3/31/26 – Since Hurricane Harvey, one of the dominant themes of more than 3,000 posts on this website has been outdated rainfall assumptions and how flood-infrastructure design lags those assumptions.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Meanwhile, parts of the region have built and will continue to build infrastructure, homes and businesses based on long-outdated rainfall predictions.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And that’s what I mean by the “design lag” problem based on outdated rainfall assumptions.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
3/30/31 – According to numerous engineering studies, the single most consistent, anthropogenic (human-created) driver of flooding worldwide is expansion of impervious cover related to urbanization.
How Impervious Cover Contributes to Flooding
Any area growing in population adds roads and rooftops. When stormwater falls on them, it runs off quickly, unlike when forests or grass covered the land. Engineers talk of friction coefficients. Less friction with concrete increases the speed of runoff.
But it’s not just the speed, it’s also the volume of runoff. Engineers also measure the permeability of different land surfaces and soil types. Is the surface clay or sand, for instance. One holds water; the other lets it sink in.
Regardless, replacing either with concrete and shingles increases the speed and volume of runoff. Think of water running off a roof and down a gutter during a torrential rain.
Quantitative example calculated by ChatGPT.
Note how volume peaks higher and faster after development.
Pre-development: CN 68, Time of concentration 2.5 hr
Post-development: CN 90, Time of concentration 0.8 hr
Results:
Pre-development peak flow: about 81 cfs
Post-development peak flow: about 402 cfs
Pre-development time to peak: about 1.75 hr
Post-development time to peak: about 0.73 hr
Runoff volume: increases from about 21.6 ac-ft to 44.8 ac-ft
This example is not site specific. It is an example for illustrative purposes only. While the numbers would change depending on soil types, slope and native ground cover, something similar happens everywhere urban growth occurs. You see:
Increased runoff volume
Faster time of concentration (peaking)
Higher peak discharges.
You see similar, though not identical, responses worldwide. For example…
Atlanta: Faster, Higher Peaks after Urbanization
A widely cited Georgia State University study of eight metro-Atlanta streams correlated population increases with flooding frequency and severity from 1986 to 2010. During that time, developed land and high-flow days (flood-like conditions) roughly doubled.
Urbanization led to a 26 percent increase in annual stream flow. The increase was not rainfall driven; the study controlled for precipitation.
It reflected: increased impervious cover, reduced infiltration, and faster concentration in channels.
“This means that during a storm event, you’ll now see more runoff, more extreme flows and more flooding than you would have seen for a similar storm event in 1986,” said Jeremy Diem, the study’s lead author and associate professor in the Department of Geosciences at Georgia State.
Another study, “The Influence of Urban Development Patterns on Streamflow Characteristics in the Charlanta Megaregion,” found similar results. “The statistical analysis revealed that increasing the extent of urban development enhanced high and low flow frequency as well as annual peak unit discharge,” said the authors. “Impervious surfaces in source areas distant from streams increased the frequency of high flows.”
“Flashiness” also increased. USGS found that urbanization increased flood magnitude most strongly for moderate storms (e.g., 2–10 year events). The difference narrowed for very large storms because everything becomes saturated and behaves as impervious cover anyway.
The American Meteorological Society found that flood severity was driven as much by runoff efficiency as rainfall magnitude. In other words, in urban areas, rainfalls that aren’t historically extreme can produce exceptional stream rises and flooding.
Dallas and the Trinity River
Studies by the U.S. Geological Survey and U.S. Army Corps of Engineers documented similar problems in Dallas. As the city expanded after World War II, an explosion of impervious cover dramatically increased the speed of runoff. Storm sewers carried water to the river much faster so the city saw steeper and earlier flood peaks.
Peak flows from tributaries stacked on top of each other rather than arriving at staggered intervals. This created higher peaks on the Trinity and more frequent “bank full” conditions. It also put greater stress on levees.
The levees enabled economic development in floodplains, but narrowed channel width, increasing water surface elevation and speed.
As a result, the system became efficient at passing moderate floods—but more vulnerable to extreme ones.
Conclusion
In city after city, hydrologists find that the growth of impervious cover creates more intense, faster, and higher peak flooding. Soil differences affect infiltration and runoff rates. But it is not uncommon to find pre-/post-development differences of approximately 2X.
The two pictures below taken within a few miles of each other in the north Houston Area speak volumes.
Part of the 5,300 acres owned by Scarborough west of Kingwoodin Montgomery County is currently being studied for development.The Preserve at Woodridge was carved out of similar forest. The developer’s plans show it was supposed to be 65% impervious cover.
All those dots in the pavement above are storm drains that act as superhighways for rainfall. They channel it straight to the nearest stream.
According to a recent New York Times article, nine of the 20 counties in the U.S. that have experienced the most development the last decade are in Texas.
Posted by Bob Rehak on 3/30/2026
3135 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2026/03/DJI_20241025151910_0036_D.jpg?fit=1100%2C619&ssl=16191100adminadmin2026-03-30 17:00:172026-03-30 17:20:19Growth of Impervious Cover: Most Consistent Driver of Flooding Worldwide
3/28/26 – At the San Jacinto River Authority (SJRA) Board Meeting on 3/26/26, HCFCD Executive Director Tina Petersen updated the board on a number of Lake Houston Area projects including the Kingwood Diversion Ditch. She confirmed it is fully funded – through construction.
However, design of the Diversion Ditch Project has not yet started. It should begin in April and finish by the end of 2027.
A year ago, the preliminary engineering study estimated the cost of the project at $40.7 million. Current estimates put the cost at $43 million, according to Petersen.
Looking N at the Kingwoodwood Diversion Ditch from over the Walnut Lane Bridge
Relationship to Bens Branch Flooding
The Diversion Ditch splits off of Bens Branch near St. Martha Catholic Church north of Northpark Drive.
Stormwater flow to Bens Branch will be restricted by pipes. That will force more stormwater into the expanded Diversion Ditch. In the process, that would take enough stormwater out of Bens Branch to improve it from a 2-year level of service to a 100-year level.
Red Diagonal = Bens Branch. White = Kingwood Diversion Ditch. Green = new outfall to river.
Diverting water from Bens Branch is important because Bens Branch runs through Kingwood Town Center where 12 people died from Harvey flooding.
Crenshaw Connection
Ironically, funding obtained by US Congressman Dan Crenshaw back in 2024 to widen the bridge shown above at Walnut Lane saved this project from the chopping block – even though it was ranked the most important project in Kingwood by the Kingwood Area Drainage Analysis.
At the time, Precinct 3 Commissioner Tom Ramsey PE warned that killing projects in Quartiles 2, 3 and 4 could have dire unintended consequences. The Diversion Ditch project fell into Quartile 3.
After the Democrats saw how much partnership funding they would lose by killing projects in the lower quartiles, they relented. In their next meeting, they voted to exempt projects in the lower quartiles that already had partnership funds committed.
That breathed new life into the Kingwood Diversion Ditch project because it included widening of the Walnut Lane Bridge which Crenshaw had already secured funding for.
HCFCD spokesperson Emily Woodell confirmed the Diversion Ditch funding today. “It was categorized as a partnership project during the bond update presented to commissioners court in August [2025] which means it is fully funded through construction.”
For Updates on Other San Jacinto Watershed Projects
It covers a lot of territory including the history of HCFCD, status of the bond program, partnership funding, maintenance programs, gauges, the flood-warning system, and more.
Other capital improvement projects in the Lake Houston Area that she discusses include:
Woodridge Village/Taylor Gully – Construction starting in April.
Jackson Bayou Detention Basin – Construction starting Q3/2026.
Barrett Station Drainage Improvements – Currently in Design Stage.
Lake Houston/East Fork/West Fork Dredging – Completed.
Lake Houston Gates – Engineering should finish by end of this year.
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
Growth of Impervious Cover: Most Consistent Driver of Flooding Worldwide
3/30/31 – According to numerous engineering studies, the single most consistent, anthropogenic (human-created) driver of flooding worldwide is expansion of impervious cover related to urbanization.
How Impervious Cover Contributes to Flooding
Any area growing in population adds roads and rooftops. When stormwater falls on them, it runs off quickly, unlike when forests or grass covered the land. Engineers talk of friction coefficients. Less friction with concrete increases the speed of runoff.
But it’s not just the speed, it’s also the volume of runoff. Engineers also measure the permeability of different land surfaces and soil types. Is the surface clay or sand, for instance. One holds water; the other lets it sink in.
Regardless, replacing either with concrete and shingles increases the speed and volume of runoff. Think of water running off a roof and down a gutter during a torrential rain.
The graph above assumed:
Results:
This example is not site specific. It is an example for illustrative purposes only. While the numbers would change depending on soil types, slope and native ground cover, something similar happens everywhere urban growth occurs. You see:
You see similar, though not identical, responses worldwide. For example…
Atlanta: Faster, Higher Peaks after Urbanization
A widely cited Georgia State University study of eight metro-Atlanta streams correlated population increases with flooding frequency and severity from 1986 to 2010. During that time, developed land and high-flow days (flood-like conditions) roughly doubled.
Urbanization led to a 26 percent increase in annual stream flow. The increase was not rainfall driven; the study controlled for precipitation.
It reflected: increased impervious cover, reduced infiltration, and faster concentration in channels.
“This means that during a storm event, you’ll now see more runoff, more extreme flows and more flooding than you would have seen for a similar storm event in 1986,” said Jeremy Diem, the study’s lead author and associate professor in the Department of Geosciences at Georgia State.
Another study, “The Influence of Urban Development Patterns on Streamflow Characteristics in the Charlanta Megaregion,” found similar results. “The statistical analysis revealed that increasing the extent of urban development enhanced high and low flow frequency as well as annual peak unit discharge,” said the authors. “Impervious surfaces in source areas distant from streams increased the frequency of high flows.”
“Flashiness” also increased. USGS found that urbanization increased flood magnitude most strongly for moderate storms (e.g., 2–10 year events). The difference narrowed for very large storms because everything becomes saturated and behaves as impervious cover anyway.
The American Meteorological Society found that flood severity was driven as much by runoff efficiency as rainfall magnitude. In other words, in urban areas, rainfalls that aren’t historically extreme can produce exceptional stream rises and flooding.
Dallas and the Trinity River
Studies by the U.S. Geological Survey and U.S. Army Corps of Engineers documented similar problems in Dallas. As the city expanded after World War II, an explosion of impervious cover dramatically increased the speed of runoff. Storm sewers carried water to the river much faster so the city saw steeper and earlier flood peaks.
Peak flows from tributaries stacked on top of each other rather than arriving at staggered intervals. This created higher peaks on the Trinity and more frequent “bank full” conditions. It also put greater stress on levees.
The levees enabled economic development in floodplains, but narrowed channel width, increasing water surface elevation and speed.
As a result, the system became efficient at passing moderate floods—but more vulnerable to extreme ones.
Conclusion
In city after city, hydrologists find that the growth of impervious cover creates more intense, faster, and higher peak flooding. Soil differences affect infiltration and runoff rates. But it is not uncommon to find pre-/post-development differences of approximately 2X.
The two pictures below taken within a few miles of each other in the north Houston Area speak volumes.
All those dots in the pavement above are storm drains that act as superhighways for rainfall. They channel it straight to the nearest stream.
According to a recent New York Times article, nine of the 20 counties in the U.S. that have experienced the most development the last decade are in Texas.
Posted by Bob Rehak on 3/30/2026
3135 Days since Hurricane Harvey
HCFCD Confirms Kingwood Diversion Ditch Project Fully Funded Through Construction
3/28/26 – At the San Jacinto River Authority (SJRA) Board Meeting on 3/26/26, HCFCD Executive Director Tina Petersen updated the board on a number of Lake Houston Area projects including the Kingwood Diversion Ditch. She confirmed it is fully funded – through construction.
However, design of the Diversion Ditch Project has not yet started. It should begin in April and finish by the end of 2027.
A year ago, the preliminary engineering study estimated the cost of the project at $40.7 million. Current estimates put the cost at $43 million, according to Petersen.
Features Included in PER Recommendation
The preliminary engineering report published last year recommended:
The bridges at Northpark Drive will also be rebuilt, but as part of the Northpark Expansion Project.
Relationship to Bens Branch Flooding
The Diversion Ditch splits off of Bens Branch near St. Martha Catholic Church north of Northpark Drive.
Stormwater flow to Bens Branch will be restricted by pipes. That will force more stormwater into the expanded Diversion Ditch. In the process, that would take enough stormwater out of Bens Branch to improve it from a 2-year level of service to a 100-year level.
Diverting water from Bens Branch is important because Bens Branch runs through Kingwood Town Center where 12 people died from Harvey flooding.
Crenshaw Connection
Ironically, funding obtained by US Congressman Dan Crenshaw back in 2024 to widen the bridge shown above at Walnut Lane saved this project from the chopping block – even though it was ranked the most important project in Kingwood by the Kingwood Area Drainage Analysis.
Crenshaw requested funding for the Walnut Lane Bridge in 2023. Congress awarded it in 2024. Then in 2025, the Democratic members of Harris County Commissioners Court passed a motion to reallocate all funding from projects that fell below the top quartile of their equity prioritization framework to projects in the top quartile. That was because inflation had eaten up 25-30% of the purchasing power in the 2018 Flood Bond.
Ramsey to the Rescue
At the time, Precinct 3 Commissioner Tom Ramsey PE warned that killing projects in Quartiles 2, 3 and 4 could have dire unintended consequences. The Diversion Ditch project fell into Quartile 3.
After the Democrats saw how much partnership funding they would lose by killing projects in the lower quartiles, they relented. In their next meeting, they voted to exempt projects in the lower quartiles that already had partnership funds committed.
That breathed new life into the Kingwood Diversion Ditch project because it included widening of the Walnut Lane Bridge which Crenshaw had already secured funding for.
HCFCD spokesperson Emily Woodell confirmed the Diversion Ditch funding today. “It was categorized as a partnership project during the bond update presented to commissioners court in August [2025] which means it is fully funded through construction.”
For Updates on Other San Jacinto Watershed Projects
See the video of the SJRA Meeting on 3/26/26 starting at about 1:05:18 into the meeting. Dr. Petersen’s presentation runs roughly 25 minutes to 1:30:00.
It covers a lot of territory including the history of HCFCD, status of the bond program, partnership funding, maintenance programs, gauges, the flood-warning system, and more.
Other capital improvement projects in the Lake Houston Area that she discusses include:
See Dr. Petersen’s entire presentation for more details.
Posted by Bob Rehak on 3/28/26
3133 Days since Hurricane Harvey