Background

In September 2019, Freese & Nichols completed the Harris County Flood Control District‘s (HCFCD’s) Phase I report on Large Diameter Tunnels for Stormwater Conveyance. Mr. Brian Gettinger, a Texas Professional Engineer, sealed the report on September 23, 2019.

The Phase I report was funded by a $320,000 grant by the U.S. Economic Development Administration and $80,000 in HCFCD funding. The $400,000 study concluded that large-diameter tunnels were a feasible option to reduce flood risks. The Phase I report and all appendices (1,700 pages) were published on the HCFCD website. The Phase I study allowed HCFCD to make a “go” or “no go” decision about conducting additional studies. Additional studies were authorized.

On June 16, 2022, Mr. Scott Elmer, P.E., Assistant Director of Operations at the Harris County Flood Control District (HCFCD) provided a virtual briefing on the results of their Phase II Feasibility Study of Stormwater Conveyance Tunnels. The report was prepared by the engineering firm Black & Veatch under contract to HCFCD. Mr. Chris Mueller, a Texas Professional Engineer, sealed the report on March 30, 2022. The Phase II report and all appendices (1,863 pages) were published on the HCFCD website.

This post provides a high-level summary of the key elements of the Phase II report after my initial review.

Identify Flood Damage Centers

The report identified flood damage centers (FDCs) using both the number of historical flooded structures and using the projected (modeled) number of flooded structures by comparing the structure floor elevation against the modeled water surface elevation during a particular design flood. We would like to use tunnels to take flood waters away from FDCs, especially when the existing channels and bayous are not able to do so effectively.

FDCs were identified both along bayous in the existing floodplain areas mapped by the National Flood Insurance Program and outside of floodplains, based on a computer model of how intense rainfall can sometimes fall on neighborhoods and streets and how it collects and ponds. This type of ponding often floods homes and structures in our community.

The engineering team identified 107 FDCs inside the existing floodplains and 34 FDCs outside the mapped floodplains, as shown below:

Here’s a map of the FDC’s identified by the report authors:

There are a few different ways to address the higher risk of flooding in the mapped Flood Damage Centers (FDCs). We could buy-out the properties and convert the area to green space; we could enlarge and deepen the channels and bayous that drain the area; we could excavate new detention basins; we could install new tunnel systems; or we could do a combination of these.

Watershed Screening

The engineering team screened all 23 watersheds in Harris County to determine if the conditions in each watershed were favorable or unfavorable for the use of a tunnel to reduce flood risks. Screening factors included the historical number of structures flooded, the projected number of instances of flooding over 100 years of operation, flood insurance claims, the social vulnerability index within the area, the distance from flood damage centers (FDCs) to any potential tunnel outlet (discharge point), number and concentration of FDCs identified by the engineering team, ground elevation differences (since height about sea level is required to use gravity to convey water), and costs relative to non-tunnel options like channel widening, detention, and buyouts. Watershed screening resulted in ten favorable watersheds, as follows:

The engineering team developed eight preliminary tunnel alternatives that reduced flooding in the identified Flood Damage Centers. Here’s a map showing all eight tunnel alignments:

Weighted Decision Matrix

To see which of the eight alternatives provide the best results, the team then conducted a detailed weighted decision matrix evaluation of the alternatives.

A weighted decision matrix allows various factors to influence a decision to different degrees. Say you’d like to attend a movie but you have three alternatives before you. Here’s a weighted decision matrix you might consider:

Factor	Weight		Theater 1	Theater 2	Theater 3
Restaurant Food Available 0 = no, or 1 = yes	3		0	0	1
Seat Comfort 1 = low, 2 = medium, or 3 = high	2		3	2	2
Distance from Home 1 = far, 2 = medium, or 3 = close	1		2	1	3
Total Weighted Score	NA		8	5	10

The weights in this example indicate that this person thinks that good food is three times more important than distance and 30% more important than comfort. This example shows they would choose to see the movie at Theater No. 3.

The engineering team used a similar approach to evaluate the eight tunnel alternatives. The tunnel evaluation factor weights were as follows:

Notice that flood risk reduction is much more important than any of the other factors. It is six times more important than the degree to which the tunnel helps socially vulnerable people. It is twelve times more important than how easy the tunnel is to construct or the likelihood of encountering adverse soil or fault conditions (geotechnical issues). The assignment of these weights is a policy decision, not an engineering decision.

The results of the evaluation of the eight tunnel alternatives are presented below:

Recommended Tunnels

Section 12 of the Black & Veatch report provides an overview of the four recommended tunnel alternatives. At first glance, one would expect to see the engineers recommend the top four scoring tunnel alternatives: Halls, Greens, Sims, and Cypress; however, the report does not recommend proceeding with these four. Black & Veatch promoted two of the lower-scoring alternatives.

Black & Veatch promoted the Brays Tunnel to a “recommended tunnel concept” because the number of instances of avoided flooding was reduced by assuming that the federal project to continue widening the bayou channel would be constructed in the future. If the federal widening project is not constructed, the construction of a tunnel would increase the number of avoided instances of flooding from 8,700 to 41,252.

Black & Veatch promoted the White Oak Tunnel to a “recommended tunnel concept” for reasons that I don’t fully understand. The report states:

The [White Oak Tunnel] rating for flood risk reduction is 1 out of 10, which reflects a relatively low number of instances of structures removed from flooding. However, the cost per instance of structure flooding is considered moderate when compared to all eight (8) tunnel alternatives. Considering this factor and that the alternative received rankings ranging between 4 and 5 for the four evaluation metrics, the White Oak Bayou Tunnel was selected as a “recommended tunnel concept” for further study and refinement.

After the promotion of Brays and White Oak, the report presents four “recommended tunnel concepts” as follows:

— Brays Bayou
— Greens, Halls, and Hunting Bayou
— Halls and Hunting Bayou
— White Oak Bayou

Here’s a summary of these four tunnel alternatives (highlighted in green) along with the other tunnels evaluated:

The total cost of all eight projects is $21.08 billion. The total cost of the recommended projects is $10.08 billion. The public presentation indicated a total cost of $30 billion. I don’t know why the total reported during the presentation was different. It might be an inflation adjustment or added costs to cover contingencies or both. It certainly would not be unexpected to have estimated costs increase as the design progresses from conceptual designs, to preliminary designs, and on to more refined design stages.

People who saw my earlier post explaining why I don’t support the Buffalo Bayou Community Plan should not be surprised by the low ranking for the Buffalo Bayou Tunnel Alternative.

Next Steps

Harris County Flood Control is planning to conduct a Phase III tunnel study in 2023. That study will include community engagement, additional planning, additional design work, additional cost estimates, estimates of benefits, and an examination of funding sources.

Harris County Flood Control is accepting comments on the report and input on the scope of their pending Phase III report which will be starting in 2023. Comments are being accepted through September 30, 2022, and may be submitted on the project website.