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.
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:
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.
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.
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.
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.
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.
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2024/05/20240522-RJR_3549.jpg?fit=1100%2C733&ssl=17331100adminadmin2026-04-08 13:04:282026-04-09 14:11:04How Subsidence Amplifies Flood Risk in Lake Houston Area
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.
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.
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.
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
Bridges, culverts, sand bars, levees, dikes, and new developments
Can constrict conveyance and back water up
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.
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.
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.
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
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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/02/LakeHoustonDamDuringHarvey.jpg?fit=1500%2C968&ssl=19681500adminadmin2026-04-06 13:24:242026-04-06 18:17:53Reservoir Coordination Still Elusive After 53 Years
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.
Where Subsidence Happens
As a result, subsidence afflicts fast-growing regions across the U.S. Several examples include:
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.
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.
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.
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.
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.
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
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:
Summary
To summarize, in flat coastal plains with low-gradients:
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:
River Authorities that Manage Multiple Dams for Flood Control
Examples of coordinated management abound. Take for instance:
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.
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.
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:
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.