Countywide Large Diameter Tunnels for Stormwater Conveyance

December 14, 2021 - Harris County Commissioners Court approved a $1,699,938 amendment to the contract with an engineering firm for additional engineering and related consulting services in support of Phase 2 of this project.

July 20, 2021 - Harris County Commissioners Court authorized an additional $4,086,702 in engineering and related consulting services for a study of the feasility of constructing stormwater conveyance tunnels (Phase 2).

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.

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 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.

Final Report

• Phase One Final Report (298 MB)
• Conceptual Tunnel Cost Analysis (4.2 MB)
• Design Criteria Memo (1.6 MB)
• Inverted Siphon Hydraulic Analysis (1.6 MB)
• Preliminary Geotechnical Desktop Study (69 MB)
• Preliminary Opinion of Tunneling Applicability (44 MB)

Phase 2 of the study focused on:

• Identifying the watersheds that met the criteria for a tunnel
• Identifying flood damage centers that presented the highest risk and determining whether the tunnels would be more cost-effective over traditional flood control measures (e.g. stormwater detention, channelization, or buyouts)
• Identifying potential strategic locations for intakes and outfalls
• Identifying potential opportunities to integrate tunnels with existing and proposed flood damage reduction systems
• Avoiding geologic and man-made hazards

Phase 2 took a step-by-step approach that started with the assumption that every watershed in the county could potentially benefit from a tunnel alignment and continued to identify, examine and rank distinct areas of flooding risk. Phase 2 considered the number of existing structures at risk in a 1% (100-year) storm using the National Oceanic and Atmospheric Administration’s Atlas 14 rainfall data and also looked at the cumulative instances of flooded structures over time in an area by calculating instances of flooding up to an 1% (100-year) storm event and over the approximate 100-year service life of a tunnel. This made sure that areas where flooding is the worst were identified. Phase 2 also included consideration of the social vulnerability index, which identifies areas where residents are less able to recover from a flood event.

Based on these criteria and others, Phase 2 identified a countywide tunnel system featuring conceptual tunnel alignments that could potentially serve areas with the deepest and most frequent flooding; that could potentially improve community resiliency, including in socially vulnerable and low-to-moderate income areas; and that could potentially mitigate large numbers of estimated instances of flooding.

Phase 2 of the analysis is funded by a $2.5 million from the U.S. Department of Housing and Urban Development’s Community Development Block Grant Disaster Recovery (CDBG-DR) program.

Final Report

• Phase Two Final Report (232 MB)