems
Landscape Contractor: Ruppert Landscape

Role of the Landscape Architect

The landscape architect worked directly and collaboratively with the University Landscape Architect and Facilities Management Department (FMD) team through years of planning studies and benchmarking to understand and establish opportunities for a high-performing landscape. The landscape architect led initial research and studies for planting regimes that would accommodate stormwater management requirements and also generate ecological and hydrological improvements. This collaboration included engagement with university faculty to specify plants and understand opportunities for educational programming.