Roofs Are a Potential Solution for Urban Stormwater-management Issues

Can stormwater management using rooftops in urban areas be the financial solution to our growing urban stormwater problem? Will public-private partnerships with building owners help to provide a government service—stormwater drainage—in a more cost-effective manner? As cities struggle with the high administrative and procurement costs and time delays to manage stormwater, should we be looking up to roofs as part of the solution? Can we avoid more regulations and instead look to market-based solutions? These questions are beginning to be discussed and tested as new, innovative approaches to solving difficult and expensive urban stormwater-management issues.

Consulting and engineering firm Geosyntec Consultants is monitoring and controlling runoff from an existing New York City Parks and Recreation facility green roof.

Consulting and engineering firm Geosyntec
Consultants is monitoring and controlling runoff from an existing New York City Parks and Recreation facility green roof.

STORMWATER MANDATES

Many cities and counties are dealing with more stringent stormwater permits issued from the Washington, D.C.-based U.S. Environmental Protection Agency (EPA) and state environmental agencies that implement the federal Clean Water Act. Many communities are operating under federal court orders and administrative consent orders from EPA to reduce stormwater runoff into rivers, lakes and streams. In addition, there are 177 communities in the U.S. where stormwater and wastewater-collection systems are combined, known as combined sewer overflows (CSOs). These CSOs result in billions of gallons per year of combined untreated stormwater and wastewater discharged into waterways during large rainfall events. Funding crises have developed in many municipalities as they create programs, hire new staff, and design and construct new infrastructure to meet these regulatory requirements.

Many cities have spent billions of dollars separating stormwater drainage from wastewater-collection systems by installing new, costly drainage systems. In addition, large underground storage tunnels and vaults have been installed by many cities at the costs of billions of dollars per installation. These tunnels and vaults are designed to collect, hold and slowly release the stormwater into the treatment network. Increasing stormwater pipe sizes and creating tunnels and vaults is extremely costly. For example, Washington, D.C., just broke ground on the construction of two stormwater tunnels that are currently projected to cost $2.6 billion dollars to construct. Just one of the tunnels will be 13-miles long and hold 157 million gallons of combined stormwater and wastewater in 23-foot-diameter tunnels, 100-feet below the surface.

Green-infrastructure approaches to stormwater issues are included in most municipal stormwater permits and orders. For example, New York City is spending $187 million on green infrastructure for stormwater control in CSO areas to control the equivalent of 1 1/2 inches of runoff from impervious surfaces by December 2015. Public and private areas are under consideration for green-infrastructure solutions, and the city expects to spend $2.4 billion in green infrastructure during the next 20 years.

As cities address urban stormwater management, stormwater fees are being assessed on private-property owners to help fund the programs to solve urban stormwater issues. Close to 1,500 stormwater utilities are now in operation in the U.S., and the number is rapidly growing. These stormwater utilities typically are assessing stormwater fees based on the amount of impervious surfaces by property owner. The fees can range from a few hundred dollars per year to tens of thousands.

Roofs are considered an impervious surface because they are designed to shed stormwater through drainage networks into the collection system beneath city streets. For example, in New York City alone roofs make up 11.5 percent of the total area, or roughly 944.3 billion square feet, according to the city’s Department of Design and Construction’s Cool & Green Roofing Manual. Rather than looking at roofs as part of the stormwater problem in cities, they should be viewed as a possible solution.

DID YOU KNOW?

Baltimore enacted a stormwater fee
in 2013. Currently a building with a
200,000-square-foot roof would be
assessed $11,400 per year.

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From Green to Blue: Making Roof Systems Sustainable in Urban Environments

Municipal storm-water managers historically have focused on controlling runoff from ground-level impervious surfaces, such as roadways, sidewalks and parking areas. However, the next frontier in storm-water management is rooftops. In urban storm-water management, roofs are part of the problem and potential solution. An exciting new technology to control rooftop runoff is known as blue roofs. Over the next several years, New York City alone will spend several billion dollars on green infrastructure solutions to address its storm-water-control problem, and blue roofs will be a key part of these efforts.

Blue-roof trays are held in place with stone ballast and hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events in a New York pilot project.

Blue-roof trays are held in place with stone ballast and hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events in a New York pilot project.

Blue Roofs

The roofing industry has become very familiar with the use of vegetated, or green, roofs. The vegetative layer grown on a rooftop provides shade and removes heat from the air through evapotranspiration, ultimately reducing temperatures of the roof surface and the surrounding air. By reducing the heat-island effect, these buildings require less energy to cool in the summer and use fewer natural resources (oil or other fuel) in the process.

However, an even newer and less-well-known sustainable technology applicable to roofs is the blue roof. A blue roof temporarily stores rainwater in any of a number of types of detention systems on the roof. They are most applicable and provide the most benefit in highly urbanized cities that are serviced by combined sewers. Combined sewers handle sewage and rainwater runoff from roofs, streets and other impervious surfaces. On dry days, these combined sewers can easily handle the amount of sewage flowing through them to the local treatment plant. However, on days with heavy rain, these combined systems can easily overflow with rainwater and raw, untreated sewage. This combined sewer overflow, or CSO, can flow into local sensitive receptors, like streams, ponds and oceans, contaminating the natural resources and killing fish and other wildlife dependent on them.

The beauty of blue roofs is they can store much of this rainwater during and immediately after a rainstorm, temporarily preventing it from reaching the sewer system. In this way, CSOs are minimized and local natural resources are protected. When the storm is over and the sewer system has the capacity to handle it, the blue-roof retention materials are designed to slowly release the stored rainwater back into the storm-drain system.

This blue roof in New York uses a check dam to retain storm water.

This blue roof in New York uses a check dam to retain storm water.

NYC Pilot Program

Our firm, Geosyntec Consultants, along with environmental engineers Hazen and Sawyer and HydroQual and water-management firm Biohabitats, designed and implemented a groundbreaking blue-roof system in New York. The New York City Department of Environmental Protection (NYCDEP) retained the team to implement a sustainable green infrastructure retrofit pilot program to demonstrate how rooftops can reduce the frequency and volume of CSOs in the city. The objective was to design and install storm-water controls to quantify the benefits of sustainable approaches as a viable solution to reduce storm-water flows to the city’s CSO system. Rainfall of less than 1/2 inch can overload the system and result in untreated discharges. The use of sustainable green infrastructure, like blue roofs, to reduce storm-water inputs to the combined system is one of many approaches New York City is considering to help solve this problem.

Geosyntec’s role on the team was to design several storm-water pilot studies, including blue roofs. Our blue-roof designs included installing risers on rooftop outlets that would result in ponding of water around the outlets, small dams on the roof surface using check dams of angle-iron to create ponding and the most successful technique—blue-roof trays. We developed specially designed trays, held in-place with stone ballast, to hold up to 2 inches of water. The tray systems resulted in a 45 percent reduction in roof runoff during rainfall events. If blue-roof trays were installed on all roofs in an entire drainage area to a CSO, the results would be significant in solving the CSO problem. In addition, trays are more practical because they can be spaced around existing equipment on roofs and moved during repairs and maintenance of other rooftop systems.

Geosyntec Consultants designed a blue roof that included installing risers on rooftop outlets that would result in ponding of water around the outlets.

Geosyntec Consultants designed a blue roof
that included installing risers on rooftop outlets that would result in ponding of water around the outlets.

Roof-system Protection

Protecting the integrity of a roof membrane is an important consideration for roofing and building contractors that are considering installing a blue roof. Blue-roof-tray systems offer the best protection because they rest on top of existing membranes and ballast systems and do not result in any membrane perforations that require additional waterproofing. Other blue-roof systems, like check dams or new drain inserts, may require additional waterproofing. The bottom line is if the roof membrane is old, compromised or currently leaking, any type of blue roof would be problematic until a new membrane is installed.

In addition, during the pilot projects, we took great care to inspect and test the roofs for load-bearing support—a step that should be conducted for all blue and green roof systems.

As we look to the future, roofs in urban areas will most definitely become a major part of the storm-water solution, and blue-roof technologies will evolve to become a common practice.

Learn More

NYCDEP has posted information about blue roofs and other urban green infrastructure for CSO control on its website.
The U.S. Green Building Council offers an online course about blue roofs for storm-water management.

PHOTOS: Geosyntec Consultants