While large buyouts and billion-dollar bond projects dominate the public’s attention, Precinct 4 has quietly started testing a low-cost method to delay the release of stormwater into the county’s easily overwhelmed channel system. The technology is designed to work on nearly any dry-bottom stormwater detention basin.
The pilot project, known as the Drainage Reuse Initiative, is managed by the Harris County Engineering Department and funded by the Harris County Flood Control District with Hurricane Harvey bond funding.
As part of the project, Steve Albert, the vice president of infrastructure with the Binkley & Barfield engineering firm, installed a redesigned outfall structure last year at the bottom of a 35-acre detention basin near Holderrieth Road and the Tomball Tollway in Tomball. The pipe modification doubles the amount of time the basin can hold water, giving creeks and bayous time to drain, which helps reduce flooding downstream.
“Before the modification, the lower portion of the basin would drain within half a day,” said Albert. “Now it will hold stormwater for up to four days.”
The Holderrieth location is the first application of this technology in Harris County. Instead of the usual 24-inch drainage pipe that allows water to enter and exit the basin, the modified outlet features three pipes. A 24-inch pipe allows water to enter the basin from Willow Creek during heavy storms. At the same time, an 8-inch pipe returns water from the basin back into Willow Creek. This design ensures that potential floodwater enters the basin 10 times faster than it drains, allowing much more water to be detained in the basin.
The third component, a 24-inch pipe attached to a swivel, can be raised above basin water levels by remote or timer to allow the basin to fill and lowered to return drainage to full capacity.
The modification comes after a two-month study by Binkley & Barfield determined that the design would not negatively impact existing drainage upstream or downstream of the basin.
To prevent basin water levels from rising too high, Albert explained that the 24-inch pipe will be lowered during large rainstorms so the basin will empty as normal.
“The modification only works for small storms,” said Albert. “During 10-year or 100-year storms, the outfall pipe will operate the same as it always did.”
The pipe modification is just one aspect of the drainage project. A 12-month study kicked off on Jan. 1 that will look at how well stormwater is absorbed into the ground at four 10,000-square-foot test locations inside the basin, one of which serves as a control.
The first test area includes an 18-inch layer of native soil mixed with sand and organic compost. The second site features 16 trenches measuring 3 feet deep and 2 feet wide filled with gravel.
The third and most expensive test area involves a system of tubes that suck water into the ground like a vacuum.
During rainstorms, the ground modifications will work with the outfall structure to filter stormwater and to replenish the groundwater supply. Because water sits inside the basin longer, the ground can absorb more stormwater.
The study will conclude at the end of 2020. Any stormwater that enters the basin and flows into the ground within the four study areas will be measured to determine the effectiveness of each ground modification.
Commissioner R. Jack Cagle hopes the technology will help the Harris County Flood Control District improve stormwater management using existing infrastructure at a lower cost to taxpayers.
“This technology has the potential to significantly increase the holding time of other existing basins in a way that brings huge benefits at a relatively reasonable cost,” said Cagle.