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Extreme Growth in Flatland With Constrained Drainage

4/12/26 – Numerous human and geologic factors drive flood risk. Where they exist in combination, flood risk degrades the most.

One of the most important “lessons learned” since Hurricane Harvey has been how A) extreme population growth in B) a flat river basin with C) a constrained outlet … combine to increase flood risk.

Extreme Population Growth in Region

Worldwide, urbanization and impervious cover (concrete, rooftops) represent the single most consistent, anthropogenic (man-made) driver of flooding worldwide. And…

Extreme population growth in the Houston region has fueled the growth of impervious cover.

According to U.S. Census Bureau figures released in January 2026, Texas ranked as the fastest growing state in the U.S. last year. And within Texas, Harris County grew more than any other county, adding an estimated 48,695 people – a 1% gain. Harris County’s population now tops 5 million people.

But the areas around Harris County are all growing rapidly, too – at least in percentage terms. Upstream from the Lake Houston Area:

Waller County grew by 5.7%
Liberty County grew by 4.4%
Montgomery County grew by 4%.

The U.S. grew only about 0.5% overall from 2024–2025. So these counties are outliers as is Texas itself. Since 2020, Texas has added more residents than any other state, with approximately 2.6 million new residents.

When you look closely at the numbers, there’s a clear bifurcation. Giant metro areas, such as Houston, lead in absolute population growth. But small, fast-developing fringe counties lead in percentage growth.

Why This Matters: Impervious cover

Waller, Liberty and Montgomery Counties lie right upstream from Harris County within the upstream San Jacinto River Basin. They rank among the fastest growing counties in the fastest growing state. That confirms rapid upstream urbanization in the watershed, which puts continued pressure on the San Jacinto West and East Forks.

Meanwhile, Harris County leads the nation in absolute growth. That means:

More downstream exposure
More assets at risk
Higher consequences when upstream peak flows stack on top of each other.

Just as concerning, to accommodate this growing population, we see floodplain encroachment throughout the watershed. For instance, right now, Scarborough and the Texas General Land Office are trying to develop more than 5,000 of the most flood-prone acres in southeast Texas near the confluence of Spring Creek and the San Jacinto West Fork.

*Scarborough Area in center from FEMA’s Flood Hazard Layer Viewer based on pre-Atlas 14 data.*

Land that once might have been dedicated to parks for flood control has become too valuable for that. Land owners want to catch the development gravy train.

Crossing at the Commons of Lake Houston Floodplains and General Plan — *Proposed Signorelli Development “Crossing at the Commons of Lake Houston.” Dotted lines represent floodplains and floodway. Original residents were promised this would be land for recreation.* *Signorelli fought the City of Houston all the way to the Supreme Court of Texas for ten years for the right to build on this land.*

Fragmented Governance Complicates Growth Factor

Fragmented governance also complicates the issue. Take, for instance, the Scarborough land that borders Harris County. If the land gets developed, Harris County would face increased flood risk while Montgomery County would reap benefits from expanding its tax base. So, the two counties have opposing interests.

Another example: Montgomery County gives tax breaks to sand mines along the San Jacinto that help fuel all this upstream growth. But sediment from those same mines washes downriver and gets deposited in the headwaters of Lake Houston. That sediment reduces conveyance and increases flood risk for the people in Harris County.

Infrastructure Bottlenecks and Peak Stacking

Drainage from all this extreme upstream growth feeds into a bottleneck with a constrained outlet – Lake Houston.

*Drainage from the 2,500 square mile upper San Jacinto River Basin all flows through Lake Houston.*

And extremely low gradients throughout this funnel means water moves slower and lingers longer. That increases the chances of peaks from different tributaries stacking on top of each other.

And finally, other bottlenecks exist, too. Like the FM 1960 Causeway and the Lake Houston Dam with its four small gates. They have a combined release capacity of just 10,000 cubic feet per second – one fifteenth the release capacity of Lake Conroe’s gates.

For More Information

See the Lessons Page of this web site. It condenses the major lessons learned from researching more than 3,000 posts since Harvey into a sort of quick guide.

Posted by Bob Rehak on 4/12/2026

3148 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 A Terminal Reservoir with Limited Gate Capacity Increases Flood Risk

6/11/26 – Those seeking to understand flooding in the Lake Houston Area need to understand the placement and construction of the dam within the larger San Jacinto River Basin. Lake Houston is what hydrologists call a “terminal reservoir.”

Flood risk in the Lake Houston area is governed less by any single upstream factor and more by how all upstream factors converge at one critical place. And that critical place (the Lake Houston Dam) has extremely limited flood-gate capacity to lower the lake in advance of approaching storms.

gates for Lake Houston and Conroe — *Lake Houston gates (l) can release 10,000 cubic feet per second (CFS). Lake Conroe gates (r) can release 150,000 CFS.*

How All Risk Factors Converge in Terminal Reservoirs

As a terminal reservoir, Lake Houston backstops everything that happens anywhere upstream. A terminal reservoir is a reservoir located at (or very near) the downstream end of a watershed, such as the San Jacinto River Basin. It receives the cumulative inflows from all upstream tributaries before water exits to a larger receiving body, like a bay or Gulf of Mexico.

Lake Houston is the tip of a funnel draining more than 2,500 square miles from seven counties. That’s an area 50% larger than all of Harris County itself. See below.

Upstream watershed percent of Lake Houston Area — *Percentages represent sub-watershed’s portion of acreage within Lake Houston’s drainage area which includes parts of seven counties. Lake Conroe controls only 13% of the drainage area.*

Because of Lake Houston’s location, peak flows from multiple tributaries (East Fork, West Fork, Caney Creek, Peach Creek, Luce Bayou, Spring Creek, Cypress Creek, Lake Creek, etc.) can stack on top of each other.

During a storm, if peak flows from those tributaries arrive at different times, they may be manageable.

But if peaks arrive simultaneously, as they likely would in a large storm like Harvey, they create an exponential spike in water levels. And that can create catastrophic flooding in a terminal reservoir with limited gate capacity.

Six years ago, the SJRA’s River Basin Master Drainage Plan recommended 10 areas for additional upstream regional detention. But not one has even been bid.

Sediment Trap for the River Basin

Terminal reservoirs catch more than water. They also trap sediment from all uncontrolled upstream tributaries. Those include the mining corridors along the West Fork, East Fork and Caney Creek.

This leads to the progressive loss of storage volume behind the Lake Houston dam. That reduces flood-buffering capacity over time. It also increases reliance on dredging.

This is a much bigger issue in the Lake Houston Area than in Lake Conroe. Lake Houston traps sediment from an area seven times larger than Lake Conroe does.

Sediment from 87% of the river basin ultimately ends up in Lake Houston.
Computed from acreage figures supplied by San Jacinto River Authority

Plus, the largest sources of sediment are between the two lakes. Virtually all sand mines in the river basin are downstream from Lake Conroe and upstream from Lake Houston.

Low Gradient and Urbanization Increase Flood Risk

The low gradient in our flat coastal plain, also means that this terminal reservoir can back water up into tributaries, such as Bens Branch, where 12 people died at an assisted living facility for seniors during Harvey.

Urbanization compounds all these risks. Incremental upstream development in Montgomery, Waller and Liberty counties creates a cumulative increase in runoff volume and speed which amplifies peaks at the terminal location.

In other posts, I showed how even if each upstream development project meets the “no-net-runoff-increase” mandate locally, a system-level effect still concentrates peak flows at the terminal reservoir.

*Peak flows in Lake Houston watershed during Hurricane Harvey.* *400,000 CFS went over Lake Houston Dam.*

The 400,000 CFS going over the Lake Houston Dam during Harvey created a wall of water 11 feet high. The volume was five times more than the volume of water going over Niagra Falls on an average day.

Implications

In summary, Lake Houston is the control point for a 2,500 square mile watershed. Lake Conroe controls only 13% of upstream drainage. Lake Houston controls 100%. The entire drainage area flows through this one control point with little help.

This heightens sensitivity to timing (when flood peaks arrive) and coordination (with Lake Conroe). Other than Lake Conroe, there is NO redundancy built into the system.

Sediment accumulation is not only inevitable, it is accelerated – by sand mining and rapid upstream development. This limits the buffering capacity of the lake for flood-control purposes. Sediment management is not optional. Safety requires it. Luckily, State Representative Charles Cunningham was able to start a Lake Houston Area Dredging and Maintenance District in the 2025 legislature.

The design of the Lake Houston dam also limits flexibility for flood management. Lake Houston has a 3,100-foot wide spillway but extremely limited gate capacity – 1/15th the capacity of Lake Conroe’s gates.

That limits pre-release capacity. It takes days in advance of a storm to lower Lake Houston enough to absorb anticipated incoming stormwater. But storms can veer away during that time. Yet millions of people depend on water from Lake Houston.

So dam managers must be extremely cautious about pre-releasing water. Before they open the gates, they must be sure the storm will replenish any water discharged.

Conclusion

Thus, gate capacity has outsized importance for flood safety. That’s why Houston Public Works and the Coastal Water Authority have studied the best way to add more gates to the dam ever since Harvey. But they haven’t yet finalized a design.

Meanwhile, people live with the flood risk of a terminal reservoir with limited gate capacity and little upstream help from other reservoirs.

For information about other factors that create flood risk, see the Lessons Page of this website.

Posted by Bob Rehak on 4/11/26

3147 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 Sediment-Induced Loss of River Conveyance Increases Flood Risk

4/10/26 – Sediment buildups reduce river conveyance everywhere. But sedimentation is especially threatening in the Lake Houston Area because:

It creates hydraulic chokepoints that increase flood risk next to major commercial and population centers
Industrial-scale sand mining upstream accelerates erosion
The design of the Lake Houston Spillway doesn’t facilitate sediment discharge.
Lake Houston provides water for millions of people and loss of storage capacity is critical

Consequences of Sedimentation

This combination of factors is not unique, but it is unusual in its magnitude and consequences. Here’s why.

*Lake Houston has a 3,160 foot long fixed height spillway for its primary discharge feature.* *While common in older water supply reservoirs, it provides few flood-control benefits.* *Studies to add tainter gates are not yet complete.*

There is no way to remove sediment from Lake Houston except by dredging. Texas Water Development Board studies show that the lake has lost about 20% of its original storage capacity.

But the total storage loss is not the only issue; also consider where the loss is. Most sediment concentrates in the headwaters of the lake upstream from the FM1960 causeway where the river slows down as it meets a standing body of water.

Deceleration causes sediment to drop out of suspension. A delta has formed at this location, creating a hydraulic chokepoint that backs water up.

The Army Corps dredged approximately 2.1 million cubic yards from the area above FM1960.

Mouth bar was dredged and is now gone. — *Looking S over West Fork mouth bar after Harvey and before dredging.*

Subsequently, the City of Houston commissioned Callan Marine for a separate dredging effort to address additional buildups in Lake Houston’s headwaters.

East Fork Mouth Bar cost $18 million to dredge. — *East Fork Mouth Bar after Imelda, but before dredging*

We have seen what happens when we ignore sediment buildups. Thousands of people can lose homes and businesses in a major flood.

Need for Ongoing Sediment Removal and Preservation

The SJRA has studied sedimentation and sand traps for years to help control such buildups. But they have yet to implement any solutions.

Thankfully, a bill by State Representative Charles Cunningham created a Lake Houston Dredging and Maintenance District during the 2025 legislature. That should help manage sediment buildups in the future before they become critical.

But it’s no excuse to ignore things that contribute to excessive sedimentation. Those include failure to observe best management practices in mining and construction. They also include loss of riparian buffers and forests.

Confluence of Spring Creek and West Fork San Jacinto — 5,300 flood-prone acres between the confluence of Spring Creek (l) and the San Jacinto West Fork (r) are slated for development by Scarborough, a Dallas-based developer and the Texas General Land Office. Note first of many upstream sand mines in upper right.

The area above borders 20-square miles of West Fork sand mines.

*The loss of forests due to mining exposes more sediment to erosion during floods.*

The loss of forests and riparian buffers leads to a triple whammy in terms of sedimentation. It increases the natural rate of erosion. It costs taxpayers hundreds of millions of dollars to remove it. And until they do, it increases flood risk.

As sand production on the West Fork winds down, now is the perfect time to discuss creating a Montgomery County Lake District or a new state park in this area. It could turn into a win/win for upstream and downstream interests.

For More Information

For more about the causes of flooding and how they can compound each other in the Lake Houston Area, see the Lessons Page.

Posted by Bob Rehak on 4/10/26

3146 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.

Role of Industrial-Scale Sand Mining in Lake Houston Area Flooding

4/9/2026 – Another “lesson learned” about flooding since Hurricane Harvey has been role of sedimentation from industrial-scale sand mining upstream from Lake Houston. It can only be appreciated from the air.

May 2024 flood at Hallett Mine on West Fork — *May 2024 flood at Hallett Mine on San Jacinto West Fork* *near Northpark Drive in Kingwood.* *Note the difference in water quality in the ditch.*

Few, if any, urban watersheds have such dense clusters of sand mines directly upstream from their water reservoirs in major river channels. This is one of the most distinctive features of flooding in the San Jacinto Watershed.

The mines not only contribute to flooding in an outsized way, they impair water quality, damage the environment and harm wildlife.

Chronic Sediment Mobilization During Floods

Most erosion happens during floods. And most sediment moves during floods.

More than 20 square miles of sand mines in the floodway and floodplains of the San Jacinto immediately upstream from Lake Houston provide a ready source of exposed sediment, ripe for mobilization.

I have photographed:

Sand mines pumping sediment over and through dikes
Dike failures.
Rivers capturing sand pits during floods
Mines dumping sediment directly into rivers

See a small sampling of photos below.

one of the prettiest places in texas — *LMI sand mine on West Fork*

breach out of sand pit — *Breach of sand-mine dike into San Jacinto West Fork*

*Breach of Triple PG mine dike into Caney Creek in East Fork Watershed*.

Hallett spill — *Mine dumping sediment in West Fork*

Hallett Mine — *Same mine overflowing onto neighboring properties and then West Fork*

New Sand Mining BMPs needed to control sediment pollution. — *Pit capture at Hallett Mine. River now flows through mine instead of around it.*

*FM2090 is now threatened by the East Fork which has rerouted itself through an abandoned sand mine.*

*Pipelines carrying highly volatile liquids undercut by erosion from sand mine on West Fork.*

*East Fork Sand Mine undercutting more pipelines and leaking into Caney Creek*

*Abandoned sand mine equipment leaking oil into West Fork*

abandoned dredge — *Rusting dredge in abandoned sand mine in Humble on North Houston Avenue*

white water caused by flaunting regulations — *Confluence of Spring Creek and West Fork in August 2019. TCEQ cited Liberty Mines for discharging 56 million gallons of white sludge into the West Fork.*

*Sand freshly deposited during Harvey blocked West Fork 90%, according to Army Corps.*

west fork mouth bar before dredging — *Farther downriver, a mouth bar blocked the West Fork backing water up into thousands of homes and businesses. Army Corps has since dredged this.*

The Houston area certainly isn’t the only place in the U.S. with alluvial floodplain mining. But rarely, if ever, do such practices take place with such impunity so close to the water supply for two million people.

For More Information

See the sand-mining page of this website for Best Practices in the U.S. and abroad; academic studies on sand mining; Texas sand mining laws/regulations; observations; and tax appraisal practices. An investigation found that virtually all of the sand mines shown above are in Montgomery County which taxed them as ag- and timberland rather than depleting assets.

Also search on the tag “sand mining” to find more than 300 posts containing thousands of pictures of mining practices in the San Jacinto River Basin.

Posted by Bob Rehak on 4/9/26

3145 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 Subsidence Amplifies Flood Risk in Lake Houston Area

4/8/26 – Water extraction from aquifers, driven by population growth, causes subsidence. It is a well-recognized phenomenon across the U.S. and in southeast Texas. Here in the Lake Houston Area, differential subsidence is creating a bowl in the landscape that amplifies flood risk for people in southern Montgomery and northern Harris Counties.

Let’s look first at how, why and where subsidence happens. That understanding will help explain how it amplifies flood risk in the Lake Houston Area.

How Subsidence Happens

Extraction of groundwater – faster than nature recharges it – can cause silt and clay layers underground to compact. That compaction is permanent. Think of smashing a brownie; it will never regain its original shape. Innumerable tiny voids in the soil (or brownie) disappear, causing the surface above to sink.

For a more scientific explanation see the Harris Galveston Subsidence District FAQ on “What is Subsidence?”

Why Subsidence Happens

Population growth creates demand for the water in those aquifers, often at the fringes of major metropolitan areas. Developers build new subdivisions faster than water authorities can build pipelines to them from local surface water supplies, such as lakes.

Drilling wells is a much faster, more cost-effective solution at that stage of development – for both the water authorities and developers.

Plus, it’s not just the cost of the pipeline. You need to consider the cost of the water treatment plant. Both together can cost billions of dollars – far more than even a large subdivision could support.

*Houston’s new Northeast Water Treatment Plant under construction in 2020*. *Projected cost was $1.7 billion.*

Where Subsidence Happens

As a result, subsidence afflicts fast-growing regions across the U.S. Several examples include:

Atlantic Coast
- NASA reports that that more than half of infrastructure in major cities such as New York, Baltimore, and Norfolk is built on land that sank, or subsided, by 1 to 2 millimeters per year between 2007 and 2020. Land in several counties in Delaware, Maryland, South Carolina, and Georgia sank at double or triple that rate.
Chicago
- 98% of the city reportedly sinks at 2 to 3 millimeters per year.
San Joaquin Valley
- To feed the hungry growing population of the U.S., agricultural interests in the California’s San Joaquin Valley began over-pumping groundwater in the 1920s. According to the U.S. Geological Survey, the land surface there had subsided 30 feet by 1980.
Las Vegas
- The city subsided 5 feet by 1980. But fast-growing Las Vegas extracts three times more groundwater than the natural recharge rate to this day. Subsidence in northern parts of the city forced residents to relocate.
Houston
- During the last 100 years, Houston has consistently ranked among the fastest-growing major U.S. cities according the Census Bureau, frequently placing in the top five and even top two. Its subsidence problems are legendary. Parts of Baytown subsided more than 10 feet before the formation of the Harris-Galveston Subsidence District, causing the Brownwood subdivision to sink beneath Galveston Bay.

Differential Subsidence in the Lake Houston Area

But subsidence is not just a coastal issue. It also can threaten areas far inland. According to Mike Turco, general manager of the Harris-Galveston Subsidence District, areas in Spring has subsided by almost 4 feet and the area around spring has subsided by about 4 feet. Recent subsidence rates in Spring have generally been between 0.5 and 1.0 foot per decade. That’s much faster than at the Lake Houston Dam. So, in effect, we’re creating a bowl in the landscape.

And that bowl amplifies flood risk.

Even though homes may be 75-100 feet above sea level, they may only be one foot above the floodplain.

As water, from say Spring Creek or the San Jacinto West Fork, goes into that bowl, it increases erosion on the upstream side and deposition on the downstream side. That deposition contributes to pooling within the bowl. A double whammy.

So, when a major storm comes along homes may have had their “freeboard factor” wiped out. In engineering and insurance, “freeboard” means your “safety margin above the floodplain.” Live in a place long enough and you may find water creeping closer and closer to your home in successive storms.

Of course, subsidence is only one of many factors that could cause that. But it amplifies those other factors and increases your flood risk.

Past catching up with Montgomery County — *Woodloch Subdivision damage near San Jacinto West Fork in Southern Montgomery County from May 2024 flood.*

To complicate matters for the poor homeowners shown in the picture above, Dallas-based Scarborough recently purchased 5,300 acres nearby between Spring Creek and the West Fork. Any new subdivisions built on that property would use well-water and further contribute to subsidence.

For More Information

Your safety ultimately depends on maintaining a healthy safety margin – much like the distance between you and the car in front of you on the freeway. We’ve all seen what can happen without enough distance.

For a discussion of other factors that contribute to flood risk, see the Lessons page of this website.

For more on subsidence and flooding, see:

Posted by Bob Rehak on 4/8/26

3144 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 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:
- Where Spring Creek converges with the San Jacinto West Fork just upstream from the I-69 bridge, makes a classic example.
- Stacking peaks created the highest flooding in Harris County during Harvey – 22 feet above flood stage!
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:

Rainfall probability estimates increase over time.
Urban growth increases the amount of impervious cover, creating faster, higher peak flows.
Upstream developments encroach on floodplains
Fragmented governance makes it difficult to know how to plan.

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.

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.

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.

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.

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 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.