Tunneling Offers a Solution to Houston's Flooding Problems
Eighteen months after Hurricane Harvey devastated the Texas Gulf Coast, recovery efforts and flood management projects continue as the region improves its resiliency for future events. Although unprecedented, Houston's catastrophic flooding from Harvey was not without foreshadowing. Tax Day 2016, Memorial Day 2015 and other storms had caused similar flooding across portions of the Greater Houston Area.
Hurricane Harvey was an unprecedented storm, resulting in $125 billion in damages and flooding 300,000 structures in southeast Texas during days of unrelenting downpours. Generations of Houstonians have recognized the need to move stormwater out of their neighborhoods. Houston's population, however, has multiplied more than 10 times over the last 80 years. Today, surface diversion canal and expanded bayou routes envisioned by the U.S. Army Corps of Engineers in the 1930s are no longer feasible. The Houston metropolitan area's continuing growth persistently challenges the region's ability to continue managing stormwater with surface infrastructure.
As a result of Hurricane Harvey, Harris County applied for and received a grant from the federal government for a study to validate the deep tunnel concept as a potential solution to regional flooding and to provide preliminary information for further development in future phases. The study is expected to begin in April 2019.
Harvey forced creative engineers, urban planners and community leaders back to the drawing board for innovative ways to reduce the impact of future storms. From the drawing board, the “flood tunnel” concept emerged with significant promise for reducing flood risk across Houston. Tunneling allows conveyance of high flow rates by gravity through dense urban areas with minimal impact to the community, businesses, traffic and the environment.
Where else are Texans tunneling?
As we consider a path forward in Houston, we can look to other Texas urban centers — San Antonio, Austin and Dallas — for direction on addressing urban stormwater challenges. All of these urban centers have chosen large-diameter, gravity flow, inverted-siphon stormwater tunnels to combat their most persistent stormwater challenges.
The San Antonio River Tunnel was completed in 1997 for $111 million and a year later protected the Riverwalk and downtown from significant flooding. The 24-foot-diameter tunnel, located 150 feet below downtown San Antonio, also protected the flourishing Riverwalk from severe rainfall events in 2002, 2013 and 2015.
The Waller Creek Tunnel in Austin, completed in 2017 for $161 million, removed 28 acres on the east side of downtown from the floodplain, encouraging reinvestment and growth. Dallas is currently building the Mill Creek Drainage Relief Project, a $206 million, 35-foot-diameter tunnel to provide much-needed drainage relief for east Dallas and the Baylor Medical Center area.
How could tunneling help?
Houston’s topography and geology are different from Texas’ other urban centers, and until recently its soft clay and sandy soils, high groundwater and flat terrain had deterred tunneling as a solution. But 30 years of advances in soft-ground tunneling technology below the groundwater table have yielded promising cost-effective solutions. The ability to manufacture larger-diameter machinery also counteracts Houston’s flat terrain by allowing a larger tunnel diameter with a larger flow cross section. In the 1980s, the largest machine capable of mining in geology like Houston’s was approximately 20 feet in diameter; today, the largest machines are almost 60 feet in diameter. This adaptability means that tunneling can provide a solution for multiple watersheds in the Houston region on both a macro and micro scale.
Let’s take a look at one of the large-scale ideas and examine the idea of connecting the Barker and Addicks Reservoirs to the Houston Ship Channel, a proposal also known as the “Super Tunnel.” This concept builds on the USACE’s two-generations-old concept for a North and South Canal using modern construction methodologies. Property owners upstream of the reservoirs and along Buffalo Bayou experienced significant flooding: during Harvey, as floodwaters were impounded in the reservoirs, and following Harvey, after floodwaters were released from the reservoirs.
Though Houston is flat, there is an elevation difference of almost 100 feet between the maximum water surface elevation (WSEL) in Barker Reservoir and the normal WSEL in the Houston Ship Channel. A large-diameter inverted-siphon stormwater tunnel connecting the reservoir to the Ship Channel could use the elevation differential to convey floodwaters into the Ship Channel by gravity. A tunnel 30 to 40 feet in diameter could move 10,000-15,000 cubic feet of water per second passively without pumps or other infrastructure. The “Super Tunnel” could have provided much needed operational flexibility during Harvey, improving reservoir reliability and reducing flood damages both upstream and downstream of the reservoirs.
What is tunneling’s bottom line?
Tunnels are inherently low-impact, moving and storing stormwater with little effect on the surface, which benefits communities and minimizes environmental impacts. Large diameter tunnels are excavated deep underground, and shaft sites can be strategically spaced miles apart to minimize impact.
Tunnels also can be routed along existing public right-of-way, minimizing property acquisition and avoiding home and business displacement. If a tunnel must cross private property, access is provided with a subterranean easement, which is less costly to acquire than a surface easement and is unlikely to interfere with future property development.
Based on large-diameter soft-ground tunnels built or under construction elsewhere in the United States, early cost estimates have centered on approximately $100 million per mile of 30-to-40-foot-diameter tunnel. That means a tunnel from the reservoirs to the Houston Ship Channel 20 miles or longer could cost more than $2 billion. It’s a big price tag, but one that must be weighed against the benefits, including flood damage reduction both upstream and downstream of the reservoirs. Also, we have to account for the cost of other project alternatives to accomplish the same goal. Widening and improving Buffalo Bayou to convey the same flow rate starts at an estimated $4 billion just to acquire property along the channel and would likely be a nonstarter with environmental and community groups.
By using tunneling as a construction method, the Greater Houston Area potentially can realize the much-needed stormwater conveyance plans conceived in the 1930s with minimal impacts to the community, business and the environment. This low-impact solution can move a significant amount of flood water passively by gravity. As part of an integrated flood management system, tunneling makes existing infrastructure more resilient and more adaptable to unpredictable future events. For the future of flood management in Houston, there may be light at the end of the tunnel.
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