How Flat Terrain In Southeast Texas Increases Flood Risk

Flat terrain is one of the most under-appreciated aspects of flood risk. It complicates virtually every aspect of flood control.

Flat terrain increases flood risk primarily by reducing the speed of runoff. It causes water to pool rather than drain away. Also, lack of gradient lets floodwaters spread wider, increases sedimentation, backs water up into storm sewers, and creates unpredictable backwater effects.

Spreading Wider

In steep, mountainous areas, a one-foot rise in a river can be relatively contained; it won’t spread out much. But in flat areas, water spreads out easily. Even a slight rise in water level can submerge numerous properties. And that water may not drain away quickly. As water fills channels, it backs up water into storm sewers and may cause street flooding.

Increases in rainfall can also shift floodplain boundaries significantly, as we saw recently with the introduction of new draft flood maps based on Atlas 14. Homes once considered outside of floodplains are now within them.

Increasing Sedimentation

Flat terrain also makes rivers move slowly. This enables suspended sediment to settle and reduces a river’s capacity to convey stormwater, leading to more frequent flooding.

It can also lead to the creation of sand bars, especially where rivers meet standing bodies of water, such as Lake Houston. During Harvey, we saw “mouth bars” grow thousands of feet on the East and West Forks above Lake Houston.

west fork mouth bar
San Jacinto West Fork Mouth Bar after Hurricane Harvey

Mouth bars are sand bars found at the mouths of rivers. Such blockages create partial sediment dams that back water up and promote even more deposition upstream. For instance, see below.

Farther upstream from the West Fork mouth bar, additional freshly deposited sand during Harvey reduced West Fork conveyance by 90% at this location, according to the Army Corps.

The Army Corps has since dredged the West Fork. However, while dredging can temporarily deepen a channel, it does nothing to increase the slope (gradient). Without a steeper slope, the river remains slow, and new sediment quickly refills the dredged areas.

That is why the Army Corps recommended a maintenance dredging program and why State Rep. Charles Cunningham’s Lake Houston Dredging District is so important.

Unpredictable “Stacking” Effects

In steep river systems, flood waves pass quickly. But in Southeast Texas, flood waves move slowly down rivers. Peaks linger and lengthen. This increases the probability that the peaks will synchronize, i.e., stack on top of each other.

That’s especially true in large storms, such as hurricanes, that may stall over an area for days and dump rain uniformly across the region. Stormwater peaks from different tributaries then stack on top of each other as they moves downstream.

New Sand Mining BMPs needed to offset sediment pollution.
The highest flooding in Harris County during Harvey occurred at the confluence of Spring Creek (L) and West Fork (R). Looking NW from over I-69 Bridge.

Other Backwater Effects

The low gradient of streams in southeast Texas makes them extremely sensitive to a variety of backwater effects.

In flat river basins like the San Jacinto, Trinity and Brazos, mild slopes amplify “backwater effects.” Examples include

  • Tributary confluences:
  • Reservoir backwater:
    • Lake Houston backs up the lower San Jacinto West and East Forks plus a number of creeks and bayous for miles upstream.
    • As peaks arriving from different streams arrive, they stack on top of each other.
    • During floods, even modest lake-level rises can push water far upstream into Montgomery County
  • Tidal influence:
    • Near-coast systems can experience tidal backwater
    • Storm surge from the Gulf can reach miles inland, blocking inland rainfall from draining to the Gulf.
  • Infrastructure bottlenecks:
    • Bridges, culverts, sand bars, levees, dikes, and new developments
    • Can constrict conveyance and back water up
Illegal fill
Same confluence as shown above but from a different angle shows how floodplain fill is constricting floodplain storage and the floodway near a critical choke point, the I-69 Bridge

Implications for Policy Making

In low gradient systems, many streams become hydraulically coupled during floods. So, basin-wide coordination is essential.

That’s why the SJRA’s Joint Reservoir Operations Study is so crucial and why fragmented governance increases flood risk. We need river-basin-wide flood control.

We must also be more sensitive to:

west fork sand mine
West Fork sand mine frequently inundated by floods illustrates need for vegetative controls to reduce erosion.

Summary

To summarize, in flat coastal plains with low-gradients:

  • Low slope = low velocity = poor drainage
  • Water spreads laterally instead of moving quickly downstream, flooding many structures
  • Stormwater from different tributaries has a higher chance of stacking up
  • Sediment accumulates faster
  • Infrastructure bottlenecks can have large spatial impacts.

Posted by Bob Rehak on 4/7/26

3143 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.

Reservoir Coordination Still Elusive After 53 Years

4/6/26 – One of the biggest “lessons learned” from Harvey is that coordinated Joint Reservoir Operations are crucial. The San Jacinto River Basin has two reservoirs controlled by dams. But coordinating their operation to reduce flooding remains elusive after 53 years.

The San Jacinto River Authority (SJRA) finished the Lake Conroe dam in 1973, but is still seeking public input on its Joint Reservoir Operations Study. They hope to have a first draft of the study by the end of 2026.

Benefits of Reservoir Coordination

Other authorities around the world have long recognized the benefits of coordinating the operations of multiple dams on their rivers. Benefits include:

  • Enhanced flood control and mitigation – By acting in tandem, dams can reduce flood peaks more efficiently than isolated dams.
  • Improved water security and drought resilience – Coordinated dam systems can manage water storage across a basin to alleviate water stress during dry seasons.
  • Reduced sediment transport –  Tandem operation can reduce peak flows that cause heavy erosion, clogging rivers and downstream lakes.
  • Increased hydropower generation – Although not a factor in the San Jacinto Basin, coordinated operations allow water to be used multiple times as it passes through a series of dams, exponentially increasing total energy output from the same water resource.
  • Environmental sustainability – Strategic releases of water can sustain downstream ecosystems, habitats, and species, as seen in the U.S. Sustainable Rivers Program.
  • Improved navigation and trade – A system of coordinated locks and dams can regulate river flow consistently, facilitating the transport of goods via barges and promoting regional economic development.
  • Water security – Upstream dams can supplement the water supply in downstream dams that may support major metropolitan areas. Lake Conroe, for instance, provides backup to the smaller Lake Houston, which is the primary water supply for more than 2 million people.

River Authorities that Manage Multiple Dams for Flood Control

Examples of coordinated management abound. Take for instance:

  • In Texas, the Lower Colorado River Authority (LCRA) provides a textbook example of coordinated dam management for flood control through the Highland Lakes system. The LCRA manages a “staircase” of six dams northwest of Austin. All six assist with flood mitigation. They operate as an integrated unit to protect downstream communities.
  • Tennessee Valley Authority manages a network of 9 main-river dams and 22 tributary dams. The system is designed to catch heavy runoff in tributary reservoirs before it reaches the main river, significantly reducing flood risks for downstream cities like Chattanooga. The TVA operates these dams as a single unit. That way, they also ensure a consistent water depth of at least 11 feet along the entire 652-mile main channel. That lets 28,000 barges transport goods annually.
  • The Columbia River System (CRS) consists of 14 federal dam projects managed as a coordinated system for power, flood control, and fish protection.
  • California Department of Water Resources found that “weather-informed reservoir operations” at Lake Oroville and New Bullards Bar Reservoir can further reduce flood risk for communities along the Yuba and Feather rivers during extreme atmospheric river storm events and potentially benefit water supply during drier periods.
  • In the Delaware River Basin, a “flexible flow management program” mitigates flooding impacts immediately downstream of reservoirs.

Two Key Houston-Area Reservoirs Have Different Missions, Management

So, why can’t the SJRA manage two dams?

For one thing, SJRA only controls Lake Conroe. The Coastal Water Authority controls Lake Houston.

For another, the two dams have slightly different goals and radically different construction.

  • Lake Conroe was conceived as a water supply and flood control reservoir (even though SJRA now claims Lake Conroe is strictly for water supply). Lake Conroe’s tainter gates can release 150,000 CFS.
  • Lake Houston, on the other hand, is primarily for water supply. It has limited flood control capability because of its fixed height spillway. Lake Houston has only four small gates with a combined release capacity of 10,000 cubic feet per second (CFS).

Engineers are currently studying ways to add more and bigger tainter gates to Lake Houston. The current plan under study would boost the release rate to 78,000 CFS, thus matching the highest release rate ever from Lake Conroe (during Harvey). That would enable better coordination between the dams.

Why It Matters

Timing of releases can materially affect downstream flooding in a densely developed floodplain. During Harvey, a wall of water 11 feet high was going over the Lake Houston spillway. 16,000 homes and 3300 businesses behind the dam flooded. It backed water up for miles. Lake Houston’s Dam had 5X more water going over it than Niagra Falls usually does – enough to fill NRG Stadium in 3.5 minutes – 425,000 CFS.

Lake Houston Dam During Harvey. Can you even see the gates at the right end of the spillway?

Twenty percent of all homes and forty percent of all businesses in the area were affected.

Lake Houston Area Flood Task Force

Getting the water out faster is crucial. But it must be done safely. In a way that doesn’t hurt downstream interests.

While Coastal Water Authority figures out how to add more gates, SJRA is building a forecasting tool for the entire watershed that has the potential to:

  • Improve coordination between the dams
  • Inform decisions about pre-releases and gate operations
  • Enhance emergency management

For More Information

See SJRA’s presentation at the Humble Civic Center on 3/5/26 for more on Joint Reservoir Operations.

See ReduceFlooding’s new Lessons page for more “lessons learned” about flooding. It’s my attempt to distill my most important findings from more than 3000 posts since Harvey.

Posted by Bob Rehak on 4/6/26

3142 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.

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.

What happens downstream when peaks from different tributaries arrive simultaneously instead of separately.

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:

  • 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.

The San Jacinto River Authority Master River Basin Plan recommended ten similar detention projects in 2020. But six years later, not one is funded.

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.

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:

  • 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.

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:

  • 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 Washout

Rapid development of Colony Ridge led to a washout of FM1010 during Harvey.

FM1010 at Plum Grove
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.

I-69 repairs
Replacement took almost a year.
UPRR Bridge over West Fork
UP Bridge
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 damage
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.

Rustic Elms Bridge on Taylor Gully
The bridge will soon be replaced as part of a HUD/GLO/HCFCD project.
Rustling Elms Bridge over Taylor Gully in May 2019
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 before dredging
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.

FM 1960 constriction

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.

HCFCD 2019 photo of blocked culverts under Kingwood Drive
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.

site of proposed gates for Lake Houston on east side of dam
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-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.

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:

  • City and County of Denver
  • Upstream suburbs (Littleton, Englewood, Lakewood, Aurora)
  • Multiple counties (Denver, Arapahoe, Jefferson, Adams, Douglas)
  • 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?

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.

Counties within the San Jacinto River Basin
Counties within the San Jacinto River Basin

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.

Bolivar after Ike
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.

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.

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.

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.

Hallett Sand Mine complex on San Jacinto West Fork.
Hallett Mine on San Jacinto West Fork

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.

Breakdown of lower estimate from Joint Economic Committee, 6/10/2024 Report

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.

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’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:

  • 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

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.

Quantitative example calculated by ChatGPT.

Note how volume peaks higher and faster after development.

The graph above assumed:

  • Watershed area: 100 acres
  • Storm depth: 5 inches
  • Method: NRCS Curve Number runoff depth plus a synthetic hydrograph
  • 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 Kingwood in 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





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.

Page 18 from Dr. Petersen’s Presentation

Features Included in PER Recommendation

The preliminary engineering report published last year recommended:

  • A diversion structure at the intersection of the Diversion Ditch and Bens Branch to reduce the volume flowing into Bens Branch
  • Channel conveyance improvements to the Diversion Ditch
  • A new outfall to the West Fork San Jacinto River, just west of Woodland Hills Drive/River Grove Park
  • A stormwater detention basin on the south side of the San Jacinto West Fork.
  • Bridge replacements at Kingwood Drive, Walnut Lane, Deer Ridge Estates Boulevard and the pedestrian bridge at Lake Village Drive

The bridges at Northpark Drive will also be rebuilt, but as part of the Northpark Expansion Project.

Kingwood Diversion Ditch
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.

Bens Branch and Kingwood Diversion Ditch
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.

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:

  • 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.

See Dr. Petersen’s entire presentation for more details.

Posted by Bob Rehak on 3/28/26

3133 Days since Hurricane Harvey