Harris County Flood Control District has received an award letter from the U.S. Economic Development Administration (EDA) in connection with a $400,000 feasibility investigation of large-diameter, deep underground stormwater conveyance tunnels.
Once Harris County Commissioners Court has accepted the $320,000 grant – to be matched by $80,000 in local funds provided through the 2018 Bond Program – the four-month investigation is expected to get underway in 2019. It will assess the potential for stormwater tunnels to help drain or increase the capacity of existing bayous and drainage channels in Harris County.
The feasibility investigation will include a review of soil and groundwater conditions, as well as other geotechnical, geological and environmental assessments. Developing better cost information for tunnel construction in our area, based on these preliminary findings, will be part of this study.
A feasibility study is an early step in the lifecycle of a flood damage reduction project. In the case of the Stormwater Tunnel Feasibility Study, the study would examine whether this technology is even possible in our area, which is characterized by soft soils and a high groundwater table. We will be looking at our soil and water table conditions and compare those to the technology that exists in the tunnel construction industry to ensure there is a fit.
If the use of tunnels is found to be feasible, future efforts could examine potential tunnel routes, complete hydraulic analyses to determine the required tunnel diameters, and determine proposed inlet and outlet shaft locations.
In this concept, tunnels would collect stormwater at upstream locations along the bayous, convey the stormwater downstream via gravity in deep underground tunnels measuring 20 feet or more in diameter, and discharge the stormwater into the Houston Ship Channel at a safe location. The goal would be to significantly reduce flood risks in Harris County watersheds where tunnels could be constructed.
The grant comes via an EDA grant program in connection with the 2017 Hurricane Harvey presidential Disaster Declaration. The grant could be submitted to Commissioners Court for authorization to accept as early as the February 26 meeting.
While costly, tunnels have been employed to carry stormwater in other locations in Texas and nationwide as an alternative to traditional stormwater conveyance improvement projects. Tunnels can utilize existing public rights-of-way – such as roadway corridors – and with relatively little disruption to developed urban areas, which helps to reduce cost. Cost per mile may vary widely – between roughly $40 million and $163 million per mile for projects in San Antonio, Dallas and Austin – depending on soil conditions and other factors. Tunnels typically are constructed using tunnel boring machines, at a depth of from 100 to 150 feet.
The 2018 Bond Program includes a total of $20 million for preliminary engineering in connection with the tunnel concept.
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