4/5/26 – Today’s “lesson learned” from almost nine years of research into flooding is about the counterintuitive “Limitations of Multiple Small Detention Basins at Watershed Scale.”
Research shows that hundreds of small ponds built during subdivision construction do little to reduce flooding at the watershed scale and may increase it in places. A 2009 National Academies study found (Page 422) that “In many cases the site-by-site approach has exacerbated downstream flooding and channel erosion problems as a watershed is gradually built out.”
Regional stormwater detention basins are superior to multiple small, on-site basins because they offer better flood control, higher water-quality treatment, and increased cost efficiency.
Regional basins effectively manage large-scale runoff from multiple developments by providing comprehensive peak flow reduction. Simultaneously, they reduce the maintenance burdens and land-use inefficiencies of scattered, small, and often poorly maintained small ponds.
With the exception of Lake Conroe, the 2,500 square miles upstream from the Lake Houston Area in the upper San Jacinto River Basin has no other regional detention basins/reservoirs as of this writing. And Lake Conroe controls only 13% of the watershed flowing into Lake Houston.
How Detention Basins Work
The goal of detention basins in general is to ensure post-development runoff is less than or equal to pre-development rates. That’s important because increases in impervious cover during development increase the speed of runoff. So floods peak faster and higher.
Detention basins do nothing to reduce the total amount of runoff. They just spread it out over a longer time. And that spread increases the probability that peak flow from one tributary will stack on top of another peak somewhere downstream in the river systems branching structure.
Peak Stacking
While detention basins effectively reduce peak flows at individual sites, they don’t necessarily reduce peak flows everywhere because of this stacking effect.
A flood-frequency analysis of large European river basins found that “If a flood peak in the main river is superimposed by a simultaneous peak from a tributary, the magnitude of the flood peak may be increased significantly downstream.”
This graph illustrates the concept.
What happens downstream when peaks from different tributaries arrive simultaneously instead of separately.
The simultaneousarrival of peak flows from different tributaries can increase the height of a flood even if total volume remains unchanged.
Factors that Contribute to Peak Stacking in Lake Houston Area
Several factors present in the Lake Houston Area increase the probability of this “peak stacking.” They include:
Convergence of many major tributaries and sub-tributaries
Low gradients, flat terrain
Rapid upstream growth
Largely uncontrolled sedimentation reducing conveyance and creating backwater
Detention basins usually have no way to delay or accelerate the timing of releases. Stream levels control timing; when they get low enough, water can start trickling out of the basin. But that’s precisely what maintains peaks longer. And that longer peak increases the probability of peaks merging at confluences instead of arriving at different times.
Regional Detention Prospects Look Bleak for Lake Houston Area
While regional detention may be preferable and more effective than hundreds or even thousands of small detention basins, it is difficult to find space for regional basins – at least with a Benefit Cost Ratio (BCR) that justifies the project.
A recent feasibility study on regional detention upstream on Spring Creek offered little hope after developers snapped up the land before the study was completed. The federal government also excluded social benefits from BCR calculations during the study.
The same land could have been purchased decades ago for a fraction of the cost when it was good for nothing but timber. But the BCR would have been even lower because few people lived in the “benefit” area at that time. Damage to structures would have been minimal.
Ten of 16 projects recommended by SJRA’s Master River Basin Drainage Plan involved regional detention basins.
Similarly, HCFCD’s promising Little Cypress Creek Frontier Program, which would have created regional detention, has been cancelled. Harris County Commissioners reportedly felt reluctant to spend money where few people lived.
These examples highlight a systemic problem: Most effective regional detention basin projects with available land are far upstream in their respective watersheds where few people live. That means the people who benefit from them may live across jurisdictional boundaries, such as city or county lines.
For instance, the Spring Creek detention basins were in Waller County. But most of those who benefited from them lived in Montgomery County. That makes financing and managing them more difficult.
Conclusion
Regional detention basins are more efficient and effective than small local detention basins. But until people of the river basin recognize the benefits of working together on flood mitigation, we must live with distributed detention and suffer the consequences.
Posted by Bob Rehak on 4/5/2026
3141 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/2026/04/flood_peaks_comparison.png?fit=1979%2C1177&ssl=111771979adminadmin2026-04-05 23:24:242026-04-05 23:50:27Limitations of Multiple Small Detention Basins at Watershed Scale
4/4/26 – States, counties and communities across the U.S. prioritize flood mitigation over flood prevention, despite FEMA studies that have found prevention costs up to 5-6X less than correction. What types of costs?
Examples of Mitigation Costs
Examples of mitigation costs include:
Post-flood buyouts: Government often buys and demolishes homes after repeated flooding.
Levees/dams/detention basins/channel improvements: Expensive to build and maintain — and they can fail.
Flood insurance subsidies: Taxpayers often foot the bill via programs like the U.S. National Flood Insurance Program (NFIP), which is deeply in debt.
Examples of Prevention Costs/Strategies
Examples of much more cost-effective Prevention Strategies include:
Zoning restrictions to keep development out of high-risk zones.
Green infrastructure like wetlands that absorb floodwaters.
Elevated buildings or flood-resistant designs where development is unavoidable.
Parks,buying out land, and conservation easements before development occurs.
Kingwood’s East End Park provides habitat and recreation while improving the value of neighboring homes and distancing them from flooding.
While development in floodplains may seem cheaper at first, the long-term economic, environmental, and social costs almost always outweigh the initial savings.
National Subsidies Distort Local Priorities
So, why do the inverted priorities persist? The developer reaps the profit, but taxpayers bear the costs. Economists call it an “externality problem” 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 this case, the availability of cheap, nationally subsidized flood insurance distorted the market for floodplain properties by insulating buyers and lenders from the true costs of flooding.
And when flooding did happen, FEMA and HUD were there to help bail out local communities with hundreds of billions of dollars of flood mitigation grants.
As a result…
The U.S. chronically underinvests in mitigation and over-relies on post-disaster funding.
We see this economic and policy pattern across the U.S. and locally.
Scarborough Example
For instance, in the Lake Houston Area, residents are fighting a 5,300+ acre development upstream from the I-69 bridge where the San Jacinto West Fork, Spring Creek, Cypress Creek and Turkey Creek all converge. It is one of the most flood-prone parcels in south Texas and large parts of it have just been reclassified as “floodway.”
Unbelievably, the Texas General Land Office (GLO) is helping bankroll the development. The GLO is also responsible for distributing billions of dollars of federal flood-mitigation aid in Texas. (Somebody needs to write President Trump!)
For More Information
To learn more about the cost of prevention versus correction, see:
For more on other causes of flooding, see the Lessons page of ReduceFlooding.
Posted by Bob Rehak on 4/4/26
3140 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/2018/11/SJR_704_048-e1612057029212.jpg?fit=1200%2C811&ssl=18111200adminadmin2026-04-04 17:34:192026-04-04 17:41:52How U.S. Prioritizes Flood Mitigation Over Flood Prevention
Infrastructure bottlenecks reveal themselves during extreme events. This is a universal phenomenon. Bridges, culverts, and channel constrictions control flood elevations behind them.
You see this everywhere around the world. It’s a well studied phenomenon. Let’s look at two areas.
Openings then catch debris floating downstream, further reducing the conveyance of the river. Forested basins supply large wood during storms. Inlet plugging is common. Debris blockage can reduce culvert capacity by 30–80%.
This is especially acute in Appalachia because of steep basins susceptible to flash flooding and the fact that roads usually follow streams. They can have many crossings per mile.
Older culverts and bridges may have been designed in previous eras when engineers anticipated smaller storms. You see scour at outlets where water churns, trying to get under the bridge. You also see jetting on the downstream side. Both lead to channel instability.
In June 2016, West Virginia experienced one of the worst floods in its history. After the event, U.S. Geological Survey, Federal Highway Administration and West Virginia Department of Transportation documented:
Hundreds of culvert and small bridge failures
Widespread road overtopping and washouts
Extensive debris blockage at inlets
Excessive erosion and sedimentation downstream.
NOAA photo from 2016 West Virginia Flood
Houston-Area Examples
In the Houston area, we see the same hydraulic constraint issues.
FM1010 culverts could not handle the drainage from new upstream development larger than Manhattan.
I-69 Bridge Over West Fork
TxDot had to replace part of the I-69 bridge over the San Jacinto West Fork while residents endure massive traffic jams because of scouring under the supports for the southbound lanes.
Replacement took almost a year.
UPRR Bridge over West Fork
Union Pacific’s Bridge over the San Jacinto West Fork was destroyed by Harvey. Trees washing downstream formed a dam that raised the flood level.
Tree Lane Bridge
Multiple floods backed water up behind the Tree Lane Bridge over Bens Branch in Kingwood. They raised water levels upstream as jetting scoured bridge supports on the downstream side. The bridge has been repaired several times since Harvey.
Tree Lane bridge after Imelda in 2019.
Given upstream development, the width was insufficient. Engineers ultimately had to widen the opening of the bridge to let high peak flows pass through.
Rustic Elms Bridge
Up to 600 homes flooded twice in Kingwood’s Elm Grove Village in 2019. The bridge below was one of the problems. The culverts convey less water than the open span design of the West Lake Houston Parkway Bridge over Taylor Gully in the background.
The bridge will soon be replaced as part of a HUD/GLO/HCFCD project.Rustling Elms Bridge underwater as school bus tries to cross it in May 2019.
West Fork Mouth Bar
Eroded sediment washed downstream during Harvey and dropped out of suspension where water slowed as it met Lake Houston. Thousands of homes and businesses flooded behind this sediment dam, which reached more than 10 feet about normal water level.
West Fork mouth bar after Harvey and before dredging. Note the openings under the FM1960 bridge in the background.
FM 1960 Bridge
Post-Harvey analyses revealed a significant constriction in the San Jacinto near the headwaters of Lake Houston. The openings in the FM 1960 causeway across the lake are half the width of upstream and downstream bridges.
John Blount, Harris County Engineer at the time of Hurricane Harvey said he noticed a difference in the water surface elevations on the upstream and downstream sides of the bridge. Downstream was lower by 1-2 feet.
During Harvey, one hundred and ten homes in Kings Forest flooded behind these culverts half clogged with sediment. The City of Houston cleaned them out in 2025.
HCFCD 2019 photo of blocked culverts under Kingwood Drive
Lake Houston Dam
The single biggest blockage in the Lake Houston Area is the Lake Houston Dam. While the dam has a spillway to handle high-water events, it does not have gates with a sufficient release capacity to lower water levels immediately before storms. The City of Houston is designing additional gates that will allow greater coordination with pre-releases from Lake Conroe. And the San Jacinto River Authority is studying ways to coordinate pre-releases.
Earthen portion of Lake Houston Dam where new gates will be added.
I-45 Bridge at Cypress Creek
Harvey’s floodwaters in Cypress Creek were so strong that they literally picked up parts of the southbound I-45 lanes and shifted the bridge. The constriction caused by the bridge backed water up into hundreds of homes.
I-45 southbound feeder over Cypress Creek showing cattywumpus bridge panels
I could go on. But you get the idea. Bridges and culverts restrict flow in large events, creating dangerous backwater. Those bridges and culverts may have been adequate early in their lives, but upstream growth rendered them inadequate.
For more information about other factors that contribute to flooding, see the Lessons page of this website.
Posted by Bob Rehak on 4/3/26
3139 Days since 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/2018/05/I69Damage-e1775243525276.jpg?fit=1100%2C733&ssl=17331100adminadmin2026-04-03 14:15:142026-04-03 14:42:25How Infrastructure Bottlenecks Constrain Peak Flows, Cause Local Flooding
Limitations of Multiple Small Detention Basins at Watershed Scale
4/5/26 – Today’s “lesson learned” from almost nine years of research into flooding is about the counterintuitive “Limitations of Multiple Small Detention Basins at Watershed Scale.”
Research shows that hundreds of small ponds built during subdivision construction do little to reduce flooding at the watershed scale and may increase it in places. A 2009 National Academies study found (Page 422) that “In many cases the site-by-site approach has exacerbated downstream flooding and channel erosion problems as a watershed is gradually built out.”
Regional stormwater detention basins are superior to multiple small, on-site basins because they offer better flood control, higher water-quality treatment, and increased cost efficiency.
Regional basins effectively manage large-scale runoff from multiple developments by providing comprehensive peak flow reduction. Simultaneously, they reduce the maintenance burdens and land-use inefficiencies of scattered, small, and often poorly maintained small ponds.
With the exception of Lake Conroe, the 2,500 square miles upstream from the Lake Houston Area in the upper San Jacinto River Basin has no other regional detention basins/reservoirs as of this writing. And Lake Conroe controls only 13% of the watershed flowing into Lake Houston.
How Detention Basins Work
The goal of detention basins in general is to ensure post-development runoff is less than or equal to pre-development rates. That’s important because increases in impervious cover during development increase the speed of runoff. So floods peak faster and higher.
Detention basins do nothing to reduce the total amount of runoff. They just spread it out over a longer time. And that spread increases the probability that peak flow from one tributary will stack on top of another peak somewhere downstream in the river systems branching structure.
Peak Stacking
While detention basins effectively reduce peak flows at individual sites, they don’t necessarily reduce peak flows everywhere because of this stacking effect.
A flood-frequency analysis of large European river basins found that “If a flood peak in the main river is superimposed by a simultaneous peak from a tributary, the magnitude of the flood peak may be increased significantly downstream.”
This graph illustrates the concept.
The simultaneous arrival of peak flows from different tributaries can increase the height of a flood even if total volume remains unchanged.
Factors that Contribute to Peak Stacking in Lake Houston Area
Several factors present in the Lake Houston Area increase the probability of this “peak stacking.” They include:
Detention basins usually have no way to delay or accelerate the timing of releases. Stream levels control timing; when they get low enough, water can start trickling out of the basin. But that’s precisely what maintains peaks longer. And that longer peak increases the probability of peaks merging at confluences instead of arriving at different times.
Regional Detention Prospects Look Bleak for Lake Houston Area
While regional detention may be preferable and more effective than hundreds or even thousands of small detention basins, it is difficult to find space for regional basins – at least with a Benefit Cost Ratio (BCR) that justifies the project.
A recent feasibility study on regional detention upstream on Spring Creek offered little hope after developers snapped up the land before the study was completed. The federal government also excluded social benefits from BCR calculations during the study.
The same land could have been purchased decades ago for a fraction of the cost when it was good for nothing but timber. But the BCR would have been even lower because few people lived in the “benefit” area at that time. Damage to structures would have been minimal.
The San Jacinto River Authority Master River Basin Plan recommended ten similar detention projects in 2020. But six years later, not one is funded.
Similarly, HCFCD’s promising Little Cypress Creek Frontier Program, which would have created regional detention, has been cancelled. Harris County Commissioners reportedly felt reluctant to spend money where few people lived.
These examples highlight a systemic problem: Most effective regional detention basin projects with available land are far upstream in their respective watersheds where few people live. That means the people who benefit from them may live across jurisdictional boundaries, such as city or county lines.
For instance, the Spring Creek detention basins were in Waller County. But most of those who benefited from them lived in Montgomery County. That makes financing and managing them more difficult.
Conclusion
Regional detention basins are more efficient and effective than small local detention basins. But until people of the river basin recognize the benefits of working together on flood mitigation, we must live with distributed detention and suffer the consequences.
Posted by Bob Rehak on 4/5/2026
3141 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.
How U.S. Prioritizes Flood Mitigation Over Flood Prevention
4/4/26 – States, counties and communities across the U.S. prioritize flood mitigation over flood prevention, despite FEMA studies that have found prevention costs up to 5-6X less than correction. What types of costs?
Examples of Mitigation Costs
Examples of mitigation costs include:
Examples of Prevention Costs/Strategies
Examples of much more cost-effective Prevention Strategies include:
While development in floodplains may seem cheaper at first, the long-term economic, environmental, and social costs almost always outweigh the initial savings.
National Subsidies Distort Local Priorities
So, why do the inverted priorities persist? The developer reaps the profit, but taxpayers bear the costs. Economists call it an “externality problem” 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 this case, the availability of cheap, nationally subsidized flood insurance distorted the market for floodplain properties by insulating buyers and lenders from the true costs of flooding.
And when flooding did happen, FEMA and HUD were there to help bail out local communities with hundreds of billions of dollars of flood mitigation grants.
As a result…
We see this economic and policy pattern across the U.S. and locally.
Scarborough Example
For instance, in the Lake Houston Area, residents are fighting a 5,300+ acre development upstream from the I-69 bridge where the San Jacinto West Fork, Spring Creek, Cypress Creek and Turkey Creek all converge. It is one of the most flood-prone parcels in south Texas and large parts of it have just been reclassified as “floodway.”
Unbelievably, the Texas General Land Office (GLO) is helping bankroll the development. The GLO is also responsible for distributing billions of dollars of federal flood-mitigation aid in Texas. (Somebody needs to write President Trump!)
For More Information
To learn more about the cost of prevention versus correction, see:
For more on other causes of flooding, see the Lessons page of ReduceFlooding.
Posted by Bob Rehak on 4/4/26
3140 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.
How Infrastructure Bottlenecks Constrain Peak Flows, Cause Local Flooding
4/3/26 – Engineers design infrastructure to handle certain amounts of rainfall. But:
Infrastructure bottlenecks reveal themselves during extreme events. This is a universal phenomenon. Bridges, culverts, and channel constrictions control flood elevations behind them.
You see this everywhere around the world. It’s a well studied phenomenon. Let’s look at two areas.
Appalachia
Appalachia has many undersized road crossings. Culverts and bridges have a finite conveyance capacity. Exceeding that capacity creates a partial dam that causes backwater to rise upstream and overtop roadways.
Openings then catch debris floating downstream, further reducing the conveyance of the river. Forested basins supply large wood during storms. Inlet plugging is common. Debris blockage can reduce culvert capacity by 30–80%.
This is especially acute in Appalachia because of steep basins susceptible to flash flooding and the fact that roads usually follow streams. They can have many crossings per mile.
Older culverts and bridges may have been designed in previous eras when engineers anticipated smaller storms. You see scour at outlets where water churns, trying to get under the bridge. You also see jetting on the downstream side. Both lead to channel instability.
In June 2016, West Virginia experienced one of the worst floods in its history. After the event, U.S. Geological Survey, Federal Highway Administration and West Virginia Department of Transportation documented:
Houston-Area Examples
In the Houston area, we see the same hydraulic constraint issues.
FM1010 Washout
Rapid development of Colony Ridge led to a washout of FM1010 during Harvey.
I-69 Bridge Over West Fork
TxDot had to replace part of the I-69 bridge over the San Jacinto West Fork while residents endure massive traffic jams because of scouring under the supports for the southbound lanes.
UPRR Bridge over West Fork
Tree Lane Bridge
Multiple floods backed water up behind the Tree Lane Bridge over Bens Branch in Kingwood. They raised water levels upstream as jetting scoured bridge supports on the downstream side. The bridge has been repaired several times since Harvey.
Given upstream development, the width was insufficient. Engineers ultimately had to widen the opening of the bridge to let high peak flows pass through.
Rustic Elms Bridge
Up to 600 homes flooded twice in Kingwood’s Elm Grove Village in 2019. The bridge below was one of the problems. The culverts convey less water than the open span design of the West Lake Houston Parkway Bridge over Taylor Gully in the background.
West Fork Mouth Bar
Eroded sediment washed downstream during Harvey and dropped out of suspension where water slowed as it met Lake Houston. Thousands of homes and businesses flooded behind this sediment dam, which reached more than 10 feet about normal water level.
FM 1960 Bridge
Post-Harvey analyses revealed a significant constriction in the San Jacinto near the headwaters of Lake Houston. The openings in the FM 1960 causeway across the lake are half the width of upstream and downstream bridges.
John Blount, Harris County Engineer at the time of Hurricane Harvey said he noticed a difference in the water surface elevations on the upstream and downstream sides of the bridge. Downstream was lower by 1-2 feet.
Graph from Post-Harvey Analysis by Charles Jones.
Culverts under Kingwood Drive
During Harvey, one hundred and ten homes in Kings Forest flooded behind these culverts half clogged with sediment. The City of Houston cleaned them out in 2025.
Lake Houston Dam
The single biggest blockage in the Lake Houston Area is the Lake Houston Dam. While the dam has a spillway to handle high-water events, it does not have gates with a sufficient release capacity to lower water levels immediately before storms. The City of Houston is designing additional gates that will allow greater coordination with pre-releases from Lake Conroe. And the San Jacinto River Authority is studying ways to coordinate pre-releases.
I-45 Bridge at Cypress Creek
Harvey’s floodwaters in Cypress Creek were so strong that they literally picked up parts of the southbound I-45 lanes and shifted the bridge. The constriction caused by the bridge backed water up into hundreds of homes.
I could go on. But you get the idea. Bridges and culverts restrict flow in large events, creating dangerous backwater. Those bridges and culverts may have been adequate early in their lives, but upstream growth rendered them inadequate.
For more information about other factors that contribute to flooding, see the Lessons page of this website.
Posted by Bob Rehak on 4/3/26
3139 Days since 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.