After the Rain

Nashville implements large-scale plan to combat stormwater control issues with green initiatives
After the Rain
Stormwater crew member James Madden cleans a culvert, part of the routine maintenance schedule. The crew uses a Vac-Con truck and blasting heads to blow out sand, gravel and debris. Then a large vacuum attachment is used to suck up the debris that was blown from the culvert.

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Nashville, Tenn., is recognized as the country music capital of the world, but the city is working hard to garner another title, that of the greenest, most livable city in the Southeast.

A mayoral ordinance directing Metro Water Services and other departments to develop a plan to install green infrastructure within the Stormwater Master Planning District has been a big part of the push. The move dovetails with the city’s consent decree with the U.S. EPA to control combined sewer overflows. The capital project, which includes some separation of sanitary and storm sewers and constructing equalization basins, will cost more than $1 billion over the next 10 years. Green infrastructure is expected to limit some need for sewer separation by reducing the volume of runoff reaching the sewers.

A stormwater user fee, effective July 2009, created a $14 million revenue stream to manage the four stormwater sections within Metro Water. Their biggest concerns were maintaining drainage in basins that occasionally experienced flash flooding.

“It was localized with water in some basements,” says Roger Lindsey, Stormwater Development Review and Permitting section program manager. “We hadn’t had a historic flood in more than 30 years.”

All that changed on May 1-2, 2010. That Saturday, 6.3 inches of rain fell on Nashville in 24 hours. On Sunday, 7.2 inches fell in 12 hours, making it the wettest day in the city’s history. Lindsey and his section faced their greatest challenge, one that would take almost two years to overcome.


Beneficial experience

The Cumberland River winds through downtown Nashville. More than 100 years ago, workers laid 20-foot brick combined sewers in the creeks feeding the river, filled in the creek beds and built over them.

Public Works managed the stormwater division when it formed in 1979. In 2002, the city transferred the responsibilities to Metro Water Services and created the Stormwater Division with four sections totaling 90 people.

“My section is an engineering group involved in reviewing calculations for stormwater drainage and infrastructure like detention basins and water-quality treatment units,” says Lindsey.

Almost half the division’s employees work in Routine Maintenance cleaning culverts and ditches, reforming ditches, and repairing clogged or eroded infrastructure. The NPDES section is responsible for permit compliance and water-quality sampling. Remedial Maintenance, another engineering group, designs remedial stormwater projects and oversees construction.

“We’ve been building large concrete equalization basins for more than 20 years to capture the flow from combined sewers and prevent it from reaching the river,” says Lindsey. “After storms, the structures release water to the interceptors and outfalls that feed back to the wastewater treatment plant.”

The division began a home buyout program 10 years ago in response to repetitive-loss properties along creeks prone to flooding.

“Our experience dealing with the Federal Emergency Management Agency (FEMA) hazard mitigation grant program (HMGP) made an enormous difference after the 2010 flood,” Lindsey says. “We knew how to apply for the funds to purchase properties. Our application packages didn’t come back for review and corrections.”

Prompt federal responses were critical for owners of homes with more than 50 percent flood damage. They had to keep mortgages current while paying to live somewhere else.

“At best, it takes eight to 20 months before they get their money out of the house,” says Lindsey. “Some can’t survive the wait.”


Silver lining

The division has 25 more flood-damaged properties to acquire for a total of 207 lots that are then deeded to the city. Workers have demolished 101 homes, removing any structure indicating human inhabitancy and returning the land to its natural state.

“We’re working with the Parks Department to tie extended stretches of buyout properties into our greenway trail system,” says Lindsey. “Parks also is considering installing picnic pavilions in places, or playgrounds on land adjacent to the floodplain. We’re also working with volunteer organizations to plant community gardens on some properties.”

After the flood receded, trees and vegetation sloughed off the saturated banks of the Cumberland River. Erosion was severe in places and many people lost significant parts of their backyards. A program to plant 20,000 trees is helping restore buffer areas, and to date, half the trees have been planted.

“During the storm, it was difficult to sense the true magnitude of what was happening,” says Lindsey. “Our Emergency Operation Center was responding to calls for swift water rescues and dealing with evacuations from institutions. Tractor-trailers swept from industrial parking lots created logjams under downstream bridges, causing the most extreme flooding.”

The Harpeth River peaked 15 feet higher than previous records. Thousands of houses in the Bellevue area not in the floodplain had five to 10 feet of water in them, and few owners had flood insurance.

“By Tuesday or Wednesday, life was back to normal for 90 percent of the city,” says Lindsey. “But if you lived along the river or creeks, your life was turned on end for years to come.”


Water in motion

After the storm, staff combed the county identifying damage to stormwater infrastructure for FEMA and drivers.

“The flood washed away culverts, then the roads,” says Lindsey. “In one case, a culvert floated out of its installation without the bed collapsing. We immediately barricaded the area.”

Replacing culverts was the most challenging of the stormwater infrastructure repairs. Temporary efforts slowed erosion, then crews returned to replace corrugated culverts with concrete pipe or constructed box culverts. The cast-in-place drainage improvements took months to complete.

As the flood ripped through backyards, it swept away tons of outdoor possessions that snagged 20 feet up in trees or mounded behind the Cheatham Dam 30 miles downstream of Nashville.

“The sea of floating debris defied description,” says Lindsey. “River cleanup organizations are still scheduling weekend excursions to pull material out of trees and brush along the banks.”

The storm happened so quickly that many people had no time to save anything except their lives. Those who chose to repair their homes immediately hauled flood-damaged possessions to the curb.

“It’s a FEMA-reimbursable cleanup,” says Lindsey. “Our Solid Waste Department quickly removed mountains of nasty, stinking debris to staging locations in our parks, then to landfills. Putting a city back together again after a flood is a monumental undertaking, made more challenging because we had to fight for every nickel we received from FEMA.”

To control normal flooding, Lindsey’s section pinpointed cleaning silt and sediment from stormwater inlets as the best practice. It was and is a major issue because people in some neighborhoods still rake leaves into the inlets. Workers from Routine Maintenance visit every structure annually and have identified those prone to stoppages.

“When a significant storm is predicted, crews make what they call a ‘rain run’ to the hot spots to remove debris,” says Lindsey.

Routine Maintenance also cleans or redefines ditches using Gradall equipment, but most brush and debris work is intensive manual labor. They also do some masonry and install culverts with headwalls in rights-of-way.

“We rent more equipment than we purchase because it is economical,” says Lindsey. “More important is that for the last two years, we have spent most stormwater revenue repairing flood-damaged infrastructure instead of building new projects. Now we’re back to where we were before the flood.”


Nashville SAFE

Six months after the disaster, the section created a computer model for more structured flood forecasting. Jennifer Higgs, Planning Department GIS director, wrote the code based on the city’s geographic information system mapping program. Staff worked with the Corps of Engineers, the U.S. Geological Survey, the National Weather Service and a consultant to develop Nashville SAFE (situational awareness for flooding events).

During emergencies, the operations center launches the interactive tool, which compiles the data from every USGS stream and river gauging station in the Metro area. It projects a map of those stations on the wall. When stations reach a defined action level, they blink.

“The computer monitor has the same image, and holding the mouse cursor over a station brings up the real-time water level,” says Lindsey. As the storm progresses, the map shows inundation levels of roads and bridges, and institutional facilities that should be evacuated.

The center activated the model 11 times in 2011. Every time the team used the tool, they learned from it.

“As Jennifer and I spent long hours watching storm systems, we’d think of fancy new things the model should do,” says Lindsey. “She could always write the code to make it happen.”

Lindsey has presented Nashville SAFE to sister cities and at the Kentucky-Tennessee Water Environment Association Watershed Conference in January 2012.

“People are amazed by what this model does,” he says. “It is probably one of the most sophisticated in the country.”


Green grows the valley

The mayor’s Green Ribbon Committee on Environmental Sustainability is equally progressive. Their efforts to ensure that Nashville is a leader in livability and environmental quality through low-impact development (LID) dovetails perfectly with stormwater and flood control programs.

To date, the city has 50 green sites that incorporate irrigation and graywater harvesting, constructed wetlands, green roofs, bioretention areas, vegetated swales, newly planted trees and permeable pavement. The Pinnacle at Symphony Place, a 29-story office building, is the first skyscraper in the state to receive LEED Gold certification for its 1-acre green roof terrace garden on top of the parking garage. According to the American Society of Landscape Architects, the roof captures 67 percent of the annual stormwater and harvests it for irrigation.

The Nashville Public Square project replaced the Metro Courthouse parking lot with a five-story subterranean garage topped by a 2.25-acre green roof. A 57,000-gallon below-grade tank stores runoff for on-site landscape irrigation. The project earned the Green Roof Award of Excellence in 2007 from the Green Roofs for Healthy Cities. The 25,000-square-foot McCabe Park Community Center with its green roof, permeable pavement and rain gardens is the city’s first LEED-certified community center.

In 2011, the Natural Resources Defense Council named Nashville one of 14 cities leading the country in using green infrastructure to reduce runoff. The council’s peer-reviewed report, Rooftops to Rivers II, contains case studies of how the cities maximized their LID investment.


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