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.
3/27/26 – On 3/26/26, the San Jacinto River Authority (SJRA) board heard the results of a feasibility study about creating “dry-bottom dams” on Birch and Walnut Creeks. The creeks are far upstream in the Spring Creek watershed in Waller County and were being studied for potential flood-mitigation benefits.
The board made no decision in the meeting on whether to pursue recommendations from the study. However, they did agree to discuss several issues with study partners. Other sponsors included the City of Humble, Harris County Flood Control District and five municipal utility districts in Harris and Montgomery Counties.
Chief among the concerns discussed:
Whether funding is available given low Benefit-Cost Ratios (BCR)
Whether land is still available to build the projects
Finding a party that could take “ownership” the projects.
But before the presentation even started, Kaaren Cambio, a former director, laid down a fiery challenge to the board. Let’s look at the study first. It will provide a context for Cambio.
Benefit/Cost Ratio Concerns
Matt Barrett, PE, SJRA’s Water Resources and Flood Management Division Director, gave a presentation that summarized the results of the 661-page feasibility study.
The feasibility study came out of the larger San Jacinto Master Drainage Plan study which identified 16 projects costing more than $3 billion in a 3000 square mile area upstream from Lake Houston.
Birch Creek had an estimated BCR of .55 to .83. That means costs exceeded benefits by almost as much as 2 to 1.
Walnut Creek had an estimated BCR of .77 to 1.04. In the best case scenario, benefits barely exceeded costs.
See estimated BCRs in blue boxes. Date of BCR calculations is not listed, but they came from 2020 Master Drainage Plan, not Feasibility Study, according to Barrett.
Design and Operation
Barrett identified the construction as something akin to detention basins. The dams would feature a long barrier that trapped water with a small opening that let water out at a slow rate.
He next described how such construction would work in four different scenarios.
On a sunny day with no rain, water in the creeks would simply pass through them unobstructed.In a small storm, not likely to cause flooding, water would still pass through the opening unobstructed.But in a moderate storm that could cause minor flooding, water would pool behind the dam faster than it could go through.In a major storm, water would also pass over the spillway at the top of the dam.
Barrett then talked about the exact locations of the dams, their widths, and land-use conflicts. The latter include a solar farm and new developments in the project footprints.
Benefits of Project(s)
Next, Barrett addressed the flood reduction benefits of the two dams in 10-, 50-, 100-, and 500-year storms.
He discussed the benefits of both dams together and individually. Below are the combined benefits of both dams.
They produce benefits measured in feet upstream and inches downstream.
Upstream, near the dams, the benefits exceed 3 to 4 feet. But downstream, near the confluence with the West Fork at the US59 bridge, benefits would only be 3 to 4 inches. That’s because the dams have a large effect on the small watersheds they directly control. But they exert no influence over the rest of the San Jacinto River Basin draining to that point.
Location of Structures Benefitted
The study found that more structures in Montgomery County Precinct 3 would benefit than anywhere else – by a factor of almost 4X compared to other jurisdictions.
Social Benefits Needed to Justify Funding
Near the end, Barrett showed what happens to the BCRs if you include “social benefits,” such as time lost from work during and after a flood. When you factor those in, the benefits exceed costs. However, Barrett also pointed out that as of 2025, the federal government no longer allows social benefits in BCR calculations.
In my opinion, this makes federal support unlikely in the current environment. And the state is unlikely to be able to make up the difference. The cost of the dams comprises a huge percentage of the balance in the Texas Flood Infrastructure Fund.
Significantly, while Barrett addressed the BCR, he did not call out total current costs. He did mention 2020 costs of $200 million in his narration. But total current estimates from the study put the cost at $298 million. And even $298 million assumes property can be acquired at market rates.
Conclusions
Barrett’s concluding slide focused on the challenges ahead based on the findings of his feasibility study. He implies the projects are still worthwhile if you consider social benefits. However, he acknowledges several additional hurdles ahead. And they are high hurdles.
For instance:
Who will take ownership of this project when the dams are in Waller County? (Editorial comment: the largest beneficiary is Montgomery County Precinct 3)
Where will the money come from when social benefits no longer apply?
For a high-resolution PDF of Barrett’s complete slide deck, click here.
Cambio Comments
You may also want to watch Kaaren Cambio during the public comment period before Barrett took the floor. Cambio starts at 11:10 into the video. She is a former SJRA director appointed by Governor Abbott.
Cambio began by reminding the board that after Harvey, the governor charged the SJRA with developing short, medium, and long range plans to ensure another Harvey would never produce so much damage again.
Time it has taken the SJRA flood management division to produce studies
Management of the studies
Absence of any benefits produced to date in any of the SJRA studies since Harvey
Pursuit of the Spring Creek study even after it became clear the land was not available
Cost per structure pulled out of the floodplain in the Spring Creek study – more than $800,000 each
SJRA’s inability to examine less expensive options, such as buyouts or elevation of those structures
Cambio closed her remarks by urging the board to “Please go back and look at the goals that this division had and make sure you’re meeting those goals. And redirect your efforts, so that we are seeing manageable solutions.”
An outspoken leader, Cambio raised some great points.
You could sense the urgency in her voice as she pled with the board to implement solutions, not just studies.
Plea for Involvement
More people from downstream areas need to testify at SJRA board meetings. We should never let the SJRA board – now heavily dominated by Lake Conroe residents – forget the destruction of lives and property caused by the massive release from Lake Conroe during Hurricane Harvey.
Posted by Bob Rehak on 3/27/26
3132 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/2026/03/5.3_Spring-Creek-Rev2_Page_10.jpg?fit=2000%2C1125&ssl=111252000adminadmin2026-03-27 19:00:522026-03-29 09:07:18SJRA Board Takes No Action on Birch, Walnut Creek Dams Feasibility Study
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
SJRA Board Takes No Action on Birch, Walnut Creek Dams Feasibility Study
3/27/26 – On 3/26/26, the San Jacinto River Authority (SJRA) board heard the results of a feasibility study about creating “dry-bottom dams” on Birch and Walnut Creeks. The creeks are far upstream in the Spring Creek watershed in Waller County and were being studied for potential flood-mitigation benefits.
The board made no decision in the meeting on whether to pursue recommendations from the study. However, they did agree to discuss several issues with study partners. Other sponsors included the City of Humble, Harris County Flood Control District and five municipal utility districts in Harris and Montgomery Counties.
Chief among the concerns discussed:
But before the presentation even started, Kaaren Cambio, a former director, laid down a fiery challenge to the board. Let’s look at the study first. It will provide a context for Cambio.
Benefit/Cost Ratio Concerns
Matt Barrett, PE, SJRA’s Water Resources and Flood Management Division Director, gave a presentation that summarized the results of the 661-page feasibility study.
The feasibility study came out of the larger San Jacinto Master Drainage Plan study which identified 16 projects costing more than $3 billion in a 3000 square mile area upstream from Lake Houston.
Design and Operation
Barrett identified the construction as something akin to detention basins. The dams would feature a long barrier that trapped water with a small opening that let water out at a slow rate.
He next described how such construction would work in four different scenarios.
Barrett then talked about the exact locations of the dams, their widths, and land-use conflicts. The latter include a solar farm and new developments in the project footprints.
Benefits of Project(s)
Next, Barrett addressed the flood reduction benefits of the two dams in 10-, 50-, 100-, and 500-year storms.
He discussed the benefits of both dams together and individually. Below are the combined benefits of both dams.
Upstream, near the dams, the benefits exceed 3 to 4 feet. But downstream, near the confluence with the West Fork at the US59 bridge, benefits would only be 3 to 4 inches. That’s because the dams have a large effect on the small watersheds they directly control. But they exert no influence over the rest of the San Jacinto River Basin draining to that point.
Location of Structures Benefitted
The study found that more structures in Montgomery County Precinct 3 would benefit than anywhere else – by a factor of almost 4X compared to other jurisdictions.
Social Benefits Needed to Justify Funding
Near the end, Barrett showed what happens to the BCRs if you include “social benefits,” such as time lost from work during and after a flood. When you factor those in, the benefits exceed costs. However, Barrett also pointed out that as of 2025, the federal government no longer allows social benefits in BCR calculations.
In my opinion, this makes federal support unlikely in the current environment. And the state is unlikely to be able to make up the difference. The cost of the dams comprises a huge percentage of the balance in the Texas Flood Infrastructure Fund.
Significantly, while Barrett addressed the BCR, he did not call out total current costs. He did mention 2020 costs of $200 million in his narration. But total current estimates from the study put the cost at $298 million. And even $298 million assumes property can be acquired at market rates.
Conclusions
Barrett’s concluding slide focused on the challenges ahead based on the findings of his feasibility study. He implies the projects are still worthwhile if you consider social benefits. However, he acknowledges several additional hurdles ahead. And they are high hurdles.
For instance:
For More Information
See Barrett’s full 38-minute presentation (including the Q&A that followed) on the SJRA website. The video starts at 27:50 and runs to 1:05:18.
For a high-resolution PDF of Barrett’s complete slide deck, click here.
Cambio Comments
You may also want to watch Kaaren Cambio during the public comment period before Barrett took the floor. Cambio starts at 11:10 into the video. She is a former SJRA director appointed by Governor Abbott.
Cambio began by reminding the board that after Harvey, the governor charged the SJRA with developing short, medium, and long range plans to ensure another Harvey would never produce so much damage again.
She reminded them of the successful lake lowering plan and said “you have abandoned a proven solution with no substantive plan” because of a “lawsuit by a non-representative organization.” She lamented the:
Cambio closed her remarks by urging the board to “Please go back and look at the goals that this division had and make sure you’re meeting those goals. And redirect your efforts, so that we are seeing manageable solutions.”
An outspoken leader, Cambio raised some great points.
Plea for Involvement
More people from downstream areas need to testify at SJRA board meetings. We should never let the SJRA board – now heavily dominated by Lake Conroe residents – forget the destruction of lives and property caused by the massive release from Lake Conroe during Hurricane Harvey.
Posted by Bob Rehak on 3/27/26
3132 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.