Atlas 14 Updates Rainfall Frequency Estimates Developed in 1960s
Developers must now design detention and storm sewers around rainfall rates that increase 16-32% compared to the old standards for Harris County.
Data included in Atlas 14:
Replaces rainfall-depth information used since the 1960s
Provides estimates of the depth of rainfall for average recurrence intervals of 1 year through 1,000 years, and durations from 5 minutes to 60 days.
NOAA collected this data in Texas through December 2017, which includes rainfall from Hurricane Harvey.
New Atlas-14 Rainfall Frequency Estimates for the Lake Houston Area
Floodplain, Detention & Fill Restrictions
The amended policy manual adopts the increased precipitation rates. It also specifies more rigorous criteria for detention basins and fill within the floodplain.
Amendments anticipate that the future Atlas-14 1% (100-year) floodplain will equal the current 0.2% (500-year) floodplain.
Harris County Flood Control District
Therefore, these amendments are considered to be interim and will be reevaluated once new floodplains have been produced as part of HCFCD’s Modeling Assessment and Awareness Project (MAAPnext) in late 2021. You can find more information on MAAPnext at www.maapnext.org.
Zero Net Fill
The old guidelines prohibited developers from adding fill only within the 100-year floodplain. Now they’re prohibited from adding fill within the 500-year floodplain, too. The policy is called “zero net fill.” It means developers cannot bring fill into the floodplains. They can, however, excavate fill from one part of their property and use it to build up another part of their property.
Under new guidelines, developers cannot bring fill into either the 100-year or 500-year floodplains.
For a 20-acre development, the average volume of stormwater within detention basins will increase by about 20%, or about 32,500 additional gallons per acre.
Effort to Harmonize Floodplain Regs with Neighbors’
Camcorp Management is building a new high-density development in Montgomery County called Brooklyn Trails on a tributary of Ben’s Branch upstream from Kingwood. The development’s detention pond is apparently 30% smaller than new Atlas-14 regulations would require for this area.
Most of Brooklyn Trails is still vacant...…but time is running out to do something. High density homes are going up quickly.
Even though plans were discussed, reviewed and revised after Atlas 14 went into effect, in Montgomery County the submission date determines which rainfall statistics apply.
Ben’s Branch cuts diagonally through Kingwood. It goes through three commercial areas: Northpark, Town Center and Kings Harbor. Bear Branch Elementary, Kingwood High School and the Humble ISD instructional center all border Ben’s Branch, not to mention hundreds of homes and St. Martha Catholic Church.
Rainfall rates that A&S used to design drainage for Brooklyn Trails vary substantially from MoCo’s new rate and Atlas-14 rates for the Lake Houston Area.
Montgomery County bases its 100-year/24-hour rainfall rate on Conroe (the County seat). Despite variations within the county from north to south, adopting the Conroe rate makes it easier for developers to calculate detention requirements. Some parts of the county have no gages. However, the uniform rate also understates the detention needed for new developments in the fast growing southern part of the county, which receives more rain.
Differences Between Three Rates
The three different rates referenced above for the 24-hour 100-year rain break down as follows:
17.3 inches = Rate for 100-year/24-hour rain for Lake Houston Area by NOAA (see below)
That means Brooklyn Trails is 25% short of MoCo’s new requirements and 30% short of NOAA’s.
NOAA Atlas 14 Rainfall Totals for the Lake Houston Area. Brooklyn Trails is 3 miles from Lake Houston but 20 miles from Conroe.
In fact, the rate A&S used (12.17 inches) corresponds to a 10- to 25-year rain by NOAA’s new standards, not a 100-year rain.
A&S Engineers Certify No Adverse Impact
A&S concluded on page 10 of its analysis that “…the proposed excavation/fill will cause no increase to the base flood elevation, and the proposed excavation/fill will have no adverse impact to the drainage on, from, or through adjacent properties.”
That may be true if you base all your calculations on rainfall that’s 30% less than NOAA’s best available statistics. Or even the new MoCo numbers. But, in fact, we get more rain.
Both Montgomery County and City of Houston signed off on the A&S plans. The City signed in January before the Elm Grove floods. Montgomery County signed after the Elm Grove floods – on 10/1/19.
There’s time to fix this before the development is built out. But that window is rapidly closing.
Posted by Bob Rehak on 10/12/2020
1140 Days since Hurricane Harvey and 389 since Imelda
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.
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The official rain gage at West Lake Houston Parkway and the West Fork San Jacinto recorded 2.32 inches of rain between 7 and 9 a.m. this morning. That was officially a 1-year rain. (See Atlas-14 chart below.) As rains go, it was not a severe test; it was more like a pop quiz.
After the rain subsided, Taylor Gully in Elm Grove was less than half full.
The good news: Taylor Gully was well within its banks and no one in Elm Grove or North Kingwood Forest flooded. The bad news: Perry’s detention ponds experienced severe erosion, enough to warrant repairs and perhaps delay the schedule.
The even worse news: Harris County’s meteorologist, Jeff Lindner predicts another one to two inches of rain tonight with isolated totals of three to four.
West Lake Houston Pkwy. Gage Showed 2.32 Inches In 2 Hours
24 hour rain totals for WLHP gage showed bulk fell in 2 hours.
2.32 inches in two hours qualifies as a one-year rain according to the new Atlas-14 rainfall precipitation frequency estimates. Even if you considered the entire 3.12 inches in 24 hours, it would still only be a one year rain.
Atlas 14 Precipitation Frequency Table for the Kingwood Area.
Aerial Images Show How Perry Detention Ponds Performed
These aerial images taken shortly after noon today when the rain stopped show that:
The detention ponds are starting to do their work and hold back water.
That kept the level in Taylor Gully manageable
The overflow spillway between S2 and the concrete-lined channel was apparently not needed.
However, the images also show that:
Portions of the detention pond walls severely eroded and appear to have collapsed in places.
The water in the N1 pond overcame temporary dirt barricades sending water and silt down to N2.
The newly excavated N2 was entirely covered with water for the first time. It also received a significant amount of erosion.
N3 merges with Taylor Gully to form one large detention pond that holds water all the way from the northern end of the pond to the county line.
Rain has halted construction for the last two days and could delay it into next week.
Expanded, giant N2 detention pond was covered entirely with water for first time.Looking West toward western border of Woodridge Village.However, erosion re-deposited large amounts of soil within the pond. Looking North along Western Border of Woodridge Village.Rainwater entering the site from Joseph street in Porter (center left) shows by comparison how much silt the Perry water held. Looking north along western border of Woodridge Village.Still looking north, but farther up western border, you can see silt slumping into ditch.Looking SE toward Elm Grove and North Kingwood Forest from the NW corner of site. Water coming in N1 pond from left exited right, down the western border. Water washed out a temporary dirt barrier that appeared designed to hold water in the pond.Looking east. Note erosion from former utility corridor on left that has turned into a new drainage ditch along northern edge of property. Looking at western wall of N3 which runs along eastern border of Woodridge Village.Another portion of the western wall of N3 shows severe erosion.Standing water from rest of property is slowly making its way into detention ponds.Looking South along eastern border toward Taylor Gully. At present, N3 (bottom left) simply merges with the concrete channel by S2 (top right). It appears to have nothing to control the outflow.Looking north along eastern border. Silt fences prove inadequate at stopping erosion. In fact, most of site has no silt fences.
More Rain Likely Tonight
Jeff Lindner, Harris County meteorologist, says that today’s wet pattern should remain in place through the weekend, contrary to earlier predictions that saw rain chances ending by Friday.
Storms currently in the Gulf near Corpus Christi are tracking toward Houston late tonight and Friday morning. They will probably not be as severe as this morning’s storms. With that said…the air mass remains tropical over the region and excessive rainfall rates of 2-3 inches per hour will be possible, warns Lindner.
As of 6 p.m. Thursday, the National Weather Service decided NOT to issue another flash flood watch for tonight, but stay alert to see if a more significant threat may develop.
Expect rainfall amounts of generally 1-2 inches tonight with isolated totals of 3-4 inches.
To Get Up-to-the-Minute Forecasts and Stream Alerts
1031 Days since Hurricane Harvey and 280 since Imelda
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.
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What causes street flooding? At the risk of clarifying the obvious, rain accumulates faster than storm sewers and drainage ditches can carry it away.
A Lack-Of-Capacity Issue
Most streets are actually designed to be part of the flood retention system in any community. That’s because most storm sewers can only handle a two-year rain (about 2 inches per hour). When we get more than that – say a 10-, 25-, 50- or 100-year rain – water is stored in the street until capacity opens up in the storm sewers, ditches and creeks.
As you can see from the new Atlas-14 rainfall chart below, a 2-year rain in this area is 2.23 inches/hour; a 25-year rain 3.88 inches/hour; and a 100-year rain 4.88.
New Atlas-14 Rainfall Data for Lake Houston area from NOAA
When evaluating rainfalls, look at the storm totals AND shorter intervals, such as 15, 30 and 60 minutes.
Street flooding usually results from short, high-intensity downpours caused by slow-moving or training thunderstorms.
From a street-flooding perspective, getting 4 inches of rain in one day is not the same as getting 4 inches in one hour.
If you get 2 inches of rain in 30 minutes, you’re already at a 5-year rain. That’s well beyond the design capacity of storm sewers. You can expect water to back up into the street at that point, even if there are no blockages in the storm sewers.
That’s why builders elevate most homes several feet above street level and above the 100-year flood plain. It gives you an additional margin of safety.
How To Determine Intensity of Rainfalls
If you flooded from your street, first determine whether the cause was simply overwhelming rainfall or whether complicating factors existed.
How can you determine how much rain you got in any given time interval during a storm? Follow these simple steps:
Click on the gage nearest you. (For me, that’s Gage #755 at the San Jacinto West Fork and West Lake Houston Parkway. I will use that in the example below.)
In the pop-up window, click on the “For More Information” button.
At the top of the next window, select date and time intervals. The Time Interval varies from One Hour to One Year. I selected September 19, 2019 (the day of Imelda) and 24 hours. That shows me 24 1-hour intervals. From this and the table above, you can see that we had three very intense hours in a row during Imelda.
HarrisCountyFWS.org shows we got almost 11 inches during Imelda, the vast majority of it in three hours.Note: selecting other time intervals displays other time increments.For instance the system breaks hours down into 5-minute increments, years into months, etc.
From the two charts above, correlate the actual precipitation with the recurrence intervals. You can see that…
We had a 10-year rain, followed by a 5-year rain, followed by a 2-year rain – all in three hours!
Every single one of those hours met or exceeded the maximum capacity of the storm sewers. So it’s easy to see WHY we had street flooding.
When Street Flooding Turns into Home Flooding
In a small percentage of cases, street flooding turns into HOME flooding – when there simply isn’t enough backup capacity in the streets. (In the following discussion, I’m EXCLUDING homes that flooded from rivers, streams, or overland sheet flow during Imelda, i.e., Ben’s Branch, Elm Grove, etc.).
Extreme events reveal the weaknesses in any system. If your home was:
At a low point on the street…
Near a clogged storm drain…
A foot or two lower than surrounding homes…
At the bottom of a hill…
In an area where water collected or converged…
Near an outfall pipe that collapsed or was blocked…
Upstream from a ditch that was blocked…
…you may have flooded.
And then there are the bizarre cases.
I visited one man in Trailwood at the bottom of a hill that had NO storm drains. Inexplicably, someone placed the nearest drain in the middle of the hill – about half a block ABOVE his home.
Another man called me who lived near Village Park Drive next to a tributary of Ben’s Branch. The Community Association had erected a fence between the end of the street and the tributary. They built the fence so low to the ground that it became clogged with weeds and grass clippings during Imelda and formed a dam. In the heavy rain, water could not get under it and backed up into his home.
What Can You Do?
Short of praying or digging up every street in Houston to enlarge the storm sewers, homeowners DO have some remedies.
Keep storm drains clear. Keep yard waste out of them.
Call 311 for a storm-drain inspection if you suspect yours have become clogged. The City is currently inspecting ALL drains in Kingwood subdivisions that had street flooding last year.
Inspect outfall pipes where your storm drains enter the nearest ditch to ensure they have not collapsed or become blocked.
Look out for new construction, such as the fence above, that may back water up. Remove or elevate the horizontal rot board if it blocks the overflow of water from your street.
If the problem recurs in less extreme events, consider flood proofing or elevating your home.
Make sure you have flood insurance; that it’s up to date; and that it reflects the true replacement value of your home.
Wide shot from farther up the block of fence shown above. Gap under fence did not exist at time of Imelda.Note how rot board has NOW been elevated to allow water collecting in street to get into creek beyond fence.
Great Options Where Possible
If your area floods repeatedly, you may also be interested in lobbying the City or County to build an overflow spillway or detention pond between your street and the nearest drainage channel. Obviously, geographic circumstances may rule this possibility out for many. But if you have a vacant lot in your neighborhood and a nearby ditch…
Example of community detention pond with overflow channel to Taylor Gully (beyond fence). This wasn’t enough to protect North Kingwood Forest in Imelda, but their problem was complicated by sheet flow from Woodridge Village.
Posted by Bob Rehak on 2/22/2020
907 Days after Hurricane Harvey and 156 after Imelda
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Usually when you make an investment, the worst thing that could happen is that you lose all your principle. But Perry Homes could loose a hundred times more than they paid for Woodridge Village land. That takes special talent.
Out-of-Pocket Costs
The land that Woodridge Village sits on didn’t cost much; much of it was wetlands and many streams converged there. Regardless, a Perry Homes subsidiary, Figure Four Partners, bought the land. Montgomery County Appraisal District values the two main parcels at less than a million dollars. Together they comprise more than 80% of the 268-acre project. (See screen captures below from Montgomery County Appraisal District website.)
Real Costs Could Be 100X Greater
Now let’s look at the real costs to Perry. Just to screw up the land, they paid for:
Excavating two detention ponds (out of five they promised)
Soil tests and a geotechnical report
A mile of pavement to the middle of nowhere
Two large box culverts
Storm drains
Let’s say that cost another five million.
But all of that contributed to the flooding of approximately 200 homes in May and 350 in September. Let’s assume the damage to each home totaled $100,000. That comes to about $55,000,000.
Furniture, appliances, rugs, window coverings and other contents? Let’s assume an average of $40,000. That would total another $22,000,000.
Let’s also assume that 300 cars flooded. Average cost – $30,000. Bingo. $9 million.
Now let’s estimate the reduced marketability of homes that flooded. To do this, let’s assume an average price of $200,000 per home and a 20% reduction. That would cost homeowners $40,000 each in the market value of their homes. That’s another $22,000,000.
If juries rule in favor of the flood victims, that million dollar investment could add up to more than $100 million in potential liabilities…before any penalties for negligence and/or gross negligence kick in.
But perhaps Perry Homes went too far. People did notice. The wetlands that they conveniently ignored fall under the jurisdiction of the Army Corps. And the Corps is now investigating potential violations of Section 404 of the Clean Water Act. That could get expensive all by itself.
Like Building Homes at the End of a Gunnery Range
It just keeps getting worse for Perry. This was kind of like buying land to build homes at the end of a gunnery range. A little risky.
But it’s too late to rethink that decision. No one will ever want to buy a home on this site. It’s less marketable than swampland near Chernobyl.
There’s another rule of thumb in business. When you find yourself in a hole, stop digging. And that’s exactly what Perry has done. They have stopped work on the site for months. Work on detention ponds that would help protect people downstream from future flooding is going undone.
That means the numbers above could balloon with the next big rain. Or a negligence ruling by a jury. Yep, we’re in double Jeopardy now.
Career-Limiting Moves
Whoever made the decision to develop Woodridge Village definitely made a CLM (career-limiting move). At this point, even Perry Homes employees not associated with the decision must worry about their Christmas turkeys. Few careers or companies survive blunders that become case studies for how not to do something.
Eroding Profit Margins
Because of faulty assumptions and corner cutting, Perry Homes put itself between a rock and a hard place. They’ve managed to turn a million dollar investment into a potential $100 million liability. They can’t develop this property profitably now. And they can’t sell it. Who would want to buy this land and inherit the liability every time a storm cloud floats by?
To protect downstream homes from flooding, they would have to expand the detention ponds by at least 40%. And that would eliminate so many homesites that costs could exceed income. I say “at least” because the issue is not just Atlas-14 compliance. While digging the S2 detention pond, contractors hit water that’s not going away.
The S2 Detention Pond has lost about 20-30% of its capacity. The bottom 3-5 feet have been filled with ground watersince contractors started digging to the target depth.
That means they can’t achieve their detention goals by going deeper; they’ll have to go wider. And that will cut into marketable land even more.
Toxic for Perry Homes
Let’s face it. When Perry Homes bought this property, Kathy Perry Britton swallowed a poison pill. Woodridge Village now has a toxic reputation that will infect the rest of Perry Homes. No one will ever be able to trust anything Perry Homes says again.
But what to do with this land? If you’re Kathy Perry Britton trying to spit shine the legacy of dear old dad, you can’t keep it. And you can’t sell it. You can’t even give it away. No land conservancy organization would take it until the damage done to wetlands and streams was remediated. That could take decades.
The Real Value of Wetlands
However, there are two pieces of good news in this mess.
If Perry Homes implodes, it won’t take a lot of investors with it; the company is private.
Perry Homes may serve as a lesson to other developers and teach them that the real value of wetlands is their downstream legal costs.
Time To Be Decisive
Just remember, Ms. Britton. Historically, 85% of Houston floods are non-tropical. So if you think you have eight more months to figure this out, think again.
Posted by Bob Rehak on 11/15/2019
808 Days after Hurricane Harvey and 57 after Imelda
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.
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