12/6/24 – When you look at Harris County Flood Control District (HCFCD) spending figures, it’s clear they are making a major flood-mitigation investment along Cypress Creek. At the end of the third quarter this year, HCFCD had spent more than $190 million on Cypress Creek projects since Hurricane Harvey. That was enough to rank it second among all Harris County watersheds. Only Brays Bayou received more funding.
New stormwater-detention basin under construction in September 2023 at Cypress Creek and TC Jester.
So I asked how much the county’s investment in stormwater detention basins along Cypress Creek could help offset future releases from Lake Conroe by the SJRA during flood events.
Of course, any detention upstream helps offset flooding downstream. So the question is really “Can additional detention offset SJRA releases significantly?”
“Not a 1:1 Storage Question”
Harris County Precinct 3 Senior Project Manager Eric Heppen, P.E., PMP, explained how a professional engineer would answer my seemingly innocent question. It’s far more complex than it looks.
“Significantly” masks several layers of complexity.
For instance, how:
Much can Lake Conroe release compared to the total detention along Cypress Creek?
Fast is the SJRA releasing from Lake Conroe?
Long would the SJRA release last?
Much does the water spread out before reaching Lake Houston?
And then there are the questions of rainfall distribution and the arrival time of peaks. Is it raining as hard along Cypress Creek as it is above Lake Conroe’s dam? What if the flood peaks don’t arrive simultaneously?
For all these reasons, said Heppen, “It’s never going to be a 1:1 storage question. From a Lake-Houston-Area perspective, you can’t say that an acre foot of water held back on Cypress Creek exactly equals an acre-foot of water released from Lake Conroe.” Then he went on to answer my question as best he could given the uncertainties involved.
Comparison of Total Stormwater Detention
Heppen began by pointing out that studies show Cypress Creek needs another 25,000 acre-feet of floodwater storage. “Our initial goal is to add another 12,000-14,000 acre feet,” he said.
Lake Conroe, on the other hand, has a surface area of 21,000 acres. And the SJRA has an easement that gives it the ability to raise the lake six feet. For comparison, that’s 126,000 acre feet of additional detention, roughly 10X more than the detention volume being added to Cypress Creek initially or 5X more than the desired 25,000 acre feet.
So additional Cypress detention, when complete, would equal 10-20% of the variable storage that SJRA has to work with at Lake Conroe. That can make a dent in Lake Houston Area flooding.
Comparison of Release Rates and Durations
However, Heppen did not stop there. He also calculated how long Cypress Creek’s additional detention could offset releases from Lake Conroe. “IF the dam releases at 6,000 cubic feet per second (CFS) then that is approximately an acre foot every 7 seconds or so,” said Heppen. “That would mean SJRA was releasing approximately 500 acre feet per hour.”
“So…very, very, very simplified, if Harris County adds 10,000 acre feet along Cypress, and Lake Conroe releases 6,000 CFS, then the additional detention along Cypress Creek could take the increased release rate for nearly 20 hours,” added Heppen.
Compare that to actual release rates during a large flood. Affidavits by SJRA engineers show that releases above 70,000 CFS from Lake Conroe lasted 26 hours during Harvey.
Shaving 6,000 CFS off those releases for 20 hours could well have saved many homes and businesses on the periphery of the flood.
Conclusion: Blunted Peaks
The initial 12,000 to 14,000 acre feet of stormwater detention being added to Cypress Creek could have blunted the peak of Harvey had it been there at the time. The desired 25,000 acre feet would have made an even greater impact.
Of course, flooding is all about timing, i.e., when peaks arrive. But if the Lake Conroe and Cypress Creek peaks arrived at the US59 bridge simultaneously, the Cypress Creek storage could have reduced the combined peak significantly in my opinion.
Additional upstream detention was one of the three main goals advocated by the Lake Houston Area Task Force after Harvey. And I, for one, am all for the improvements being made to Cypress Creek stormwater-detention capacity.
Posted by Bob Rehak on 12/6/2024
2656 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2023/09/20230925-DJI_0502.jpg?fit=1100%2C733&ssl=17331100adminadmin2024-12-06 17:09:092024-12-06 19:40:58Can Cypress Creek Stormwater Detention Offset SJRA Releases from Lake Conroe?
12/5/2024 – Tsunamis are huge waves caused by earthquakes. Storm surge is a huge wave caused by hurricane winds. Both have unimaginable destructive power.
Destruction on Bolivar Peninsula from Hurricane Ikestorm surge in 2008.
West Coast Tsunami Warning This Morning
This morning, a magnitude 7.3 earthquake struck Northern California. That caused the National Weather Service to issue tsunami warnings from Oregon down to San Francisco and beyond.
About three hours later, the warning was cancelled without explanation. Since then, significant aftershocks ranging from 3.0 to 5.3 magnitude have occurred. We will learn more in coming days.
Different Types of Earthquakes Trigger Different Tsunami Threat Levels
Massive earthquakes don’t always trigger massive tsunamis. The magnitude 7.9 quake that destroyed much of San Francisco in 1906 reportedly produced a wave height of only three inches. Small localized, underwater landslides created 1-2 foot waves, but their destruction was confined to small areas.
Geology determines, in large part, the height of a tsunami. Imagine two different types of geologic faults.
In a thrust fault, one tectonic plate is forced under another, pushing it up and causing a vertical displacement of the seafloor. This type of fault commonly generates a tsunami.
However, a strike-slip fault causes a lateral displacement. Land along a fault moves in opposite directions without causing elevation changes. Strike-slip faults typically do not generate tsunamis because they generally do not displace large volumes of water. (Contrast the animations in the two links above.)
Other factors may also affect tsunami formation. They include:
Depth: Shallow earthquakes (occurring at depths less than 70 km) are more likely to cause tsunamis than deep earthquakes, as the energy has a more direct impact on the seafloor.
Seafloor Topography: The configuration of the ocean floor, including ridges and trenches, can amplify or reduce tsunami formation.
Underwater Landslides or Underwater Volcanic Activity triggered by the earthquake can also contribute to tsunami generation.
In summary, while massive offshore earthquakes increase the risk of tsunamis, they do not guarantee one will occur.
In this case, officials issued the tsunami warning out of an abundance of caution when seismographs first detected the earthquake. Later, they cancelled the warning when it became clear no massive waves threatened. They made the right call initially. Here’s why.
Record Waves, Record Destruction
Some massive earthquakes, like the magnitude 9.0, 2011 Tōhoku earthquake in Japan, did produce devastating tsunamis. One wave reached more than six miles inland. A 48-foot high tsunami wiped out a nuclear power plant at Fukushima. And the death toll ultimately reached almost 20,000 people.
So you can imagine why California officials immediately issued warnings today for residents to evacuate to higher ground.
During the great Galveston hurricane of 1900, a giant wave of water caused by storm surge, not a tsunami, inundated the island with 8- to 12-feet of water. It killed an estimated 6,000 to 12,000 people.
One cubic foot of seawater weighs 64 pounds. Imagine getting stuck by tens of thousands of cubic feet. That’s why storm surge is the leading cause of death associated with tropical events, according to the National Weather Service.
Don’t take either storm surge or tsunamis lightly.
I notice the same spike in traffic every time floods threaten. People want to know whether they live higher than the expected crest of the flood, storm surge or, in this case, tsunami.
Published in 2020, the post has received more than 150,000 page views already this year. The biggest peaks were during the May storm, Beryl, Helene and Milton.
Ironically, when I wrote that post about elevation, tsunamis were the furthest thing from my mind.
Even more ironic, shortly before the warning, someone emailed me asking whether I thought a development near the San Jacinto West Fork was safe from flooding. I replied to her that if she had a concern about flood risk, she should buy on the highest ground she could afford, as far from the water as she could get.
Posted by Bob Rehak on 12/5/2024
2655 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2024/12/Earthquake-sites-e1733446450615.png?fit=1100%2C654&ssl=16541100adminadmin2024-12-05 18:58:092024-12-05 18:58:10Tsunamis, Storm Surge Demonstrate Power of Moving Water
12/2/24 – System capacity maps for each of Harris County’s 23 watersheds show widespread problems and raise questions about whether the county’s most severe needs are confined to historically underserved areas.
Harris County Flood Control District (HCFCD) released the maps in response to a Freedom of Information Act (FOIA) request.
What “System Capacity” Means
System capacity reflects the ability of a stream or channel to handle rainfall of different intensities before coming out of its banks. The way system capacity is expressed also reflects the expected frequency of over-bank events.
A channel that can hold a:
100-year rainfall has a 1% system capacity
50-year rainfall has a 2% system capacity
25-year rainfall has a 4% system capacity, etc.
“1% system capacity” indicates that the infrastructure is designed to handle the peak flow or volume associated with a 1% event without causing overflow or flooding.
One Key Part of Comprehensive Flood-Risk Picture
System-capacity maps are one piece of information used in determining a comprehensive view of flood risk. They help you quickly spot areas that need closer examination.
Other key factors used to determine flood risk include: topography around the channel, the elevation of a structure, degree of development in an area, building codes in effect when an area was built, population density, and more.
Impact of Atlas 14
The maps released today rely on pre-Atlas 14 data. Harris County did not provide current maps. Nor do they reflect improvements made to channels recently. But the older maps are still instructive because they formed the starting point for spending billions of flood-mitigation dollars. However…
Because these maps are based on old rainfall standards, today’s true system capacity is actually lower than shown.
Said another way, the situation on the ground is worse than the maps show. That’s because the transition from pre-Atlas 14 to Atlas-14 data in Harris County resulted in significant increases in estimated rainfall depths for various storm events.
Notably, the 1% annual exceedance probability (AEP) 24-hour rainfall depth—commonly referred to as the “100-year” event—experienced substantial changes.
Prior to Atlas 14, Harris County was divided into three hydrologic regions with the following 100-year, 24-hour rainfall depths:
Region 1: 12.4 inches
Region 2: 12.8 inches
Region 3: 13.5 inches
With the implementation of Atlas 14, these values increased to:
Region 1: 16.3 inches
Region 2: 16.9 inches
Region 3: 18.0 inches
This represents increases of approximately 31%, 32%, and 33% for Regions 1, 2, and 3, respectively.
These updated figures reflect a more accurate understanding of rainfall patterns, incorporating additional years of data and improved analytical methods. Consequently, infrastructure design and floodplain management practices in Harris County have been adjusted to align with these revised estimates, enhancing resilience against flooding events.
Value of Maps Based on Old Rainfall Standards
Still, these maps have value. They are a starting point for the $2.5 billion 2018 Flood Bond. They also show that:
All watersheds have problem areas
Channel capacity in some areas is extremely low
Some watersheds that are not “historically underserved” have more severe system-capacity issues than those that are underserved.
System Capacity Maps
Below are low-resolution maps for each of the 23 watersheds in Harris County. For high-resolution maps of all watersheds in one file, click here. Caution: [26-meg download.]
Correlation of Maps with Spending
It’s instructive to correlate HCFCD flood-mitigation spending with these maps. Below is where more than $2 billion has gone since Hurricane Harvey, in large part, to address the problems shown above.
Data supplied by HCFCD. Shows relative spending by watershed since Harvey through Q3 2024.HCFCD spending by watershed through Q3 2024 since Harvey in dollars.
Compare the maps with the spending. And use the contact form of this website to let me know if you feel your area is not getting its fair share of flood-mitigation funding.
Note on Next Update with Atlas 14 Data
I have tried to get accurate flood risk data for years. However, HCFCD says it does not routinely update these maps every time it completes a new construction project. The District says it may update them again as part of the MAAPnext project after FEMA approves new maps based on Atlas-14 data.
Posted by Bob Rehak on December 2, 2024
2652 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/2024/12/20241202-Brays-System-Capacity.jpg?fit=1100%2C850&ssl=18501100adminadmin2024-12-02 18:55:522024-12-03 13:20:44System Capacity Maps for Harris County Channels Reveal Widespread Problems
Can Cypress Creek Stormwater Detention Offset SJRA Releases from Lake Conroe?
12/6/24 – When you look at Harris County Flood Control District (HCFCD) spending figures, it’s clear they are making a major flood-mitigation investment along Cypress Creek. At the end of the third quarter this year, HCFCD had spent more than $190 million on Cypress Creek projects since Hurricane Harvey. That was enough to rank it second among all Harris County watersheds. Only Brays Bayou received more funding.
So I asked how much the county’s investment in stormwater detention basins along Cypress Creek could help offset future releases from Lake Conroe by the SJRA during flood events.
Of course, any detention upstream helps offset flooding downstream. So the question is really “Can additional detention offset SJRA releases significantly?”
“Not a 1:1 Storage Question”
Harris County Precinct 3 Senior Project Manager Eric Heppen, P.E., PMP, explained how a professional engineer would answer my seemingly innocent question. It’s far more complex than it looks.
“Significantly” masks several layers of complexity.
For instance, how:
And then there are the questions of rainfall distribution and the arrival time of peaks. Is it raining as hard along Cypress Creek as it is above Lake Conroe’s dam? What if the flood peaks don’t arrive simultaneously?
For all these reasons, said Heppen, “It’s never going to be a 1:1 storage question. From a Lake-Houston-Area perspective, you can’t say that an acre foot of water held back on Cypress Creek exactly equals an acre-foot of water released from Lake Conroe.” Then he went on to answer my question as best he could given the uncertainties involved.
Comparison of Total Stormwater Detention
Heppen began by pointing out that studies show Cypress Creek needs another 25,000 acre-feet of floodwater storage. “Our initial goal is to add another 12,000-14,000 acre feet,” he said.
Lake Conroe, on the other hand, has a surface area of 21,000 acres. And the SJRA has an easement that gives it the ability to raise the lake six feet. For comparison, that’s 126,000 acre feet of additional detention, roughly 10X more than the detention volume being added to Cypress Creek initially or 5X more than the desired 25,000 acre feet.
So additional Cypress detention, when complete, would equal 10-20% of the variable storage that SJRA has to work with at Lake Conroe. That can make a dent in Lake Houston Area flooding.
Comparison of Release Rates and Durations
However, Heppen did not stop there. He also calculated how long Cypress Creek’s additional detention could offset releases from Lake Conroe. “IF the dam releases at 6,000 cubic feet per second (CFS) then that is approximately an acre foot every 7 seconds or so,” said Heppen. “That would mean SJRA was releasing approximately 500 acre feet per hour.”
“So…very, very, very simplified, if Harris County adds 10,000 acre feet along Cypress, and Lake Conroe releases 6,000 CFS, then the additional detention along Cypress Creek could take the increased release rate for nearly 20 hours,” added Heppen.
Compare that to actual release rates during a large flood. Affidavits by SJRA engineers show that releases above 70,000 CFS from Lake Conroe lasted 26 hours during Harvey.
Shaving 6,000 CFS off those releases for 20 hours could well have saved many homes and businesses on the periphery of the flood.
Conclusion: Blunted Peaks
The initial 12,000 to 14,000 acre feet of stormwater detention being added to Cypress Creek could have blunted the peak of Harvey had it been there at the time. The desired 25,000 acre feet would have made an even greater impact.
Of course, flooding is all about timing, i.e., when peaks arrive. But if the Lake Conroe and Cypress Creek peaks arrived at the US59 bridge simultaneously, the Cypress Creek storage could have reduced the combined peak significantly in my opinion.
Additional upstream detention was one of the three main goals advocated by the Lake Houston Area Task Force after Harvey. And I, for one, am all for the improvements being made to Cypress Creek stormwater-detention capacity.
Posted by Bob Rehak on 12/6/2024
2656 Days since Hurricane Harvey
Tsunamis, Storm Surge Demonstrate Power of Moving Water
12/5/2024 – Tsunamis are huge waves caused by earthquakes. Storm surge is a huge wave caused by hurricane winds. Both have unimaginable destructive power.
West Coast Tsunami Warning This Morning
This morning, a magnitude 7.3 earthquake struck Northern California. That caused the National Weather Service to issue tsunami warnings from Oregon down to San Francisco and beyond.
About three hours later, the warning was cancelled without explanation. Since then, significant aftershocks ranging from 3.0 to 5.3 magnitude have occurred. We will learn more in coming days.
Different Types of Earthquakes Trigger Different Tsunami Threat Levels
Massive earthquakes don’t always trigger massive tsunamis. The magnitude 7.9 quake that destroyed much of San Francisco in 1906 reportedly produced a wave height of only three inches. Small localized, underwater landslides created 1-2 foot waves, but their destruction was confined to small areas.
Geology determines, in large part, the height of a tsunami. Imagine two different types of geologic faults.
In a thrust fault, one tectonic plate is forced under another, pushing it up and causing a vertical displacement of the seafloor. This type of fault commonly generates a tsunami.
However, a strike-slip fault causes a lateral displacement. Land along a fault moves in opposite directions without causing elevation changes. Strike-slip faults typically do not generate tsunamis because they generally do not displace large volumes of water. (Contrast the animations in the two links above.)
Other factors may also affect tsunami formation. They include:
In this case, officials issued the tsunami warning out of an abundance of caution when seismographs first detected the earthquake. Later, they cancelled the warning when it became clear no massive waves threatened. They made the right call initially. Here’s why.
Record Waves, Record Destruction
Some massive earthquakes, like the magnitude 9.0, 2011 Tōhoku earthquake in Japan, did produce devastating tsunamis. One wave reached more than six miles inland. A 48-foot high tsunami wiped out a nuclear power plant at Fukushima. And the death toll ultimately reached almost 20,000 people.
So you can imagine why California officials immediately issued warnings today for residents to evacuate to higher ground.
During the great Galveston hurricane of 1900, a giant wave of water caused by storm surge, not a tsunami, inundated the island with 8- to 12-feet of water. It killed an estimated 6,000 to 12,000 people.
One cubic foot of seawater weighs 64 pounds. Imagine getting stuck by tens of thousands of cubic feet. That’s why storm surge is the leading cause of death associated with tropical events, according to the National Weather Service.
Don’t take either storm surge or tsunamis lightly.
Elevation: Key to Survival
The National Weather Service’s Tsunami Warning Center offers an excellent FAQ page concerning tsunami warnings.
Interestingly, shortly after NWS issued the tsunami warning for the San Francisco area this afternoon, I had a huge spike in traffic on ReduceFlooding.com. Everyone was going to a post with the headline “Easy way to find the elevation of your home and the slopes around it.“
I notice the same spike in traffic every time floods threaten. People want to know whether they live higher than the expected crest of the flood, storm surge or, in this case, tsunami.
Published in 2020, the post has received more than 150,000 page views already this year. The biggest peaks were during the May storm, Beryl, Helene and Milton.
Ironically, when I wrote that post about elevation, tsunamis were the furthest thing from my mind.
Even more ironic, shortly before the warning, someone emailed me asking whether I thought a development near the San Jacinto West Fork was safe from flooding. I replied to her that if she had a concern about flood risk, she should buy on the highest ground she could afford, as far from the water as she could get.
Posted by Bob Rehak on 12/5/2024
2655 Days since Hurricane Harvey
System Capacity Maps for Harris County Channels Reveal Widespread Problems
12/2/24 – System capacity maps for each of Harris County’s 23 watersheds show widespread problems and raise questions about whether the county’s most severe needs are confined to historically underserved areas.
Harris County Flood Control District (HCFCD) released the maps in response to a Freedom of Information Act (FOIA) request.
What “System Capacity” Means
System capacity reflects the ability of a stream or channel to handle rainfall of different intensities before coming out of its banks. The way system capacity is expressed also reflects the expected frequency of over-bank events.
A channel that can hold a:
“1% system capacity” indicates that the infrastructure is designed to handle the peak flow or volume associated with a 1% event without causing overflow or flooding.
One Key Part of Comprehensive Flood-Risk Picture
System-capacity maps are one piece of information used in determining a comprehensive view of flood risk. They help you quickly spot areas that need closer examination.
Other key factors used to determine flood risk include: topography around the channel, the elevation of a structure, degree of development in an area, building codes in effect when an area was built, population density, and more.
Impact of Atlas 14
The maps released today rely on pre-Atlas 14 data. Harris County did not provide current maps. Nor do they reflect improvements made to channels recently. But the older maps are still instructive because they formed the starting point for spending billions of flood-mitigation dollars. However…
Said another way, the situation on the ground is worse than the maps show. That’s because the transition from pre-Atlas 14 to Atlas-14 data in Harris County resulted in significant increases in estimated rainfall depths for various storm events.
Notably, the 1% annual exceedance probability (AEP) 24-hour rainfall depth—commonly referred to as the “100-year” event—experienced substantial changes.
Prior to Atlas 14, Harris County was divided into three hydrologic regions with the following 100-year, 24-hour rainfall depths:
With the implementation of Atlas 14, these values increased to:
This represents increases of approximately 31%, 32%, and 33% for Regions 1, 2, and 3, respectively.
These updated figures reflect a more accurate understanding of rainfall patterns, incorporating additional years of data and improved analytical methods. Consequently, infrastructure design and floodplain management practices in Harris County have been adjusted to align with these revised estimates, enhancing resilience against flooding events.
Value of Maps Based on Old Rainfall Standards
Still, these maps have value. They are a starting point for the $2.5 billion 2018 Flood Bond. They also show that:
System Capacity Maps
Below are low-resolution maps for each of the 23 watersheds in Harris County. For high-resolution maps of all watersheds in one file, click here. Caution: [26-meg download.]
Correlation of Maps with Spending
It’s instructive to correlate HCFCD flood-mitigation spending with these maps. Below is where more than $2 billion has gone since Hurricane Harvey, in large part, to address the problems shown above.
Compare the maps with the spending. And use the contact form of this website to let me know if you feel your area is not getting its fair share of flood-mitigation funding.
Note on Next Update with Atlas 14 Data
I have tried to get accurate flood risk data for years. However, HCFCD says it does not routinely update these maps every time it completes a new construction project. The District says it may update them again as part of the MAAPnext project after FEMA approves new maps based on Atlas-14 data.
Posted by Bob Rehak on December 2, 2024
2652 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.