How Pavement Can Help Flooding

Nashville’s recent surge in development has led to a substantial increase in impervious surfaces (e.g., surfaces that do not absorb water). Rainwater that used to be able to infiltrate the pervious ground locally, now must flow somewhere. That “somewhere” is to our creeks, rivers, or combined sewer system. This runoff likely includes pollutants such as chemicals, fertilizers, and bacteria from pet waste, as well as sediment and plastics.

In contrast, green infrastructure such as green roofs, bioretentions, urban trees, and permeable materials helps maintain the hydrologic function of the landscape by allowing water infiltration, evapotranspiration. In some cases, that water can be reused onsite. Green infrastructure has many benefits such as decreasing flood risk, protecting water quality by reducing pollutant loads to water bodies, and protecting local flora and fauna.

Metro Nashville Government has several examples of green infrastructure on city properties, many of which are managed by the Department of General Services. At Fulton Campus (a central campus which serves as a hub for several Metro agencies) alone, there are bioretentions, water quality units, a dry pond, water underground detention units, and the largest Metro-owned example of a permeable paver parking lot. All this infrastructure protects our local waterways in a variety of ways: allowing stormwater to infiltrate, filtering out debris before the water flows into the sewer system, and storing stormwater temporarily so that our water treatment systems don’t get overwhelmed.

If you’ve been to Fulton Campus to renew your driver’s license with the County Clerk or to visit Metro Codes, you’ve probably parked in the permeable paver parking lot in front of Metro Office Building (800 2nd Ave South 37210). The permeable interlocking concrete pavers allow stormwater runoff to filter through the voids and gravel in between the pavement surface to an underlying layer of stone. There, water is temporarily stored and then safely infiltrates into the ground to recharge the groundwater. Thousands of gallons of rainwater can be absorbed during a storm event, that would otherwise have rushed into our stormwater system. When the permeable pavers were installed in 2012, the Metro Fire Department performed a demonstration to show how all the water “disappears” between the pavers.

Stormwater Control Measures (SCMs) reduce the amount of polluted stormwater that enter our creeks and streams. Stormwater Control Measures such as permeable pavers and the other features listed above are important to inspect regularly to identify problems such as cracking of the pavers and clogging of the system with sediment or mulch. Maintenance may be needed to stabilize adjacent vegetation to prevent this sediment or mulch runoff from clogging the surface. Once the surface becomes clogged, a vacuum-type sweeper is used to suck up the debris, leaves, sediment, cigarette butts, etc. This also vacuums up the gravel in between the pavers, which must then be replaced. The permeable pavers at Fulton were recently repaired in the places where it had clogged and shifted over time from heavy use. Some of the pavers were taken up, the rock layer below repaired, and the pavers put back. The water now once again filters through and the permeable parking lot works again as intended.

As you can see, even a parking lot can be an example of low impact development and sustainable design! At General Services, we work to ensure that both the city buildings we manage and the surrounding grounds are functional, beautiful, and sustainable.