Tunneling: A Potentially Valuable Flood Mitigation Tool
Engineering firm Freese & Nichols claims that “Incorporating tunneling into Houston’s stormwater portfolio could significantly reduce flood damages and improve the reliability of existing conveyance and detention infrastructure.” Tunneling technology, the firm says, has improved dramatically in the last 30 years, making projects possible that were once deemed impossible.
Rapid Growth Limits Mitigation Possibilities
Houston’s exponential population growth (16X during the last 100 years) has made both flooding and flood mitigation more difficult to deal with. However, tunneling, says Freese & Nichols bypasses the urban sprawl issue – especially in dense neighborhoods, such as those inside Beltway 8. Tunneling’s low-impact can move stormwater with very little effect on the surface, benefiting communities and addressing environmental concerns.
Tunnels Expand Both Conveyance and Storage
Tunnels, they point out, expand conveyance capacity within a watershed. They can also store stormwater during floods. A 30-50 foot tunnel can store 50 to 150 acre-feet of storm water per mile. More important, it can convey 10,000 to 15,000 cubic feet of water per second. To put that in perspective, that’s about 40-60% of the flow coming from Cypress Creek during Harvey. Or almost 20% of the flow coming from the Lake Conroe Dam.
Thus, tunneling could significantly reduce total flow coming down rivers and streams during floods by providing an alternative means of conveyance.
How Tunnels Are Built
This detailed video shows how a modern tunneling machine works. It can construct up to 350 meters of tunnel in a week. That’s between a fifth and a quarter of a mile per week. The machine continuously cases the hole with precision, pre-caste, concrete segments as it excavates through loose sandy soil. It also dynamically balances pressure in the tunnel along the cutting head face to prevent cave ins. Working a hundred feet or more below the surface, it can even evacuate ground water.
Editor’s note: This 13-minute industrial video is among the best-produced videos of its kind that I have ever seen. It should satisfy professionals as well as non-technical types. If you have students who lean toward science and engineering, make sure they see this; it shows how human ingenuity can fill the gap between problem and solution.
Once built, gravity drives the system during floods.
Success Stories in Other Parts of Texas
Here in Texas, engineers have used the technology successfully to reduce flooding potential in Dallas, San Antonio and Austin. As urban centers grow, the need to move infrastructure underground grows with them. Disruption to life and the environment on the surface are simply too costly otherwise.
This presentation gives an overview of the technologies involved several case studies in Texas and the U.S. Here’s a shorter two-page summary. And a link to the Freese & Nichols blog that provides a more detailed discussion of the possibilities.
Weighing Expense Against Flood Cost
Because of the expense, tunneling isn’t the first technology you would consider for flood mitigation. But it can be a valuable addition to the tool chest…especially when weighed against the $125 billion that Harvey cost Houston residents.
Numerous discussions have been held at the county, state and federal levels re: the potential applications of this technology.
Community Impact reported last week that Brian Gettinger—tunneling services leader with Freese & Nichols —said he thinks the concept could work on Cypress Creek.
The newspaper said that Gettinger pitched the tunnel system to the Cypress Creek Flood Control Coalition in March. He said if the tunnel becomes a reality, it could cost $2 billion-$3 billion, would take years to build, and would require federal support because of the high price tag.
Feasibility and Alignment Studies
Harris County Flood Control should soon begin Phase 1 of a $400,000 study. Once started, it could take four months to confirm whether tunneling is feasible in this area. Future phases of the study will dig deeper into specific alignments (Buffalo, Brays, Cypress, etc.), evaluating inflow and outflow points, and specific routes.
Posted by Bob Rehak on 4/22/2019 (Earth Day)
601 Days since Hurricane Harvey