The sky darkens. Thunder grows louder. Suddenly, the rain comes down so hard you can barely see. Soon, water starts coming over the curb toward your house. Your weather radio crackles with warnings. And when it’s over, the stormwater has stopped just feet from your door. Close call! But then you wonder. How intense was that storm? And does it really matter?
The answers affect disaster planning, flood insurance, mitigation, and more.
Consider the intensity question first.
How Intense Was That Storm?
Was it a 1-, 2-, 5-, 10-, 25-, 50- or 100-year storm?
Determining the intensity requires more than just reading your rain gage. That’s because intensity depends on volume within a certain time period. Five inches of rain in an hour should concern you far more than in a day.
Start by going to the Harris County Flood Warning System. It shows all official gages in the region. Hover over any one. A box with a hyperlink will pop up. For this example, let’s assume you live near the East Fork San Jacinto and FM1485.
Hovering over the gage by New Caney pulled up box with the blue link.
Click on the More Information link. It will lead you to a search screen that lets you specify date(s). You can also specify time ranges from one hour to one year.
In this case, I selected 2 days and adjusted the date until I captured all the significant rainfall.
By experimenting with different time ranges, you can narrow down the start and stop times. This is crucial, because intensity depends on both quantity and duration.
At our sample gage, I learned that rainfall from Tropical Storm Imelda in 2019 happened over a two-day time period.
Now, look up the total (28.68 inches) in the Atlas-14 table below. This table shows the rainfall precipitation frequency estimates for the Lake Houston Area.
So, how intense was that storm? The total rain for two days – 28.68 inches – qualifies Imelda as a 500-year storm at that gage. Look down to the 2-Day line and then across to the 500-year column.
If you captured more or less rain in your rain gage at home, use that total instead. At least now you know that you’re working with a 2-day event. (Note: some rain fell outside this range, but not much.)
Check other Gages to See Geographic Rainfall Distribution
While you’re exploring the Flood Warning System, check several different gages, especially those upstream. Rainfall can vary considerably within short distances.
For instance, during Imelda, a gage at the West Fork and SH242 showed only 13 inches of rain during an identical period – less than half the rain just a few miles west. That ranks as a 25-year storm, not a 500!
When people say that “We got X number of 500-year storms in Y years,” it’s important to remember that the 500-year designation applies only to a particular gage, not an entire region.
Do 500-Year Storms Mean 500-Year Floods?
Also remember this. As the example above shows, having a 500-year storm somewhere near you does not automatically mean you will experience a 500-year flood. It depends on where the rain falls in your watershed. If it falls upstream, it could result in a 500-year flood. But if it falls downstream, it will not.
During Imelda, the gage(s) at:
Greens Bayou and US59 got 15 inches – between a 25- and 50-year rain.
Spring Creek and I-45 got 8 inches – a 5-year rain.
Cypress Creek and US290 got 2.52 inches, not even a 1-year rain.
Lake Creek at Dobbin got 1.52 inches.
Lake Conroe at 1375 got .84 inches.
Use the drop-down list of gages to explore rainfall rates elsewhere.
A few miles in each case separated Armageddon from something far more manageable.
What Can You Do with This Information?
This exercise can help protect you in several ways. For instance:
If you didn’t flood during Imelda and you heard it was a 500-year rain, you might assume you didn’t need flood insurance. That could be a financially devastating miscalculation.
If you get little rain upstream and still flood, you should investigate why.
If you flood frequently on small rains, you need to investigate the causes and mitigation. Engage with HCFCD, your county engineer or floodplain manager. Perhaps the floodplain has been altered by upstream development. Perhaps the stream near you has become clogged. Perhaps you need to elevate your home or seek a buyout.
The more you know, the safer you’ll be.
Posted by Bob Rehak on 1/18/23
1968 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/2023/01/CallbackPanelPartial-2.png?fit=820%2C400&ssl=1400820adminadmin2023-01-18 17:14:232023-01-18 17:24:09How Intense Was That Storm? And Why Does It Matter?
One third of Harris County is now impervious cover. According to the Harris County Infrastructure Resilience Team, green roofs are one of several tools that can help mitigate flooding by converting impervious cover back into green space.
Also known as ‘vegetated roofs’ or ‘living roofs,’ they can reduce runoff by soaking up part of the rainfall during a storm and holding it back.
Part of 19,200 square foot green roof installed on EPA headquarters in Denver
Green roofs consist of a waterproofing membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof.
The U.S. General Services Administration (GSA) manages more than 2 million square feet of green roofs. Some date back to the 1930s. So, they have extensive experience.
GSA points out that “Most urban and suburban areas contain large amounts of paved or constructed surfaces which prevent stormwater from being absorbed into the ground. The resulting excess runoff damages water quality by sweeping pollutants into water bodies. Green roofs can reduce the flow of stormwater from a roof by up to 65% and delay the flow rate by up to three hours.
Green roofs mimic natural hydrological processes as part of a watershed management approach to drainage. They are just one of many tools to reduce impervious cover.
Cost/Benefit Analysis Shows Payback
Cost/benefit analysis shows that green roofs can provide payback in 6.2 years. Their longevity has the greatest effect on savings.
They last twice as long as conventional roofs.
U.s. General services administration
Of course, they cost more to install and maintain. But GSA says that the other benefits they provide (stormwater and energy reduction) more than compensate for the premium owners pay.
But any cost/benefit analysis for owners/investors depends on many factors, such as the square footage and height of buildings. Energy and stormwater management benefits increase with size of a roof. Energy savings primarily benefit the higher floors in a multi-story building, i.e., those closest to the roof.
GSA notes that any increased market value of buildings with green roofs was not included in its cost/benefit analysis. GSA believes that if it were, it would show even greater benefit.
Benefits to Owners AND Community
Regardless, even though they provide a positive payback to owners, benefits to the community have the greatest positive impact. They provide net present value savings of almost $38 per square foot of roof, according to the GSA.
It is important that green roofs have a suitable drainage path for excess water that is not absorbed during larger storms. All green roofs should be designed to move excess water away from the building. This water can then be directed to rain gardens or cisterns.
However, green roofs won’t work for everyone. The increased weight requires additional structural support. And the slope must be gentle. They’re primarily for flat or slightly sloping roofs. All those factors make retrofitting them to pre-existing buildings difficult. Generally, engineers must plan for them before construction of a building.
Posted by Bob Rehak on 1/14/23, first in a series on green infrastructure
1964 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2023/01/20230114-Screenshot-2023-01-14-at-6.25.25-PM.jpg?fit=1200%2C680&ssl=16801200adminadmin2023-01-14 18:29:142023-01-14 18:52:11Green Roofs Help Reduce Runoff and Energy Use
On October 28, Royal Pines flooded a neighbor on less than an inch of rain. Two months later, on December 29th, the same thing happened again. The video below provided by the homeowner shows the volume of water funneled across her property by the developer.
Video from NW corner of Royal Pines
This video and the previous one from October demonstrate the dangers of clearcutting and redirecting drainage without first constructing sufficient stormwater detention capacity.
The homeowner who shot the video lives adjacent to the left border in the photo below. Royal Pines has apparently sloped its property toward that corner where contractors will eventually build a stormwater detention basin.
Looking N across Royal Pines. This and other photos below taken on 1/3/23.
Land now slopes toward where video was filmed at left corner. But that area used to slope in the opposite direction. See details below from the USGS NATIONAL MAP and the developer’s plans.
Green arrow on left shows location of homeowner’s property. Red X within V-shaped contour shows exact location of low point (graph on right) before clearing and grading the land.
There used to be an 8-foot drop east of the homeowner’s property. But now, instead of water flowing directly north to White Oak Creek, it flows northwest.
The general plan for Royal Pines (below) shows the same V-shape in the proposed detention basin (upper left). The line represents the edge of the floodplain and confirms that the developer A) knew about the slope and B) changed it.
Royal Pines General Plan.
Silt Fence, Trench Ineffective Against That Much Water
The video above and the photos below show that silt fence makes a terrible dam against even small rains funneling toward a point from such a large area.
Exercise in futility. A series of silt fences have done little to catch and slow the water...or the silt. Note erosion deposited in woods.Looking south. The developer apparently tried to divert runoff racing toward the homeowner with a trench. But erosion from the barren land rapidly filled it in. Runoff also collects at the entrance to Royal Pines. Looking ENE from the entrance at the northern end of West Lake Houston Parkway.
Unfortunately, the developer plans to build homes there, not another detention basin.
0.88 Inches of Rain Fell in Two Hours
The graph below from the Harris County Flood Warning System shows that .88 inches of rain fell in the two afternoon hours before the homeowner shot the video.
Homeowner shot video after first two bars on left.
The table below shows that that much rain in two hours constitutes less than a 1-year rainfall event.
Atlas 14 rainfall probabilities for this area.
That’s consistent with actual observed events and climate records. According to the National Weather Service, on average, we can expect rainfalls greater than 1 inch 14 times per year in Houston. That’s about once per month.
Woodridge Village Revisited
The Montgomery County Engineer’s Office has reportedly asked the developer’s engineering company to revise its plans. The homeowner says that according to the engineer’s office, not even a 6-7 foot tall berm around that portion of the property would be enough to stop all the water flowing in that direction.
So, what lessons can we learn from this example? As with Woodridge Village, don’t clear and grade this much land before constructing detention basins!
The first sentence of Section 11.086 of the Texas Water Code states that “No person may divert … the natural flow of surface waters in the state, or permit a diversion … to continue, in a manner that damages the property of another…”
Posted by Bob Rehak on 1/13/2023
1963 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/2023/01/Keyframe-Royal-Pines-Flood-2.jpg?fit=1200%2C675&ssl=16751200adminadmin2023-01-13 10:01:062023-01-13 10:38:08Royal Pines Floods Neighbor on Less Than 1″ of Rain … AGAIN
How Intense Was That Storm? And Why Does It Matter?
The sky darkens. Thunder grows louder. Suddenly, the rain comes down so hard you can barely see. Soon, water starts coming over the curb toward your house. Your weather radio crackles with warnings. And when it’s over, the stormwater has stopped just feet from your door. Close call! But then you wonder. How intense was that storm? And does it really matter?
The answers affect disaster planning, flood insurance, mitigation, and more.
Consider the intensity question first.
How Intense Was That Storm?
Was it a 1-, 2-, 5-, 10-, 25-, 50- or 100-year storm?
Determining the intensity requires more than just reading your rain gage. That’s because intensity depends on volume within a certain time period. Five inches of rain in an hour should concern you far more than in a day.
Start by going to the Harris County Flood Warning System. It shows all official gages in the region. Hover over any one. A box with a hyperlink will pop up. For this example, let’s assume you live near the East Fork San Jacinto and FM1485.
Click on the More Information link. It will lead you to a search screen that lets you specify date(s). You can also specify time ranges from one hour to one year.
By experimenting with different time ranges, you can narrow down the start and stop times. This is crucial, because intensity depends on both quantity and duration.
At our sample gage, I learned that rainfall from Tropical Storm Imelda in 2019 happened over a two-day time period.
Now, look up the total (28.68 inches) in the Atlas-14 table below. This table shows the rainfall precipitation frequency estimates for the Lake Houston Area.
So, how intense was that storm? The total rain for two days – 28.68 inches – qualifies Imelda as a 500-year storm at that gage. Look down to the 2-Day line and then across to the 500-year column.
If you captured more or less rain in your rain gage at home, use that total instead. At least now you know that you’re working with a 2-day event. (Note: some rain fell outside this range, but not much.)
Check other Gages to See Geographic Rainfall Distribution
While you’re exploring the Flood Warning System, check several different gages, especially those upstream. Rainfall can vary considerably within short distances.
For instance, during Imelda, a gage at the West Fork and SH242 showed only 13 inches of rain during an identical period – less than half the rain just a few miles west. That ranks as a 25-year storm, not a 500!
When people say that “We got X number of 500-year storms in Y years,” it’s important to remember that the 500-year designation applies only to a particular gage, not an entire region.
Do 500-Year Storms Mean 500-Year Floods?
Also remember this. As the example above shows, having a 500-year storm somewhere near you does not automatically mean you will experience a 500-year flood. It depends on where the rain falls in your watershed. If it falls upstream, it could result in a 500-year flood. But if it falls downstream, it will not.
During Imelda, the gage(s) at:
Use the drop-down list of gages to explore rainfall rates elsewhere.
A few miles in each case separated Armageddon from something far more manageable.
What Can You Do with This Information?
This exercise can help protect you in several ways. For instance:
The more you know, the safer you’ll be.
Posted by Bob Rehak on 1/18/23
1968 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.
Green Roofs Help Reduce Runoff and Energy Use
One third of Harris County is now impervious cover. According to the Harris County Infrastructure Resilience Team, green roofs are one of several tools that can help mitigate flooding by converting impervious cover back into green space.
Also known as ‘vegetated roofs’ or ‘living roofs,’ they can reduce runoff by soaking up part of the rainfall during a storm and holding it back.
Green roofs consist of a waterproofing membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof.
Well-designed, engineered and maintained green roofs provide multiple environmental, social, economic, and aesthetic benefits. They help:
Can Reduce Runoff by up to 65%
The U.S. General Services Administration (GSA) manages more than 2 million square feet of green roofs. Some date back to the 1930s. So, they have extensive experience.
GSA points out that “Most urban and suburban areas contain large amounts of paved or constructed surfaces which prevent stormwater from being absorbed into the ground. The resulting excess runoff damages water quality by sweeping pollutants into water bodies. Green roofs can reduce the flow of stormwater from a roof by up to 65% and delay the flow rate by up to three hours.
Extensive green roofs intercept and retain the first 1⁄2 to 3⁄4 inch of rainfall, preventing it from ever becoming runoff.
Green roofs mimic natural hydrological processes as part of a watershed management approach to drainage. They are just one of many tools to reduce impervious cover.
Cost/Benefit Analysis Shows Payback
Cost/benefit analysis shows that green roofs can provide payback in 6.2 years. Their longevity has the greatest effect on savings.
Of course, they cost more to install and maintain. But GSA says that the other benefits they provide (stormwater and energy reduction) more than compensate for the premium owners pay.
The City of Houston provides tax abatements and discounts for green stormwater infrastructure, such as green roofs, in recognition of the stormwater management benefits. Such savings can also provide significant value for owners.
But any cost/benefit analysis for owners/investors depends on many factors, such as the square footage and height of buildings. Energy and stormwater management benefits increase with size of a roof. Energy savings primarily benefit the higher floors in a multi-story building, i.e., those closest to the roof.
GSA notes that any increased market value of buildings with green roofs was not included in its cost/benefit analysis. GSA believes that if it were, it would show even greater benefit.
Benefits to Owners AND Community
Regardless, even though they provide a positive payback to owners, benefits to the community have the greatest positive impact. They provide net present value savings of almost $38 per square foot of roof, according to the GSA.
Green roofs are being increasingly used in urban areas where space constraints limit the use of other stormwater management practices. Such roofs can protect the roofing materials below by shielding harmful UV rays and protecting them from adverse weather events.
Importance of Drainage Paths, Right Plants
It is important that green roofs have a suitable drainage path for excess water that is not absorbed during larger storms. All green roofs should be designed to move excess water away from the building. This water can then be directed to rain gardens or cisterns.
In Texas, such roofs require special plant selection that can hold up to the region’s extremes in heat and humidity.
Not for Everyone
However, green roofs won’t work for everyone. The increased weight requires additional structural support. And the slope must be gentle. They’re primarily for flat or slightly sloping roofs. All those factors make retrofitting them to pre-existing buildings difficult. Generally, engineers must plan for them before construction of a building.
Posted by Bob Rehak on 1/14/23, first in a series on green infrastructure
1964 Days since Hurricane Harvey
Royal Pines Floods Neighbor on Less Than 1″ of Rain … AGAIN
On October 28, Royal Pines flooded a neighbor on less than an inch of rain. Two months later, on December 29th, the same thing happened again. The video below provided by the homeowner shows the volume of water funneled across her property by the developer.
This video and the previous one from October demonstrate the dangers of clearcutting and redirecting drainage without first constructing sufficient stormwater detention capacity.
Altering Landscape Accelerates Runoff Toward Homeowner
The homeowner who shot the video lives adjacent to the left border in the photo below. Royal Pines has apparently sloped its property toward that corner where contractors will eventually build a stormwater detention basin.
Land now slopes toward where video was filmed at left corner. But that area used to slope in the opposite direction. See details below from the USGS NATIONAL MAP and the developer’s plans.
There used to be an 8-foot drop east of the homeowner’s property. But now, instead of water flowing directly north to White Oak Creek, it flows northwest.
The general plan for Royal Pines (below) shows the same V-shape in the proposed detention basin (upper left). The line represents the edge of the floodplain and confirms that the developer A) knew about the slope and B) changed it.
Silt Fence, Trench Ineffective Against That Much Water
The video above and the photos below show that silt fence makes a terrible dam against even small rains funneling toward a point from such a large area.
Unfortunately, the developer plans to build homes there, not another detention basin.
0.88 Inches of Rain Fell in Two Hours
The graph below from the Harris County Flood Warning System shows that .88 inches of rain fell in the two afternoon hours before the homeowner shot the video.
The table below shows that that much rain in two hours constitutes less than a 1-year rainfall event.
That’s consistent with actual observed events and climate records. According to the National Weather Service, on average, we can expect rainfalls greater than 1 inch 14 times per year in Houston. That’s about once per month.
Woodridge Village Revisited
The Montgomery County Engineer’s Office has reportedly asked the developer’s engineering company to revise its plans. The homeowner says that according to the engineer’s office, not even a 6-7 foot tall berm around that portion of the property would be enough to stop all the water flowing in that direction.
So, what lessons can we learn from this example? As with Woodridge Village, don’t clear and grade this much land before constructing detention basins!
The first sentence of Section 11.086 of the Texas Water Code states that “No person may divert … the natural flow of surface waters in the state, or permit a diversion … to continue, in a manner that damages the property of another…”
Posted by Bob Rehak on 1/13/2023
1963 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.