Stormwater Tunnel on Buffalo Bayou Will Not Prevent Flooding

Plans Still Evolving

Comments and Questions

Please note that the Houston Chronicle has published a highly useful explanation of the coastal protection plan for Galveston Bay known as the Ike Dike. Unfortunately the paper has not figured out how to market to nonsubscribers.

Oct. 2, 2022

Large stormwater tunnels will likely not prevent flooding on Buffalo Bayou, according to the recent report from the Harris County Flood Control District.

Tunnels draining the federal flood control dams on upper Buffalo Bayou would not even be adequate to prevent a catastrophic overtopping of the dams or flooding of properties behind the reservoirs if a Harvey-like storm parked on top of the reservoirs, according to an engineering analysis prepared for Houston Stronger, a west Houston-based group formed in the wake of Harvey in 2017.

The Harris County Flood Control District is considering a $30 billion, 133-mile system of eight large-diameter stormwater tunnels to manage flood risk in the county. The district issued its Phase 2 feasibility report at the end of March and updated it in September. The district has been holding public meetings and taking public comments in preparation for the next phase of analysis to begin in the spring.

Proposed route of the Buffalo Bayou tunnel, including tunnels draining Addicks and Barker reservoirs. From the Phase 2 report, p. 82.

Limited Capacity

The probability that the proposed Buffalo Bayou tunnel would not prevent flooding downstream on the bayou is based on its limited capacity. The main tunnel would connect to two short 40-foot diameter tunnels, less than two miles long, draining Addicks and Barker dams far upstream. They would have a combined capacity of 11,600 cubic feet per second (cfs). This would drain into the much longer 40-foot diameter bayou tunnel which would have a capacity of only 12,240 cubic feet per second. Inlets or intakes draining the entire bayou downstream would have to be closed to accommodate stormwater flowing into the main tunnel from the two dams. The main tunnel would traverse the city deep underground for some 22 miles all the way to the ship channel east of downtown. (p. 127) (See also p. 1130)

That means that Buffalo Bayou would still flood because the bayou floods from urban rain runoff below the dams even when the dam floodgates are closed and no stormwater is draining out of the reservoirs. (See also here.)

Even though the district is considering possible branching tunnels draining neighborhoods on Buffalo Bayou (p. 27), it would seem that those intakes too would have to be closed if the main tunnel was already filled to capacity by draining the reservoirs.

Possibly the four downstream bayou intakes (p. 83) would be opened to siphon flow out of the bayou during a storm while the reservoir tunnels were closed. These are questions that remain to be answered by the flood control district.

The conclusion that the tunnels would not be sufficient to resolve the problem of too much stormwater flowing into the federal reservoirs during a lengthy heavy storm is the result of a pro bono study by the engineering firm BGE prepared for Houston Stronger and announced at the Sept. 9 quarterly meeting.

Phase 2 of the tunnel study was done for the flood control district by the engineering firm Black and Veatch.

The entire Buffalo Bayou tunnel, including the short reservoir tunnels, (p. 82) would be 24.5 miles long and cost $4.6 billion. It would be the longest and most expensive tunnel and take the longest to build (some 17 years). It scores the lowest in all categories, including flood risk reduction, environmental impact, social vulnerability, operations and maintenance, and more. (p. 1185)

The report also notes also that 40-foot diameter tunnels as proposed for Buffalo Bayou are much more costly, difficult, and risky to build. (p. 76)

Other Comments and Concerns

Intake Operations

Would the tunnel intakes prevent the stream flow from reaching the upper banks of the bayou during ordinary rainfall or minor storms? The upper banks need to be soaked and replenished, even replanted by the natural functioning of the stream. Vegetation on the banks holds the bank in place, cleanses pollutants from the water, and helps slow and absorb stormwater.

The intakes would be located just above what’s known as the Ordinary High Water Mark. Would they normally remain open or closed? How would they be operated? Who is going to pay for the maintenance and operation of tunnel intakes, the tunnels, and outfalls?

The problematic issue of opening and closing the four intakes downstream of the dams is still being studied, according to Scott Elmer, assistant director of operations for Flood Control, who gave a presentation on the flood tunnels study to the Houston Galveston Area Council Regional Flood Management Committee during the summer.

Drought, Faults, Sediment, Water Quality

The report itself describes concerns about the potential for drought and faults to damage the concrete tunnels as well as sediment buildup in the tunnels and in the ship channel. (p. 23 and  pp. 1802-1812 and 1837)

The natural function of a river is to distribute sediment from one side to the other, collecting it from one bank, laying into down further downstream, with trees and vegetation collecting sediment, rebuilding the banks. Eventually the naturally-cleansed sediment ends up as beach sand on the coast. One has to wonder about the impact to our coast from continual and costly removal of sand and sediment from our rivers. Note the plan, detailed in the Chronicle article on the Ike Dike, to rebuild sand dunes on the beach where they once were as part of a project to protect Galveston Island and the ship channel from a storm surge.

Where and how would all this sediment collected in the tunnels be disposed of? (For that matter, what happens to all the valuable sand and sediment repeatedly dredged up by private contractors working for the flood control district?) The Corps of Engineers, among others, including the Port of Houston, has expressed concern about the impact of sediment as well as the polluted water flowing out of the tunnels. (p. 19)

Would the outfalls from the tunnels not be blocked by high flows in Buffalo Bayou and the ship channel and other receiving streams such as Spring Creek and Sims Bayou? Or is the idea that the tunnels would prevent high flows? And what would be the impact of so much stormwater emptying into the ship channel from outfalls basically concentrated in the same place upstream of the Turning Basin? (p. 23)

Joint Studies, Limited Options

The Galveston District of the US Army Corps of Engineers, in conjunction with the flood control district, is also investigating the big stormwater tunnel idea as part of its Buffalo Bayou and Tributaries Resiliency Study. The Corps is under heavy pressure to do something about the problem of too much stormwater flowing too fast into the federal reservoirs behind the earthen dams they built almost eighty years ago.

Both the Corps and the district give only examples of much shorter and shallower stormwater tunnels built elsewhere. (p. 18) They never mention the much longer and more problematic tunnel in Chicago, where there are still flooding problems.

In response to continuing flooding problems in Chicago, activists developed a RainReady program to help “individuals, businesses, and communities find solutions to the problem of urban flooding.”

Neither the Corps nor Flood Control  (p. 24) in their tunnel studies considers the option or regional impact of widespread individual and community actions to slow rainwater runoff, planting or preserving trees, creating or restoring more greenspace, installing or retro-fitting green infrastructure to soak up the rain, which actually improves property value. Both agencies, the district and the Corps, are under orders and significant public pressure to use nature-based solutions. Somehow green stormwater infrastructure seems to apply only to new development. Or stormwater detention basins. Never existing shopping malls or giant empty parking lots, etc.

However, the Corps does recommend a variety of enlightened flood risk reduction measures at the household and neighborhood level, as well as the bayou and regional scale, in its 2021 Metropolitan Houston Regional Watershed Assessment. (pp. 77-80)

And the flood control district is now looking at a demonstration rain garden project and claims to plant thousands of trees annually (not including how many they cut down or how many they plan to cut down to build the tunnel intakes and shafts).

SC