Stormwater detention basins work by storing excess stormwater temporarily until channels can safely carry it away. Water enters the basin quickly during heavy downpours. But the basin releases it slowly at a steady rate that channels are designed to carry. This helps reduce the risk of flooding.
Harris County is so flat that dams are not often options. Therefore, virtually all of our stormwater storage has to be excavated.
Harris County Flood Control District
Willow Water Hole Example
The Harris County Flood Control District (HCFCD) detention basins typically cover several hundred acres and service regions. Willow Water Hole just outside the southwest corner of Loop 610 on a tributary of Brays Bayou is an excellent example.
The 279-acre Willow Waterhole has six compartments. Willow is part of the Brays Bayou Federal Flood Damage Reduction Project (Project Brays), a multi-year, $550 million project that substantially reduces flooding risk in the Brays Bayou watershed. The project is a cooperative effort between the Harris County Flood Control District and the U.S. Army Corps of Engineers.
Willow Waterhole Detention Basin Complex in SW Houston has six compartments.
Why the Need?
As areas develop, buildings and concrete cover up soil, so stormwater can’t sink into the ground. Water runs off concrete faster than it does from native grasslands. When that water all hits channels, streams and bayous simultaneously from different directions, it exceeds the carrying capacity of the channel. And homes flood.
Many, but not all, new developments use stormwater detention basins to offset that negative effect.
When full, detention basins often resemble lakes. When dry, detention basins look like large excavated open space areas. The Willow Water Hole is normally dry. Yesterday, however, it contained water from recent rains and the low (seasonal rate of evaporation).
Two of the compartments bracket South Willow Drive. See location above. Note the weir (discussed below) leading to the channel.
Some systems have water in them permanently, so they resemble small lakes. These provide flood storage between the normal surface of the lake and the top of the bank. See the difference in the photograph below.
Willow Water Hole southwestern pond. Note extra capacity between the top of the water and the top of the banks.
Detention? Retention? Which is It?
A detention basin normally has a dry bottom. It holds excess stormwater temporarily.
A retention basin always has a wet bottom. It stores water indefinitely. Retention basis normally have no outlet. Evaporation and infiltration usually keep the lake levels manageable.
The Harris County Flood Control District always builds and uses detention basins. Developers more likely will use retention ponds and market the resulting “lakes” as residential amenities.
HCFCD owns approximately 70 large regional detention basin sites throughout Harris County. They supplement hundreds of smaller developer-built basins. Countywide, these basins hold billions of gallons of stormwater during heavy rainstorms.
Two northeastern retention ponds within Willow Water Hole complexon either side of South Post Oak Road.
How Water Gets In
Sometimes HCFCD designs stormwater detention basins with a weir (visible in the first and second drone photos above). The weir, or low dam, lets stormwater rising in the channel spill into the detention basin when it reaches a certain height. Other detention basins have no weirs. They are simply open to a channel. In this case, stormwater fills the basin as it rises in the channel.
But there’s also a third alternative for stormwater detention basins, i.e., those not near a channel. Storm sewers and/or sheet flow fill these detention basins. “Big pipes in – little pipes out” is the rule in this instance. The basin gets the water away from streets and homes quickly. Then lets it drain off slowly.
How Water Gets Out
HCFCD typically designs detention basins to drain by gravity, as opposed to using pumps. This lets basins function when power goes out, a frequent occurrence during floods.
In ponds that drain by gravity, depth of the drain (outfall) is dictated by the depth of the receiving channel. The rate at which stormwater drains depends on the stormwater level in the receiving channel. Typically, stormwater drains out of the detention basin after channel levels recede.
Complex engineering calculations determine the volume of stormwater that a detention basin must hold to protect surrounding homes and businesses. That volume, usually measured in acre-feet, determines the width, length and depth of a basin. The amount of time stormwater stays in a basin depends on levels in the receiving channel and how full the basin got. In Harris County, detention time is usually measured in hours, not days.
How the Process Works
Normal Flow
When there is normal flow in a bayou or channel, the detention basin is generally empty.
Initial Storm Effects
Basins begin to fill as bayous or channels rise, or as surrounding developments drain into them through storm sewers.
Capturing the Flow of a Heavy Storm
As water continues to fill the detention basin, it spreads out into the excavated area. Often culverts connect multiple “compartments” within a larger basin, as above.
Detaining the Flow
By holding water in the detention basin, it does not flood homes and businesses downstream.
Draining Detained Water
As the level of the channel recedes, the channel water level drops and lets the basin drain, but only as fast as the channel can handle it.
Back to Normal Flow
With the water level in the channel normal, the basin is once again empty and ready for the next rainstorm.
End Result
Often, HCFCD partners with local groups, such as the Houston Parks Board, to build trails around these ponds that provide a retreat from busy city life. Areas such as Willow Water Hole also provide habitat for birds. People out for a stroll or a jog may think they are in a beautiful park and not even realize the role it plays in reducing flood risk.
The City of Houston has scheduled a repaving project for West Lake Houston Parkway from Dec. 6-27, 2021. This is a bit off-topic for a flood blog, but it affects thousands of readers. This follows several previous repaving and bridge repair projects on West Lake Houston Parkway. The following is based on a press release by Mayor Pro Tem Dave Martin’s office.
Mayor Pro Tem Dave Martin would like to make Kingwood area residents aware Houston Public Works will start a panel replacement project on West Lake Houston Parkway beginning Monday, December 6, weather permitting.
The scope of work includes replacing damaged concrete panels along the north and southbound lanes of W Lake Houston Parkway from Kingwood Drive to Magnolia Cove Drive.
The cost of the project is $38,200 and is funded through Mayor Pro Tem Martin’s Council District Service Funds.
Crews will mobilize on-site beginning Monday, December 6 and should finish the project by Monday, December 27, weather permitting.
Construction activities are expected to take place Monday through Friday from 8:00 a.m. to 5:00 p.m., and on Saturdays from 7:00 a.m. to 12:00 p.m.
Please stay alert for flagmen and orange traffic cones that will help traffic flow through the construction zone. The project may require a one-lane closure at times, but two-way traffic will be maintained at all times.
Businesses and residents will have access to driveways and sidewalks at all times, but may experience an increase in noise levels.
On the Harris County Commissioner’s Court agenda for today are two Harris County Flood Control District (HCFCD) “transmittals.” One will update commissioners on flood-bond spending to date. The other will update commissioners on the progress of new flood maps (the MAAPnext program). They are items 269 and 270 on today’s agenda.
Transmittals are reports by departments. Commissioners don’t usually discuss them unless one of the commissioners wishes to make comments for some reason. So, I’m calling them to your attention here.
Flood-Mitigation Spending Through Third Quarter Reaches $865 Million
About half of the $865 million spent on flood mitigation since voters passed the bond in 2018 has come from bond funds. The rest has come from grants and local partnerships. See pie chart below on left.
The left pie chart underscores the importance of partnership funding.
The map below shows where flood-bond spending has occurred.
Flood-mitigation spending by watershed since approval of flood-bond in 2018.
The winner in the $weep$take$: HCFCD spent almost $154 million on Brays Bayou.
Other leading watersheds (rounded to nearest million) in flood-bond spending included:
$81 million in Addicks Reservoir
$76 million on Greens Bayou
$76 million on Cypress Creek
$50 million on Little Cypress Creek
$46 million on White Oak Bayou
$32 million on Clear Creek
With a few exceptions, this spending reflects the influence of the Harris County Flood-Bond Equity Prioritization Framework implemented in 2019. That framework gives highest priority to low- to middle-income watersheds with a high social-vulnerability index. Thus, tiny Halls Bayou has received more assistance than the largest watershed in the county – the San Jacinto River. And Brays Bayou has received almost 11 times more assistance than Buffalo Bayou.
Two notable exceptions are:
Vince Bayou which is almost totally inside the City of Pasadena and is therefore primarily Pasadena’s responsibility.
Little Cypress Creek which is part of HCFCD’s experimental Frontier Program. The Frontier Program aims to prevent future flooding by buying up land on the cheap before it’s developed. HCFCD then sells detention basin capacity to developers to help make back its investment.
Other Insights Gained from Report
Most projects are ahead of schedule and on budget. Good news!
More than half of buyouts have been completed and enough funding apparently remains to complete the rest.
Progress continues on the $124 million FEDERAL Flood Damage Reduction project on White Oak Bayou, where six stormwater detention basins will hold almost a billion gallons of stormwater. That’s equivalent to about a foot of stormwater falling over almost 5 square miles.
An earlier version of this report generated some controversy. People in some watersheds didn’t believe the reported expenditures. Members of the Northeast Action Collective questioned whether any projects had started in their watersheds. They demanded immediate cancellation of projects in Kingwood and transfer of Kingwood’s funds, so that projects in Halls and Greens Bayou could start immediately.
That’s, in part, why I wrote “How to Find and Verify Flood-Related Information: Part I.” Flood-mitigation projects are hard to spot from the ground. Construction almost always happens out of sight behind tall fences and dense tree lines. After construction, the projects are often disguised as parks. For those who doubt, I recommend confirming the existence of projects from the air.
I haven’t confirmed every project in the county, but I have spot-checked many. And I have yet to find discrepancies between what HCFCD reports and what I can see from the air.
C-25, a Halls Bayou Detention pond now under construction by HCFCD. The bayou runs through the trees in the foreground.New basin at Hopper and US59 on a tributary of Halls Bayou.Lauder Detention Basin on Greens Bayou as of 10/12/2021. Phase One of a two-phase project is nearly complete.Cutten Road detention basin on Greens Bayou continues its relentless expansion.Phase 1 of the Greens Bayou Aldine-Westfield Basin on left is complete. Phase 2 on right is now beginning.
For more information that includes watershed spending data before the flood-bond, check out the funding page.
MAAPnext Effort About to Be Turned Over to FEMA
Harris County Flood Control (HCFCD) estimates it has completed 86% of its part of the flood-map updates. HCFCD will deliver drafts of the new maps to FEMA in January for review and kick off a campaign of public meetings at the same time. The public will see draft maps in February. A public comment period of 90 days will follow. And FEMA hopes to release preliminary flood insurance insurance rate maps by mid-year next year.
I have had a peek at the new maps and reports. And I must say, the effort should result in a dramatic leap forward in flood-risk understanding. Individualized reports will inform homeowners of their flood risks from a variety of different sources, including street flooding. The prototype of the website is very user friendly.
After receiving preliminary maps from HCFCD, it typically takes FEMA another 18-24 months to release final, official flood maps. That gives affected property owners time to comment and appeal. The process looks like this.
MAAPnext milestones as of the end of 2021.
Posted by Bob Rehak on 11/30/2021
1554 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2021/11/20211130-Screen-Shot-2021-11-30-at-10.31.04-AM-2.jpg?fit=1200%2C836&ssl=18361200adminadmin2021-11-30 12:47:472021-11-30 12:47:51Where Flood-Bond Spending Is Going, When New Flood Maps Will Be Released
How Stormwater Detention Basins Work
Stormwater detention basins work by storing excess stormwater temporarily until channels can safely carry it away. Water enters the basin quickly during heavy downpours. But the basin releases it slowly at a steady rate that channels are designed to carry. This helps reduce the risk of flooding.
Willow Water Hole Example
The Harris County Flood Control District (HCFCD) detention basins typically cover several hundred acres and service regions. Willow Water Hole just outside the southwest corner of Loop 610 on a tributary of Brays Bayou is an excellent example.
The 279-acre Willow Waterhole has six compartments. Willow is part of the Brays Bayou Federal Flood Damage Reduction Project (Project Brays), a multi-year, $550 million project that substantially reduces flooding risk in the Brays Bayou watershed. The project is a cooperative effort between the Harris County Flood Control District and the U.S. Army Corps of Engineers.
Why the Need?
As areas develop, buildings and concrete cover up soil, so stormwater can’t sink into the ground. Water runs off concrete faster than it does from native grasslands. When that water all hits channels, streams and bayous simultaneously from different directions, it exceeds the carrying capacity of the channel. And homes flood.
Many, but not all, new developments use stormwater detention basins to offset that negative effect.
When full, detention basins often resemble lakes. When dry, detention basins look like large excavated open space areas. The Willow Water Hole is normally dry. Yesterday, however, it contained water from recent rains and the low (seasonal rate of evaporation).
Some systems have water in them permanently, so they resemble small lakes. These provide flood storage between the normal surface of the lake and the top of the bank. See the difference in the photograph below.
Detention? Retention? Which is It?
A detention basin normally has a dry bottom. It holds excess stormwater temporarily.
A retention basin always has a wet bottom. It stores water indefinitely. Retention basis normally have no outlet. Evaporation and infiltration usually keep the lake levels manageable.
The Harris County Flood Control District always builds and uses detention basins. Developers more likely will use retention ponds and market the resulting “lakes” as residential amenities.
HCFCD owns approximately 70 large regional detention basin sites throughout Harris County. They supplement hundreds of smaller developer-built basins. Countywide, these basins hold billions of gallons of stormwater during heavy rainstorms.
How Water Gets In
Sometimes HCFCD designs stormwater detention basins with a weir (visible in the first and second drone photos above). The weir, or low dam, lets stormwater rising in the channel spill into the detention basin when it reaches a certain height. Other detention basins have no weirs. They are simply open to a channel. In this case, stormwater fills the basin as it rises in the channel.
But there’s also a third alternative for stormwater detention basins, i.e., those not near a channel. Storm sewers and/or sheet flow fill these detention basins. “Big pipes in – little pipes out” is the rule in this instance. The basin gets the water away from streets and homes quickly. Then lets it drain off slowly.
How Water Gets Out
HCFCD typically designs detention basins to drain by gravity, as opposed to using pumps. This lets basins function when power goes out, a frequent occurrence during floods.
In ponds that drain by gravity, depth of the drain (outfall) is dictated by the depth of the receiving channel. The rate at which stormwater drains depends on the stormwater level in the receiving channel. Typically, stormwater drains out of the detention basin after channel levels recede.
Complex engineering calculations determine the volume of stormwater that a detention basin must hold to protect surrounding homes and businesses. That volume, usually measured in acre-feet, determines the width, length and depth of a basin. The amount of time stormwater stays in a basin depends on levels in the receiving channel and how full the basin got. In Harris County, detention time is usually measured in hours, not days.
How the Process Works
Normal Flow
When there is normal flow in a bayou or channel, the detention basin is generally empty.
Initial Storm Effects
Basins begin to fill as bayous or channels rise, or as surrounding developments drain into them through storm sewers.
Capturing the Flow of a Heavy Storm
As water continues to fill the detention basin, it spreads out into the excavated area. Often culverts connect multiple “compartments” within a larger basin, as above.
Detaining the Flow
By holding water in the detention basin, it does not flood homes and businesses downstream.
Draining Detained Water
As the level of the channel recedes, the channel water level drops and lets the basin drain, but only as fast as the channel can handle it.
Back to Normal Flow
With the water level in the channel normal, the basin is once again empty and ready for the next rainstorm.
End Result
Often, HCFCD partners with local groups, such as the Houston Parks Board, to build trails around these ponds that provide a retreat from busy city life. Areas such as Willow Water Hole also provide habitat for birds. People out for a stroll or a jog may think they are in a beautiful park and not even realize the role it plays in reducing flood risk.
Posted by Bob Rehak on 12/2/2021 based on information provided by HCFCD
1556 Days since Hurricane Harvey
West Lake Houston Parkway Repaving Project Scheduled
The City of Houston has scheduled a repaving project for West Lake Houston Parkway from Dec. 6-27, 2021. This is a bit off-topic for a flood blog, but it affects thousands of readers. This follows several previous repaving and bridge repair projects on West Lake Houston Parkway. The following is based on a press release by Mayor Pro Tem Dave Martin’s office.
Mayor Pro Tem Dave Martin would like to make Kingwood area residents aware Houston Public Works will start a panel replacement project on West Lake Houston Parkway beginning Monday, December 6, weather permitting.
The scope of work includes replacing damaged concrete panels along the north and southbound lanes of W Lake Houston Parkway from Kingwood Drive to Magnolia Cove Drive.
The cost of the project is $38,200 and is funded through Mayor Pro Tem Martin’s Council District Service Funds.
Crews will mobilize on-site beginning Monday, December 6 and should finish the project by Monday, December 27, weather permitting.
Construction activities are expected to take place Monday through Friday from 8:00 a.m. to 5:00 p.m., and on Saturdays from 7:00 a.m. to 12:00 p.m.
Please stay alert for flagmen and orange traffic cones that will help traffic flow through the construction zone. The project may require a one-lane closure at times, but two-way traffic will be maintained at all times.
Businesses and residents will have access to driveways and sidewalks at all times, but may experience an increase in noise levels.
For more information, please contact Mayor Pro Tem Dave Martin’s office at (832) 393-3008 or districte@houstontx.gov.
Posted by Bob Rehak on 12/1/2021
1555 Days since Hurricane Harvey
Where Flood-Bond Spending Is Going, When New Flood Maps Will Be Released
On the Harris County Commissioner’s Court agenda for today are two Harris County Flood Control District (HCFCD) “transmittals.” One will update commissioners on flood-bond spending to date. The other will update commissioners on the progress of new flood maps (the MAAPnext program). They are items 269 and 270 on today’s agenda.
Transmittals are reports by departments. Commissioners don’t usually discuss them unless one of the commissioners wishes to make comments for some reason. So, I’m calling them to your attention here.
Flood-Mitigation Spending Through Third Quarter Reaches $865 Million
About half of the $865 million spent on flood mitigation since voters passed the bond in 2018 has come from bond funds. The rest has come from grants and local partnerships. See pie chart below on left.
The map below shows where flood-bond spending has occurred.
Other leading watersheds (rounded to nearest million) in flood-bond spending included:
With a few exceptions, this spending reflects the influence of the Harris County Flood-Bond Equity Prioritization Framework implemented in 2019. That framework gives highest priority to low- to middle-income watersheds with a high social-vulnerability index. Thus, tiny Halls Bayou has received more assistance than the largest watershed in the county – the San Jacinto River. And Brays Bayou has received almost 11 times more assistance than Buffalo Bayou.
Two notable exceptions are:
Other Insights Gained from Report
Additional maps in the full report show:
Also, a massive GANNT chart shows the stages of every project in every watershed and county-wide projects.
Check out the full report here.
Controversy over Previous Version of Report
An earlier version of this report generated some controversy. People in some watersheds didn’t believe the reported expenditures. Members of the Northeast Action Collective questioned whether any projects had started in their watersheds. They demanded immediate cancellation of projects in Kingwood and transfer of Kingwood’s funds, so that projects in Halls and Greens Bayou could start immediately.
That’s, in part, why I wrote “How to Find and Verify Flood-Related Information: Part I.” Flood-mitigation projects are hard to spot from the ground. Construction almost always happens out of sight behind tall fences and dense tree lines. After construction, the projects are often disguised as parks. For those who doubt, I recommend confirming the existence of projects from the air.
I haven’t confirmed every project in the county, but I have spot-checked many. And I have yet to find discrepancies between what HCFCD reports and what I can see from the air.
For more information that includes watershed spending data before the flood-bond, check out the funding page.
MAAPnext Effort About to Be Turned Over to FEMA
Harris County Flood Control (HCFCD) estimates it has completed 86% of its part of the flood-map updates. HCFCD will deliver drafts of the new maps to FEMA in January for review and kick off a campaign of public meetings at the same time. The public will see draft maps in February. A public comment period of 90 days will follow. And FEMA hopes to release preliminary flood insurance insurance rate maps by mid-year next year.
I have had a peek at the new maps and reports. And I must say, the effort should result in a dramatic leap forward in flood-risk understanding. Individualized reports will inform homeowners of their flood risks from a variety of different sources, including street flooding. The prototype of the website is very user friendly.
After receiving preliminary maps from HCFCD, it typically takes FEMA another 18-24 months to release final, official flood maps. That gives affected property owners time to comment and appeal. The process looks like this.
Posted by Bob Rehak on 11/30/2021
1554 Days since Hurricane Harvey