February 23, 2016

It’s no secret that urban areas are booming across the nation. My city, Nashville, Tenn., is growing so quickly that you can’t drive far at all without seeing cranes towering overhead. Apartments, live-work-play developments, office buildings, new neighborhoods, and retail stores are popping up in all corners of the city during our post-recession rebound.  This progress is exciting (and challenging), but it’s important to remember that our natural environment has been paying a price for these structures replacing open spaces.  Traditional development reduces green space, resulting in rainwater that cannot infiltrate the ground, and therefore runs offsite at volumes and velocities higher than would naturally occur.  The collective effects of this rainwater can overwhelm existing storm water systems, scour streams, erode stream banks, and introduce other pollutants into our creeks, rivers, and lakes.  It’s not a new issue, but what is new are more aggressive solutions mandated by policy makers as we become more aware of our planet’s limited resources.

Low impact development (LID) is the latest approach to addressing storm water runoff.  Previous approaches focused on water-quality treatment and reduction of peak runoff rates, but LID approaches runoff on a volumetric basis.  They aim to mimic the storm water speed, quality, and volume that would run off a typical small residential lot during most rainfall events (approximately 20% of the rain amount).  A couple years ago, the EPA, along with other federal agencies, non-governmental organizations, and members of the private sector, created the Green Infrastructure Collaborative to assist communities with green infrastructure implementation.

At the beginning of 2016, Nashville introduced a Municipal Separate Storm Sewer System (MS4) permit requirement for all new projects, both redevelopment and new development, to infiltrate, evapo-transpire and harvest/reuse the first inch of water from each rainfall event. But even without EPA prodding, LID had become a priority in Nashville development in response to our 2010 flood.

LID was often incorporated voluntarily where possible and with the added benefit that a majority of LID design solutions also help projects earn credits towards LEED certification.  (In addition, the Institute for Sustainable Infrastructure recently introduced a parallel certification program called Envision®, which is a holistic approach to sustainable infrastructure design.)  Now that LID is mandatory in Nashville and other cities, design goals will focus on conserving natural vegetation, reducing nutrient and pollutant loads,  managing storm water runoff volume close to its source and reducing and “disconnecting” impervious surfaces.  Examples of some of the most popular LID methods include bioretention areas, green roofs, permeable pavement, downspout disconnection, reforestation, vegetated bioswales, infiltration planters, rain barrels and cisterns.

In addition to the long-term sustainable benefits, the city of Nashville also provides incentives for LID implementation that include waivers of plan review fees, monthly storm water fee reductions, runoff reduction credits, green roof credits, and reduced storm water detention requirements.  Plus, a recent report by the EPA found a majority of LID practices reduce overall project costs.

The green infrastructure trend will only continue to evolve.  As more cities adopt similar mandates, the EPA will likely enact additional regulations as part of its Green Infrastructure Collaborative.  Implementing LID solutions in Nashville is now a reality: this will inspire designers and their clients to think outside the box pursuing innovative techniques that not only fulfill the new requirements, but also create a more sustainable environment for our city.

*This blog post is an adaptation of Mike’s guest column, “Keeping a LID on stormwater,” in the Nashville Post.