Z-08 Preliminary Engineering Study for Large Diameter Tunnels for Stormwater Conveyance

Z-08 Preliminary Engineering Study for Large Diameter Tunnels for Stormwater Conveyance

Last Modified: 09/10/2020 03:11 PM

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April 28, 2020 - Harris County Commissioners Court approved a $2.5 million agreement with an engineering firm to conduct Phase 2 of the feasibility study.

February 11, 2020 - Harris County Commissioners Court authorized negotiations with an engineering firm for engineering and related consulting services for Phase 2 of a feasibility study of constructing stormwater conveyance tunnels.

December 3, 2019 – Harris County Commissioners Court approved a request for qualifications from engineering consultants in connection with the second phase of a feasibility study on the use of large diameter deep tunnels to move stormwater.

September 30, 2019 – Final Report submitted to Department of Commerce, Economic Development Administration for review. 

September 23, 2019 – Final Report delivered to HCFCD. 

September 17, 2019 – Initial Project Criteria report completed. 

August 22, 2019 – Preliminary Inverted Siphon Analysis completed. 

August 16, 2019 - Preliminary Tunnel Cost Analysis completed. 

August 16, 2019 – Preliminary Geotechnical Desktop Study completed. 

August 15, 2019 - Preliminary Opinion of Tunneling Applicability completed. 

June 25, 2019 – Houston Faulting Workshop held with local fault experts. 

June 10, 2019 – Project Kickoff meeting with project consultant held.

PROJECT DESCRIPTION 

This project is now in Phase 2 of an investigation to determine the feasibility of constructing large-diameter deep tunnels to help our bayous move excess stormwater out of Harris County. The purpose of the Phase 2 study, which began in the spring of 2020 and will take about one year to complete, is to identify unmet flood mitigation needs in Harris County’s watersheds and – if possible -- develop distinct tunnel project concept(s) to meet those needs that provide sufficient benefit to justify their construction.

Phase 2 will:

  • Consider how to measure tunnel success,
  • Recommend tunnel alignments that reduce flooding risks along major bayous and creeks while not causing negative impacts at the discharge locations,
  • Estimate cost – including the cost of community disruption -- as compared to potential flood risk reduction benefits,
  • Determine an estimated timeline for building tunnel projects,
  • Identify environmental impacts and permit requirements, and
  • Compare tunnel costs, benefits and implementation timelines to those of other flood risk reduction projects.

Following this work, Phase 2 will recommend and continue to refine no more than three potential “actionable” tunnel projects in distinct locations in Harris County. An “actionable” tunnel project is defined as one that – based on the opinion of the Flood Control District after input from the public – provides adequate flood damage reduction benefits to justify its construction and operation, as well as its implementation timeline.

This stage of the analysis will cost $2.5 million and is funded by a Community Development Block Grant for disaster recovery.

Phase 3 of Preliminary Engineering will follow if there are actionable recommendations as a result of Phase 2.

Phase 1 Preliminary Engineering

This short-term study was intended to take a high-level look into the feasibility of constructing large-diameter deep tunnels to help move stormwater out of Harris County. The study was the first phase of a multi-phased approach and was focused on determining the applicability of tunneling considering the soil types and geotechnical challenges specific to Harris County, evaluating hydraulic capacity and impacts, scheduling and cost projections, and comparing geotechnical conditions in Harris County with other active and completed tunnel projects around the United States and the world. This Phase 1 study was not watershed specific nor scoped to focus on any particular alignment/location.

Phase 1 findings include but are not limited to:

  • Geotechnical conditions do not appear to present any remarkable nor non-negotiable concerns.
  • Geologic faults may require special design and construction considerations if crossed by the tunnel; not considered fatal flaws.
  • Tunnels can move a significant rate of stormwater operating by gravity as an inverted siphon. Tunnel cost, including a 50 percent contingency, for a representative 10-mile long, 25- and 40-foot diameter tunnel is approximately $1 billion and $1.5 billion respectively.

Harris County Flood Control District funded this study through a $320,000 grant from the U.S. Economic Development Administration and an $80,000 cost-share from the HCFCD 2018 Bond Program. Harris County 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.

Phase 1 Final Report

Project stage

This project is currently in the FEASIBILITY STUDY stage. Phase 1 of the study was completed in 2019.

Project Life cycle

Bond Project Listing

Total allocation for this multi-phase project is currently $20 million on the Bond Project List.

Project Life cycle