The plight of Detroit, which has been prominently covered both in the media and in urban planning circles, may be an extreme example of urban decay, with its reported (albeit now falling) 25% vacancy rate and startlingly eerie images, but it is hardly a unique phenomenon. Legacy cities, those once prosperous centers of industry and culture are now struggling to survive in a new knowledge-based economy. So what does green infrastructure have to do with this conversation? It’s all about a changing concept of resiliency.

Before diving into this further, here’s how Alan Mallach and Lavea Brachman of the Lincoln Institute of Land Policy define “Legacy Cities”: 

American legacy cities were once industrial powerhouses and hubs of business, retail, and services scattered across New England, the Mid-Atlantic, and the Midwest. Their factories provided jobs, and downtown areas contained department stores, professional offices, and financial institutions that served large regions. Since the mid-twentieth century, however, these cities have seen sustained loss of jobs and population, and now face daunting economic, social, physical, and operational challenges.

These are some major challenges. Imagine a city drained of population and tax base. In addition to the ghostly abandoned buildings scattered throughout the city, with essentially no money to repair or demolish, what about the systems once designed to service this bustling metropolises? There are the hard infrastructure systems: the water lines, sewer and stormwater collection networks, the electrical grid. What happens as a city disintegrates, density drops and municipal funding to maintain and repair aging, and now oversized (due to population loss), systems is the lowest its ever been?

The term “resiliency” actually comes from ecological theory where it refers to an ecological system’s ability to either:

  1. Return to its equilibrium state after sustaining an external shock
  2. Ability to adapt and adjust to changing internal or external processes. (Pickett et al 1992)

Interestingly, traditional engineering of urban systems is usually more in line with the first (older) definition of ecological resilience. As engineers, we want to be able to absorb the external “shocks” of large storms, earthquakes, fires. More abstractly, we also work to provide consistent levels of service of electricity (which is kind of like evening out the “shock” of the sun not being available all the time for all our needs). This is all well and good, except that there is also a good reason why ecologists have generally rejected this first definition of resilience: it represents a very, very, very small subset of what systems actually do.

Systems are not self-contained and they are not unchanging, therefore return to “equilibrium” as quickly as possible is not a great model to adopt, for either ecologists or engineers. Take stormwater management for example. Te conventional method of managing urban runoff is to design for a specific “storm event” (for example, the 100-year storm, or the storm that has the likelihood of occurring once every 100 years). You size your pipes to deal with this rain intensity and runoff volume, design the network, put the pipes in the ground, bury them, and then forget about them. When it rains, the system acts exactly how you’ve designed it to. If the storm is smaller than one you’d experience once every 100 years, it works. If you get the 100-year storm, then the place floods, also the way it’s supposed to work. But what if you start getting 500-year storms multiple times in a lifetime? Pipes buried in the ground will not adapt to climate change, a non-equilibrium external change. The first model of resiliency has failed you.

The same failure can be said of cities in general, though of course cities are much more complex. Here’s a (simplified)  example: Detroit’s prior resilience was based on accommodating growth associated with growing industrial manufacturing– it “absorbed” the “shock” of population growth by building more suburbs, taller buildings, and larger infrastructure systems. What it did not anticipate was the rules of the game are not equilibrium. The exterior change in process was a national-shift away from manufacturing altogether.

Vision Rendering of Philadelphia's Green Infrastructure proposal. Source:
Vision Rendering of Philadelphia’s Green Infrastructure proposal. Source:

The way green infrastructure fits into this conversation is actually quite critical. While ecologists use these resiliency models to explain natural phenomena, engineers and planners can use them to design interventions or solutions to problems. What is so tricky about the non-equilibrium model of resiliency is that it is impossible to know what future changing process our urban areas will need to adapt to. For this reason, infrastructure needs to be modular, easily and naturally activated/decommissioned, multifunctional, and distributed within communities for maintenance.

Green infrastructure absorbs stormwater where it falls into natural impervious, vegetated areas that can be used as parks, playground and community spaces. Instead of a large, centralized, buried storm drain network that cannot be changed, and is empty and underused when it’s not raining, green infrastructure can still add value to the community, even when the community goes through changes. Green infrastructure on vacant lots has been shown to reduce violent assaults (Branas 2012); green streets have been shown to reduce drug violations (Kondo 2013); extremely relevant functions in our depopulating legacy cities.

Broadening the possibilities of green infrastructure to other distributed infrastructures, imagine the modular, potentially low-tech and environmentally-based technologies of solar panels, small-scale, decentralized wastewater treatment through natural wetlands, or rainwater capture for non-potable uses. If we acknowledge the important new definition of “resiliency”, in light of the challenges of legacy cities, or even climate change, then green infrastructure and all its characteristics, becomes even more appealing. Indeed, it seems that Detroit is already starting to look to green infrastructure as a solution

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