[On October 25, 2017 Harris County Judge Ed Emmett released a list of post-Harvey recommendations relating to flooding. This document presents those recommendations in bold text followed by responses and suggestions in regular text. The responses reflect the views of the author. They don’t necessarily reflect the views of his employer or any of the organizations with which he volunteers.]

Harvey was the worst flood in our history, but Harvey was not a singular event. It followed two other major rain events in recent years. This area has seen three of these so-called 500-year rains in the past two years. Either our definition of a 500-year rain needs updating or we can hope that we are safe for the next 1,500 years.

This illustrates how civil engineers and meteorologists have failed to adequately communicate the likelihood of large amounts of rainfall (inches in a certain period of time) to the public and to our elected officials. This lack of effective risk communication not only leads to misconceptions about rainfall probabilities, it also leads to misconceptions about the likelihood of high bayou flowrates and flood stages occurring (resulting from rainfall runoff). It also leads to misconceptions about what depth of rain our infrastructure is designed to handle over a certain time period.

To clarify, a 100-year storm has a 1 in 100 chance of occurring every year. That’s a 1% chance of occurrence during a 1 year time period. In the Harris County region a 100-year storm is about 13 inches of rain in 24 hours. Over a 30 year mortgage there is a 26% chance of that size rain event occurring.  Over a 50 year period there is a 40% chance of occurrence. Over a 100 year period there is a 63% chance of occurrence. The likelihood of this size rain event occurring approaches 100% as the time period hits 900 or 1,000 years.  The longer the time period the higher the likelihood. The graph below illustrates how the likelihood increases with time. The red line is the 100-year event for the southeastern part of Harris County.

Policy makers across the United States selected the 1% annual chance event as the design basis for drainage infrastructure in second half of the 20^th century – sometime between 1960 to 1990, depending on the location. This policy decision strikes a balance between benefit and cost. As we enhance drainage system designs to achieve lower risk levels — the desired BENEFITS; the associated COSTS of these systems go up.  This is because lower risk levels require bigger, deeper, and wider channels; bigger pipes; larger inlets; bigger dams; and higher structure elevations.

In the immediate aftermath of Harvey people appear willing to pay almost any amount of money to achieve risk reduction benefits. How much are we willing to pay to reduce the risk of flooding any particular structure from 1.0% a year to 0.2% a year? The risk reduction price tag (upfront costs plus operational costs and debt financing) must be compared to the avoided damage costs times their probability of occurring.

Following Harvey, government officials, academics, private institutions and the general public have all weighed in with ideas of what our area needs to do to protect life and property from future floods.

We should not frame post-Harvey decision-making as binary choices: protect life and property or not protect life and property. This is choice of what level of risk we are willing to accept. The choice should be framed as “pay $X for Y 30-year cumulative risk level, which equals an annualized cost of $Z of avoided damages and additional benefits” or “pay $A for B 30-year cumulative risk level, which equals an annualized cost of $C of avoided damages and additional benefits.”  We should assertively pursue projects with a favorable benefit to cost ratio.

(1) Create a regional flood control/water management organization similar to the Transportation Policy Council at the Houston-Galveston Area Council. This will allow for multi-county coordination of flood control and water management.
–
This already exists. It is called the Regional Flood Management Council. Information about this body is on the Houston-Galveston Area Council webpage: http://www.h-gac.com/board-of-directors/advisory-committees/regional-flood-management-council/default.aspx Perhaps the H-GAC board of directors and the member governments need to update its charge, duties, and authorities?–

(2) FEMA flood plain maps need to be revised immediately to reflect the impact of Harvey. Development rules should focus on restricting development in the 500-year flood plain instead of the 100-year flood plain — or the 100 year flood plain needs to be vastly redefined and updated.

Floodplain maps definitely should be updated, but the update should only occur after the National Weather Service rainfall statistics study called Atlas 14 is completed in May 2018. Updating development regulations should be considered, but it would be prudent for all stakeholders to recognize the benefits (the value of avoided future damages times the likelihood of the loss) and the costs.

(3) A third reservoir should be built to protect the west and northwest sections of Harris County. Rather than waiting on federal funds, the reservoir should be funded by the State of Texas’ “rainy day fund.” The reservoir should be part of a larger project to create a state or national park for the Katy Prairie.

Consideration of a third reservoir is a worthy effort, however, the benefits and costs as outlined above, should be determined prior to moving forward. All stakeholders need to fully understand that a new reservoir will not “protect” the west and northwest sections of the county.  It WILL reduce the risk of flooding for some, increase the risk of flooding for others, and will cost a substantial amount of money.  If the cumulative benefits (include recreational value, aesthetics, and flood risk reduction) are greater than the costs (capital and operations) than let’s do it!

(4) The U.S. Army Corps of Engineers should immediately fund the four Harris County Flood Control District projects that are now ready for completion. Those four are Brays Bayou, White Oak Bayou, Hunting Bayou and Clear Creek. New flood maps showing the impact of these projects should be released so homeowners will know if their property will remain flood prone.

This is a no-brainer. Let’s do this.

(5) Old watersheds in developed areas in Harris County should be identified. For example, those areas downstream from Addicks Reservoir need to know where an “uncontrolled release” over the spillway would flow.

This is a worthy effort and it could be reasonably accomplished using historic aerial photographs. It might help in planning efforts and in floodplain modeling efforts to some extent.

(6) The Harris County Office of Emergency Management, working with municipalities and special districts, should develop a state-of-the-art flood warning system and localized evacuation plans. Such plans should use recognized volunteer organizations to assist first responders. The Harris County Sheriff’s Office and the Community Emergency Response Teams should have a defined water rescue effort featuring private boats and high-water vehicles.

A new or enhanced flood warning system should be designed like an Amber alert to cell phones and should be implemented using an “op-out” approach. Some areas of the county and city with a low percentage of residents with cell phones should deploy public sirens or alarms to warn those areas without ubiquitous cell phone use.

(7) Lake Houston and Lake Conroe should be converted to serve as Flood Control facilities in addition to serving as water supplies. Lake Houston should be restored to maximum storage capacity, and the San Jacinto River Authority should create retention/detention capacity upstream of Lake Houston. And the San Jacinto River Authority should have representation from Harris County.

These proposals should also be evaluated using a robust benefit / cost analysis. The analysis should consider all costs, including construction costs, right of way acquisition costs (for a large amount of the privately only land around both lakes might be required), dam and gate modifications, dredging costs, dredge spoil disposal costs, design and permitting costs, operational costs, and the value of lost drinking water. These costs should be compared to the monetized risk reduction benefit of fewer homes, of a certain value, flooding less often.  These proposed projects don’t appear, at first glance, to have a favorable benefit / cost ratio.It would seem fair and appropriate for Harris County to appoint a board member to serve on the SJRA Board.

(8) The Harris County Emergency Operations Center should be expanded to assist emergency operations for smaller surrounding counties.

Yes.

(9) The roles and responsibilities of municipal utility districts and other special districts should be clarified to include flood control and storm water management, in cooperation with the Harris County Flood Control District. Existing districts should be studied for untapped capacity, and new districts developed with flood control in mind. Until a true 100-year flood level is defined, the 500-year level should be used for detention purposes.

Clarification of the purpose of local government entities is prudent. Harris County Flood Control District evaluates bayous and watersheds for opportunities to add detention facilities and to otherwise reduce flood risks. If they see opportunities they often work with land owners and MUDs to build new facilities and reduce flood risks. The use of the 500-year storm should be considered as discussed in Item 2 above.

(10) All underpasses that have the potential for drowning should be identified and equipped with automatic barriers or be part of a comprehensive manual plan for closures. In addition, vehicle manufacturers should be encouraged to develop technology to detect high water.

Yes, this should definitely be implemented. It will probably save more lives than most of the other suggestions on this list (except the Amber-alert flood warning system, which will likely save many lives during an evacuation from a hurricane storm surge).

(11) The Harris County Flood Control District should develop comprehensive plans for every major watershed in Harris County, with immediate attention given to the entire length of Buffalo Bayou and to plans to divert storm water around downtown Houston, either through a canal or tunnel system.

These projects are worthy of investigation using the benefit / cost ratio approach previously outlined.

(12) Federal, state and local governments should implement a buyout and/or elevation program for all homes located in the 100-year flood plain or that have flooded repeatedly. Such a buyout/elevation program should use traditional government funding and private funding, such as social impact bonds.

This is great suggestion that typically has a very good benefit to cost ratio, especially if the buyouts are contiguous and flood damage reduction detention or conveyance can be constructed in the area after the buyouts are completed.

(13) The State of Texas should institute clear rules for approval of development plats in unincorporated areas, specifically those areas in the extraterritorial jurisdiction of a city. Additionally, there should be clear requirements for disclosure of flood risk to homebuyers and renters.

The Texas Legislature should grant counties additional development and floodplain regulatory powers and clarify how county and city rules should be applied in the extraterritorial jurisdictions.  I strongly support a statewide law mandating flood risk disclosure in a real estate transaction (buying, renting, or leasing) involving an individual or family. This disclosure should provide the risk of flooding over durations of 1, 30, and 50 years for all properties – not just those located in the 100-year regulatory floodplain.  Since the risk of flooding outside the 100-year regulatory floodplain is less than 1% annually but is NOT zero, the disclosure must include a statement indicating that the risk of flooding is less than 1% per year or less than 26% over 30 years for those homes.

(14) The U.S. Army Corps of Engineers should restore the dams and detention areas of Addicks and Barker reservoirs to first-class condition including, if necessary, removing dirt and vegetation within the reservoirs.

The gate and berm repair project should be completed as quickly as possible.  Removing dirt and vegetation within the reservoirs should be evaluated using the benefit / cost ratio approach outlined earlier in this document.

(15) Given the population of unincorporated Harris County and the restrictions on incorporation and annexation, Harris County should be allowed some ordinance making power and should receive a portion of the sales tax collected in unincorporated areas. To continue to exclusively rely on the property tax is fundamentally unfair and unsustainable.

I support additional rule-making authority for the county to help with development and floodplain management issues.

[This is verbatim from the document provided by his office. Bold text added to help identify the key issue addressed in each suggestion.]

Two months ago, the remnants of Hurricane Harvey dumped unprecedented amounts of rain on Southeast Texas, including Harris County. People died, homes and businesses flooded, families were displaced and our lives changed.

Harvey was the worst flood in our history, but Harvey was not a singular event. It followed two other major rain events in recent years. This area has seen three of these so-called 500-year rains in the past two years. Either our definition of a 500-year rain needs updating or we can hope that we are safe for the next 1,500 years. Obviously, we cannot assume the latter.

Following Harvey, government officials, academics, private institutions and the general public have all weighed in with ideas of what our area needs to do to protect life and property from future floods. Special committees have been formed, and significant amounts of money have been committed to finding solutions.

However, now is not the time for a piecemeal approach. The sense of urgency created by Harvey will fade, so we must quickly commit ourselves to a comprehensive plan to redefine Harris County and the surrounding region as a global model for living and working in a flood-prone area.

I do not pretend to have all the right answers, nor do I see myself as an expert in flood control. My purpose today is to present a broad vision of what is needed and to challenge those involved in seeking solutions to think boldly. There is a general rule in politics that you should focus on fewer, narrowly defined items, since the more ideas that are presented, the more people will disagree with something. Harvey should make all of us suspend politics as usual and take ownership of flood control.

As county judge, I am the Director of Emergency Management. One thing I know for sure is that the best time to manage an emergency is before it happens. With that in mind, I offer the following suggestions.

Rivers, bayous, creeks and flood mitigation areas should be viewed as positive features and treated as preserves or recreational and tourist areas. We should turn a vulnerability into an asset. That should be our overriding vision. With that vision in mind, there are numerous specific ideas that need to be present.

1. Create a regional flood control/water management organization similar to the Transportation Policy Council at the Houston-Galveston Area Council. This will allow for multi-county coordination of flood control and water management.
2. FEMA flood plain maps need to be revised immediately to reflect the impact of Harvey. Development rules should focus on restricting development in the 500-year flood plain instead of the 100-year flood plain — or the 100 year flood plain needs to be vastly redefined and updated.
3. A third reservoir should be built to protect the west and northwest sections of Harris County. Rather than waiting on federal funds, the reservoir should be funded by the State of Texas’ “rainy day fund.” The reservoir should be part of a larger project to create a state or national park for the Katy Prairie.
4. The U.S. Army Corps of Engineers should immediately fund the four Harris County Flood Control District projects that are now ready for completion. Those four are Brays Bayou, White Oak Bayou, Hunting Bayou and Clear Creek. New flood maps showing the impact of these projects should be released so homeowners will know if their property will remain flood prone.
5. Old watersheds in developed areas in Harris County should be identified. For example, those areas downstream from Addicks Reservoir need to know where an “uncontrolled release” over the spillway would flow.
6. The Harris County Office of Emergency Management, working with municipalities and special districts, should develop a state-of-the-art flood warning system and localized evacuation plans. Such plans should use recognized volunteer organizations to assist first responders. The Harris County Sheriff’s Office and the Community Emergency Response Teams should have a defined water rescue effort featuring private boats and high-water vehicles.
7. Lake Houston and Lake Conroe should be converted to serve as Flood Control facilities in addition to serving as water supplies. Lake Houston should be restored to maximum storage capacity, and the San Jacinto River Authority should create retention/detention capacity upstream of Lake Houston. And the San Jacinto River Authority should have representation from Harris County.
8. The Harris County Emergency Operations Center should be expanded to assist emergency operations for smaller surrounding counties.
9. The roles and responsibilities of municipal utility districts and other special districts should be clarified to include flood control and storm water management, in cooperation with the Harris County Flood Control District. Existing districts should be studied for untapped capacity, and new districts developed with flood control in mind. Until a true 100-year flood level is defined, the 500-year level should be used for detention purposes.
10. All underpasses that have the potential for drowning should be identified and equipped with automatic barriers or be part of a comprehensive manual plan for closures. In addition, vehicle manufacturers should be encouraged to develop technology to detect high water.
11. The Harris County Flood Control District should develop comprehensive plans for every major watershed in Harris County, with immediate attention given to the entire length of Buffalo Bayou and to plans to divert storm water around downtown Houston, either through a canal or tunnel system.
12. Federal, state and local governments should implement a buyout and/or elevation program for all homes located in the 100-year flood plain or that have flooded repeatedly. Such a buyout/elevation program should use traditional government funding and private funding, such as social impact bonds.
13. The State of Texas should institute clear rules for approval of development plats in unincorporated areas, specifically those areas in the extraterritorial jurisdiction of a city. Additionally, there should be clear requirements for disclosure of flood risk to homebuyers and renters.
14. The U.S. Army Corps of Engineers should restore the dams and detention areas of Addicks and Barker reservoirs to first-class condition including, if necessary, removing dirt and vegetation within the reservoirs.
15. Given the population of unincorporated Harris County and the restrictions on incorporation and annexation, Harris County should be allowed some ordinance making power and should receive a portion of the sales tax collected in unincorporated areas. To continue to exclusively rely on the property tax is fundamentally unfair and unsustainable.

In this “post-Harvey” period there are many suggestions about how best to spend the $10 or $20 billion we hope to get from the federal government, the state of Texas, local property taxes, potentially new sales tax collections, and (of course) cash proceeds from new local government bond sales.

For example, some folks are suggesting we should spend $6 billion to buy out homes in the western fringe of the Addicks and Barker flood pools and along Buffalo Bayou from Highway 6 to downtown, and channelize or modify the bayou corridor so it can convey 15,000 cubic feet per second. Others are pushing for the so called coastal spine to protect the region from storm surge.  Still others are suggesting that we build a third flood control reservoir in the Cypress Creek area.

I’m concerned that we are letting our emotions and a few cognitive biases harm our decision-making. Our emotions are telling us that flooding is terrible. People’s homes were destroyed. Their lives were horribly disrupted.  Photo albums destroyed.  Mold grew in bedrooms and living rooms. People died. This high level of emotion impacts decision making.  This high level of emotion increases our willingness to pay for projects that will (supposedly) cost-effectively reduce flood risks and damages. This is known as the “affect heuristic” cognitive bias, which basically means emotions can take over decision-making.

In addition, the “availability heuristic” cognitive bias, makes us all feel that flooding has a much higher likelihood of occurring in the near future if a flooding event has happened recently, regardless of the mathematical probability of the future occurrence.  A vivid example of this relates to how we perceive the risk of dying from a shark attack compared to the risk of dying from a falling airplane part.  Most people mistakenly believe that a shark death is more likely than a death from an airplane part because stories about shark attacks are widely reported and deaths from airplane parts are not.  What we hear about or experience recently is weighted more heavily in decision-making.

To avoid falling into the trap of these and other cognitive distortions we should decide how best to use post-Harvey funds using a risk-based decision making framework.  Here’s how I think it should work.

We need to think about potential projects in two distinct ways.  First, we need to think about the risk of a particular flooding event being addressed (over an appropriate period of time) and, second, we need to think about the consequence of that particular event occurring at any point in time. To help illustrate this approach we will consider two hypothetical projects, each with their own risk level, consequence, and cost.

But first we need to talk a bit more about risk and probability.  I created a graph that displays an array of various risk levels that we will use to help make the best decision.

The graph was constructed using basic probability calculations and it shows the probability of various rainfall events being exceeded during various time periods.  The vertical axis shows the probability from 0% chance to 100% chance.  The horizontal axis shows various time periods ranging from 1 to 10 to 10,000 years. It is plotted using a log scale, which allows us to see a very long period of time in a reasonable graph width. The various colored lines illustrate the probability of a particular 24-hour rain event being exceeded during any time period (duration) of interest.  The smaller events are much more common. The larger events are less common.  By examining the red line (13.2 inches in 24-hours) you can see that the likelihood of that storm falling on your home during your 30-year mortgage is about 26%.

The important thing to notice about this chart is that the longer we are willing to wait, the more likely any storm event becomes.  Let’s take a look at the very rare 18.9 inch rain storm in light blue. Over a 1 year time period we feel safe, because that much rain only has a 0.2% of happening during that time period. But the likelihood of that much rain falling on your home during your 30 year mortgage is about 6%. The chance of that storm hitting your home during 250 years is about 50%. So the crazy thing is that ALL of Houston has a non-zero probability of getting hit with a Harvey type storm or larger. It’s just a matter of time and the longer we wait the more certain the storm becomes. All of the curves eventually hit 100%.

These curves describe the risk of a certain rainfall depth hitting a certain location. Another similar set of curves could be created to illustrate the risk standing or flowing water achieving a certain elevation as a result of runoff from a rain event. This could be from bayou flooding, inundation of a low area, coastal surge, or reservoir pools filling up.

Ok, back to our two hypothetical projects. Here are the estimated or calculated facts about each of the projects. In real life all of this information would be estimated or calculated using engineering principles, computer models, construction cost estimating techniques, appraised property values, land values, and other sources and methods.

Project One – Conveyance Improvements: 

• Description: Acquire land and enlarge bayou channel to increase the channel’s conveyance capacity.
• Design Basis: Improve conveyance of stormwater runoff. Change stormwater carrying capacity to handle runoff from a 9.6 inch storm (4% annual chance) to 13.2 inch storm (1% annual chance).
• Cost: $450 million
• Benefit: Reduces flooding risk for 1,350 structures worth $675 million from a 9.6 inch storm (4% annual chance) to 13.2 inch storm (1% annual chance).

Project Two – Buyouts: 

• Description: Acquire land, demolish structures, regrade land to provide detention, and re-landscape to make park area.
• Design Basis: Remove home from high risk area which floods from 6.2 inches of rain (20% annual chance of flooding).
• Cost: $720 million
• Benefit: Reduces flooding risk for 1,350 structures worth $675 million from 4% per year (rain depth of 9.6 inches) to 0% forever.

Let’s look at each project from a benefit / cost perspective, while factoring in the cumulative risks.

Before Project One the risk of loss over 100 years can be determined from the graph by finding the intersection of the 9.6 inches curve with the 100 year line. This risk is 98%. The value of the loss (the consequence) is $675 million in present dollars. Multiplying the risk times the consequence provides the risk weighted loss for a 100 year period, which, in this case is $661.5 million.

After Project One the risk of loss over 100 years can be determined from the graph by finding the intersection of the 13.2 inches curve with the 100 year line. This risk is 63.5%. The value of the loss (the consequence) is still $675 million in present dollars. Multiplying the risk times the consequence provides the risk weighted loss for a 100 year period, which, in this case is $428.6 million.

Project One reduces the risk weighted loss by the difference between $661.5 million and $428.6 million. This reduction is the project benefit, which equals $232.9 million. The benefit to cost ratio is calculated by dividing the risk weighted benefit of $232.9 million by the project cost of $450 million. For this project the benefit to cost ratio is 0.52, which would not justify doing the project. Most project sponsors would only move forward if the benefit / cost ratio was greater than 1.0.

Now let’s look at Project Two.

Before Project Two the risk of loss over 100 years can be determined by multiplying the 20 year risk weighted loss by five. The 20 year risk can be obtained from the graph by finding the intersection of the 6.2 inches curve with the 20 year line. This risk is 99%. The value of the loss (the consequence) is $675 million in present dollars. Multiplying the risk times the consequence provides the risk weighted loss for a 20 year period, which, in this case is $668.3 million. Multiplying this by five, provides the 100 year risk weighted loss of $3.34 billion. (This assumes that the homes are repeatedly rebuilt and flooded.)

After Project Two the risk of loss over 20 years is zero because the structures are gone. The value of the loss (the consequence) is also zero because the structures are gone. This means the 100 year risk weighted loss is also zero.

Project Two reduces the risk weighted loss by the difference between $3.34 billion and $0.00. This reduction is the project benefit, which equals $3.34 billion. The benefit to cost ratio is calculated by dividing the risk weighted benefit of $3.34 billion by the cost of $720 million. This division yields a benefit to cost ratio of 4.6, which is an excellent ratio which indicates the project should proceed.

These two examples are admittedly simplistic, but we really need to use the methods illustrated to make smart decisions.  Let’s use the change in the risk weighted loss (pre to post project) to derive the project benefit value. Let’s compare that to the project cost.  Let’s calculate the project benefit to cost ratio over an appropriate time frame. Let’s do the projects with benefit to cost ratios of more than 1.0.

In June 1940 the United States Army Corps of Engineers released its Flood Protection Plan for Houston (Buffalo Bayou). I was able to find the original “Definite Project Report” and the associated drawings at the Rice University library.  This post provides some highlights from the documents.

I’ve posted the plan overview before, but its worth reposting here for context.  The green items are levees which form flood damage reduction reservoirs; the blue items are canals or channels; and the yellow text provides identification labels.

The Design Storm

The Corps reviewed a number of different “storms of record” and decided to use a design storm created by combining two of the worst-case scenarios they had seen up to that point.  They derived the design storm by taking the total depths of rain observed from June 27 to July 1, 1899 at Hearne, Texas and combining it with the rainfall intensities (inches per hour) observed in September 9-10, 1921 at Taylor, Texas.  They then placed this hypothetical worst-case storm directly over the planned reservoirs and facilities.

The Hearne storm produced 31.4 inches of rain in 3 days and the average depth of rain over 1,000 square miles was 25.8 inches.  The Taylor storm produced intensities ranging from 4.4 to 0.44 inch per hour.

The combination of these two events is depicted in the graphic below, which shows the hypothetical cumulative rainfall in inches vs. time for the design storm.

The design storm has the total rainfall amount (the horizontal end of the curve in the upper right) of the Hearne event coupled with the crazy steep slope, indicating high rainfall intensity (from 36 to 42 hours) of the Taylor event.  Yikes.

The Design Storm Runoff

To size reservoirs and channels the Corps needed to convert the rainfall depth to actual runoff rates and volumes. This is critical to determining how much of the rain falling from the sky actually runs off and either flows harmlessly around homes and businesses or inundates them.

The Corps conservatively assumed that 90% of the rainfall from the design storm described above would runoff.  To estimate the runoff flowing to each element of the overall plan, the design storm was then moved to be directly above each element.  Runoff timing, total volume, and flow rates (depicted in curves called hydrographs) were estimated using Franklin P. Snyder’s Synthetic Unit Graphs, published in the Transactions of the American Geophysical Union (1938).

Runoff hydrographs for each of the key definite plan elements are presented below.  Note that “second-feet” is equivalent to the flowrate expressed in “cubic feet per second.”  There are about 7.48 gallons in each cubic foot, so you can multiply by 7.48 to get gallons per second and then by 60 to get gallons per minute.  Also, an “acre-foot” is a volume measurement. It is 1 acre of area covered by 1 foot of water or about 325,829 gallons.

Definite Plan Elements

The Corps devised the definite plan to accommodate the design storm and the predicted runoff described above.  The definite plan included the following elements:

• White Oak Reservoir
• Brickhouse Gully Bypass Channel
• North Canal
• Cypress Creek Levee
• Addicks Reservoir
• Barker Reservoir
• Rectification of Buffalo Bayou Above South Canal
• South Canal
• Improvement of Buffalo Bayou Through City

A few details of each of these plan elements are provided below.

White Oak Reservoir

• Peak Design Inflow:                30,800 cubic feet per second
• Total Design Inflow:                103,900 acre-feet
• Levee Length:                         4.7 miles
• Levee Max. Height:                 35 feet
• Levee Max. Elevation:            90 feet above mean sea level
• Storage:                                  24,400 acre-feet
• Max. Pool Elevation:               85 feet above mean sea level

Brickhouse Gully Bypass Channel

• Channel Length:                     1.9 miles
• Lining:                                     Grass / Rip Rap in Selected Locations
• Top Width:                               From 70 to 100 feet
• Bottom Width:                         6 feet
• Side Slopes:                            2 to 1
• Depth:                                      From 10 to 25 feet
• Conveyance:                           1,500 cubic feet per second

North Canal

• Channel Length:                     20 miles
• Lining:                                     Concrete Paving
• Top Width:                               From 140 to 200 feet
• Bottom Width:                         25 feet
• Side Slopes:                            2 to 1
• Depth:                                     From 28 to 38 feet
• Conveyance:                           22,000 cubic feet per second

Cypress Creek Levee

• Levee Length:                         14.9 miles
• Levee Max. Height:                 12 feet
• Levee Max. Elevation:             180 feet above mean sea level
• Top Width:                               10 feet
• Max. Bottom Width:                 82 feet
• Side Slopes:                            3 to 1

Addicks & Barker Reservoir System (Combined)

• Max. Combined Outflow:15,000 cubic feet per second (based on navigation in the Houston Ship Channel)
• Release Conduits: Four 8 feet diameter (Addicks); Two gated “spare” conduits of equal size (Addicks); Five 8 feet diameter (Barker); Two gated “spare” conduits of equal size (Barker)

 Addicks Reservoir

• Drainage Area:                         134 square miles
• Peak Design Inflow:                  50,500 cubic feet per second
• Levee Length:                          10.4 miles
• Levee Max. Height:                  43 feet
• Levee Max. Elevation:             115 feet above mean sea level
• Max. Pool Elevation:                108.3 feet above mean sea level
• Design Storage:                       134,000 acre-feet

Barker Reservoir

• Drainage Area:                        152.8 square miles
• Peak Design Inflow:                 40,300 cubic feet per second
• Levee Length:                          13.8 miles
• Levee Max. Height:                  37 feet
• Levee Max. Elevation:             109 feet above mean sea level
• Max. Pool Elevation:                101.7 feet above mean sea level
• Design Storage:                       135,800 acre-feet

Rectification of South Mayde and Buffalo Bayou Above South Canal

• Channel Length:                     7.2  miles
• Lining:                                     Grass / Rip Rap in Selected Locations
• Top Width:                              From 140 to 200 feet
• Bottom Width:                       15 feet
• Side Slopes:                            3 to 1
• Depth:                                     From 5 to 22 feet
• Conveyance:                           15,000 cubic feet per second

South Canal

• Channel Length:                      39.2 miles
• Lining:                                     Concrete Paving
• Top Width:                               From 120 to 270 feet
• Bottom Width:                          From 30 to 163 feet
• Side Slopes:                            2 to 1
• Depth:                                      From 16 to 42 feet
• Max. Conveyance:                  15,000 cubic feet per second (above Brays Bayou)
• Max. Conveyance:                  28,800 cubic feet per second (below Middle Bayou)

Total Cost

The total cost of all plan elements, plus utility work and other associated details was $13,243,700 (in 1940 dollars).

If you are interested in reading more from the 1940 Definite Plan, I’ve made a scan of it available here.

My crazy idea about land development projects being “hydrologically invisible” is featured on the Texas Living Waters Project blog. Shout out to Keiji Asakura and Abigail Phillips of Asakura Robinson for doing the rendering of my wacky idea.

Check out the post here: http://texaslivingwaters.org/natural-drainage-low-impact-development-houston/

[Reposting a worthwhile white paper prepared by Wendell Cox and Tory Gattis, with the Center for Opportunity Urbanism, that provides some valuable context to support informed decision-making during our post-Harvey policy discussions.

My one quibble with this paper relates to their lack of precision in describing how current detention requirements work. They state that our detention regulations “require no net increase in runoff from new developments.” It would be more precise to say that Harris County detention regulations generally restrict the flow rate of runoff leaving the site to the pre-development flow rate, but they don’t generally restrict the total amount (volume) of runoff leaving the site. The City of Houston’s detention regulations are not based on runoff volume or rates, but rather on the site’s imperviousness.]

In the aftermath of Hurricane Harvey, and the disastrous flooding, Houston has come under extreme scrutiny. Some in the global, national as well as local media assaulted the area’s flood control system and its development model, criticisms that were echoed by some in the local area.

Much of the current debate starts from a firm misunderstanding of the region’s realities. This could lead to policies that ultimately undermine the keys that have propelled the region’s success. Below is a primer to inform future discussions of Houston’s future trajectory.

Did Harvey reflect Houston’s failure or a remarkable resiliency?

Harvey was a remarkable event for which there is little precedent. The Harris County Flood Control District estimates the four-day rainfall from Hurricane Harvey to be a once in 500 to 40,000 year flooding event. Whether such events are more likely in the future, the region’s systems worked remarkably well, although they should be bolstered considerably in the future.

Read more…

I was poking around the Atlas 14 website and found a quarterly report from July 2017. Atlas 14 is the national effort to update rainfall statistics based on the very best rain gage data with good geographic coverage and the longest history (time period) available.  The work for most of the country has been completed, except for Texas and some states in the Pacific Northwest. Two key items from the report are discussed in this short post.

First, the results for Texas will be published in May 2018. As I posted earlier, this will likely revise upwards our estimate of the 1% annual chance, 24-hour event, from around 12 or 13 inches to perhaps 14 to 16 inches (depending upon what area of Harris County you are interested in).  This will require bayou modeling updates and updates to the regulatory floodplains – they will get wider and deeper.

Second, the progress report includes a section entitled: “Analysis of Impacts of Non-Stationary Climate on Precipitation Frequency Estimates.” This can be roughly translated to mean: “How might climate change alter rainfall statistics in the future?”

The Atlas 14 effort is a backward-looking effort. The study team is applying standard statistical tools to the available rainfall records to predict the likelihood of rain events of various depths in the future. This assumes that history is good predictor of the future.

The “non-stationary climate” analysis is starting to look at how a changing climate might alter things. The quarterly progress report indicates that the Federal Highway Administration tasked the study team to conduct a pilot project to look at how a changing climate might alter the future. The report indicates that “preliminary findings were inconclusive and the pilot project ended with more questions than answers.”

The project team promises to work with academia (Penn State and University of Illinois, Urbana-Champaign) and to report on further work in the next quarterly report.

So stay tuned.

I’ve written many times about the depth of the current, statistically-derived, 1% annual chance, 24-hour storm in the Houston area. Its currently defined by the Harris County Flood Control District (HCFCD) as ranging from 12.4 to 13.5 inches in 24 hours, depending on the location. As you move closer to the coast (towards the southeast) the depth of the 1% event increases. This variation is addressed in HCFCD’s Hydrology and Hydraulics Guidance Manual by dividing the county into three rainfall regions as shown below:

The National Oceanic Atmospheric Administration (NOAA) is updating the nation’s storm statistics to include a longer period of rainfall records.  This effort is known as “Atlas 14.” The work is being accomplished on a state by state or region by region basis. The Texas analysis is currently underway, however, the work won’t wrap up for a year or two.

To get an idea of what the updated 1% event might be for Houston I browsed to the NOAA Atlas 14 website. This website has a map that let’s you pick any point in the United States. The website then provides the storm event statistics for that point based on the most recent data available. Since the update for Texas is not yet complete, you’ll get a message below the location picker map pointing you to the three older documents that provide information on Texas. (For the Houston area, folks should rely on the HCFCD Hydrology and Hydraulics Guidance Manual, which is more detailed and up to date.)

I picked a location along the Sabine River on the border of Texas and Louisiana, about 50 miles in from the coast (about equivalent to the distance from the Galveston coast to Memorial Park) to get an idea of what the Houston area Atlas 14 rainfall amounts might be when the NOAA study is finished.

After clicking on the map near the right-hand star above I was presented with the following table:

The Atlas 14 table for Starks, Louisiana shows that the 1% annual chance, 24-hour rainfall (equivalent to the 100 year recurrence interval in the table) is 14.3 inches, with a 90% confidence interval ranging from 10.9 inches to 19.0 inches.  Here’s what that looks like in graphical format.

The rainfall information from this point in Louisiana should be a pretty good predictor of what the rainfall statistics will show for the Houston area.  Based on this, I’d expect the 1% annual chance, 24-hour event for Houston will increase from 12.4 to 13.5 inches to something closer to 14 to 16 inches in a 24-hour period.

If the Atlas 14 analysis was updated to include rain events from 2016 and 2017 these 1% annual chance depths might increase a bit more.

One important outcome of this pending change: HCFCD will need to re-run our bayou models to determine the water surface elevation that results from the new, larger, 1% annual chance event and update all the regulatory floodplain maps to match this new elevation. This will likely enlarge our regulatory floodplains and increase the number of structures in the floodplain.

On September 26, 2017 the Harris County Commissioners Court and the Board of Directors of the Harris County Flood Control District will conduct a special meeting at 9 am to review mid-year progress.  At 10 am they will be conducting their regular bi-monthly meeting.

The mid-year review includes a lot about Hurricane Harvey and possible responses. The report highlights significant damage to Harris County facilities, including the Criminal Justice Center and Jury Assembly facility. There is some discussion of how the county is tracking expenses and labor costs to help ensure reimbursement for those costs from the Federal Emergency Management Agency (FEMA).  The report indicates that recovery will require additional capital dollars to help make infrastructure and facility repairs. The report provides some insight regarding potential impact on county revenues from lower total property value appraisals. The report indicates that about 50,000 single family homes in the county were flooded. This reduced revenue will be absorbed with some departments reducing spending by 5%.

The mid-year report includes a discussion of some possible solutions to flooding events, including new reservoirs, changes or enhancements to waterways, changes in development rules, and buyouts. It calls for a Master Plan to be developed and local, state, and federal funding. No increase in the property tax rate for HCFCD is proposed.

Read the entire Harris County Mid-Year Review Report.

The regular meeting agenda has one noteworthy item; The County Engineer seeks authorization to hold a “workshop to discuss regulatory changes needed to the flood plain management and infrastructure regulations in response to Hurricane Harvey.”

By Brian Bledsoe September 13, 2017, Washington Post

[Re-posted from the Washington Post]

As the tragedies of Harvey and Irma continue to unfold, it’s disconcerting but hardly surprising to hear a chorus of voices from all walks of life — laypeople, politicians and technical experts — perpetuate our ignorance of extreme weather and flood events.

Harvey is extreme by any measure, but we don’t really know how rare Harvey is because we simply don’t have long enough records, nor enough knowledge of how storms will behave in the future to pin down a number. Undeterred, we extrapolate and get what Mark Twain called “wholesale returns of conjecture out of such a trifling investment of fact.”

Our inability to understand and effectively communicate flood hazards is especially well-established. For decades, researchers have asked: Do the public and policymakers understand the 100-year flood concept? Do we understand what flood plain maps are telling us (and not telling us)?

The answer is a resounding no. The 100-year flood, the flood that has a 1 percent chance of being exceeded in any given year, simply does not register in our consciousness. Just the sound of it — a 100-year flood, longer than a lifetime — lulls us into believing such events are rare and immutable acts of God. But to paraphrase the geographer Gilbert White: Floods are acts of God — flood losses are largely acts of people. Harvey is no exception.

Cities in the United States have tried to regulate development in “100-year floodplains” and provide maps of flood hazard zones to the public for several decades. Yet most people are still surprised, if not astonished, to learn that the 100-year flood at a given location has more than a 1 in 4 chance of occurring within the term of a 30-year mortgage. For most of us, this 26 percent chance our home will be flooded before we have a chance to pay it off is troubling if not unacceptable.

This 26 percent is the risk if one lives at the upper edge of the 100-year flood “fringe,” the regulated outer zone of a flood plain. Structures at lower elevations are at even greater risk. To reduce the likelihood of flooding to a more tolerable 1 in 10 chance over a 30-year mortgage, structures need to be above the height of the 285-year flood. That is, if floods behave in the future as they did in the past records used to estimate these probabilities.

Will flooding behave in the future as it has in the past? Most of us intuitively grasp that intense rainfall interacts with increases in impervious surfaces such as roads, sidewalks, parking lots and rooftops to amplify the volume and speed of storm runoff. We are less inclined to think about where the water goes from there — how flood mitigation measures like ponds and detention basins can become less effective over time or fail if not properly maintained, or how the ability of streams and rivers to carry runoff changes due to sediment movement and other natural processes. It’s not just the rain that can change. It’s the rain, the urban footprint and the drainage systems all changing together.

Engineers and hydrologists are trained to look to the past to understand the future. They analyze records of past events (where the information exists) to assign probabilities to future events. This approach will always be necessary and useful; however, it generally presumes that the future will continue to behave within the same range of variability observed in the past.

In the context of warmer seas, mounting urbanization and intensifying rainfall, solely relying on the past becomes questionable at best. Asserting that rain and floods will continue to behave as they have in the past is akin to building a taller water tower that increases the pressure on a town’s pipes but believing that the flows from showers and faucets will remain the same.

Flood hazards are moving targets that often reflect a mix of circumstances. A 50-year rainstorm may produce a greater than 100-year flood if it falls on soaked or burned ground, melts a snowpack or coincides with a coastal storm surge or dam failure. Yet even the most up-to-date insurance maps portray the edges of flood hazard zones as a bright line in the sand — you’re either in or out. This clarity is an illusion. Basic engineering analysis reveals that there is substantial uncertainty as to where the lines should be drawn. But flood plain maps still depict it as a precise line, regardless of whether the potential error is 10 feet or 1,000 feet.

What can we do? Let’s start talking about flood risks over time frames we truly care about — for example, over the term of a mortgage, a lifetime or other planning horizon of tangible concern.

When one realizes their home has a 1-in-4 chance of experiencing a 100-year flood in 30 years, or a 15 percent chance of a 500-year flood over an 80-year lifetime, then perceptions shift and denial becomes more conspicuous. We need to invest in better flood hazard maps and update them to transparently show the uncertainty in flood levels due to model inaccuracies and potential changes in weather, urbanization and drainage. Simply knowing whether the margin of error in mapped flood extents is a big or little number would help.

Ultimately, improved communication of natural hazards must translate to vision and action if we are to avoid repeating the mistakes of the past. To some extent, our language and misconceptions around floods have enabled a status quo in which policy decisions are driven by election cycles and business cycles rather than by analysis of the potential costs of underinvesting in flood resilience over the long run.

The future is unknowable, but investments in hybrid systems of traditional “gray” and natural “green” infrastructure that work together along with nonstructural measures such as insurance reform, zoning, buyout and relocation can improve outcomes across a wide range of future scenarios. Building such redundancy and resilience into flood mitigation projects should boost their benefit to cost ratios, not decrease them as is the case when we focus on a single, oversimplified version of the future.

Regardless of which solution one favors, they all hinge on better communication. Crises open windows of opportunity for doing things better — windows that all too often close before we get unstuck from the status quo. Let’s hope Harvey and Irma will inspire many communities to rethink how they talk about floods, and to begin preparing in earnest for when their window opens.

The author, Brian Bledsoe, is a professor of civil and environmental engineering at the University of Georgia. His research focuses on the interface of hydrology, ecology and urban water sustainability.