In Part 1 of this two-part post, I outlined how I would create a heat map of the county showing areas of high inundation risk, low prior investments, and high vulnerability.

This month, in Part 2 of this two-part post, I will describe how we might build the composite heat map and how it could be used to identify areas for future flood resilience investments.

Building the Composite Heat Map

Last month I described three input variables that I would use to create a composite heat map of flood risk reduction needs. The three layers included:

— The Flood Risk (annual chance of inundation);

— The Flood Mitigation Benefit Index (FMBI); and,

— The Social Vulnerability Index (SVI).

These are useful data, but they are not integrated or combined yet. Each variable can be used to create a separate heat map. How can we combine these three maps into one composite map?

Overhead Transparency Projectors

I attended high school in the 1980s. At that time many teachers used an overhead transparency projector to present information to the class. They would place transparent sheets of plastic on a light box, light would shine up through the sheet, hit a mirror, shine through a lens, and get projected onto a screen for the class to see. The sheets would sometimes have pre-printed information or the teacher might actually write with a marker on the sheet as they presented the information. Here’s a photograph of one of these old devices.

Transparencies for Planning

We can create a transparent sheet for each of the layers we’d like to consider when creating the composite heat map. Today we can do this with Geographic Information System (GIS) mapping software, but I thought it would be helpful to illustrate the idea using clear sheet transparencies.

I created a hypothetical area of the county with nine U.S. Census Blocks. I also created three hypothetical transparencies for consideration; one for flood risk, one for FMBI, and one for SVI. In each map, the red color indicates a greater need for future resilience investments and the green color indicates a lower need for future resilience investments. The numbered rectangular shapes are hypothetical U.S. Census Blocks.

The flood risk map shows that Blocks 1, 2, 3, 7, and 8 have very high risks, while 5 has a very low risk.

Here’s the Flood Mitigation Benefit Index (FMBI) map:

The FMBI map shows a wide range of index values within the very high flood risk areas on the left side of the image.

Here’s the SVI map:

The SVI map shows very highly vulnerable people in Block 8 with high to moderate vulnerability in Blocks 2 and 7.

Equal Weight Example

If we place all three transparencies on the overhead transparency projector at the same time and without any adjustment, we see an equal weight composite. This means that all three variables – flood risk, FMBI, and SVI are considered equally. Take a look:

The equal weight composite heat map shows that additional resilience investments are needed in U.S. Census Blocks 7 and 8 (and perhaps 2 if we have sufficient funding). The legend colors don’t exactly match up anymore because of the way the layered colors blend.

Variable Weight Example

Let’s say we decide the current flood risk is the most important factor we should consider, followed by SVI, and then followed by FMBI. Let’s also say we think that flood risk should be six times more important than the FMBI and three times more important than the SVI. This logic would yield the following weight factors and transparency levels. Note that the transparency level goes down with a high weight to allow that layer to influence the composite more strongly. Also, note that the assignment of the weights is a policy choice – not an engineering choice.

	Weight Factor	Transparency Level
Flood Risk	60	40
Flood Mitigation Benefit Index	10	90
Social Vulnerability Index	20	80

Here are the new transparencies with the weight factors and transparency levels applied:

Here is the new composite heat map we see after we place all three of the weighted transparencies on the overhead projector:

The weighted composite heat map shows that future resilience investments are needed in U.S. Census Blocks 1, 2, 3, 7, and 8 (and perhaps 6 and 9 if we have sufficient funding). The legend colors don’t exactly match up anymore because of the way the layered colors blend.

Discussion of Results

Compare the weighted result to the equal weight result.

The equal-weighted composite heat map showed that future resilience investments are needed in U.S. Census Blocks 7 and 8 (and perhaps 2 if we have sufficient funding). The weighted composite heat map showed that future resilience investments are needed in U.S. Census Blocks 1, 2, 3, 7, and 8 (and perhaps 6 and 9 if we have sufficient funding). What accounts for this difference?

In the first example, all three input variables were considered equally. In the second example, flood risk was considered to be six times more important than FMBI and three times more important than SVI.

Implications for Planning

This article shows how policy decisions regarding the relative importance of various factors will lead to different decisions and different outcomes. It shows why elected officials and the public should stay engaged in planning efforts to let planners know what factors and weights they should be considering.

I believe that Harris County elected officials and Harris County Flood Control District (HCFCD) managers have not intentionally set out to make inequitable flood risk reduction investments. I believe that some inequitable investments were a side-effect of relying on the Civil Works program of the U.S. Army Corps of Engineers (USACE) for the vast majority of funding. Why would this reliance lead to some inequitable investments?

By federal law and policy, the USACE can only study a project if the study directive is included in federal legislation passed by both the House and the Senate and signed by the President. The USACE then must evaluate project alternatives using a planning guidance document published in 1983, known by the short title: “Principles and Guidelines” (P&G).

Under the P&G document, planners must calculate the benefits and costs of all project alternatives and, in a broader sense, all projects that are competing for federal funding. Engineers like numbers so we calculate the ratio of benefits over costs and call it the Benefit-Cost Ratio or BCR. Projects (or project alternatives) that deliver higher BCRs are much more attractive to the federal government than those that deliver lower BCRs.

I’m told that in recent years the Executive Branch of the federal government typically only supports projects with a BCR of more than 3.0.

Section IV of the Principles and Guidelines – relating to urban flood damage – requires planners to calculate the benefit portion of the BCR from the value of the avoided “physical damages to … buildings or parts of buildings; loss of contents … loss of roads, sewers, bridges, power lines, etc.”

To illustrate this, consider two projects that each cost $100 million dollars. One project removes 1,000 homes each worth $350,000 from the 100-year floodplain (the 1% annual chance floodplain). The other project removes 1,000 homes each worth $85,000 from the 100-year floodplain. Let’s also assume that both projects benefit 2,300 people (2.3 people per household).

For simplicity let’s compare both projects for just one 100-year flood event and assume that the 100-year flood would completely destroy all of the homes. (I realize this is not true in the real world, but just go with it for the example.)

The first project would avoid $350 million in property damage (1,000 homes times $350,000 for each home). This would yield a BCR of 3.5 ($350 million / $100 million) and benefit 2,300 people.

The second project would avoid $85 million in property damage (1,000 homes times $85,000 for each home). This would yield a BCR of 0.85 ($85 million / $100 million) and benefit 2,300 people.

Which project do you think would secure federal support? In this example, the project that removed the higher-value homes from the floodplain would be supported and would attract federal funding, while the other project would not attract any federal funding. When federal funding is secured, it often pays for 90% of the project cost. So in this case, the federal government – using income taxes from citizens all around the nation – would pay $90 million for the project while HCFCD would pay $10 million using local property tax revenue.

To illustrate just how important property values are to the process, the P&G document provides a flowchart to illustrate the process.

The federal reliance on BCRs determined using the value of avoided property damages outlined above, means that when we rely on federal funding to do projects, we must identify projects that have a high BCR so that the projects can get a favorable recommendation from the USACE to federal lawmakers. This encourages flood mitigation planners and engineers to think up projects that protect higher-value properties, regardless of the number of people helped.

Federal law and policy also direct the USACE to only design and build projects with both a local sponsor – in our case HCFCD – and authorization and appropriated funding by Congress and the President.

A quick review of the Texas law that created the Harris County Flood Control District (HCFCD) in 1937 shows why HCFCD was created in the first place. Section 2.g. of House Bill No. 1131 of the 44th Texas Legislature states:

To cooperate with and contract with the United States of America or with any of its agencies now existing, or which may be created hereafter, for grants, loans, or advancements to carry out any of the powers or to further any of the purposes set forth in this Act and to receive and use said moneys for such purposes; or to contribute to the United States of America or any of its agencies in connection with any project undertaken by it affecting or relating to flood control in Harris County;

HoUSE BILL NO. 1131, 44TH TEXAS LEGISLATURE

So it is clear that we have relied on federal funding to reduce flood risks in Harris County. Maximizing the use of federal dollars is important. Why not take advantage of available federal funding? Many people, myself included, appreciate how HCFCD staff has been able to maximize the federal investments in our region. But I think it’s fair to acknowledge that this reliance on federal funding did inadvertently lead to some inequitable investments.

Today, one important policy question for Harris County is this: “To what extent, if any, will we rely on federal funding to further reduce flood risks?”

Since this question depends upon federal policy and regulations, in another post, I will provide an update on pending changes to federal policies and regulations that may allow “counting” other benefits — not just the value of avoided property damages — in the benefits part of the BCR. Other benefits might include favorable social, environmental, and economic outcomes – some that can be converted to a monetary value and some that are qualitative.

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:

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:

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.

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 Now, 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:

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.

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.

Part II of this discussion is posted here.

I recently posted a short summary of the 1,800+ page tunnel study prepared by the engineering firm Black & Veatch under contract with Harris County Flood Control District. In today’s post, I will highlight some of the key questions I have about the study.

Before we get to the questions, I wanted to go on record as supporting the use of tunnels to reduce flood risks in Harris County. I support studying them and I support using them to reduce flood risks for areas of the county with the highest current flood risk and the highest social vulnerability.

I also support the use of the January 2021 U.S. Army Corps of Engineers (USACE) Policy Directive known as “Comprehensive Documentation of Benefits in Decision Document.” This policy “emphasizes and expands upon policies and guidance to ensure the USACE decision framework considers, in a comprehensive manner, the total benefits of project alternatives, including equal consideration of economic, environmental and social categories.” I support the use of this approach to help make the case for flood risk reduction projects in areas of the county with the highest current flood risk and the highest social vulnerability.

Now, on to the questions.

Changes to Watershed Screening

Section 3.0 of the report presents the results of a screening process of all 23 watersheds in the county. Black & Veatch screened each watershed to determine if watershed characteristics were favorable for a tunnel system. They considered the number of flooded structures, the social vulnerability of the residents, the location and distance of suitable tunnel discharge points (outfalls), ground elevations needed to allow gravity flow, and the relative cost of tunnels versus traditional flood risk reduction approaches. The table below presents the screening results in summary format.

During their June 16, 2022 presentation, the Harris County Flood Control District (HCFCD) presented a slide that showed the watershed screening results in a map format (below).

While the report indicates that Addicks and Barker Reservoirs were unfavorable for a tunnel system, the map indicated that they both would receive benefits from the installation of a tunnel in those watersheds. Why were these two watersheds presented this way? How can those watersheds receive any benefits from tunnels if those watersheds each have unfavorable characteristics for a tunnel system?

Selection of Evaluation Criteria

As outlined in my prior post, each of the eight tunnel alternatives was evaluated using ten criteria, each with its own weight. The ten criteria were: flood risk, environmental impact, social vulnerability index, operations and maintenance, constructability, community impacts, integration with traditional flood control systems, permitting, land acquisition, and geotechnical. How were the ten criteria selected? Why these ten? Why do some of them include duplicate scoring elements?

Evaluation Criteria Scoring

I have questions about how six of the evaluation criteria were defined and scored.

Environmental Impact: I would expect alternatives with a higher degree of anticipated impacts prior to mitigation would score lower (worse) and those with a lower amount of anticipated impacts prior to mitigation would score higher (better). Was the information in Appendix G – Environmental Constraints Analysis used to estimate the anticipated impacts for each alternative? Why did this criterion include permitting time? Doesn’t that duplicate another evaluation criterion (see below)? Wouldn’t permitting time be similar for all alternatives? Why include potential fines for non-compliance? Wouldn’t HCFCD follow all laws and environmental regulations while implementing any of the alternatives? How was each alternative scored to address the possibility of encountering hazardous materials during construction? Wouldn’t the likelihood of this be the same for all alternatives? Unweighted scores for this criteria range from 12.5 to 17.25. What factors were used to come up with this range of scores?

Operations and Maintenance: All tunnel options are very similar, except for their length and perhaps the number of inlet shafts. The criterion is scored based on the frequency and complexity of operations and maintenance activities. How would these activities differ for each alternative? Unweighted scores for this criteria range from 7.5 to 15.0. What factors were used to come up with this range of scores?

Constructability: All tunnel options are very similar, except for their length and perhaps the number of inlet shafts. The criterion addresses construction risks such as a flood event during construction or constrained surface site access. How would these risks differ for each alternative? Unweighted scores for this criteria range from 15 to 39. What factors were used to come up with this range of scores?

Community Impacts: This criterion includes consideration of the amount of traffic disruption, noise pollution, the number of displaced businesses and homes, and the amount of public outreach “that will be required.” It appears that this criterion considers impacts during project development, construction, and operation. Doesn’t displaced businesses and homes duplicate the Land Acquisition criterion described below? How was the amount of required public outreach scored? Did the report’s authors consider more required outreach a positive or a negative? Most engineers I know consider more outreach a negative, although many planners and elected officials might consider more outreach a positive.

Permitting: This criterion considers “the level of permitting requirements” for each tunnel alternative. This seems like a duplicate of the Environmental Impacts criterion. It appears to be scored higher if less permitting is required and lower if more permitting is required. How would permitting requirements differ for each alternative? Unweighted scores for this criteria range from 8.75 to 18.75. What factors were used to come up with this range of scores?

Land Acquisition: This criterion appears to use at least one of the same scoring elements as Community Impacts. Why duplicate this?

Promotion of Alternatives

As I noted in my summary post, Black & Veatch promoted the Brays Tunnel as 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. Why does the report assume that the federal project won’t proceed?

As I noted in my summary post, Black & Veatch promoted the White Oak Tunnel as 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.

Why was the White Oak Tunnel selected as a recommended tunnel concept?

Sediment

Black & Veatch considers sediment accumulations in Appendix Q of the report (pp. 1,1818 to 1,849). Heavy storms more easily cause soil erosion and higher runoff flows can more easily move the sediments into our channels, bayous, and any future tunnel systems. This means that sediment will collect in the tunnels and will need to be removed unless we are prepared to accept reduced tunnel capacity as the sediment reduces the effective diameter. In the appendix, Black & Veatch attempts to answer some of the key questions we all should be asking.

Black & Veatch reviewed prior studies and reports sediment concentrations ranging up to 3,180 milligrams of sediment in each liter of water (mg/L). As expected, lower concentrations are associated with lower channel or bayou flow rates and higher concentrations occur with higher flow rates.

This concentration value may not be very meaningful to some of my readers, so I will provide some context. More frequent and smaller rain events generate urban stormwater runoff with sediment concentrations of about 100 mg/L. Imagine pouring some volume of cloudy water with this concentration of sediment floating in it into a collection container. How much water would you need to pour to collect one pound of wet sediment? Using conversion factors we can figure out that you would need to pour about 120,000 gallons of water to collect about 1 pound of sediment. That’s about 5 or 6 days of a garden hose flowing at 15 gallons per minute.

The higher sediment concentration in flood waters would deliver sediment much more quickly. You would only need to pour 4,000 gallons of water with a sediment concentration of 3,000 mg/L to collect about 1 pound of sediment. Since 3,000 mg/L is 30 times larger than 100 mg/L it only takes 1/30th of the flow to get 1 pound collected.

The water flow in the thought experiment above is one way, just into a container. The tunnels will have water flowing in and out at the same time until the floodwaters near the inlets recede. This means the collection of sediment in the tunnel will depend upon other factors, such as sediment particle size (bigger particles settle more easily) and water velocities (higher velocities keep particles from settling).

Black & Veatch notes, in Section 10.3 of the main report, that “sediment will … accumulate in the tunnel” at a rate of “less than a few inches per year.” Black & Veatch suggests that water could be diverted into the tunnel to resuspend the sediment (by moving water over the bed sediment fast enough to exceed the forces that keep it in a nice pile). This water could then be pumped out along with the suspended sediment to keep the tunnel from silting up and losing conveyance capacity over time.

My questions include the following: Where would this water come from? How much water would be needed? What flow rate and volume of water would be needed for each tunnel? To what location would the water and sediment be discharged without harming surface water quality or damaging dredged navigation channels? How much energy would be required to pump the water into and out of the tunnel? What would happen if a large storm occurred while the sediment removal process was underway? How do other tunnel systems handle sediment?

If you think of any questions you’d like to ask, please leave a comment.