With growing populations and the need for more water for agriculture and industry, clean potable water is becoming scarce. With the advent of LEED and the thrust for water conservation, many in the building community are looking for ways to conserve our most precious resource. The technology to achieve this goal with stormwater is not so much new as are the latest applications of the technology.
In the buildings industry, stormwater capture is the practice of intercepting stormwater from roofs and sites and preventing all or a large portion of it from entering the municipal stormwater system. Stormwater capture can be broken into two approaches: detention systems that manage the stormwater runoff from building sites, and retention systems that utilize the stormwater for other purposes, such as irrigation, toilet flushing, and cooling tower makeup.
Detention systems are a common solution for urban drainage problems. A detention basin or reservoir acts as a temporary storage area, allowing runoff to be released at a slower, predetermined rate to reduce the impact of the new development on the downstream drainage system. The basin does not reduce the total volume of runoff, but it does increase the flow duration by slowing it down, thereby reducing peak runoff rates.
Within a building, stormwater detention is accomplished by directing the water from the roof to a detention tank. The tank is then allowed to overflow, or the water flow is controlled through an orifice, or the water is pumped from the tank on a controlled basis. On a building site, detention can also be accomplished by using a detention basin, such as a drainage swale that floods during wet weather flow but is dry at other times, or by using an architectural feature, such as a decorative pond or lake, where the dry weather level is lower than the wet weather level, which is maintained in the “free board” of the basin.
Calculating Flow Rates for Detention Systems
A detention basin holds the difference between the flow from the developed and the undeveloped portions of the building site. The rate of rainfall as measured by the time of concentration is reduced by the time required to fill a basin or tank.
In New York City, the 5-year design storm has an intensity of 5.95 inches per hour (I) based upon a 6-minute time of concentration. For a building with a 30,000-square-foot roof, the flow rate (Q) would be Q = CIA, where C is the runoff coefficient (for a roof assumed at 1), I is the rainfall intensity, and A is the area in acres (30,000 square feet/43,560 square feet per acre).
For this example, the flow rate would equal 4.1 cubic feet per second (cfs) or 1,845 gallons per minute (gpm). If a 30,000-gallon tank were provided, the time to fill the tank would be 16 minutes excluding the time required for the rainwater to flow through the pipes to the tank. Therefore, the new time of concentration for the rainfall event would be 6 +16 for the travel time from the roof to the tank, or 22+ minutes total. The new calculated storm would then have a rainfall intensity rate of 4.5 inches per hour based on the formula of i-140/(t+15), where i is the initial rainfall rate and t is time in minutes. If a constant outflow of 2 cfs from the tank utilizing a fixed orifice or constant pump rate were used instead, the new storm duration would still calculate to 31 minutes but the tank volume would be reduced to 10,030 gallons.
Another method of detaining stormwater on site is “controlled flow roof drainage.” This system permits rainwater to drain off a roof at a controlled rate (usually equal to or less than the previous undeveloped rate); after a storm has abated, the accumulated water drains off within a prescribed period of time. For most locales, the building code dictates how quickly the water must be drained off, but usually no longer than 24 hours after the rain has ceased. In effect, this system uses the roof as a tank.
Roof drains with notches or weirs are sized to control the water flow into the drain. These weirs are designed to limit or control the flow for storms of varying intensities and return periods. In New York City, for example, a single notch can drain a roof area of 7,600 square feet with a resultant build up of 3 inches on the roof and 15 hours of draindown time after the rain stops. In the event a storm occurs that is greater than the design storm, the excess water is handled by scuppers within the parapet walls or by overflow drains. This system has lost its popularity in commercial buildings due to difficulties in roof maintenance.
Sizing a Retention System
If all the water falling on a 30,000-square-foot roof in New York City were captured, a total of 900,000 gallons per year of water would be available. Nevertheless, it is not practical to design a tank large enough to capture the water from all anticipated storm events. However, a retention system can be sized to provide a workable source of stormwater for irrigation, toilet flushing and cooling tower makeup.
The first step is to calculate the amount of water that could be captured on average. Then calculate how much rainwater would overflow a given tank size and then estimate the total potential water capture.
For One Bryant Park, a 55-story, LEED platinum building in New York City, stormwater tanks were sized to capture 87 percent of the average annual rainfall. Harvesting the stormwater from the building’s roughly 80,000-square-foot roof yields a potable water savings of 2.3 million gallons annually. Four 7,000-gallon tanks in the tower collect the rainwater, and gravity supplies it immediately to toilets for flushing, eliminating the pumps and energy that would be needed to pump it upward from a collection point in the base.
For municipalities like New York City that have combined sewers, a retention system not only reduces a building’s impact on the sewer system but also the wet weather flow on sewage treatment plants.
Robert Benazzi is a consultant and a former partner with Jaros Baum & Bolles, an electrical and mechanical engineering firm based in New York City. Chris Olson is Chief Content Director of BUILDINGS.
