Flood Control Looking at Countywide System of Flood Tunnels
System Could Be One of Largest in the World
- Tunnel Drains Would Take 10 Acres of Memorial Park, 14 in T. Hershey Park
- Buffalo Bayou Tunnel Longest, Most Expensive, Longest Time to Build
- Public Comment Encouraged Through Sept. 30
July 5, 2022
Updated July 6 with question about total acreage required for the tunnels.
It’s difficult to think about flooding during record heat and drought, when it’s too hot to go outside, the roads are melting and water lines are bursting. And when you do go outside your cell phone shuts down and burns up in your hand and the utility poles stink of toxic creosote and the shadeless, empty concrete parking lot radiates like a stovetop and scorches your face. And it’s not even the hottest time of the year yet.
But maybe there’s a way to think about all that and flooding at the same time.
The Harris County Flood Control District is thinking big. They are looking at digging a $30 billion, 133-mile system of 8 large underground tunnels to drain stormwater out of our bayous and streams anytime it rains enough to, say, push water levels over the ordinary high flow. The tunnels would also have the potential to collect rain runoff from neighborhoods and streets.
The scope of the project, the result of Phase 2 of the tunnel study, was announced during a public meeting in June. The details are contained in a $2.5 million report conducted for Flood Control by the global engineering firm Black and Veatch. It’s 1,860 pages, which is why it’s taken us so long to explain it. Sorry.
If built, the ambitious project might just be the largest stormwater tunnel system in the country, if not the world.
However, one can’t help but wonder about the environmental impact and unintended consequences of regularly siphoning flow from our streams, particularly in Buffalo Bayou. There are sediment (pp. 1836-1848), maintenance and water quality issues to explore. Mosquitoes. Muck. Ground faults, leaking oil tanks, wells, pipelines, bridge and building support structures underground, the impact and operation of the outfalls. (p. 191) Drought and shrinking, buckling soils. (p. 12) And what if a heating climate changes rainfall patterns?
The Houston Climate Impact Assessment predicts a hotter Houston climate (p. 48), longer and hotter summers (pp. 31-32) with little change in annual rainfall. (p. 37) However, extreme precipitation events are expected to increase (p. 42) as is the risk of drought. (p. 43) More rainfall all at once and less rainfall for longer periods. But where in the Houston region this extreme rain will fall is difficult to predict. (p. 47)
The Tunnel System
The benefits of a stormwater tunnel system, according to Flood Control, include using less total land compared to the district’s usual methods, presumably for achieving the same level of reduced flood risk. Total land use for the proposed tunnels is 34 acres, compared to 377 acres for “channel improvements,” 12,882 acres for stormwater detention basins, and 3,145 acres for buying out homes and properties in flood-prone areas, according to a presentation by Scott Elmer, assistant director of operations for Flood Control, during the public meeting last month.
Update: It’s unclear how Flood Control calculates the land use for the tunnels since the 10 shafts along the route of the Buffalo Bayou tunnel alone require over 76 acres of mostly wooded land, including wetlands. (p. 960)
Flood tunnels also would be faster to construct “than multiple traditional projects of equivalent combined benefit,” said Elmer.
Note that the land use comparison doesn’t include prairies, wetlands, forested parks or undeveloped greenspace, street trees, conversion of parking lots, rooftops, and lawns or other “sponge city” approaches with multiple co-benefits, in addition to reducing flooding, like reducing urban heating (which also attracts storms) and global warming, increasing healthiness, economic activity, and the attractiveness of our urban environment. Could be a worthwhile alternative investigation.
The 8 concrete tunnels would be 30-40 feet wide, some 80 to 180 feet underground (p. 1104) with 39-140 feet of earth covering the top of the tunnel. (p. 19) Between 8.2 and 24.5 miles long, they would be constructed starting shallow and getting deeper as they approach the point where they would discharge the stormwater. Most would discharge into (p. 70) Buffalo Bayou east of downtown, into the Houston ship channel below the turning basin, or, in the case of the Cypress Creek tunnel, into Spring Creek (p. 92). The Sims Bayou tunnel would empty into Sims Bayou near Milby Park. (p. 94)
Most would operate by gravity, though pumps would be required to pump out water between storms and prevent accumulation of sediment. (p. 69) Even though Houston is famously flat (excluding its many sloping ravines, gullies, and creeks), there is a 240-foot drop in elevation from the highest point (Hockley Mound in northwest Harris County) to Galveston Bay. (p. 37) Which is why our rivers and streams drain that way.
Though the Phase 2 report frequently refers to 4 tunnels selected for further study, Flood Control’s Elmer confirmed in an email that all 8 tunnel concepts, potentially benefiting 11 of the 23 watersheds in Harris County, are being investigated in the next phase of the process.
Tunnels could be located in these watersheds: Brays, Buffalo (including Addicks and Barker), Clear Creek, Berry and Vince; Greens, Halls and Hunting; Halls and Hunting; Little Cypress Creek-Cypress Creek, Sims, and White Oak. (p. 17) However, further study will be conducted during Phase 3, which is to start in Spring 2023. The Phase 3 study will be funded with $20 million from the 2018 Harris County flood bond program and is expected to take three years.