Low-impact design minimizes urban runoff pollution
DALLAS — In the city, rain doesn’t so much clean things up as sweep them under the rug. And in Dallas, the “rug” is the Upper Trinity River watershed, according to a Texas A&M AgriLife Extension Service expert.
This is because so much of the land is paved or covered by structures and impervious to water penetration, said Dr. Fouad Jaber, AgriLife Extension specialist for integrated water resources management.
Because of this imperviousness, the typical filtration and purification process done by soil and plants is sidestepped, and bacteria, sediments, oil, grease, pesticides and fertilizer nutrients from yards, roads and parking lots end up running off directly into the watershed, Jaber said.
“In the Upper Trinity River watershed, legacy pollutants (chemicals no longer in active use), including chlordane and PCBs, continue to find their way into the river and reservoirs as part of the sediment load, where they continue to accumulate in fish tissue and contribute to public health concerns,” he said.
Worse, recent urban development in the Dallas area continues to exacerbate the problem, according to Jaber.
However, there are sustainable designs that offer a way to turn this problem around. Jaber is showing how relatively simple measures can drastically reduce the amount of pollutants that wind up in the watershed. And he’s doing this in what essentially is his own backyard, the Texas A&M AgriLife Research and Extension Center in Dallas on Coit Road, just south of the President George Bush Turnpike.
Jaber’s project is funded by an by a Clean Water Act 319 (h) EPA grant administered by the Texas Commission on Environmental Quality.
Jaber’s demonstration project at the center has five components: a detention pond, a bio-retention area, a comparison of permeable pavements in parking lots, rainwater harvesting demonstration and green-roof design tests.
The first of these measures is a bio-retention area, commonly known as a rain garden. Part of the water leaving the 22,000 square-foot public parking adjoining Coit Road drains into the 3-foot deep, 1,550-square foot rain garden. The rain garden has a perforated pipe under-drain and a corrugated overflow pipe. Jaber designed the rain garden to retain a 1.5-inch rain.
The rain garden bed is composed of 50 percent compost, 25 percent native soil and 25 percent expanded shale. The rain garden will eventually be planted in a mixture of perennial and annual native plants to further promote water absorption and filtration run-off water absorption.
Part of the overflow from the rain garden goes to the detention area, a unique design reminiscent of a meandering river, Jaber said. Runoff from the eastern side of the center complex goes directly into the detention pond. The detention area terracing will also have vegetation, mostly perennials, and will retain part of storm runoff, reducing the volume of water that enters the drainage and improving its quality, Jaber said.
The lot upon which the detention area was constructed was about 7.6 acres, but the meandering-river design increases the effective drainage area to about 10.5 acres, he said.
Some might consider the detention area a wetland as it performs many of the same functions, but technically it’s not, Jaber said. Water in a wetland generally is shallow, only a few inches deep. In contrast, the detention area will hold water 12-feet deep in some areas.
“We’d need a lot more acreage to do the same thing with a wetland,” Jaber said.
To the east of the center, Jaber is conducting permeable pavement tests on a 54-space parking lot. He’s comparing five types of parking lot pavement: interlocking concrete blocks, grass pavers, permeable concrete, permeable asphalt and regular concrete. All areas are underlain with layers of aggregate fill stone.
The idea with permeable paving materials is that rainwater and sediments pass through the surface. The sediments, along with pollutants, are trapped in the fill material underneath the pavers before the water reaches outlet pipes that lead to a sewer or stream. Automatic sampling instruments connected to sensors under the pavement record the sediments and amount of water reaching the storm drains.
Farther to the east, Jaber designed the rainwater harvesting roofs and corresponding turfgrass areas to reflect the proportion to average-size Dallas private homes and lots. The roof of each shed has an area of 150 square feet, and the turfgrass lawn associated with each shed is 225 square feet. Each shed has three 55-gallon barrels. Rainwater is collected in guttering from three of the small roofs, and drains into 55-gallon barrels. A fourth shed is also equipped with three barrels, but they are filled with city water, not rainwater.
“Rounded off, each station is in the same proportion of a 1,500 square foot private dwelling with a 2,250-square foot lawn,” Jaber said.
The water from the barrels is used to irrigate the turfgrass lawn by three different methods. In one, soil moisture probes are used to determine how much water is needed. In another, evapotranspiration monitoring is used. In the third, rainwater is also used, but is controlled by an irrigation timer regardless of actual water need, as many Dallas homeowners do. The fourth turf area is also irrigated by timed irrigation but uses city water.
Water volumes collected and applied by the various systems are recorded, as are measurements of water quality.
In the fifth site, Jaber is testing various green-roof designs. A green roof, or “living roof,”refers to a roof partially or completely covered by a growth medium such as topsoil and planted with vegetation. The green roof has a waterproof membrane, root barrier and drainage area.
There are four green roofs, each 100 square feet in area and divided into four sections. On each green-roof section, three types of green-roof growth medium are tested, plus one area of conventional roofing. The three sections with green-roof medium are planted with plants native to the North Texas area.
“From each of the sections, we measure how much water drains out or overflows, so we can evaluate how a green roof reduces runoff and if it adds pollution to storm water,” Jaber said.
Jaber emphasized that the Upper Trinity-White Rock Creek Watershed, with its clayey soils overlaying an underlying calcareous layer, is representative of many urban watersheds in the U.S. Because of this, his test results should be applicable to many cities.
“While recent studies have evaluated the effectiveness of low-impact design practices in various regions in the United States, there is still a great need to evaluate these practices in the field and to collect quantitative data on their performance,” he said. “This is especially true for the Southern United States.”
Jaber welcomes tours of the sites, either individuals or small and large groups. To arrange a tour or for more information, contact him at 972-952-9672, firstname.lastname@example.org.