Unlocking Stormwater Solutions: Case Study on Chicago Blue-Green Roofs
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Critical Side Wall calculation adds 25% of tower area to overall watershed of project and increases stormwater volumes. Photo: American Hydrotech
Introduction
Water. The single most precious substance on this planet. And yet this substance that, arguably, can cause the most consternation. Its varying distribution and drinkability create issues for people all over the Earth. Changes in climate and storm frequencies have created deep challenges to civilizations for epochs. These issues are becoming increasingly critical as the march of urbanization continues unabated and storms become more intense. Unable to soak into original natural vegetated areas, rainfall falling on paved and built surfaces must go somewhere. This water is concentrated in ever-decreasing spaces. Pipe systems, often designed more than a century ago, can no longer transport the increasing amount of rainfall.
Catching the Rain Where it Falls
As in any Hydrotech Garden Roof, the LiteTop® growing media is lightly compacted to the specified thickness before planting. Low parapets in background (highlighted with blue flashing tape required special wind considerations in Garden Roof component configuration. (Note: this is the third Garden Roof image)Photo: Garden Roof
Green infrastructure has been an important tool for handling this increase in stormwater. While effective in areas where access to at-grade techniques, rooftops have been largely ignored. In urban areas, rooftops constitute a much higher percentage of horizontal spaces than green spaces. Financial objectives encourage dense developments; many urban projects do not even include green spaces. Spaces in developments that were once allocated for underground and below-grade stormwater management are now so valuable that they are being used for leasable parking decks and below-grade development spaces.
Green roofs have been incorporated into projects for many years here in North America as well as Europe and other places. While green roofs provide stormwater retention, that capability only addresses part of the stormwater equation. The advent of blue roofs provides the capability of stormwater detention by providing space for temporarily holding back (detaining) a specified volume of stormwater and releasing it in a controlled manner and over a specific amount of time.
Combining stormwater retention and detention capabilities is one of the most important advancements in utilizing the roof space on buildings. The following is a case study on just how this combined capability can be deployed.
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Northwestern Bio-Med Project Blue Roof / Green Roof Cross Section. Photo: American Hydrotech
Northwestern University
Located in the Streeterville area on the north side of Chicago, the Northwestern University Simpson Queery Biomedical Research Center is the newest medical building in the Northwestern University medical campus.
A privately funded cancer research center, the major addition to the Chicago medical community was designed by the Chicago office of Perkins+Will and its sub-consulting team. The project is a two-phase project. Phase 1 consists of the base building of 14-stories plus two underground basement stories. Phase 2 will consist of a 20-story tower constructed on top of the 250-foot tall Phase 1 structure.
Located in a dense urban area on the north side of Chicago, this project occupies a very large percentage of its site. All parking is located remote to this building. In addition, this building is near Lake Michigan which creates a high-water table issue around the building.
The City of Chicago required a solution to the ultimate project’s total stormwater generation which required accounting for the total building mass and footprint. A minimum total of 10,000 cubic feet of stormwater needed to be managed on the project. Given the high-water table and the very valuable below-grade spaces, P+W came to Hydrotech for solutions to how to incorporate that stormwater on the top two roof surfaces of the Phase 1 structure.
Given the stormwater and green roof requirements, Hydrotech’s Garden Roof / Blue Roof Assembly was selected to address these conditions.
The project was required to accommodate the stormwater volume generated by the “critical side wall” of the future Phase 2 tower of the project. Hence, the Phase 2 stormwater volume is incorporated within the stormwater management plan for Phase 1.
Northwestern Bio-Med Project Blue Roof / Green Roof Cross Section. Photo: American Hydrotech
The blue roof assemblies covered the majority of roof areas of Levels 13 and 14. Pavers and Garden Roof assemblies cover the Blue Roof components on these two levels.
Starting at the Roof Deck
The roof decks for the blue roof areas were constructed in concrete with a zero-slope. This created optimum water detention volume across the roof surface.
Hydrotech’s Monolithic Membrane 6125 (MM6125-FR) was used to waterproof the roof surfaces to protect the building. This membrane is not negatively affected by standing water that is common on green and blue roofs.
Blue Roof / Green Roof Cross Section
The following major components and assemblies were installed for this project.
Making a place for the Water
Completed Blue Roof component installation over DuPont Styrofoam, Hydrotech’s MM6125 membrane and zero-slope concrete deck. Photo: American Hydrotech
Creating a blue roof involved providing a place for detaining the stormwater. For this project, Hydrotech provided Permavoid (manufactured by ABT Plastics). Permavoid consists of a plastic voiding component, approximately 14” x 28” x 6” thick, which generates a voided volume of approximately 90-95%.
The structure of Permavoid allows for detention of “free water” (water not bound-up with growing media or drainage layers). Stormwater can move freely through the Permavoid units across the roof space.
The Permavoid units are installed in a single layer directly on top of the finished Styrofoam thermal insulation. The Permavoid units are connected to create a firm surface on which pavers and Garden Roof components can be installed. The Permavoid units were easily cut to fit with the odd spaces typical on any rooftop installation.
When completed, the Permavoid layer created a continuous voiding space for stormwater detention and a firm platform on which the next component layers would be installed.
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Releasing the Water - The Flow Control Drain
Standard roof drains were modified and installed with standpipes and orifices required to create detention for blue roof. The orifice is sized to limit stormwater release to not exceed the prescribed rate. Photo: American Hydrotech
To achieve the required maximum release rate for the project, flow control drains components were included at each drain point on Levels 13 and 14. These were standard roof drains that were modified to create the detention in the blue roof and the required release rate set by the design team and the City officials.
The orifice at the base of the standpipe is sized to regulate the volume of stormwater to be released at a rate prescribed by the stormwater permit documents.
When the influx of stormwater exceeds the outflow rate of the orifice, the water backs up into the open Permavoid space where it is held until it can drain back through the orifice in each drain. The combination of all the drains with these orifices determines the total release rate for the roof surfaces.
In larger rain events, the water level may rise to the height of the standpipe. In that instance, the excess water will enter the top of the standpipe where it flows unrestricted into the drain. This protects the roof assembly and structure on the building from excessive amounts of stormwater associated weight.
Garden Roof components being installed over filter fabric and Permavoid Blue Roof elements. Special pavers in background were used for added wind resistance at roof’s altitude and wind exposure. Photo: Garden Roof
Finishing with the Green
Upon completion of the Permavoid layer and concurrently with the paver installation, a standard Hydrotech extensive Garden Roof with 6-inches of LiteTop® growing media was constructed over the Permavoid.
Hydrotech’s Instagreen® Carpet was used to create a complete sedum planting on the roof.
Conclusion
Finished Garden Roof in bloom several months after installation. Parapets are not completed and blue, Styrofoam encased concrete structures are where the future Phase 2 tower will be attached to Phase 1 base structure. Photo: Garden Roof
The combination of blue and green roofs provides additional performance benefits for stormwater management, at a time when stormwater is becoming a bigger challenge for many communities. Keeping stormwater onsite and where it initially lands on the building has many advantages to conventional concrete pipe and below-grade approaches to dealing with water. Since its completion, this project provides an important stormwater and environmental resource for the City of Chicago while also providing a watertight roof for the building. It is a prime example of how to combine several different assemblies together to address an important stormwater issue.
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Richard Hayden, GRP ASLA-Emeritus, Retired Garden Roof and Blue Roof Department Manager, currently Advisor to American Hydrotech, Inc. (A Sika Company) based in Chicago, Illinois
