City Council members are generally pretty tight-fisted stewards of taxpayer dollars. So the council's 8-1 decision last week to allocate $341,000 for a new roof -- this time, a green roof -- for the city hall annex that houses the council's public meetings and offices raised some eyebrows.
In fact, the decision should be applauded. It's far more cost-efficient over the 50-plus year life of the green roof than conventional roofs. More to the point, it also shines a light on just one of the water detention systems that are expected to save the city many millions of dollars -- probably hundreds of millions -- over coming decades as the city shifts from a combined sewer system to a system that diverts massive amounts of storm-water-runoff out of the sewers and into better use in myriad other ways.
Putting up a green roof -- a roof lined with planters and vegetation that soaks up rain and drains into an on-site detention system rather into the city's combined sewage -- will be three to four times cheaper than a rotation of 15-year conventional roofs over the next 50 to 60 years. On-site storage of rain water also can provide water for the roof's plants when its dry, and gray-water for sanitary systems.
The math in favor of a green roof is fairly simple. The conventional roof would cost around $100,000 and last about 15 years, and, with a low R-2 insulation, would raise the building's heating and cooling costs. Over the next 50 to 60 years, sticking with a conventional roof system would cost another $600,000 in constant dollars.
The green roof --minus about $40,000 in life-cycle monitoring and analysis equipment that the city will use to document the roof's efficiency -- would initially cost around $305,000. But it would last more than 50 years, and save 20-to-30 percent in addition on heating and cooling. Indexed against inflation in conventional roof systems and utilities, it would ultimately save the city between $1.2 million to $1.5 million over its life-span, or three-to-four times its upfront costs. Plus, it would help reduce the city's huge storm-water run-off burden, and that's perhaps its greater value.
The roof equation is an apt metaphor for the larger problem of storm-water run-off management. Over a year, the city's average 52-inch rainfall generates 1.4 million gallons of water per impervious acre, according to David Crockett, head of the city's Office of Sustainability. The city cannot afford to continue putting such volumes of storm-water into the sewer system, nor can it pay the penalties it will face if it doesn't remedy the problem.
Why? The city's old combined sewer system floods the regional Moccasin Bend sewer treatment plant every time there's a hard rain. Such flooding sends periodic overflows of barely treated sewage -- and sometimes raw sewage -- down river, just as sewer treatment plants upriver send their tainted sewage effluent downriver to us.
This can't continue. And under the current phase-in by federal and state authorities of tighter clean water standards, the cost to the city and county governments of handling sewage and storm-water is about to take a quantum leap. The county's Water and Waste Water Treatment Authority already foresees over $100 million for required sewer work to meet current standards. The city's sewer cost will be a multiple of that.
The city has already built more than a dozen huge underground storage tanks at a cost of millions of dollars in an attempt to reduce storm-water overflow until the stored water can be slowly pumped through the treatment plant and into the river. The city's underground tank near the Tennessee Aquarium, for example, holds as much storm-water as the Aquarium. The one in Coolidge Park is equally large.
Beyond the cost of this stop-gap method, moreover, there is no above-ground benefit for taxpayers for the huge investment of building more underground tanks. Nor is there a value-added benefit for continually enlarging storm-water run-off systems into sewers and the sewer treatment plant.
To reduce the costs of storm-water run-off management and to add new civic value, the city must adopt a citywide green strategy for storm-water detention. This will require more than green roofs and onsite water detention; it also must include permeable paving, new parks with earth berms that create swales to capture storm-water runoff, and more sidewalk and center-lane planting areas, among other things.
The city's Office of Sustainability is presently planning for such a system, and its taking lessons from a number of cities that are already implementing such strategies. Philadelphia, for example, expects to reduce its new storm-water and sewer management cost from $16 billion for a sewer-based storm-water infrastructure to $2 billion for a green-strategy infrastructure. Readers interested in seeing how its multi-faceted system works can go online to
A representative from Philadelphia will join leaders from Milwaukee, metro Chicago and New Jersey in Chattanooga this week for the city's 2012 Clean Water Summit, which will discuss green strategies for storm-water detention and their cost-savings and community benefits.
The larger focus won't be on green roofs, to be sure. It will be on the larger civic benefits of greening a city in a way that improves residents' daily lives and civic amenities as it converts storm-water run-off from a problem to a benefit.