Countywide Large Diameter Tunnels for Stormwater Conveyance

Countywide Large Diameter Tunnels for Stormwater Conveyance

Recent Actions

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.


Following Hurricane Harvey in 2017, county leaders challenged the Harris County Flood Control District to consider all innovative technology for reducing the risk of flooding. Voters in 2018 approved a Bond Program that includes Bond ID Z-08, with $20 million for Feasibility Study of Stormwater Conveyance Tunnels.

Phase 1: Go/No-Go

Phase 1 of this multi-phase investigation was completed in Fall 2019. It confirmed that stormwater tunnels can be feasible in the soil types and with the geotechnical challenges specific to Harris County. Phase 1 evaluated hydraulic capacity and impacts, considered scheduling and cost projections, and compared geotechnical conditions in Harris County with other active and completed tunnel projects around the United States and the world.

Phase 1 was not watershed-specific nor intended to focus on any particular alignment or tunnel location; it was focused on whether the tunnels concept deserved additional study. The Flood Control District funded Phase 1 through a $320,000 grant from the U.S. Economic Development Administration and an $80,000 cost-share from Flood Control District operational funds.

Phase 1 Final Report

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 2: Identifying Potential Tunnel System Alignment Concepts

In Spring 2022, the Flood Control District completed Phase 2 of this feasibility investigation. The purpose of Phase 2 was to identify unmet flood mitigation needs in Harris County’s watersheds and – if possible – develop distinct tunnel concept(s) that would meet those needs, while providing sufficient benefit to justify their construction. During Phase 2, it was determined that a tunnel system, rather than one or more individual tunnel alignments, should be the focus of further study. This additional study would be needed before a final decision is made on whether to move forward with adding tunnels to Harris County’s stormwater management network. The Appendix A addendum to the Phase 2 report recommends eight tunnel alignments in the following watersheds (in alphabetical order): Brays, Buffalo, Clear/Berry/Vince, Greens/Halls/Hunting, Halls/Hunting, Little Cypress/Cypress, Sims, and White Oak.

Phase 2 Final Report

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 3: Additional Investigation

The Flood Control District will determine the scope elements of Phase 3 of the feasibility study in the Fall of 2022 and could begin Phase 3 itself in the Spring of 2023. Ahead of this Phase 3 kickoff, the Flood Control District is requesting public input from community engagement meetings and additional community outreach to solicit the community’s feedback and identify key topics that the community would potentially like to see addressed in the Phase 3 investigation.

It is expected that Phase 3 would consider further engineering analyses (validation of assumptions, hydrologic and hydraulic modeling, geotechnical investigations, and evaluation of active vs passive systems). It will examine the countywide economic benefits of adding a tunnel system to our current stormwater management network, as well as the process needed to integrate the tunnel system within existing neighborhood drainage, bayous and stormwater detention basins. Additionally, Phase 3 includes further environmental analysis and exploring funding strategies to pay for a tunnel system.

Community engagement will continue to be a priority, so additional meetings are planned. Once begun, Phase 3 of the feasibility study is expected to take approximately three years.

Phase 3 will be funded by $20 million provided by Bond ID Z-08 in the 2018 Bond Program.

Frequently Asked Questions from the Community

Where can I get more or future information?

To get project updates as they become available, please sign up for notifications on the Tunnels project webpage at

What is the cost of a tunnel system? What is the average cost per mile, per foot?

The preliminary total project cost for a potential tunnel system is estimated at about $30 billion. This would include anticipated engineering, environmental permitting, right-of-way (ROW) acquisition and construction costs. The cost to design and build a single representative 10-mile tunnel section is estimated to be in the range of $3-$4 billion. This is equivalent to $300 million to $400 million per mile, or $57,000 to $76,000 per linear foot. We will continue to refine cost estimates as part of Phase 3.

Which costs less to achieve the same benefits – tunnels or traditional flood risk reduction methods?

Monetary costs are difficult to quantify at this early stage of the study process. However, work done to date suggests that the cost of tunnels can be comparable to traditional flood risk reduction projects that would achieve the same level of benefits. The answer to which method costs less depends on the specific project site, what traditional method(s) would be most effective at that location, and how much undeveloped land is available. Monetary costs are not the only costs we will examine, however. For example, there is also the cost in community disruption, especially in areas that are already heavily developed with homes and businesses.

How long will it take to design, permit and build? When will the construction start? Wouldn’t a traditional project be quicker?

A tunnel system like the one outlined in the presentation is a multi-decade project. At this time, a detailed schedule cannot be provided, since we need to complete our study process.

Individual traditional projects might be quicker than a tunnel but would not offer the same flood risk reduction benefits. Large-scale federally funded projects such as Project Brays and the Sims Bayou Federal Project typically include multiple individually bid and constructed projects over many years or decades.

The Flood Control District continues to deliver traditional flood mitigation projects while progressing through the remaining phases of the tunnel study. For updates to current projects in your area please visit

Funding: Where will the money come from? Will FEMA pay for this? Can recovery money be used to build the tunnels? How much will my taxes go up?

In our Phase 3 work, we will be exploring funding across local, state and federal resources. At this time, it is too early to estimate what the impact might be on individual property owners’ tax bills.

Can’t the tunnels be larger (100’ in diameter?), smaller, or can we have two tunnels side by side?

We currently estimate that tunnels within a future system would be at most between 30 and 45 feet in diameter. The maximum diameter limit is based on the current construction technology for soft soils. However, as technology advances, larger diameter tunnels may become a potential option in the future. Dual-diameter tunnels are also potentially being considered, especially where multiple alignments may converge. A dual diameter section may help bridge the gap where a much larger single diameter tunnel may not be a viable option.

Smaller tunnels are also possible. However, the potential tunnel boring machine would be limited to one size for the life of its use, so minimizing size changes along the alignment could potentially be beneficial in managing costs. This optimization of diameter sizes will be examined further during Phase 3 investigation.

Pumps: Can we use pumps to maximize capacity? Can all tunnels use pumps?

Phase 3 of the study will examine the use of active stormwater management and pumps to increase capacity, maintain the system and improve stormwater quality.

Galveston Bay: How will storm surge impact tunnel performance?

Storm surge is a rising of the sea as a result of atmospheric pressure changes and wind associated with a storm (for example, an incoming hurricane). Traditionally, this information is based on historical gage data along the receiving stream(s). In Phase 2, we evaluated this historical data to determine the preliminary tunnel outfall structure elevations. We also recommended the use of a potential gating system that works by closing the tunnel outlet or intakes when storm surge pushes up against the outward flow of stormwater from the tunnels. In Phase 3, tunnel performance under storm surge conditions will be evaluated.

There are other mega-projects occurring along the Gulf Coast, including the U.S. Army Corps of Engineer’s Coastal Barrier project. Are you coordinating with those teams to ensure there are no conflicts with a tunnels project?

The project team has already started communicating with different project teams across Harris County. This coordination (including for the Coastal Barrier project) will continue during the Phase 3 investigation and extend through the life of the project.

How will stormwater release impact water quality in Galveston Bay?

We know that stormwater quality impacts must be avoided, such as the potential for pushing low-quality water into the receiving body of water (including the Houston Ship Channel and Galveston Bay). We have and will continue to engage the Port of Houston and will be exploring the use of pumps and other stormwater quality mitigation in our Phase 3 study.

Why can’t the water be stored and treated for drinking water, or released broadly across the region in a way that could potentially combat drought conditions?

The mission of the Flood Control District is flood risk reduction, which is driven by the immediate need to move excess stormwater through flood infrastructure, including the proposed tunnels, to reduce the loss of life and structural damage. Water supply has not been identified as a benefit of flood risk reduction projects. Goals and methods for water supply-focused projects could conflict with our primary flood risk reduction goals for tunnels.

Why not make new reservoir(s) to hold stormwater?

The Flood Control District has and will continue to build stormwater detention basins to temporarily hold excess stormwater during rain events. These are not “reservoirs” built to hold water for longer periods, or as a community water supply. They are designed to temporarily store excess stormwater and allow stormwater to quickly drain back out into the bayous and creeks after a storm.

The U.S. Army Corps of Engineers has jurisdiction over the two federal reservoirs in Harris County, Addicks and Barker.

Won’t the silt pile up and block the tunnel? How will you manage silt?

Maintenance is a factor for all stormwater management systems. This includes Flood Control District channels which are subject to erosion repair and desilting, as needed. For tunnels, maintenance would likely include routine inspections, dewatering between storms, and periodic desilting and debris removal. This will be explored more during Phase 3. Historically, other tunnel systems have not had significant problems with silt build-up.

Isn’t Harris County very flat and not very far above sea level? How can gravity alone push the stormwater uphill to sea level from deep underground?

Studies show the change in elevation (244 feet from the highest point in the county to sea level) is enough to convey a substantial amount of stormwater. This means that the force of gravity has the ability to move stormwater into and along a tunnel system, and out into the Ship Channel and Galveston Bay – although not from every area of the county. As part of our next phase, we will weigh the benefits of optimizing the performance of the system by including pump conveyance.

What is the Phase 3 study timeline?

Phase 3 work is expected to begin in 2023 and is estimated to take a minimum of three (3) years to complete. Prior to Phase 3 work beginning, the Flood Control District would need to develop a detailed scope of work (i.e., what work efforts will be included for study during Phase 3) and hire an engineering firm with appropriate qualifications to conduct the work. Part of the process of developing the scope is taking into account the feedback from the public during the open commenting period.

Will there be a tunnel near me?

No decision has been made as to the location of any particular tunnel alignment. Preliminary study has identified areas where tunnels would be most feasible. Based on preliminary evaluation, the eight (8) recommended alignments were in watersheds with characteristics that included but were not limited to:

  • A significant number of identified damage centers, which are concentrated areas that have and will continue to flood repeatedly, with water in homes and businesses
  • Sufficient slope to allow for gravity flow
  • Areas where it is difficult to find sufficient undeveloped land for current project approach
  • Areas with greater population density and less open land
  • Location outside the high-risk zone for storm surge

One of the most difficult challenges the Flood Control District faces is constructing effective projects that are sensitive to community and natural values in a highly urbanized area. Location and population density are key considerations. This information can also pinpoint areas that are less likely to recover quickly or be resilient after major storm events.

Virtual Community Engagement Meeting

A virtual Community Engagement Meeting for this project was held on:

Date: Thursday, June 16, 2022
6:30 p.m. - 7:30 p.m.

You can submit a comment and see the meeting presentation and video by clicking the links below.

Presentation Virtual Meeting Video Submit a Comment

Project Stage

This project is currently in the FEASIBILITY STUDY stage. Phase 1 of the study was completed in 2019. Phase 2 of the study was completed in 2022.

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