Tag Archive for: flood mitigation benefit index

Response to Concerns About Flood Mitigation Benefits Index (Part II)

The letter below expresses disagreement with two recent ReduceFlooding.com posts about a proposed Flood Mitigation Benefits Index. It is from Michael Bloom, P.E. While I disagree with almost all of his claims, I am reprinting his letter verbatim because I encourage healthy debate. Compare the posts and draw your own conclusions. – Bob Rehak, Host, ReduceFlooding.com.

This is Part II of my two-part article providing responses to concerns raised by my colleague on the Community Flood Resilience Task Force (CFRTF)Mr. Bob Rehak about the FMBI. If you missed Part I, you can read it here.

Why are we Using the Index When it Produces Inconsistent Results that are Not Intuitive? Mr. Rehak provides an example that holds the current population and current risk the same, but changes the total prior investment amounts, as illustrated in the table below:

(% Annual Chance)
Area A100,0005,000102
Area B1,000,0005,0001020

Mr. Rehak looks at these results and writes: “So, spending more money to get the same results increases benefits? Shouldn’t it be the opposite? That’s both depressing and confusing. You spend 10X the money; flood risk remains the same; and the “benefit” increases!!!??? You would think spending less money to achieve identical results would be more beneficial. It certainly is for taxpayers.”

Everyone should be depressed and confused by this result if the FMBI was illustrating the results for the same location. Mr. Rehak appears to make that inference when he writes: “spending more money to get the same results increases benefits.”

But Area A and Area B are two different locations. The FMBI is just telling us what the current conditions are at two different locations in the county. One location had 10 times the prior investment than the other – but both locations still have the same current risk.

Worse, in this case, BOTH locations have risks that are ten times the current standard of care for new developments – which require structures to have less than a 1% annual chance of inundation. Clearly, both locations need more flood risk investment. The FMBIs of 2 and 20 both are extremely low, meaning they need help, regardless of the prior investments. A high FMBI indicates that no additional help is needed in that location. A low FMBI indicates that additional help is needed in that location.

The table included in the middle of my February 17, 2022, post entitled “How Should We Decide Where to Invest in Flood Risk Reduction?” presents additional examples showing how the FMBI changes from location to location with only one changed variable. It also provides narrative explanations of each sequence. Notice how the index values are greater than 3,000 (sometimes greater than 20,000 or 100,000) in locations where the current annual chance of inundation is less than 1%? Again, a high FMBI means we don’t need to make more investments in that location. A low FMBI means that location needs more help.

Isn’t the FMBI Trying to Prove Inequitable Investments in Flood Risk Reduction? To some extent, partially, yes, it is. This was always an important aspect of the FMBI, when it was originally proposed as the “Flood Benefits Index (FBI)” by Dr. Erthea Nance and Iris Gonzalez in May 2021. I have continued to advocate for its use as one of four input variables we should use to create our county-wide “heat map.” This is explained in more detail in my other article. Mr. Rehak is concerned about the taxpayer. I am also. I don’t think the taxpayers of Harris County should pay for flood risk reduction projects in areas that already have a high FMBI. Said another way, it is a waste of taxpayer money to invest in additional flood risk reduction projects in areas currently with less than a 1% annual chance of inundation.

Isn’t the FMBI Measuring per capita Investment Associated with a Certain Level of Flood Risk and Mistakenly Calling that a “Benefit?” Mr. Rehak writes: “The more people you help with any given sum, the more the benefit goes down. Voila! That makes it look as though the highly populated watersheds (that have received the overwhelming majority of prior investments) have received little benefit. And that may be the point of this formula. It will send even more money to those same areas.”

This interpretation again seems to stem, I think, from Mr. Rehak’s belief that the index will be used to compare the same location at different times – before and after various investments. This is not the proposed use of the index. The proposal is to use the index to describe the current conditions at all locations in the county at the same time.

I’m not sure I understand Mr. Rehak’s concern about the index being a per capita value. The more people in an area who benefit from prior investments the better. Wouldn’t we want to invest in areas that help the most people?

The blue-shaded area of the table in my earlier post illustrates how population differences between locations will change the index value among those locations. For convenience I’ve repeated the table below:

Hypothetical examples.

Mr. Rehak accurately notes that the index goes up in locations with fewer people and down in locations with more people; this will incentivize planners to direct future investments in those higher population areas. He writes: “The more people you help with any given sum, the more the benefit goes down.” This is true, but Mr. Rehak’s statement doesn’t connect it to the past and it omits how the index will be normalized by area size. Index values will be calculated for similarly sized areas. This will allow an apples-to-apples comparison of per capita investments. The index is intended to incentivize future investments in areas with more people in cases where risk and prior investments are equal because we want to help as many people as possible.

By Michael Bloom, P.E.

Posted by Bob Rehak on 7/10/2022

1776 Days since Hurricane Harvey

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Response to Concerns About Flood Mitigation Benefits Index (Part I)

The letter below expresses disagreement with two recent ReduceFlooding.com posts about a proposed Flood Mitigation Benefits Index. It is from Michael Bloom, P.E. While I don’t agree with all of his claims, I am reprinting his letter verbatim because I encourage healthy debate. Compare the posts and draw your own conclusions. – Bob Rehak, Host, ReduceFlooding.com.

One of my colleagues on the Harris County Community Flood Resilience Task Force (CFRTF) – Mr. Bob Rehak, as well as Commissioner Tom Ramsey, P.E., and others have asked me about the proposed Flood Mitigation Benefits Index (FMBI). After hearing some of the questions, and reading two recent blog posts by Mr. Rehak at Reduce Flooding, one on July 2, 2022 – Questionable Validity of Flood-Mitigation Equity Formula and on July 6, 2022 – Formula for Allocating Future Flood-Mitigation Funding Deceives, I figured I should provide a more detailed explanation of the index and directly address some of Mr. Rehak’s concerns.

In addition to getting to know Mr. Rehak while attending CFRTF meetings, Mr. Rehak and I have sat down, in person, a few times since both being appointed to the Task Force in order to discuss difficult issues, in particular the FMBI. I appreciate his candor and our ability to respectfully debate things – one might say – politely argue. This post (and Part II) are extensions of those discussions so others can benefit from the exchange.

I first described the FMBI in an article I published on February 17, 2022, entitled “How Should We Decide Where to Invest in Flood Risk Reduction?” The FMBI is explained about halfway down the post. To recap, the index is equal to:

Variation on formula presented to Commissioners Court on 6/28/22. It omits the word “Density” after population. See discussion below.

The index is intended to be calculated for all locations in the county at one particular time to help define the baseline conditions. The index will be used to help plan where additional flood risk reduction investments should be made. An area with a high FMBI has already received higher per capita investments, has a low risk, and therefore doesn’t need additional help. An area with a low FMBI has received little per capita prior investments, has a high risk, and therefore does need additional help.

Responses to Specific Concerns

Which Type of Project Costs Are Included? Does including construction costs, but excluding design, right-of-way acquisition, and operational costs skew the data? Since this is an index that will be calculated for all areas of our county, costs included or excluded will not adversely impact the results.  Using the index to compare conditions in various areas within our 1,700 square mile county will still be valid if the index is calculated in all areas of the county the same way. This is an example of “normalizing” the data. It allows for an apples-to-apples comparison among and between locations. It will help us pick where to invest in the future. Since land acquisition, design, and other non-construction costs are often a similar percentage of the construction costs, their exclusion from all index calculations will keep things consistent and unskewed.

Which Agency Investments are Included? Will excluding investments from Harris County Commissioner Precincts, cities, municipal utility districts, and other entities skew the data. I actually agree with this, the investment dollars will be slightly low, but only by a little bit. I anticipate that the total amount of flood risk reduction investment dollars made by these entities will be very, very, very small compared to those made by the Harris County Flood Control District (HCFCD) and the Civil Works program of the U.S. Army Corps of Engineers (USACE). Because of this difference in the size of these investments, I anticipate that the impact on the index calculation will be negligible. HCFCD has agreed to provide their investments from 2000 to 2020. Dr. Denae King and I have submitted a Freedom of Information Act (FOIA) request to the U.S. Army Corps of Engineers, the Federal Emergency Management Agency (FEMA), and the Natural Resource Conservation Service (NRCS) to identify all flood risk reduction investments going back to 1937 – the year the HCFCD was created to serve as the “local partner” to help secure federal investments through the USACE. These requests exclude repair and recovery dollars since those expenditures don’t permanently reduce flood risks.

What Risk is Included in the Index? Does the risk used in the calculation reflect the risk before or after mitigation efforts? The risk value used is the current risk. It is the risk remaining after accounting for all risk reduction investments “counted” in the numerator. The index reflects one point in time and should be recalculated every five years or ten years, much like the Social Vulnerability Index published by the Centers for Disease Control. The population and risk values will be based on the same snapshot in time. The investment value will be based on the sum of all investments made prior to that moment in time (adjusted for inflation).

Why Include Investments Back to 1937? Why consider investments made in areas of the county that were undeveloped back then? Won’t this radically skew the comparisons? Including all investments back to 1937 is vitally important because the vast majority of the flood risk reduction investments made in the county were made by the federal government through the Civil Works program of the USACE. HCFCD was CREATED in 1937 to be the local sponsor for USACE projects. Addicks, Barker, Buffalo Bayou, Brays, White Oak, Sims, Clear Creek, and many other projects, many of them initiated prior to 2000, all significantly reduced flood risks for structures that exist today. Even if the project was initially constructed in an undeveloped area, it still benefits structures that were built later and that exist today. That’s why the investment amount is a cumulative value (inflation-adjusted) and the risk value is today’s value. This approach won’t radically skew comparisons because all three of the values will be determined for all parts of the county in the same way.

Why only Consider Mitigation Investments? Doesn’t flood risk depend on many factors – not just mitigation investments? Yes, current flood risk depends on many factors, including development rules, building codes, finished floor elevations, development locations, and improvements to our understanding of rainfall statistics. The risk value in the index is not intended to measure the risk reduction obtained from prior investments. The risk value in the index is intended to present the current risk. The current risk reflects all factors, including prior mitigation investments, development, rainfall, and everything else. The risk value is not a measure of the change in risk, it is a statement of the current risk, no matter the cause or the contributing factors.

Why Use US Census Tracts? Don’t they change over time? US Census Tracts do periodically change, however, that will not diminish the value of the index. US Census Tracts are areas that can more closely match the scale of typical flood risk reduction projects; watersheds are too large to be informative; and smaller areas would be too complex for our planning work.

Harris County outlined in red, census tracts in blue. Map supplied by Michael Bloom.

The originally proposed FMBI used the population density in the denominator. This, admittedly, would cause issues when comparing index values between large US Census Tracts and small US Census Tracts. To address this issue, the CFRTF and the Infrastructure Resilience Team (IRT) have agreed to proceed with the calculation using just population. This will make the index a per capita value. Prorating investment amounts and risk to each Census Tract can be reasonably accomplished using area ratios or other methods. This will be useful as the CFRTF and IRT work together to prepare the 2050 Flood Resilience Plan.

How Can We Use Information From 1937 When the County is So Different Now? How can this approach work without considering the construction of Lake Houston in 1954, the interstate system, Beltway 8, and the conversion of farmland and prairies into entire communities? The risk value captures all of this. The risk value used in the index reflects the current risk of any part of the county. It will be based on state-of-the-art modeling being conducted as part of the MAAPNext project. The current risk is the current risk, regardless of past changes in the watershed.

Why are we Using the FMBI Formula to Reduce Flood Damage when it Doesn’t Measure Flood Damage? The FMBI is not a tool to directly reduce flood damage and it’s not designed to measure flood damage. The FMBI is a tool to better understand past investment patterns and current risk. The FMBI is proposed to be one of four datasets used to create a baseline conditions heat map. The other three under consideration include current inundation risk, social vulnerability index, and community resources. The baseline conditions heat map will then be used to figure out WHERE flood risk reduction and flood damage reduction projects should be located. 

How Can the FMBI Compare Benefits  without Using Before and After Comparisons? The index is not intended to compare the flood mitigation benefits of the same location at different times. The index is intended to show how different locations across the county at the same time vary when compared to each other. This will help us identify WHERE we have neighborhoods that desperately need help and WHERE we have neighborhoods that don’t.

By Michael Bloom, P.E.

Posted by Bob Rehak on 7/9/22

1775 Days since Hurricane Harvey

Part II will be posted tomorrow. If you have thoughts you would like to share on this subject, please send them via the contact form on this website.

Questionable Validity of Flood-Mitigation Equity Formula

The results of an apparently invalid flood-mitigation equity formula could be used to steer billions of dollars in future flood-mitigation funding. Multiple data quality and collection issues may compound errors and the formula itself sometimes renders inconsistent, counter-intuitive results.

In the 6/28/22 Harris County Commissioners Court meeting, the Community Flood Resilience Task Force presented its first annual report. The report contains a lengthy discussion of a flood-mitigation equity formula developed by several Task Force members to “objectively” compare the “equity” of flood-mitigation investments (project costs). See the appendix starting on Page XVI and ending on Page XX.

The attempt to create objective comparisons between investments in different areas is well intentioned. However, I fear the proposed formula will create the appearance of objectivity while skewing data and producing misleading results. Here’s why.

The Formula

The formula is…

Flood Mitigation Benefits Index = Total Cost to Date/(Population Density X Risk)


  • Population Density is the number of people per square mile, calculated at the US Census Tract level.
  • Flood Risk is the current annual chance of inundation. For instance, a 1% chance = 1. Or a 10% chance = 10, etc.
  • Total Cost to Date shows cumulative dollars spent on flood-risk-reduction projects (construction only, adjusted for inflation) over the longest time period for which records are available, calculated at the US Census Tract level.

The report claims that a higher index means people have received more investment and therefore have less flood risk (i.e., more benefit). Conversely, a low index indicates less investment, more risk and less benefit.

The focus on census tracts is designed to make the data more granular than watersheds. Flood risk estimates will be averaged across the census tract and updated after MAAPnext data becomes available.

Here are several issues I have articulated to the Task Force.

Data Collection and Quality Issues

  1. Calculating only construction costs excludes other capital improvement costs such as engineering, design and right-of-way acquisition. Since 2000, construction costs have comprised only 40% of capital improvement costs. See below. And those costs don’t even reflect maintenance and repairs, which are crucial in reducing flooding.
All Flood Control and partner spending on all capital improvement projects from 1/1/2000 through the end of Q3 2021. Data obtained via FOIA Request from HCFCD.

2. According to the report, costs factored into the formula will include those from City of Houston projects and Harris County Flood Control projects. But they don’t include other municipalities’. There are at least 33 other cities in the County. The formula will reflect street-flooding risk, but not all spending to reduce that risk.

3. Likewise, it’s not clear whether the risk reflects pre- or post-mitigation spending, or both. Every time I ask about that, I get silence not an answer. Flood Control has spent more than $1.5 billion on flood mitigation since Harvey, while simultaneously developing new flood maps. Will the numerator of the formula sometimes reflect that investment but not the denominator?

4. Readily available digital spending data goes back only to 2000. But the Task Force committee chairman insists on getting data going back to the start of the Flood Control District – in 1937. If those records still exist, they will radically skew historical comparisons between watersheds, many of which were farms or forests until much more recently.

5. Flood risk depends on more than just mitigation investments. It’s a shifting target that has changed multiple times since 1937 as our understanding of rainfall probabilities has improved, and as different jurisdictions recognize that risk at different times. Flood risk also depends on upstream growth. That has been exponential. In the 2020 Census, Harris County had 4.7 million people. But in the 1930 Census, Harris County had only 359,328 – one thirteenth of today’s population, and presumably one thirteenth of the census tracts. So, attributing all change in risk to investment is fallacious.

6. Many of the census tracts have changed since the 1930s. Census tract boundaries are only “relatively” permanent. They often change based on Census results. For instance, when a census tract’s internal population grows over 8,000 persons, it may split into two or more smaller census tracts. Also, census tract boundaries may cross watershed boundaries. Major thoroughfares usually define census tract boundaries, not the direction of flowing water.

7. HCFCD has said they do not collect spending data by census tract. They calculate how much it costs to remove structures from the floodplain. So census tract data will have to be estimated manually – something that makes data-quality experts nervous.

8. Many neighborhoods outside Beltway 8 didn’t exist back in the 1930s. Beltway 8 didn’t even exist then. Nor did Lake Houston; the City began impounding water only in 1954.

9. The formula – designed to reduce flood damage – doesn’t measure flood damage.

10. So much data in this study won’t be directly comparable that I worry the authors won’t be able to highlight areas worthy of future investment. Final results will include compounded error on multiple levels. It doesn’t compare apples to apples; it compares apples to oranges, bananas, blueberries, cherries, strawberries, coconuts, Monty Python’s elderberries and more.

Questionable Validity

In fairness, I’m sure the final report, when it becomes available, would disclose these problems in an appendix or footnotes. But how many people dig into those? And who will “peer review” this study?

I have worked with market research my entire career and know the painstaking extents to which researchers go to ensure validity of their studies.

Validity has to do with accuracy. Are you really measuring what you purport to measure? For instance, is flood risk influenced ONLY by mitigation investment? Or is it ALSO influenced by other factors, such as:

The answer is a resounding YES to all those questions and more.

Good research studies typically measure the impact of one variable on another variable. For instance, in Harris County, what was the death rate last year among adults over 50 who contracted Covid among vaccinated and unvaccinated groups? Researchers carefully match the two groups being studied for factors such as randomness of subject recruitment, age, living situation, and history of other diseases. There is only one variable: vaccination. That way, they can tell whether the death rate varies among vaccinated people.

But the Flood Mitigation Benefit Index wasn’t designed with that kind of rigor. For example:

While purporting to compare ‘benefits’ of flood-mitigation to different areas, it doesn’t even employ pre- or post-measurements.

Further reducing comparability of results during the period studied:

  • Census tracts changed.
  • Population density changed.
  • Building codes changed.
  • Channels filled up with sediment, but maintenance won’t be measured.
  • AND the data does not measure street-flooding mitigation investments in almost HALF the county.

Because the flood-equity formula doesn’t control for such factors, we won’t know what caused variation in the results.

Formula Produces Inconsistent Results

The flood-equity formula does not even yield results that vary intuitively. For instance, when you hold population density and flood risk constant, but increase investment, the benefit goes up.

  • Example A: If Density = 5000, Risk = 10 and Investment = $100,000, then Benefit Index = 2
  • Example B: If Density = 5000, Risk = 10, and Investment = $1 million, then Benefit Index = 20

So, spending more money to get the same results increases benefits? Shouldn’t it be the opposite?

That’s both depressing and confusing. You spend 10X the money; flood risk remains the same; and the “benefit” increases!!!???

You would think spending less money to achieve identical results would be more beneficial. It certainly is for taxpayers.

Although the Task Force won’t admit it, the formula is really trying to prove “historical disinvestment,” a claim tossed around frequently in Task-Force and Commissioners Court meetings without data to back it up.

But if the goal is to protect the most people from future flooding, why not just invest in projects where the highest risk remains for the greatest number of people? Both of those are simple, unambiguous direct measurements. But those might not produce the results that the authors of this formula hope to get.

I believe we should look forward, not back, with our flood-mitigation dollars. We can’t change the past…whatever it was. We can only affect the future by what we do today.

Posted by Bob Rehak on 7/3/2022

1769 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.