4/2/2026 – Across the U.S., fragmented governance increases flood risk by creating a patchwork quilt of local interests that makes regulation, compliance and enforcement difficult.
One might think that our multi-level system of government – federal, state, county, city, improvement districts, etc. – creates defense in depth. In reality, each governmental entity plays by its own rules and is responsible to different groups of voters with different priorities and interests.
The problem is especially visible in large metro regions that involve numerous cities and counties with upstream/downstream conflicts of interest. Watersheds fragment into jurisdictional silos.
People downstream may experience flooding issues decades before upstream residents. And those upstream residents have no incentive to increase their taxes to pay for downstream mitigation.
Fragmented governance, therefore pits groups against each other as development spreads outward.
It also creates an accountability gap – “Not our job!”
And regulatory inconsistency (different rules for detention, rainfall and fill) make it almost impossible to measure the cumulative impact of hydrological changes throughout the watershed.
In any given river basin, we may all be part of the solution. But we don’t all feel the problem. At least, not yet.
Denver’s South Platte Basin
Fragmented governance in the Denver metro—especially in the South Platte River basin—has been a persistent, structural driver of flood risk. A patchwork of cities, counties, special districts, and state/federal agencies manage the basin. The result: coordination gaps that translate directly into flood risk.
Special districts like the Mile High Flood District (MHFD)
MHFD develops regional drainage criteria.However, it does not regulate land use; local governments do. As a result:
Different jurisdictions adopt different stormwater standards, detention requirements, and update cycles
Upstream communities can:
Allow higher impervious cover
Use less conservative rainfall assumptions
Provide insufficient detention
Flows arriving at Denver often stack on top of each other and higher in volume
Cumulative peak flows exceed what any one jurisdiction modeled
No entity is responsible for system-wide flood control.
Need for Basin-Wide Master Planning
According to the EPA, each jurisdiction optimizes locally, not basin-wide. Upstream cities capture the tax base from new development and export their runoff downstream. Meanwhile, downstream, Denver bears the flood risk and mitigation costs.
Denver has acknowledged the need for basin-wide master planning. But historically, many tributaries fell outside FEMA mapping and were handled locally. Imagine every city along a freeway each designing the freeway to meet its own needs. That’s the system in flood control.
Storm sewers, channels, and detention systems do not align across boundaries and have mismatched capacities.
Encroaching development has already narrowed the South Platte floodplain. But no single entity has tracked the loss of floodplain storage.
Furthermore, no one jurisdiction coordinates infrastructure development of bridges and highways where they intersect drainage. Undersized crossings create system-wide backwater effects.
In addition, the South Platte is heavily regulated by upstream reservoirs, which the Army Corps operates. But urban stormwater systems are operated locally and not fully synchronized. Sound familiar?
In Chicago, suburban expansion has increased runoff, but regional drainage capacity has not kept pace.
The Chicago area has even more extreme jurisdictional fragmentation than Denver. Cook County and three surrounding counties have more than 200 municipalities, plus a variety of state and federal agencies all sharing responsibilities for parts of the drainage.
Historically, each county has had different design storms, release rates, and detention requirements. Previously developed areas have had trouble keeping pace with upstream expansion.
The Chicago Area Waterway System also has three main rivers: the Chicago, Des Plaines and Calumet rivers. No single authority controls basin-wide runoff timing, impervious cover, or development in flood-prone areas.
But after several early successes, the program seems to have quietly dropped out of the headlines.
Likewise, the San Jacinto Region 6 Flood Planning Group proposed minimum floodplain management practices throughout the river basin in 2025. But it is strictly an advisory group.
Texas 2036 and the American Flood Coalition hosted an informative seminar on 2/17/26. It emphasized lessons learned from other states about the need for river-basin-wide flood control to help ensure flood resilience.
Everyone seems to recognize the need. But no one seems to have the power to address fragmented governance.
In Texas, we even have a state agency charged with flood mitigation investing in the development of property in floodplains and floodways…and denying FOIA requests to keep the investment secret. That’s how entrenched the problem of fragmented governance has become.
To see how fragmented governance compounds other factors that contribute to flooding, see the Lessons Page of this website.
Posted by Bob Rehak on 4/2/2026
3138 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.
4/1/26 – In my quest to summarize the most important “lessons learned” since Hurricane Harvey, here’s Lesson #3: Floodplain Encroachment. Floodplain encroachment is consistently rated one of the most important drivers of flooding worldwide. Think about it. If people didn’t build in floodplains, no one would flood. But that’s only part of the story. Floodplain development also changes flood assumptions for communities downstream.
Why People Build in Floodplains
Despite the risks, people worldwide build in floodplains. The land usually costs less. And it can yield extraordinary profits to developers lucky or persistent enough to obtain building permits.
After all, people pay premiums to live near water. Water views are prestigious, beautiful and soothing. Plus, historically, living near water translated to “security.” Water sustains life. The need for water is hard-wired into our DNA, our culture, and even our economy.
Once a thriving community. Destruction on Bolivar Peninsula in 2008. Storm surge 15 feet high during Hurricane Ike washed away homes and ripped storm sewers right out of the ground.
Yet during Hurricane Ike in 2008, storm surge reached 30 miles inland in places. And despite being leveled, within 10 years, homes on the vulnerable Bolivar Peninsula had built back.
While I have focused primarily on the Houston Area, floodplain encroachment is a global problem. Nearly all growing metro areas encroach on floodplains – coastal or riverine.
Loss of Natural Storage Can Increase Downstream Flood Elevations
The problem isn’t just “putting people in harm’s way.” It’s also about the loss of natural floodplain storage. In riverine systems, floodplains function as temporary storage reservoirs during overbank flows.
Insufficiently mitigated development can remove that storage volume or prevent it from being accessed.
Apartments and commercial development along Houston’s Brays Bayou
That’s why after Harvey, Houston and Harris County changed floodplain regulations so developers couldn’t bring fill into floodplains. Fill displaces water. Nature compensates for the fill by increasing water surface elevations elsewhere.
Floodplain development can also reduce the duration of floodplain storage, resulting in faster, higher peaks downstream.
See examples below.
Mississippi Floodplain Development
Historically, the Mississippi River occupied a broad alluvial valley tens of miles across in places. It inundated seasonally . Floodplains functioned as massive, temporary storage reservoirs.
Encroachment occurred primarily through federal levee construction under the U.S. Army Corps of Engineers and urban/industrial development that occurred later behind the levees.
As a result, floodplain width narrowed dramatically. The levees confined the river’s flow into a narrower channel where floods moved higher and faster, often breaking through levees with devastating consequences.
Levee failure also played a major role in the inundation of New Orleans during Hurricane Katrina in 2005.
Floodplain encroachment can turn into a vicious feedback loop. Levees reduce frequent flooding. That attracts more investment and development behind the levees. But as the consequences of levee failure increase, there’s more pressure to build higher, stronger levees.
West Fork San Jacinto/Spring Creek Confluence
Closer to home for most of my readers, developers have recently been trying to figure out ways to develop 5,300 acres between the West Fork San Jacinto River and Spring Creek. Virtually the entire area is in floodplain or floodway. Though current flood maps don’t fully reflect the danger, FEMA’s new draft flood maps for the area show part of the property. See below.
Dark blue/gray = floodway. Dark green = 100-year floodplain. Light green = 500-year floodplain.From HCFCD MAAPnext.
A leading hydrologist in the area told me that developing this area would be like “aiming a fire hose at the Humble/Kingwood Area.”
It’s not clear yet what the developer has planned for the site. Both the Texas General Land Office (GLO) and the Texas Attorney General have denied FOIA requests for the plans.
An “externality problem” occurs when the production or consumption of a good, such as housing, imposes unintended costs or benefits on third parties not involved in the transaction. In economic terms, this leads to market inefficiencies. It is a form of market failure. Private costs/benefits differ from social costs/benefits.
For instance, sand mines help produce a raw material needed for concrete. It generates profit for producers. But in their zeal to maximize their profit, they mine too close to rivers and in a manner that exacerbates erosion and sedimentation.
It’s the same way with flooding. Developers profit from building in floodplains. And the vast majority try to do it safely.
Regardless, in 2023, the Joint Economic Committee of Congress estimates that each year flooding costs Americans between $179.8 and $496.0 billion. The total depends on the types of damage included, i.e., structural, lost economic output, infrastructure repairs, insurance losses, decreased tax revenues, transit, deaths, etc.
Assuming the higher flood-damage estimate of nearly half of a trillion dollars, that represents 7% of last year’s entire federal budget. I’d sure like a 7% tax cut. April 15th is two weeks away!
For More Information
For more “lessons learned” about flooding since Harvey, see this website’s Lessons Page.
Posted by Bob Rehak on 4/1/2026
3137 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/11/Hurricane-Ike-363.jpg?fit=1500%2C993&ssl=19931500adminadmin2026-04-01 19:10:362026-04-02 08:47:26Floodplain Encroachment: Another Consistent Driver of Flooding Worldwide
3/31/26 – Since Hurricane Harvey, one of the dominant themes of more than 3,000 posts on this website has been outdated rainfall assumptions and how flood-infrastructure design lags those assumptions. That design lag contributes to flooding nationwide.
How Infrastructure Design Lags Rainfall Estimates
Almost nine years after Harvey, Harris County finally has new “draft” flood maps issued by FEMA based on Atlas-14 rainfall assumptions. But they won’t take effect for another two to three years. Meanwhile, parts of the region have built and will continue to build infrastructure, homes and businesses based on long-outdated rainfall predictions.
Different regulatory agencies and jurisdictions update at different speeds.
Imagine members of a choir each singing from different songbooks.
These rainfall predictions affect many aspects of flood control and building codes, including:
Stormwater detention basin sizing
Floodplain mapping
Subdivision drainage criteria, i.e., size of storm sewers
Elevation of homes and businesses above the 100-year floodplain and street level
Acceptable foundation types in flood- hazard areas
Building in floodways including width, depth, bracing and other requirements for piers
Where fill can and cannot be used
In 2009, Harris County revised its floodplain development standards. After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.
Subdivisions built with the updated standards experienced one twentieth the amount of damage.
EVA and the Difficulty of Developing Rainfall Estimates
So getting the rainfall right is important. But why do we get it wrong so often? Rainfall predictions stem from an obscure branch of mathematics, known as EVA (Extreme Value Analysis). With EVA, forecasters try to predict the probability of unobserved future events based on the frequency of somewhat smaller past events.
But there’s a problem. All predictions (500-year storms, etc.) are based on extremely small data sets. EVA may produce the best numbers possible, but predicting 500-years into the future based on 100 years of data takes a lot of guess-work. And we design entire cities based on these probabilities!
And that’s what I mean by the “design lag” problem based on outdated rainfall assumptions. Assumptions change with major new storms, such as Harvey. But we can’t just wave a major wand and change trillions of dollars of infrastructure already in the ground. We are systematically under-designed for current rainfall statistics.
This isn’t just a Houston problem. And it’s not just “in the ground” infrastructure. For example…
Mississippi Levees
Engineers typically design levees to contain a 100-year (1% annual chance) storm. Even small increases in in rainfall when levees constrain a river can produce huge increases in water surface elevation and push crests above levees. We’ve seen this happen recently in 2008 and 2019.
Design lag manifested itself as systemic underestimation of river stage, leading to overtopping of levee defenses.
Street Flooding in New York City
In 2023, extreme rainfall (up to 2.5 inches per hour) snarled the New York metropolitan area. It knocked out subways, commuter rail lines, flooded basements and closed a terminal at LaGuardia. It was the City’s wettest day since 1960 and came just two years after Hurricane Ida killed at least 13 people.
Engineers from previous generations designed storm sewers for much lower peak intensities. Usually, they design urban systems around peak intensities for short time periods (minutes to a few hours). But pre-Atlas 14 standards underestimated those short duration bursts.
Storm sewers were simply designed for much lower peak intensities. Systems failed almost instantly. Subway entrances became inlets for street flooding.
The Lesson Learned
Flooding results not just from rainfall. It results from under-designed infrastructure for rain that falls. For example:
Homes not high enough.
Channels not wide enough.
Storm sewers not big enough.
As a result, NOAA hopes to update rainfall frequency estimates much more frequently in the future. NOAA developed Atlas 14 region-by-region from approximately 2000–2018.
Texas lagged other regions by years. Before Atlas 14, the prior standard (TP-40) dated to 1961. And during that time, Houston grew exponentially. So most of our infrastructure is built to older standards.
NOAA is trying to eliminate the “outdated by the time it’s adopted” issue. But:
Local adoption will still likely lag availability
Regulatory inertia will remain the bottleneck.
See Lessons Page For More Information
For more information about other causes of flooding, see the Lessons page of this website.
Posted by Bob Rehak on 3/31/26
3136 Days since Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2024/05/NOAA-Atlas-14-Rainfall-Rates-for-Kingwood.png?fit=976%2C828&ssl=1828976adminadmin2026-03-31 17:07:382026-03-31 17:28:03Outdated Rainfall Assumptions: A Systemic Design-Lag Problem
How Fragmented Governance Boosts Flood Risk
4/2/2026 – Across the U.S., fragmented governance increases flood risk by creating a patchwork quilt of local interests that makes regulation, compliance and enforcement difficult.
One might think that our multi-level system of government – federal, state, county, city, improvement districts, etc. – creates defense in depth. In reality, each governmental entity plays by its own rules and is responsible to different groups of voters with different priorities and interests.
The problem is especially visible in large metro regions that involve numerous cities and counties with upstream/downstream conflicts of interest. Watersheds fragment into jurisdictional silos.
People downstream may experience flooding issues decades before upstream residents. And those upstream residents have no incentive to increase their taxes to pay for downstream mitigation.
Fragmented governance, therefore pits groups against each other as development spreads outward.
It also creates an accountability gap – “Not our job!”
And regulatory inconsistency (different rules for detention, rainfall and fill) make it almost impossible to measure the cumulative impact of hydrological changes throughout the watershed.
In any given river basin, we may all be part of the solution. But we don’t all feel the problem. At least, not yet.
Denver’s South Platte Basin
Fragmented governance in the Denver metro—especially in the South Platte River basin—has been a persistent, structural driver of flood risk. A patchwork of cities, counties, special districts, and state/federal agencies manage the basin. The result: coordination gaps that translate directly into flood risk.
Alliance for the Great Lakes produced a fascinating case study about Denver. The South Platte basin includes:
MHFD develops regional drainage criteria. However, it does not regulate land use; local governments do. As a result:
Need for Basin-Wide Master Planning
According to the EPA, each jurisdiction optimizes locally, not basin-wide. Upstream cities capture the tax base from new development and export their runoff downstream. Meanwhile, downstream, Denver bears the flood risk and mitigation costs.
Denver has acknowledged the need for basin-wide master planning. But historically, many tributaries fell outside FEMA mapping and were handled locally. Imagine every city along a freeway each designing the freeway to meet its own needs. That’s the system in flood control.
Storm sewers, channels, and detention systems do not align across boundaries and have mismatched capacities.
Encroaching development has already narrowed the South Platte floodplain. But no single entity has tracked the loss of floodplain storage.
Furthermore, no one jurisdiction coordinates infrastructure development of bridges and highways where they intersect drainage. Undersized crossings create system-wide backwater effects.
In addition, the South Platte is heavily regulated by upstream reservoirs, which the Army Corps operates. But urban stormwater systems are operated locally and not fully synchronized. Sound familiar?
If that’s not bad enough, funding is also fragmented, leading to competition for funds. So projects advance unevenly across the basin, according to the EPA.
Chicago’s 200+ Drainage Jurisdictions
In Chicago, suburban expansion has increased runoff, but regional drainage capacity has not kept pace.
The Chicago area has even more extreme jurisdictional fragmentation than Denver. Cook County and three surrounding counties have more than 200 municipalities, plus a variety of state and federal agencies all sharing responsibilities for parts of the drainage.
Historically, each county has had different design storms, release rates, and detention requirements. Previously developed areas have had trouble keeping pace with upstream expansion.
The Chicago Area Waterway System also has three main rivers: the Chicago, Des Plaines and Calumet rivers. No single authority controls basin-wide runoff timing, impervious cover, or development in flood-prone areas.
Fragmented Governance Defies Attempts to Unify It
In the Houston area, efforts to overcome similar problems have met with mixed success.
In 2022, Harris County Commissioners Court reaffirmed the need for minimum drainage standards in the region. The program was started in 2020 by the Harris County Engineering Department. The idea: to get all municipalities and other counties that drain into Harris County to adopt the same minimum drainage standards.
But after several early successes, the program seems to have quietly dropped out of the headlines.
Likewise, the San Jacinto Region 6 Flood Planning Group proposed minimum floodplain management practices throughout the river basin in 2025. But it is strictly an advisory group.
State Representative Dennis Paul introduced bills in the last two legislatures that would have established a river-basin wide flood control district. But each time, the bills have failed to gain traction and died in committees.
Texas 2036 and the American Flood Coalition hosted an informative seminar on 2/17/26. It emphasized lessons learned from other states about the need for river-basin-wide flood control to help ensure flood resilience.
Everyone seems to recognize the need. But no one seems to have the power to address fragmented governance.
In Texas, we even have a state agency charged with flood mitigation investing in the development of property in floodplains and floodways…and denying FOIA requests to keep the investment secret. That’s how entrenched the problem of fragmented governance has become.
To see how fragmented governance compounds other factors that contribute to flooding, see the Lessons Page of this website.
Posted by Bob Rehak on 4/2/2026
3138 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.
Floodplain Encroachment: Another Consistent Driver of Flooding Worldwide
4/1/26 – In my quest to summarize the most important “lessons learned” since Hurricane Harvey, here’s Lesson #3: Floodplain Encroachment. Floodplain encroachment is consistently rated one of the most important drivers of flooding worldwide. Think about it. If people didn’t build in floodplains, no one would flood. But that’s only part of the story. Floodplain development also changes flood assumptions for communities downstream.
Why People Build in Floodplains
Despite the risks, people worldwide build in floodplains. The land usually costs less. And it can yield extraordinary profits to developers lucky or persistent enough to obtain building permits.
After all, people pay premiums to live near water. Water views are prestigious, beautiful and soothing. Plus, historically, living near water translated to “security.” Water sustains life. The need for water is hard-wired into our DNA, our culture, and even our economy.
One in five Texans lives in a floodplain. And the World Meteorological Society estimates that 40% of the world’s population lives within 100 kilometers (62 miles) of a coastline.
Yet during Hurricane Ike in 2008, storm surge reached 30 miles inland in places. And despite being leveled, within 10 years, homes on the vulnerable Bolivar Peninsula had built back.
While I have focused primarily on the Houston Area, floodplain encroachment is a global problem. Nearly all growing metro areas encroach on floodplains – coastal or riverine.
Loss of Natural Storage Can Increase Downstream Flood Elevations
The problem isn’t just “putting people in harm’s way.” It’s also about the loss of natural floodplain storage. In riverine systems, floodplains function as temporary storage reservoirs during overbank flows.
Insufficiently mitigated development can remove that storage volume or prevent it from being accessed.
That’s why after Harvey, Houston and Harris County changed floodplain regulations so developers couldn’t bring fill into floodplains. Fill displaces water. Nature compensates for the fill by increasing water surface elevations elsewhere.
Floodplain development can also reduce the duration of floodplain storage, resulting in faster, higher peaks downstream.
See examples below.
Mississippi Floodplain Development
Historically, the Mississippi River occupied a broad alluvial valley tens of miles across in places. It inundated seasonally . Floodplains functioned as massive, temporary storage reservoirs.
Encroachment occurred primarily through federal levee construction under the U.S. Army Corps of Engineers and urban/industrial development that occurred later behind the levees.
As a result, floodplain width narrowed dramatically. The levees confined the river’s flow into a narrower channel where floods moved higher and faster, often breaking through levees with devastating consequences.
Levee failure also played a major role in the inundation of New Orleans during Hurricane Katrina in 2005.
Floodplain encroachment can turn into a vicious feedback loop. Levees reduce frequent flooding. That attracts more investment and development behind the levees. But as the consequences of levee failure increase, there’s more pressure to build higher, stronger levees.
West Fork San Jacinto/Spring Creek Confluence
Closer to home for most of my readers, developers have recently been trying to figure out ways to develop 5,300 acres between the West Fork San Jacinto River and Spring Creek. Virtually the entire area is in floodplain or floodway. Though current flood maps don’t fully reflect the danger, FEMA’s new draft flood maps for the area show part of the property. See below.
A leading hydrologist in the area told me that developing this area would be like “aiming a fire hose at the Humble/Kingwood Area.”
It’s not clear yet what the developer has planned for the site. Both the Texas General Land Office (GLO) and the Texas Attorney General have denied FOIA requests for the plans.
According to State Rep. Steve Toth, the GLO invested $140 million in the development of the property. Ironically, the GLO also administers billions of flood-mitigation dollars in Texas. That creates not only a conflict of interest but a classic externality problem.
What is an Externality Problem?
An “externality problem” occurs when the production or consumption of a good, such as housing, imposes unintended costs or benefits on third parties not involved in the transaction. In economic terms, this leads to market inefficiencies. It is a form of market failure. Private costs/benefits differ from social costs/benefits.
For instance, sand mines help produce a raw material needed for concrete. It generates profit for producers. But in their zeal to maximize their profit, they mine too close to rivers and in a manner that exacerbates erosion and sedimentation.
Remediating that excess sedimentation has cost taxpayers more than $200 million for dredging. Miners have literally externalized their cleanup and safety costs.
It’s the same way with flooding. Developers profit from building in floodplains. And the vast majority try to do it safely.
Regardless, in 2023, the Joint Economic Committee of Congress estimates that each year flooding costs Americans between $179.8 and $496.0 billion. The total depends on the types of damage included, i.e., structural, lost economic output, infrastructure repairs, insurance losses, decreased tax revenues, transit, deaths, etc.
Assuming the higher flood-damage estimate of nearly half of a trillion dollars, that represents 7% of last year’s entire federal budget. I’d sure like a 7% tax cut. April 15th is two weeks away!
For More Information
For more “lessons learned” about flooding since Harvey, see this website’s Lessons Page.
Posted by Bob Rehak on 4/1/2026
3137 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.
Outdated Rainfall Assumptions: A Systemic Design-Lag Problem
3/31/26 – Since Hurricane Harvey, one of the dominant themes of more than 3,000 posts on this website has been outdated rainfall assumptions and how flood-infrastructure design lags those assumptions. That design lag contributes to flooding nationwide.
How Infrastructure Design Lags Rainfall Estimates
Almost nine years after Harvey, Harris County finally has new “draft” flood maps issued by FEMA based on Atlas-14 rainfall assumptions. But they won’t take effect for another two to three years. Meanwhile, parts of the region have built and will continue to build infrastructure, homes and businesses based on long-outdated rainfall predictions.
Before Atlas-14-based flood maps become effective in Harris County, FEMA will issue Atlas 15 predictions throughout the U.S. in 2026.
Imagine members of a choir each singing from different songbooks.
These rainfall predictions affect many aspects of flood control and building codes, including:
In 2009, Harris County revised its floodplain development standards. After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.
EVA and the Difficulty of Developing Rainfall Estimates
So getting the rainfall right is important. But why do we get it wrong so often? Rainfall predictions stem from an obscure branch of mathematics, known as EVA (Extreme Value Analysis). With EVA, forecasters try to predict the probability of unobserved future events based on the frequency of somewhat smaller past events.
But there’s a problem. All predictions (500-year storms, etc.) are based on extremely small data sets. EVA may produce the best numbers possible, but predicting 500-years into the future based on 100 years of data takes a lot of guess-work. And we design entire cities based on these probabilities!
And that’s what I mean by the “design lag” problem based on outdated rainfall assumptions. Assumptions change with major new storms, such as Harvey. But we can’t just wave a major wand and change trillions of dollars of infrastructure already in the ground. We are systematically under-designed for current rainfall statistics.
This isn’t just a Houston problem. And it’s not just “in the ground” infrastructure. For example…
Mississippi Levees
Engineers typically design levees to contain a 100-year (1% annual chance) storm. Even small increases in in rainfall when levees constrain a river can produce huge increases in water surface elevation and push crests above levees. We’ve seen this happen recently in 2008 and 2019.
Design lag manifested itself as systemic underestimation of river stage, leading to overtopping of levee defenses.
Street Flooding in New York City
In 2023, extreme rainfall (up to 2.5 inches per hour) snarled the New York metropolitan area. It knocked out subways, commuter rail lines, flooded basements and closed a terminal at LaGuardia. It was the City’s wettest day since 1960 and came just two years after Hurricane Ida killed at least 13 people.
Engineers from previous generations designed storm sewers for much lower peak intensities. Usually, they design urban systems around peak intensities for short time periods (minutes to a few hours). But pre-Atlas 14 standards underestimated those short duration bursts.
Storm sewers were simply designed for much lower peak intensities. Systems failed almost instantly. Subway entrances became inlets for street flooding.
The Lesson Learned
Flooding results not just from rainfall. It results from under-designed infrastructure for rain that falls. For example:
As a result, NOAA hopes to update rainfall frequency estimates much more frequently in the future. NOAA developed Atlas 14 region-by-region from approximately 2000–2018.
Texas lagged other regions by years. Before Atlas 14, the prior standard (TP-40) dated to 1961. And during that time, Houston grew exponentially. So most of our infrastructure is built to older standards.
NOAA’s new Atlas 15 is designed to be the nation’s first unified data set. And it is built from the ground up to incorporate climate trends and be updatable as new data becomes available.
NOAA is trying to eliminate the “outdated by the time it’s adopted” issue. But:
See Lessons Page For More Information
For more information about other causes of flooding, see the Lessons page of this website.
Posted by Bob Rehak on 3/31/26
3136 Days since Harvey