Building a Resilient City Through Design and Analytics

Imagine a city covered in green. Tree canopies intermingle with the cityscape, wheat and other crops grow on the roofs, vertical gardens bring buildings to life, and streetscapes integrate with long linear gardens. These cities are not a current reality, but our Chicago office, along with OMNI Ecosystems, took a step towards imagining what that would look like in in the future.

On April 20th, the USGBC Illinois Chapter hosted the Chicago Thrives! Resilience Symposium, a collection of presentations by industry experts on resilience in Chicago. We presented findings from a mapping exercise conducted with OMNI Ecosystems’ Molly Meyer (Founder and CEO) and Michael Skowlund (Dir of Landscape Architecture). The central question of our presentation was simple: what would happen if Chicago aggressively expanded its green roof program to as many buildings as possible? What would this do to the region’s hydrology?

Chicago, like nearly every city in the United States, has a combined sewer system. This means that stormwater and sewage mix before going to a water treatment plant. In previous decades, these systems would fill beyond capacity during heavier rain events, and raw sewage mixed with storm water would be discharged directly into a local body of water. In recent years, cities have opted for tunnel or reservoir systems to act as large holding tanks for this overflow water. The overflow is then treated at a later date when the rain subsides. Chicago’s Tunnel and Reservoir Plan or TARP does exactly this and is very effective at improving water quality in the region. However, it doesn’t particularly address the issue of flooding on smaller, localized scales.

Typically, during dry weather (left) sewage safely is sent to a water treatment plant. During a large rain event (middle) a combined sewer can reach capacity and overflow into local waters. Chicago’s current system (right) includes large reservoirs and tunnels to capture this overflow.

The region around Chicago was originally swamp land and often floods. Flooding, especially flash flooding, is a major issue for cities and home owners.  It causes significant property damage, endangers motorists, creates environments for mosquito breeding (a major public health crisis), and causes erosion, among other problems. The primary culprit of urban flooding is impervious surfaces, materials that don’t allow water to penetrate them and return to the water table. More simply put: most buildings and roads., If these surfaces are altered to become pervious, they will act as “tiny sponges” to hold water and slowly release it back into the atmosphere or soil or passively reach the water table.

OMNI develops, amongst many services, green roof media. These technologies allow roofs to grow small plants, crops, grasses, and even trees in some cases. Between the water consumption of these plants and the retention capacity of the media they are planted in, they can absorb a significant volume of water. For instance, a typical 1,500 square foot roof for a three story apartment building in Logan Square (neighborhood in Chicago) could absorb up to 2,400 gallons of water if retrofit with a 12-inch deep media green roof.

Workflow used to calculate outcomes,

The Perkins+Will team from Chicago took OMNI’s figures and processed the existing GIS data for Chicago for every building in Chicago that had viable data. Through a few statistical methods and some data analytics, the team reduced over 800,000 data points to 497,868 buildings with viable data and applied some calculations to determine a total building area that could host a green roof (see figure 2 for workflow). The high estimator used a slightly different statistical method than the low estimator, hence generating a different outcome of total square feet. Minimum saturation represents a media’s performance when it is completely dry, while typical saturation looks at performance when the media has some moisture already present (see figure 3 for analysis, units are in gallons).

To add some context to these nebulous numbers, 1.8 billion gallons of water is the equivalent of 50 Willis Towers or 2,733 Olympic swimming pools. Alternatively, 1.8 billion gallons is about 20% of the total volume of TARP’s current capacity. This figure is encouraging, because flood waters never reach TARP due to storm sewer backups or poor drainage on localized levels, ultimately creating more flood resistant communities. Furthermore, this translates to 6.7 – 9.8 million pounds of wheat flour.

It is estimated that the construction costs would range from $1.3 billion to $5.7 billion. This may sound prohibitively expensive. In 2014 alone, it is estimated that Cook County filed 181,000 flood claims, amounting to an estimated $733 million in damages. In only a few years’ time, the roofs make up their outlay cost.

The potential doesn’t stop at roofs. We want to investigate how roads and parks perform when we apply OMNI retrofits. Building rooftops only make up an estimated 13% of the total land area of Chicago (based on available GIS data). Using that same dataset, roads make up about 16% of land area in Chicago meaning that the remaining 71% is made up of parks, lawns, driveways, sidewalks, train tracks, vacant lots, and so forth. It’s entirely possible for Chicago to better equip all of these spaces to become part of a living infrastructure network in the city. This would better serve communities by absorbing storm water, generating arable land, improving air quality, creating job opportunities, bettering public health, and enhancing urban aesthetics. Through actively stitching together multiple forms and layers of green infrastructure, large metropolitan cities could improve their environmental, fiscal, medical, and social resilience, ultimately improving the quality of life for urban dwellers.

Leave a Comment