Countywide Large Diameter Tunnels for Stormwater Conveyance

Countywide Large Diameter Tunnels for Stormwater Conveyance
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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.

PROJECT DESCRIPTION

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 system.

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.

Virtual Community Engagement Meeting

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

Date: Thursday, June 16, 2022
Time:
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.

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