As densities increase, environmentally advanced infrastructure and systems can help developers manage runoff in cities.
In the 1990s, urban planning and real estate professionals began to witness the advent of increased densities, mixed-use development and a renaissance in urban centers. Compact, walkable and diverse land uses were becoming favored, and this approach was a marked change from the low-density development that had characterized land use in much of the U.S. during the previous 30 to 40 years.
Higher-built densities also led to new ways to think about managing stormwater. More buildings and the resultant increase in impervious surfaces spurred new approaches. Examples include a renovated building owned by Coakley Brothers, a moving and storage company in Milwaukee, Wisconsin, which recently installed a below-grade collection system in a courtyard, and International High School in New Orleans, Louisiana, which in 2017 added multitiered smart stormwater management systems that run from the roof to a below-ground cistern.
Furthermore, as a result of regulatory changes, cities across the country now require property developers to manage a significant percentage of stormwater on site. And unlike suburban or rural regions where land is more abundant, urban areas generally can’t depend on constructed ponds to do that.
Additionally, the cost of land in densely populated areas means developers must build the most efficient stormwater management systems possible. Downtown land values can be 4 to 30 times higher per acre in comparison to land 10 miles from a city center. In major urban areas, land costs can be exorbitant. According to a 2017 study on metropolitan land values, an acre of land in the center of New York City is worth about $123 million. In downtown Chicago, Washington, D.C., and San Francisco, this same acre is about $30 million. Even in the more affordable cities of the South and Midwest such as Houston or Minneapolis, prices of $3 million to $5 million an acre are common.
The Rise of the Blue Roofs
Green infrastructure integrated with smart stormwater systems can help protect developers’ ROI by maximizing the building’s footprint in proportion to the size of the parcel of land.
“A blue roof is an innovative stormwater management strategy for our region that makes sense for urban infill projects, as on these projects every square foot of space is crucial,” said Ben Ptacek of DJR Architecture. The firm designed the JAX in Saint Paul, Minnesota, a conversion of an older industrial structure into an apartment building with a smart stormwater system.
A blue roof ponds water at the top level of a structure. It has a waterproof membrane system combined with smart valves that use sensors to control the pace and amount of stormwater released over time. The smart systems can be integrated into building management systems to continuously tabulate data. For ease of operation, they can also be controlled through cellular devices.
Consider a hypothetical seven-story multifamily structure near a major city’s downtown area. Tenants demand amenity courtyard space with trees, patios and maybe a bocce ball court or dog run. The courtyard sits above a parking deck. Because the building footprint fills the entire site, there is no land left for managing the stormwater.
There is an ordinance in this city that requires the first several inches of stormwater to be managed on site. Adaptations to the building design are required. Now the developer’s ROI is in jeopardy.
To overcome this problem, the developer converts half of a seventh-floor roof into a blue roof while still providing ample space for mechanicals. This blue roof is designed to pond 2 inches of water and will drain to the third-floor amenity courtyard. On this courtyard, there is about 3 inches of water-holding capacity. A smart valve system is integrated into all roof drains and scuppers. These roofs have smart stormwater systems that interact with weather satellites to maximize stormwater management based on predictions of rainy or dry weather, as well as a sensor-driven smart leak-detection system to prevent water intrusion.
Through these integrated systems, the stormwater-management requirements for this site can be handled efficiently and effectively. By strategically locating and moving stormwater based on the drawdown needs of evaporative irrigation and weather forecasts, the system knows when trees and plants need water or when it is best to take advantage of evaporation. It knows when there will be wet or dry patterns, sunny or cloudy conditions, and hot or cold temperatures.
Smart systems are able to manage two to three times the amount by volume of stormwater as static underground cisterns. More important to the ROI, in these urban settings, smart systems can cost half as much as comparable alternatives. As an example, if an underground infiltration or collection system costs about $5 per gallon of water managed, then a comparable smart system would cost about $3.75 per gallon. Consequently, for a blue roof, the incremental costs for smart system is about $2-$3 per gallon.
In most urban areas, where land comes at a premium, blue roofs and other smart systems will continue to gain prevalence in design due to the rapid evolution of the technology, the efficiencies it provides and ultimately its cost-effectiveness.
Green infrastructure and smart stormwater systems represent an opportunity to not just preserve but to amplify ROI and set the tone on how to meet and exceed the ever-changing stormwater-management requirements of the 21st century.