HARRIS COUNTY FLOOD CONTROL DISTRICT COMPLETES PHASE-ONE STUDY OF LARGE DIAMETER TUNNELS FOR STORMWATER CONVEYANCE

The Harris County Flood Control District worked with a team of underground construction experts to take an initial look at the feasibility of constructing large-diameter, deep tunnels to convey stormwater. Underground tunnels could supplement existing bayou and creek capacity to mitigate flooding in Harris County. The team was led by Freese and Nichols with support from Parsons, Brierley Associates, Terracon, HVJ, Sowells Consulting Engineers, and Middleton Brown.

The Harris County Flood Control District worked with a team of underground construction experts to take an initial look at the feasibility of constructing large-diameter, deep tunnels to convey stormwater. Underground tunnels could supplement existing bayou and creek capacity to mitigate flooding in Harris County. The team was led by Freese and Nichols with support from Parsons, Brierley Associates, Terracon, HVJ, Sowells Consulting Engineers, and Middleton Brown. 

The study was the first phase of a multi-phased approach and focused on determining the applicability of tunneling by evaluating the hydraulic capacity of tunnels, developing schedules and cost projections, and by comparing geotechnical conditions in Harris County with other active and completed tunnel projects around the United States and the world.

Washington, D.C., currently has two large underground tunnels that were built in part for flood mitigation purposes.

“Our analysis of the geotechnical conditions indicate that the soil and groundwater conditions in Harris County are similar to Washington, D.C. - and could potentially be even better for tunneling,” said Brian Gettinger, P.E., Tunneling Services Leader at Freese and Nichols. “This is a key factor in the phase-one study as it validates that large-diameter, deep tunnels are constructable in Harris County.”

The deep tunnel concept has similarities to a 1996 Flood Control District feasibility study that considered a trenched box culvert system along the Interstate 10 right-of-way. Unlike the 1996 study, the large-diameter tunnel concept has the potential to convey much larger flow rates and to convey the flow all the way into the Houston Ship Channel. The benefits include avoiding impacts to surface structures and property and potentially offering flood damage reduction benefits to a larger region.

“Geotechnical conditions do not appear to present any remarkable nor non-negotiable concerns for the large diameter tunnels at this time,” said Scott Elmer, P.E., Engineering Division Manager at Harris County Flood Control District. “Geologic faults in Harris County may require special design and construction considerations if crossed by a tunnel, but it is not a fatal flaw.”

Preliminary estimates prepared during the phase-one study show that a 10-mile-long, 25- and 40-foot diameter tunnel would cost approximately $1 billion and $1.5 billion, respectively, to construct.

Harris County Flood Control District funded the phase-one study through a $320,000 grant from the U.S. Economic Development Administration and an $80,000 cost-share from the Flood Control District’s 2018 Bond Program. Considerations for the next phase of the study are currently in development.

ABOUT THE HARRIS COUNTY FLOOD CONTROL DISTRICT
The Harris County Flood Control District (HCFCD) provides flood damage reduction projects that work, with appropriate regard for community and natural values. With more than 1,500 bayous and creeks totaling approximately 2,500 miles in length, the Flood Control District accomplishes its mission by devising flood damage reduction plans, implementing the plans and maintaining the infrastructure.