Collaborative Low-impact Development:Harvesting Water for Barnard Trace Urban Residential...

Collaborative Low-impact Development: Harvesting Water for Barnard Trace Urban Residential Redevelopment Authors: Jeanne Homer, Dr. Thomas Spector, Dr. Khaled Mansy, Dr. Todd Halihan, Nicholas Nelson, and Thomas Wallace, P.E.

Students Andres Patrignani, Jordan Beehler, Qualla Ketchum, Scott Meier, Laree Oden, Nelly Ruiz, Bharat Sharma Acharya, Kyle Spears, Armando Zarco

Oklahoma State University

Energy and Systems

COLLABORATIVE LOW-IMPACT DEVELOPMENT: HARVESTING WATER FOR BARNARD TRACE URBAN RESIDENTIAL REDEVELOPMENT

Collaborative Low-impact Development: Harvesting Water for Barnard Trace Urban Residential Redevelopment

Jeanne Homer, Dr. Thomas Spector, Dr. Khaled Mansy, Dr. Todd Halihan, Nicholas Nelson, and Thomas Wallace, P.E.

Students Andres Patrignani, Jordan Beehler, Qualla Ketchum, Scott Meier, Laree Oden, Nelly Ruiz, Bharat Sharma Acharya, Kyle Spears, Armando Zarco

Oklahoma State University

Abstract

When designing a competition entry for a low-impact residential development in Tulsa, the multidisciplinary team faced a variety of challenges. On the one hand, the team was asked to present a solution that could be easily implemented by creating a marketable development. On the other hand, the design was required to meet the low-impact design requirements of minimized stormwater runoff. The contextual parameters of the competition included providing single-family residences and lots sized similarly to those in the area, maintaining historic Works Progress Administration walls on the southern edge and across the center of the site, and providing a solution to stormwater runoff with poorly draining soil. The team’s goal was to minimize environmental impacts while providing a feasible, contextual design. Typical residential development in Tulsa involves stripping a site clean, building repetitive houses with superficial utilization of sustainable practices, and loading the aging infrastructure by creating higher runoff-generating urban land, ultimately resulting in contaminated water and polluted waterways. The design solution should combine water reuse, filtration, and replenishment with sustainable, contextual architecture and landscape, while providing marketability for the developer.

The multidisciplinary team consisted of three licensed architects, one with an expertise in sustainability and environmental systems and another in management, a professional engineer, a landscape architect, a hydrogeophysicist, and a class of graduate geology students. A typical collaborative process in developing a design solution might involve creating a solution,

evaluating it qualitatively from each member’s perspective, and finalizing it with revisions. The team worked within this cooperative collaborative structure in which each expert incorporated his or her experience and knowledge into the overall solution, but more specifically, the collaborative process for the team’s approach was based on a feedback loop that generated and adjusted dozens of scenarios, ultimately closed by objective assessment comparing six final scenarios. The team assessed the schemes based on parameters related to stormwater replenishment, quality of planning, sustainability, marketability, and cost. At the heart of each low-impact development scheme is an inverted road system in which water is harvested: collected, stored, and reused or filtered, recharging the natural water table. The final solution saves money, incorporates sustainable practices and passive technology, and not only cleans the stormwater runoff that replenishes the underground water table and dramatically reduces the load on the city infrastructure, but also drains it more efficiently than it would as an undeveloped plain. Ultimately, integrating architecture, landscape design, and below-grade improvements achieved a design that minimizes environmental impact.

Design

Defining the Problem

The primary problem the multidisciplinary team faced in the competition design for an urban residential redevelopment was the need for a balance between practicality, marketability, and design that minimized long-term negative impacts on the environment. Dealing with water in a sustainable way was the


primary challenge, one which is commonly ignored in much traditional development. Water usage per family is very high, as an average family can use 400 gallons of water per day with 30% of that used for landscape irrigation.1 In our development, 18 families would use 7200 gallons of water per day with over 2100 gallons used for irrigation alone. Water usage was not the only concern; the source of the water being brought to the site needed to be considered. In Tulsa, water can travel in some instances from 50 miles away from Lake Eucha and Spavinaw Lake. These sources have served Tulsa well, but low-impact development calls for more localized solutions that minimize infrastructure. Tulsa has plenty of rain, more than most central and southern areas of Oklahoma, which ordinarily goes unharvested. According to the National Weather Service, the 77-year average rainfall in Tulsa is 39.4 in/year, and in a 100-year storm event, which is predicted to become more common, a heavy downpour of 4.41 in/hour of rain is produced. To complicate matters, Tulsa is known for having poorly draining soil, and our Barnard Trace site is no exception with Dennis Complex soil series 17, classified as poorly drained. Underlying the site is fine sandstone which has moderate permeability and shale rock rich in clay with low permeability.2 Even in predevelopment condition, millions of gallons of water would not fully infiltrate the area due to poorly draining soils. Clearly, a heavy downpour with this soil in a traditional development can pose flooding problems and overload the stormwater infrastructure.

The total amount of water input on the Barnard Trace site can be as much as 4.25 million gallons per year. Many options for handling all of this water were considered. In traditional development, sites are not only stripped of elements like trees and vegetation that might slow stormwater runoff, but also elements like grass lawns are introduced that require watering, which is usually brought in from offsite. In a rain event, millions of gallons of water are contaminated by roof and roadway hydrocarbons, as well as a myriad of other contaminants, and then rushed to storm sewers via gutters and ditches. The runoff overloads the 50-year-old city infrastructure which is currently at the end of its lifespan. Over the next 25 years, the United States will spend one trillion dollars ($1,000,000,000,000) to provide and maintain that infrastructure.3 Low-impact developments that

eliminate the need for stormwater runoff systems could save municipalities many millions of dollars in a short period.

Context

The context of the Barnard Trace Development in Tulsa is a welcoming historic neighborhood in which lots are 50’-55’ wide and around 150’ deep, and craftsman houses have porches facing the street with detached garages serviced by driveways or rear alleys. The immediate site of the development, however, has its challenges. It was the site of the former 1925-era Barnard Trace elementary school which burned due to an accidental fire. Much of the 4-acre site was covered with impervious asphalt and concrete, sitting on poorly draining soil, creating load on the city stormwater infrastructure in Tulsa’s heavy rainfalls. The WPA stone wall, while offering an opportunity to create a distinct character for the development, limited options. Due to the slope of the street south of our site, the height of the wall of the south and east edges of the site varies from 4’ to 12’. It was determined that it was likely less expensive to build the wall and build up the earth to level the school site, especially since the geology class discovered there is rock just below the soil for a significant portion of the lot. Most of the north side of the site’s elevation matches the street, so vehicles are able to access the site and houses more directly relate to the surrounding historic neighborhood.

Process

The collaborative process the team incorporated had a significant impact on the final solution. In a continuous feedback loop structure, as its name suggests, there is a closed loop cycle of the identification of values, generation of an idea or ideas, assessment of the ideas, and response to the assessment, and then it is repeated. Within a design project, there is typically a process of research, identification of goals, design, assessment, and modification. Often that assessment is qualitative, sometimes performance-based, but in either case, it must narrow and result in one submission. In this project, the team followed a feedback loop for two cycles, directed by research and qualitative analysis of the different team members. See Figure 1.


Fig. 1 Team Collaborative Diagram

Roles of each member were not strictly prescribed, and all had equal say. In the end each member contributed in a manner with which they were familiar. Some stepped into the generator roles, some more reactionary, while others provided qualitative feedback until the options were narrowed, at which point they filled assessment roles.

The team began by researching and identifying the goals within a group meeting. The final goals were a result of a typical process of brainstorming ideas and digesting and interpreting the research. These ideas were checked and refined by each team member’s perspective and approach, which led to numerous design parameters. An overarching goal emerged, however, that the changes to traditional residential development should employ current feasible technologies and well-established construction techniques in an integrated approach to create an extraordinary low-impact development. The central goal of the design was to collect, purify, harvest, recharge, and use the on-site stormwater runoff in order to virtually eliminate, or at least filter and delay, any overflow peak discharge into the city’s underground stormwater network with the design solution. This led to another goal of connecting the architecture and landscape architecture directly with the site, so the team strove to integrate provisions for sustainable measures in the architecture and lot design such as geothermal technology, daylighting design, and reuse of water. In addition, the team wanted to relate to the existing context by maintaining

the existing visual character of the surrounding craftsman architecture and lot layouts of desirable area and width, which would improve neighborhood acceptance and contribute to the marketability of the development. They sought to integrate science in a way that would look natural to the neighborhood; it was determined that the project would not wear its sustainability on its sleeve. In addition, the team preserved the historic Works Progress Administration stone wall on the site: as much as 12’ tall on the southern and eastern edges of the site and a 4’ tall wall through its center. Lastly, the goal was to reduce costs by minimizing site preparation by means of using the existing infrastructure, minimizing dirt removal, and minimizing the area of the street surface. To ensure developer incentive, the number of lots needed to be maximized at eighteen. These mutually reinforcing goals of stormwater retention and reuse, low-impact landscaping, and energy efficiency can be achieved with a modest investment that yields real economic benefits to homeowners.

After goals were established, members separated and took some time to explore options. The lead architect created five potential layout approaches with alternatives. The team reacted to the layout options according to the established goals and their own experience, or they simply asked questions that placed the project in its larger context. Revisions were incorporated by the architect. The team went through two cycles of this feedback loop of design, analysis, and modification to arrive at a solution, when they received a message from the competition director that the solution was required to follow the given layout created by developers sponsoring the competition and approved by the surrounding historic neighborhood residents. The team was at an impasse, and members discussed whether to adhere to the provided layout or not. Because of the ultimate goal of proposing a scheme that could be readily implemented, the team decided to work within the narrow window of the provided plan, and eventually narrowed to four options. These four schemes were then quantitatively assessed in terms of the goals, comparing them to the developer’s original plan with no low-impact development measures, and even to original grassland. See Figure 2.


Fig. 2 Different scenarios the team assessed.

During the layout design process, the team reviewed literature about easily implemented low-impact development features. For example, Chicago developed a program to reduce water runoff in alleys and published its guidelines in the Chicago Green Alley Handbook. Four different approaches are outlined that rely on existing conditions, including local soil properties4. For the competition, the team did incorporate several strategies that were presented in the handbook that one will find in most low-impact development designs: permeable paving, water collection from roofs, water reuse, and bioswales. The approaches in the handbook, however, rely on the presence of soil that has some drainage ability. The Barnard Trace site has poorly draining soil, so the reservoir would need to be larger to improve water collection and filtration.

Design Features of the Low-impact Schemes

For all of the LID schemes, the hydrogeophysicist designed an “inverted street” in which water is collected, versus a traditional road in which water is shed and directed toward storm sewers. This important design feature is adaptable to all of the schemes. In the design, a majority of the street pavement is pervious, drawing water into below-grade cisterns. Below the site’s middle street is the community cistern that filters and recharges the ground water. This cistern would be shaped with

standard excavator equipment to maximize surface area exposure to uncompacted soil. See Figure 3.

Below the pervious concrete street surface is fine gravel and within the cistern is coarse gravel. In a standard rainfall, thousands of gallons can be collected - this is a number we assessed per scheme. In a major rain event, an overflow drain tile allows excess water to connect to the city’s storm sewer.

Fig. 3 The inverted street concept diagram.


Fig. 4 Integrated design of the street cistern system.

In addition, there is a cistern on each lot under the driveway. See Figure 4. Construction is similar to that of the street cistern. It is sloped at the bottom and below the coarse gravel, a geotextile layer contains the water for future reuse by the homeowner. It is connected to a well with a pump so that this water can

be reused for landscaping irrigation and car washing. This cistern also collects water from the roofs of the house and garage. Again, an overflow drain is connected to the street cistern for heavy rains.

Because the water will pick up contaminants from the road and roofs, the coarse gravel in the lot cistern holding the reusable water would have microbial aggregates to clean the water. “Rainfall moves into a rock chamber below the pavement. A bio-film develops on the aggregate of the rock chamber where microbes live. The microbes capture pollutants such as hydrocarbons and break them down.”5 In our low-impact schemes for the Harvest Water Development, millions of gallons of water are collected, filtered, and recharged or reused essentially due to inverting the street. This design solution improves on what Jeff Huber of University of Arkansas Community Design Center calls “hard engineering”, which involves draining, directing, and dispatching to a storm sewer.6 This proposal’s hard engineering mimics the behavior of “soft engineering”, usually only achieved with natural landscaping elements.

The architectural features, developed by the lead architect, are similar among the schemes. The architectural style is classic craftsman, contextual to the historic neighborhood, and includes porches and terraces that connect people to the outdoors and to neighbors; each room connects with an outdoor space. Garages are detached and located in the rear to further encourage pedestrian activity and interaction on the urban street. See Figure 5. The houses are designed to maximize passive daylighting and heating and cooling strategies. The south and north exposures have large windows that are protected by louvers or deep overhangs to block

Figure 5. Perspective of architecture and landscape design.


summer sun and reduce cooling load, but to allow sun in the winter for passive heating. Concrete roof tiles are used for their durability and to lower water contamination. Due to the slope and orientation of the roof, photovoltaic panels could easily be installed. Roof gutters are connected to drain tiles that surround the foundation. These not only move unwanted water away from the house foundation, but also connect to the lot cisterns. Refer again to Figure 4. Each house uses a 3-ton ground source heat pump system and highly insulated construction.

In terms of lot design, the approach is to minimize building footprints, so the houses are two stories tall, to minimize turf areas, reducing irrigation, and to maximize rain garden landscaping and trees. The team preserved the existing trees on the site, as a mature medium-sized tree will intercept over 1,000 gallons of stormwater every five years. Trees of course can provide crucial shade in the Oklahoma climate, so for this and other reasons, a medium tree might provide owners an average net value of over $400 per year over the course of 40 years. The overall value of 54 medium trees (3 trees per lot, 18 lots) over 40 years is over $850,000.7 This is an example of soft engineering. Generally speaking, the team incorporated both hard engineering that imitates soft engineering and natural soft engineering.

Final Design Assessment

The primary differences among the final schemes’ layouts are a result of varying vehicle circulation flow, amount of pavement, and lot size. See Figure 2. Scheme A represents the non-LID developer’s plan built with traditional methods. Scheme B is the developer plan built with low-impact features. In scheme C, a drive is added to avoid too much dead-end vehicular circulation. Scheme D includes common garden areas, and Scheme E features a double cul-de-sac for vehicular turn-around. Scheme F has no development on it, as the team wanted to compare the low-impact design performance to that of undeveloped grassland. The team finalized its proposal by assessing these scenarios.

Peak Discharge

The hydrogeophysicist and his class of geology students assessed peak discharge, or the highest

amount of runoff at a particular time, of the six scenarios. They used the Rational Method, which is commonly used to estimate peak discharge from an area that is less than 200 acres having a consistent ground surface. On the existing Barnard Trace site, the surface is impermeable paving. The Rational Method is based on data from the site and on potential rain events.

Table 1. Rational Method for estimating peak discharge.

Peak discharge is dependent on saturation of the soil before an event, the amount of rain (in), the intensity of rain (in/hr), length of rain (hr), and any other flow coming onto or through the site. They used the formula:

Q = C i A

Where: Q = peak discharge (cubic ft/second, or cfs) C = runoff coefficient i = average intensity of rain A = drainage area Our schemes varied in terms of areas of permeable paving, roof areas, area of impermeable paving, and green area, so this created the variables to test. This method allows for estimation in 5, 10, 50, and 100-year rain events. See Table 1. Scheme C performed best in all of the rain events with the lowest peak discharge. A primary reason for this performance is that it has the lowest area of impermeable paving. See Figure 6.

Plan Assessment

To screen the proposed alternative design schemes and to identify the few with the highest potential to achieve the pre-defined design goals, the architect with expertise in sustainability and environmental systems performed an overall assessment of the scenarios. Due to the nature of the project with


Table 2. Assessment of planning alternatives.

diverse design objectives that cover both the physical and metaphysical, design objectives are diverse and some cannot be quantitatively evaluated in order to precisely compare to the other alternatives. Scheme F is not included because it is grassland. Measurable objectives were quantities of water, site sizes, energy savings, and cost. Unmeasurable objectives included neighborhood design factors. A point system was adopted to perform the evaluation. For that purpose, each of the design objectives was given the same relative weight of 3 points. Scoring was based on (3 points) successfully achieving the objective, (2 points) somehow achieving the objective, (1 point) marginally achieving the objective, or (0 points) for not achieving the objective. Table 2 explains the assessment method. It shows the scores given in each category to each of the alternative site plans considered. Design objectives were divided into five categories, (1) cost reduction measures, (2) market value measures, (3) low-impact development measures, (4) neighborhood design, and (5) 21st Century-ready solutions. This initial assessment was not meant to be perfectly accurate. It was rather to help eliminate less successful alternatives and select the best alternative that was worth more precise evaluation. In this process, scheme C tied for the best overall score. Proceeding with scheme C in Figure 6, because it has the lowest peak discharge, the final step was to perform a life-cycle cost analysis on this design alternative.

Cost Assessment

When calculating the costs and benefits of a low-impact development, it was useful for the architect with management expertise to calculate the costs and benefits that accrue to the site work separately from those that accrue to the individual houses—although there is some overlap—because some costs can both be charged to and benefits accrue directly to individual homeowners while other benefits can only accrue if everyone in the development makes the investment. The costs of ground-source heat pumps, for example, can be straightforwardly charged to individual homeowners because both the benefits and maintenance responsibilities go to the individual. One home’s failure to invest in the system does not affect the neighbors’ costs and benefits. The increased front-end cost of the concrete tile roof, however, while generating benefits to the homeowner in the form of lowered insurance and utility bills, is also an important component of the groundwater recharging system, as roof runoff from concrete tiles is far cleaner than from asphalt or fiberglass shingle roofs, and for the proper functioning of the cisterns needed for landscape watering. It won’t do for one homeowner to save front-end costs by going with an asphalt shingle roof, since the toxins emanating from the runoff spoil the groundwater for everyone. Furthermore, while each home may have its cistern, all the cisterns must be maintained and properly functioning for the development as a whole to make claims against the local government for lower sewer and stormwater management bills. The anticipated savings in costs of city services from low-impact development requires some education of local government. Even though it is clearly in the local government’s best interest not to have to build and maintain stormwater drainage infrastructure, in many cases the cost may have already been sunk—thus causing a disincentive for governments to grant property tax and sewer system maintenance reductions. The government may have to agree to a lower fee structure, even though initially its costs are not reduced, to encourage low-impact development which, in the future, will reduce costs.

The cost increase per house for ground source heat pump, upgrade to concrete tile roofs, and strategic landscaping, which should generate approximately $1,115.00 in annual savings, was estimated at


Figure 6. Scheme C plan chosen based on assessment.

$14,000.00 per home. The cost increase per lot for site development upgrades; pervious paving and a cistern system, was estimated at $70,000.00 or $3,900.00 per lot. This investment could yield about $315.00 per year (5% return over 20 years) in savings on sewer usage and stormwater control fees assuming an amenable local government.

The initial costs for this development were kept low due to the design’s constructability with standard equipment and materials and level of maintenance. In addition, the lot design maximized the size and amount of private lots that the developer could sell. So, for the developer, although there is an initial cost increase from traditional development, the costs are not prohibitive and the developer could market the annual and long-term savings for the homeowners.

Conclusions

With low-impact development’s need to address and integrate a wide variety of design factors, a collaborative process should be incorporated early on. On the competition team, each member’s having equal say and investment in defining the initial goals led to a long list of parameters to incorporate. The competition parameters in the program and allowance of a narrow window of deviation from a given plan by the competition chair further pushed the team to propose a realistic, fully integrated scheme that could be incorporated more readily into practice and appeal to a wide audience.

Codes are in place to some degree in requiring water retention, but there is no state guidance for minimum standards for stormwater recharge and no incentive

for stormwater harvesting by eliminating or reducing the yearly stormwater management fee. Changes in terms of standardizing low-impact design would occur through adjustments to the code, but policy-making is not always possible or is slow to happen. According to the USGBC, a force in green building investment is the capital market. Energy efficiency and sustainability in the built environment are increasingly enhancing and protecting the assets held by investors. This directly affects the financial performance of real estate investments.8 Appealing to the market’s increasing sensibility toward progressive, earth-friendly design balanced with contextual architecture and low maintenance could prove to maximize the impact of low-impact development versus reliance on policy alone.

Notes: 1 “Indoor Water Use in the United States”, Environmental Protection Agency, WaterSense, accessed January 30, 2015, http://www.epa.gov. 2 “Soil Survey Supplement of Tulsa County, Oklahoma,” United States Department of Agriculture, Natural Resource Conservation Service, accessed January 30, 2015, http://www.nrcs.usda.gov. 3 “Buried No Longer: Confronting America’s Water Infrastructure Challenge,” American Water Works Association, accessed January 30, 2015, http://www.awwa.org.

4 Daley, Richard Michael. Chicago Green Alley Handbook: An Action Guide to Create a Greener, Environmentally Sustainable Chicago. Chicago Department of Transportation, 2010. 5 “Urban Conservation Best Management Practices,” Iowa Department of Agriculture, accessed January 30, 2015, http://www.iowaagriculture.gov.

6 Jeff Huber, “Low-impact Development: A Design Manual for Urban Areas,” UACDC, Fayetteville, Arkansas , 2010, http://uacdc.uark. 7 Kelaine E. Vargas, E. Gregory McPherson, James R. Simpson, Paula J. Peper, Shelley L. Gardner, and Qingfu Xiao, “Interior West Community Tree Guide: Benefits, Costs, and Strategic Planning,” United States Department of Agriculture, December 2007, accessed January 30, 2015, https://www.itreetools.org. 8 Nils Kok, “Market Forces and Sustainability in the Real Estate Sector,” USGBC, September 17, 2012, accessed January 30, 2015, http://www.usgbc.org.

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