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Buffalo Bayou and Tributaries Study (Again)

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I posted about this study back in June of 2019, when the United States Army Corps of Engineers (USACE) was asking for up front public input on the study before they got too far along. I helped the Houston Branch of the American Society of Civil Engineers (ASCE) provide some input.

Back then, many regional stakeholders chimed in, most supportive of additional federal investment to update the original federally authorized project that built the Addicks and Barker Dams. Many stakeholders encouraged USACE to be creative and to evaluate nature-based systems to reduce flood risks.

At the urging of the Harris County Flood Control District (the most likely local sponsor of any federally funded project to update the Buffalo Bayou federal project) last month the USACE released an interim version of the report for public input. An executive summary of the report is available here. For more detail, the whole report is here.



The main stakeholder reaction? Disappointment.

There are two main reasons for the public’s disappointment.

First, the report includes very traditional detention and conveyance alternatives along with buy-out options. Nothing in the report could be described as creative. There is not much in the way of nature-based solutions. The use of tunnel conveyance facilities were ruled out due to cost.

Second, most of the presented alternatives have very low benefit cost ratios (BCRs). This is very important to note because in order to attract federal support (congressional authorization) and funding (congressional appropriation), projects must have BCRs of much more than 1.0. This is embedded in laws and policies governing the White House Office of Management and Budget (OMB). Limited federal dollars most compete with scores of other projects across the country, each with calculated BCRs. Only the projects with the highest BCRs have a chance at federal funding.

So why did USACE publish a report with these two problems?

Here is my theory.

After Hurricane / Tropical Storm Harvey dropped its unprecedented rain amounts on the Buffalo Bayou system, many upstream and downstream homes and businesses were flooded. This triggered a public outcry and litigation over the federal government’s use of private property to store and convey water. It also triggered support for the resilience study and created expectations that the study would find a “silver-bullet” that would “protect” everyone from a Harvey type event in the future.

Well, the situation is not favorable to identifying projects with high benefit cost ratios (BCRs). The vast majority of homes and businesses in the Buffalo Bayou system have a very, very low likelihood of flooding during any particular time period, even with Atlas 14 rainfall used to map floodplains. This low existing risk is a direct result of the large federal investment to build the existing Addicks and Barker system.

Yes, there are homes in the inundation pools of Addicks and Barker. Yes, there are homes in the floodplain of Buffalo Bayou downstream. Yes, the Cypress Creek overflow does make drainage and land use challenging upstream. But even counting them in, it is still very challenging to identify any infrastructure investment in the Buffalo Bayou watershed that would generate a high enough monetary value of avoided damage — the benefit part (the numerator) in the BCR — to justify the required costs.

To explain this further, let’s dive into how the monetary value of avoided damage is estimated. The monetary value of avoided damage is determined by the difference between the value of avoided damages from a particular flood after a new project is built, call it post-project conditions (ADPost) minus the value of avoided damages from a particular flood before a new project is built, call it pre project conditions (ADPre) with the result multiplied by the likelihood of that particular extreme flood occurring during the study time period. If the avoided damage arising from a particular flood with a new project is not much different from the “no new project” alternative, the difference won’t be very large. This is the numerator in the BCR. If the project costs are high (Cost), the BCR denominator will be high, thus reducing the BCR. Shown as an equation, it looks like this:

Where: BCR is the benefit cost ratio; ADPost is the monetary value of avoided damages after any proposed new project; ADPre is the monetary value of avoided damages today, without any new investment in the existing Addicks and Barker system; P is the probability of the modeled extreme storm occurring during the study time period; and Cost is the cost to design, build, operate, and maintain the project during the study time period.

So let’s pretend we can devise a project that generates $8 billion in avoided damages (ADPost) from a particular extreme, but rare, storm event. Because of the prior investment in the Addicks and Barker system, the ADPre is also a pretty high number, easily $6 billion. This means the difference is about $2 billion in this hypothetical.

To account for the probability of the rare extreme event occurring during the study time period, we need to multiply the damage estimate by the likelihood of that extreme storm actually occurring during the study time period. Let’s assume that over a 100 year period the rare and extreme event has a 10% chance of occurring. That means we would have to multiply the difference in damage estimates, $2 billion, by 0.10 to compute the benefit portion of the BCR fraction. Ten percent of $2 billion is $200 million.

So to obtain a BCR of 1 or more, if the benefit portion of the fraction (the numerator) is $200 million, we need the total cost to design, permit, build, and operate the project for 100 years (the denominator) to be $200 million or less. If you’ve read the report’s executive summary, or know anything about how much infrastructure costs, you know that is pretty much impossible. (The dam safety projects – spillway fixes – should obviously be done as soon as possible. Those smaller projects cost less and yield high benefits.)

Folks who flooded and who live in the Buffalo Bayou watershed, who really want an additional federal project investment to further reduce flood risks in their area may be upset with this post. But they should know that the 1940 “Definite Plan” was a very large federal investment that reduced their flood risks when it was built. The prior federal risk reduction investment makes it harder to justify additional risk reduction investments.

Unrelated to the watershed-specific study discussed in this post, additional local, state, and federal investments should go to areas of Harris County with many more homes, structures, and businesses exposed to higher inundation risks than those in the Buffalo Bayou watershed. Think Greens Bayou, Halls Bayou, and others.

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Buffalo Bayou and Tributaries Resiliency Study

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In the aftermath of Hurricane Harvey, the federal government appropriated $6 million and authorized the U. S. Army Corps of Engineers (USACE) to conduct the Buffalo Bayou and Tributaries Resiliency Study.

According to the USACE, the study will: Identify and evaluate the feasibility of reducing flood risks on the Buffalo Bayou, both upstream and downstream of Addicks and Barker Reservoirs in Harris County, Texas, while simultaneously completing a Dam Safety Modification Evaluation (DSME) on the two dams. Three primary problems will be addressed: (1) Flooding downstream of the reservoirs on Buffalo Bayou; (2) Performance and risk issues related to flow around and over the uncontrolled spillways; and (3) Flooding upstream of the reservoirs.

Map of the study area. The Cypress Creek watershed is included only to evaluate the overflow from that watershed into Addicks. Brays Bayou will not be considered during the development of risk reduction options but it will be considered when determining potential adverse impacts.

The Corps requested public input on the scope of the study and comments were due on May 31, 2019.

I helped coordinate the development of comments on behalf of the Houston Chapter of the Environment & Water Resources Institute of the American Society of Civil Engineers. The text of the submitted comments is provided below:

The Houston Branch of the Texas Section of the American Society of Civil Engineers appreciates the opportunity to comment on the above referenced resiliency study.  Our comments are provided below.

  1. Sustainable Infrastructure: Alternatives should be evaluated using the Institute for Sustainable Infrastructure’s ENVISION rating system.  Alternatives with the highest score in the rating system should be considered further for implementation.  See sustainableinfrastructure.org for additional information about the rating system.
  2. Non-Stationary Climate: Alternatives should be developed to handle rainfall amounts that have a 1% annual chance (or greater) occurring in the year 2100.  Rainfall depths appear to be trending upwards and the 1% annual chance event will likely be larger at that time.
  3. Nature-Based Alternatives: Alternatives should be developed and evaluated that include nature-based approaches, such as land acquisition and preservation, wetland creation, natural stable channel design approaches, and similar concepts.
  4. Two-Dimensional Modeling of Non-Riverine Areas: Alternatives should be evaluated using 2-D modeling approaches, especially in areas not adjacent or near bayous or channels.
  5. Triple-Bottom-Line Net Cost/Benefit Estimations:  Alternatives should be evaluated using a more comprehensive assessment of net benefits and costs. Net costs should be estimated for traditional engineering economics inputs, such as construction costs, operations costs, maintenance costs, land acquisition costs, and labor cost.  But environmental costs should be estimated as well. These should include the value of any diminished ecosystem services, lost habitat, lost carbon sequestration, lost oxygen production, lost heat island mitigation, lost recreational opportunities, and similar well studied metrics.  Social costs should also be estimated for each alternative. These should include displaced cultural or historical features, lost recreational opportunities, lost or diminished employment opportunities, diminished views and character, light pollution impacts, diminished social equity, and similar aspects. Net economic, social, and environmental benefits should also be estimated for each alternative.  These would include the value of avoided property damage (times the likelihood of loss), the number of people benefiting from a reduced risk of inundation, the value of any increase in social values or benefits (recreation, views, safety, equity), the value of any increase in environmental values or benefits (habitat, ecosystem services, etc.).  The net present value of all economic, social, and environmental BENEFITS minus the net present value of all economic, social, and environmental COSTS should be calculated for all alternatives and the alternative with the highest net present value of total triple bottom line NET BENEFITS should be recommended for implementation.

Again, we appreciate the opportunity to comment on the scope of the study.  If there are any questions about our comments, please don’t hesitate to contact us.

Very truly yours,

AMERICAN SOCIETY OF CIVIL ENGINEERS – HOUSTON BRANCH

Leave a comment about what you or your organization thought the study should consider.