Deep Diving Into Flooding

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

July 5, 2022

Updated July 6 with question about total acreage required for 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.

Map of proposed deep stormwater tunnels, Harris County Flood Control District

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.

Elevation graphic, Harris County Flood Control District

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.

For further description of the possible alignment of these tunnels, see p. 77. However, the alignment of the tunnels is not final, Tommi Jo Scott, Flood Control’s project manager for the tunnel study, pointed out at the public meeting last month.

Tunnel construction could take from 5 years and 3 months for Sims Bayou to 17 years for Buffalo Bayou. (p. 20) The tunnels would have a 100-year life span, according to Elmer.

Note that despite more than 50 years of stripping, straightening, deepening, widening, and covering Brays Bayou in concrete, with more than $550 million spent on Brays flood projects in the last 20 years alone, all this engineering “is not sufficient to eliminate major flooding issues in this watershed,” according to the Phase 2 study. (p. 40) Brays has the greatest number of historical damage claims and second largest number of damage centers. (p. 40)

Maybe a different approach would work better?

To see Flood Control’s map of flood damage centers, go to p. 32.

Buffalo Bayou Tunnel

Route of the Buffalo Bayou tunnel, Phase 2 tunnel study, p. 82. Note that the shaft numbers do not correspond with the descriptions or photos included elsewhere in the report, which are numbered simply 1 through 10.

The Buffalo Bayou tunnel would be 24.5 miles long, cost $4.6 billion, and take some 17 years to build. It would be the longest and most expensive tunnel and take the longest to build. It also seems to provide the second least (and most expensive) benefits, depending on how you look at it.

As engineer Michael Bloom pointed out in a recent meeting of the Harris County Community Flood Control Task Force, Flood Control’s recently released Phase 2 tunnel study measures impacts to structures rather than to people. The task force has developed a Flood Mitigation Benefits Index which is meant to guide Flood Control in developing projects.

However, Flood Control claims in the Phase 2 study that the Buffalo Bayou tunnel’s unique benefits include the fact that it “can serve as a trunk line to which other tunnels are connected,” is the only tunnel that “reduces the risk of a dam breach,” and “reduces impacts across the spectrum, including impacts to critical care facilities, wastewater and water treatment, power generation and transmission.” (p. 27)

“Dam breach” refers to the potential of stormwater overtopping and collapsing the 75-year-old federal flood control dams on the Addicks and Barker reservoirs in west Houston, a nightmare scenario presented by the enormous amounts of rainfall associated with Hurricane Harvey in August 2017. Stormwater backed up behind the dams and flooded thousands of homes built in the flood pools (on land never purchased by the federal government). Thousands more properties were flooded when the Corps opened the floodgates and stormwater began spilling around the northern end of Addicks dam. The federal government has been sued by property owners behind and below the dams.

As a result, in addition to the flood control district’s tunnel studies, the US Army Corps of Engineers, which owns and operates the dams, is also conducting a separate study of a flood tunnel draining the reservoirs. That federal study, in collaboration with Flood Control, is funded with $11.167 million in federal and county funds. Completion of that study is scheduled for Dec. 10, 2023.

Most of the flood damages that occurred in the Buffalo Bayou watershed, which includes Addicks and Barker, cited in the district’s Phase 2 study were the result of stormwater rising in the reservoir flood pool behind the dams and the subsequent release of stormwater from the dams, which flooded properties to just below Beltway 8. This was a stretch of Buffalo Bayou that had been stripped and straightened by the Corps in the 1950s, effectively reducing its capacity.

The general calculation of flood damages includes both riverine and land flooding (ponding, backed-up storm drains, etc.) outside official floodplains (p. 55), as well as historical flooding over a period of 100 years. (p. 14))

Taking Up Ten Acres in Memorial Park. Passing Under the Park and Around Leaking Oil Tanks.

The Buffalo Bayou tunnel (map, p. 82) would start with two 30-foot diameter tunnels draining Addicks and Barker reservoirs. The large, normally dry reservoirs, some 26,000 acres in total, double as playgrounds and natural parks. Interestingly they are filled with abandoned gas and water wells and pipelines. This could pose a problem for proposals to excavate the reservoirs in order create more stormwater storage capacity.

Abandoned pipeline in Barker Reservoir. Photo June 2022 by Adam G.

Below the dams the Buffalo Bayou tunnel would be larger, 40-feet in diameter, and carry a flow of 11,000-13,000 cubic feet per second (cfs) (p. 128), which is major flood level in the bayou (though flood stage varies along the stream) and nearly double the volume of flow that flooded homes on the banks immediately downstream of the dams during Harvey even before the Corps of Engineers was forced to open the dam floodgates in the early morning of Aug. 28, 2017.

The tunnel would be buried some 90 to 180 feet underground (measured from the bottom or invert) and have 7 intake structures among a total of 10 shafts. (p. 19) The land required for these structures includes several large (5-10 acres) areas of woods and wetlands within the reservoirs, in Terry Hershey Park below the dams, and in Memorial Park. (p. 959)

Here are some images of potential shaft sites from the report. (pp. 1057-1063) Site 3 is in Terry Hershey Park.

One of these massive concrete intake structures would be located in Memorial Park and would level some 10 acres of woods in the historic Old Archery Range, taking up nearly half of the 23-acre section west of Loop 610. (pp. 78 and 959)

This photo of the woods of the Old Archery Range in Memorial Park is from the Phase 2 study, p. 1063.

This is also the site of a bizarre project proposed by the Memorial Park Conservancy that would have similarly stripped and graded some 10 acres of wooded public land, cut artificial channels through the meanders (also apparently cutting off public access to parts of the park), and artificially reinforced the natural banks. Apparently this pointless, costly project has been put on hold.

Flood Control’s proposal to convert land use in this little-known section of the public park, site of a popular boat ramp and massive stormwater outfall draining Post Oak Road, would require public notice and a public hearing under Chapter 26  of the Texas Parks and Wildlife Code.

The massive tunnel would then pass underneath Memorial Park, continue north of downtown, and include a potential drain north of the bayou in the Fifth Ward to accommodate future drainage projects. (p. 78)

There are some two dozen leaking petroleum storage tanks along the route of the Buffalo Bayou tunnel from Barker Reservoir at Highway 6 through Voss Road, Bering Drive, Washington Avenue near Westcott, Yale and Taylor, and where White Oak joins Buffalo Bayou downtown. (pp. 1012-1020)

Public Support, Public Comment Needed

The district needs public support for hoped-for federal and state funding and is eager for the public to comment. Comment is accepted through Sept. 30, which is before the district begins the next phase of its tunnel study. The next phase will include community meetings.

Here is how to comment. And here is a link to the slides from the June 16 presentation. You can watch a video of the public meeting here.

As usual Flood Control’s recent presentation included a slide listing the causes of flooding in Harris County: big storms, flat landscape, clay soils. Nothing to do with impervious surface, apparently. Or the many sloping gullies, ravines, and creeks filled in and flattened by developers.

Opting for Trees Instead

Just going to note upfront that the City of Philadelphia, after a cost benefit analysis, opted against a similar massive tunnel project in favor of planting more trees and becoming more of a sponge city. There is also debate about the benefits of the lengthy Chicago tunnel and underground reservoir begun 50 years ago, which like many big city tunnel projects is a combined stormwater and sewage system. The problem: Chicago still floods.

Many of the large tunnel and underground reservoir systems built in major cities around the world drain from the surface, acting as super storm sewers or even reservoirs, rather than from their rivers and streams. (See, for example, Mexico City and Japan. Also Dubai, Istanbul, Malaysia, Hong Kong and Milwaukee.) Flood Control often cites shorter stormwater tunnels in Austin, Dallas, and San Antonio.

The flood control district has so far spent $2.9 million in federal and county funds on Phases 1 and 2 of on its own flood tunnels study and, as noted above, plans to draw on the $20 million made available through the 2018 flood bond issue to fund the upcoming Phase 3.

For Phase 1 of the study, which was to determine whether such tunnels are possible in our local ground conditions, the flood control district worked with an underground construction technology expert team led by Freese and Nichols with support from Parsons, Brierley Associates, Terracon, HVJ, Sowells Consulting Engineers, and Middleton Brown.

Freese and Nichols is also providing technical support to the San Jacinto Regional Flood Planning Group, which is working on a regional flood plan under the aegis of the two-member Texas Water Development Board. Providing public engagement assistance to the regional flood planning group is Hollaway Environmental, which, along with Fugro, also worked with Black and Veatch on Phase 2 of the district’s tunnel study.