Tag Archive for: Atlas 14

Here Come New Precipitation-Frequency Estimates…Again

NOAA’s new Atlas 15 precipitation-frequency estimates will soon replace recently introduced Atlas 14 estimates – even before the Atlas 14 estimates have been fully adopted and integrated into local regulations.

About Precipitation-Frequency Estimates

Engineers use precipitation-frequency estimates to design, plan and manage infrastructure under Federal, State and local regulations. For instance, to ensure homes are built X feet above the 100-year floodplain, engineers must “know” how much rain will fall in a 100-year storm. Predicting that is one of NOAA’s jobs.

However, haphazard adoption of the new estimates has created a patchwork quilt of regulations across Texas and the U.S. One of the dirty, little secrets in the flood mitigation business is that many jurisdictions fail to adopt the new estimates and update their regulations accordingly. It’s costly, time-consuming, and raises the bar for developers.

So, many jurisdictions continue to use lower estimates to help attract development.

But designing infrastructure around artificially low rainfall estimates can lead to insufficient mitigation that increases flood risk for everyone.

Some Estimates Now in Effect Go Back 60 Years

In 2018, NOAA introduced Atlas 14 precipitation-frequency estimates for Texas. They replaced earlier estimates published by NOAA as early as the 1960s. Some parts of the Houston region still use those earlier estimates today.

Atlas 14 estimates superseded those published in:

  1. Weather Bureau Technical Paper No. 40, Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years (Hershfield, 1961)
  2. Weather Bureau Technical Paper No. 49, Two- to Ten-Day Precipitation for Return Periods of 2 to 100 Years in the Contiguous United States (Miller, 1964).

Newest Estimates Will Incorporate Climate Change

Compared to those, Atlas 14 estimates are more accurate. They incorporate data from newer technologies and more data collected over longer periods. Atlas 14 totals increased 30-40% for the Lake Houston Area.

The Atlas 15 estimates are just getting underway and have not yet been developed. NOAA expects to release them in 2027.

Atlas-15 timeline by NOAA

NOAA claims its Atlas-15 update will improve precipitation-frequency estimates by leveraging non-stationary climate estimates. Previous estimates, such as Atlas 14, have assumed a stationary climate.

In statistics, “non-stationary” means the underlying environment changes, say due to some strong trend or seasonality. Many people believe climate is changing and hence the desire to build that into the new precipitation-frequency estimates.

National Funding, New Updates Every 10 Years

Historically, NOAA precipitation-frequency estimates have been funded by states and other users, on a cost-reimbursable basis. However, that is changing.

Moving forward, the Federal government will fund precipitation-frequency updates. Under the Floods Act, signed into law in December 2022, NOAA will update precipitation-frequency estimates every 10 years.

Goals include:

  • Updating standards
  • Incorporating climate change
  • For the entire country.

Voluntary, Local Participation

But there’s a dirty little secret that not many people know about. Nothing forces individual cities, counties or states to adopt the estimates and work them into their regulations.

That’s a big job. And an expensive one. So, not all jurisdictions do it. Many areas surrounding Houston still plan infrastructure using data developed 60 years ago.

If you plan on less rain, channels can be narrower and stormwater-detention basins smaller. But residents are not protected as much as they should be.

Posted by Bob Rehak on 9/5/2023 based on information from NOAA.

2198 Days since Hurricane Harvey

New Data Suggests Houston’s Expected 100-Year Flood Is More Likely to Happen Every 8 to 23 Years

First Street Foundation, a non-profit risk-research group, estimates (based on what it says are “well known” Atlas-14 flaws) that a so-called 100-year flood event in Houston could likely happen every 8 to 23 years.

NOAA’s Atlas 14, a massive, years-long effort, which hasn’t even been fully implemented yet, may already be seriously out of date according to First Street.

As a result, First Street claims the design standards for infrastructure projects based on erroneous Atlas-14 data are likely to fail. Trillions of infrastructure investment dollars hang in the balance.

NOAA Replacing Atlas 14 with Atlas 15 Already

NOAA expects to release its latest Atlas-15 rainfall probability statistics for the U.S. sometime in 2027. Like Atlas 14 below, they will contain probabilities for every location in the country – for durations ranging from 5 minutes to 60 days and recurrence intervals from 1 year to 1000 years.

atlas 14 rainfall probabilities
Atlas 14 Probabilities for the North Houston area.

Moreover, for the first time ever, Atlas-15 probabilities will come in two flavors: with and without estimates for the impact of climate change.

First Street Foundation, a non-profit research and technology group, specializes in environmental risk assessment. They position their system, RiskFactor.com, as a stopgap until NOAA releases Atlas 15.

Time Lags Cause Confusion, Create Danger

FEMA still has not released flood maps based on the Atlas-14 probabilities above. The MAAPNext Group within Harris County Flood Control District has been working on those since Harvey. MAAPNext’s latest timeline (below) shows that FEMA may not make Atlas-14-based flood maps official for another 3+ years.

Engineers and government officials use this data when designing new subdivisions, industrial facilities, bridges, highways and other infrastructure.

For instance, they need to know, how high the bridge must be to let water flow under it during a flood to avoid catastrophes like the one below.

I-69 repairs
Old data led TxDoT to inadequately design the I-69 bridge over the San Jacinto West Fork. Repairs took more than a year after Harvey to complete while residents endured massive traffic jams.

Atlas 15 Underway Before Atlas 14 Implemented

A copyrighted article in the New York Times this morning by Raymond Zhong was titled “Intensifying Rains Pose Hidden Flood Risks Across the U.S.” In it, Mr. Zhong claims that new calculations show hazardous storms can dump significantly more water than previously believed.

“One in nine residents of the lower 48 states, largely in populous regions including the Mid-Atlantic and the Texas Gulf Coast, is at significant risk of downpours that deliver at least 50 percent more rain per hour than local pipes, channels and culverts might be designed to drain,” says Zhong.

Compounding the problem, “NOAA’s estimates are ‘the floor, not a ceiling,'” said Zhong, quoting Abdullah Hasan, a White House spokesman.

“That means millions of homeowners might be making decisions with an incomplete understanding of the true physical and financial risks they face,” said Zhong.

145,000 Houston Homes and Billions in Infrastructure Caught in Time Lag

Zhong quoted First Street Foundation, which said that in Houston alone, as many as 145,000 homes may be in the 100-year flood zone, but that they are not shown that way in current FEMA flood maps.

To put Atlas 15 and its climate change corrections into perspective…

First Street estimates that in Houston, what we currently think of as a 100-year flood may actually be an 8- to 23-year flood.

First Street Foundation Press Release

All this comes as the nation gears up to spend more than $1.2 trillion dollars on infrastructure which Congress and President Biden approved in 2021.

And that $1.2 trillion doesn’t even include the money homebuyers spend each year. About 30% of all household income in the U.S. goes toward housing. And the average American moves once every seven years.

That means virtually everyone is likely not making home-buying decisions based on the most current (accurate) flood probabilities. By the time FEMA releases Harvey-based Atlas-14 flood maps, Atlas-15 revisions will already be available to a select few.

While the Association of State Flood Plain Managers finds First Street data useful, it emailed a report at the close of business today about First Street. The report says that “ASFPM has and will continue to support NOAA’s work on Atlas 14 and 15, which will remain the gold standard within our profession.”

Problems Caused By Lack of Timely Updates

The vast majority of developers, homebuilders and engineers are ethical. But some less scrupulous developers can exploit confusion caused by irregular update policies.

Likewise, engineers who designed a bridge to one set of specs, may find their work out-dated before construction starts. What are the ethical obligations in a case like that?

Just this year, we’ve seen numerous instances of developers trying to get their plans grandfathered under pre-Atlas-14 regulations even as the U.S. moves toward Atlas 15. Little wonder that when a flood happens, few can explain where the system went wrong.

“The fact that the Nation will not have the most accurate estimates of extreme precipitation likelihoods available at the time of the design of these projects means that many of them will be out of date on the day they are opened to the public,” said Matthew Eby, Founder and Executive Director of First Street Foundation.

Governments at all levels need to work better together to shorten the data supply chain. Doing so could save Americans trillions of dollars.

Posted by Bob Rehak on 6/26/2023

2127 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.

MoCo Updating Drainage Criteria Manual, Subdivision Rules

Montgomery County (MoCo) Commissioners voted on 8/23/22 to update the County’s Drainage Criteria Manual and its Subdivision Rules and Regulations. Precinct 2 Commissioner Charlie Riley made the motion (item 16.C on the 8/23/22 Commissioners Court agenda).

Montgomery County Commissioners Court discusses new drainage and subdivision manuals in 8/23/22 meeting.

See the discussion in the MoCo Commissioner’s Court video. Select Item 16. The discussion starts at 3:12.

The previous Drainage Criteria Manual posted on the MoCo Engineer’s site is dated 1989, but appears to have some minor updates from 2019. The Subdivision Rules and Regulations for new developments date even further back, to 1984, although they too had new amendments and addenda incorporated in July, 2021.

MoCo hired Halff Associates to do the updating. Their fee: $302,000.

Welcome News

This is welcome news for people in northern Harris County. Drainage and engineering standards in MoCo have lagged those in Harris. That has created adverse downstream impacts even though developers may technically meet MoCo requirements. But the lower standards enable them to claim “no adverse impacts” when, in fact, there may sometimes be some.

Changes Could Reduce Flooding in MoCo and Harris Counties

Since Harvey, the Harris County Engineering Department and Flood Control District have worked to get surrounding counties to adopt five minimum drainage standards. They include:

Scope of Content Updates

The Scope of Work approved by MoCo Commissions last week shows that Halff will examine most, if not all, of these issues and more. The effort will evaluate and potentially update, at a minimum:

  • Hydrologic methodology (this includes hydrographic timing but is broader)
  • Detention sizing and outfall design
  • Open channel design frequency and requirements
  • Floodplain analysis.

Process for Updates

The scope of work also defines the process that Halff will follow. It includes:

  • Coordination with County engineering staff
  • Evaluation of existing manuals
  • Identifying dated criteria/information
  • Comparisons with neighboring counties‘ practices (see below)
  • Revisions
  • Development of the new documentation
  • Stakeholder review and reporting
  • Presentation to Commissioners Court
  • Reporting approved changes to adjacent counties.

Work should take about a year.

Comparison with Regs in Other Entities

For the drainage Criteria Manual, Halff will compare criteria from TxDOT, Harris County, HCFCD, Waller County, Fort Bend County, and Brazoria County.

Halff will compare MoCo’s Subdivision Rules and Regulations to those in Harris, Waller, Fort Bend, and Walker Counties.

This is more good news for those in northern Harris County.

About Halff Associates

A source in the engineering community characterized Halff as a good company. He said, “The Montgomery County manual is in good hands….as long as they let Halff do the right things.”

Halff will work with the MoCo Engineer Jeff Johnson on the updates.

Subdivision Rules and Regulations

Neither the Scope of Work, nor Commissioners discussed specific recommendations for updates to Subdivision Rules and Regulations. But Commissioners did request an opportunity to discuss and review updates on both manuals before they came back to Commissioners Court for final approval.

Immediate Impact

One former MoCo employee said, “There is still the hurdle of the court adopting the updated standards. Expect a rush of drainage studies to be submitted in the next year so they can be grandfathered in.”

We saw this in the City of Houston (CoH), for instance, with the Laurel Springs RV Resort. The detention pond in that development is half the size required by new standards. CoH permitted it one day before the new standards went into effect.

Related News: MoCo Floodplain Administrator Office

At about 40 seconds into the video for Items 17 and 18 on the agenda, the Commissioners approved a motion to have Precinct 2 Commissioner Charlie Riley oversee MoCo’s Office of the Floodplain Administrator. Reasons for the change were not clear. Discussion happened in Executive Session.

All we have to go by is the outcome. And the outcome shows that MoCo is bringing the Office of the Floodplain Administrator – for the whole county – under the direct, political control of one precinct commissioner. Interesting.

Posted by Bob Rehak on 8/28/2022

1825 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.

Giving Thanks to the Women and Men of Harris County Flood Control

A reader who visited a trade show recently in Las Vegas sent me several links to news stories about flash flooding there. 1.24 inches of rain caused widespread flooding, killed at least two people, and resulted in dozens of high water rescues!

CBS Video posted on YouTube of Las Vegas Flooding

See also:

His comment: “Imagine if those types of videos were in Houston – for less than two inches of rainfall. You can’t, because it doesn’t happen.”

Why Houston Doesn’t Flood On Two Inches

While HCFCD employees take a lot of heat every time someone floods, we should remember that it takes far more rain for people to flood here. There are several reasons for that.

First, Harris County formed a flood control district in 1937. Clark County, Nevada (Las Vegas) didn’t start its until 1985. So, we had an almost 50-year head start on them.

But sadly, some fast-growing counties around Harris County STILL don’t have flood control districts! (We’ll save that discussion for the next legislative session.)

Second, our topography is different. Because Harris County is so flat, rainfall spreads out and starts soaking into the ground before flooding starts. Rainfall in Las Vegas is funneled by the rugged landscape. It picks up velocity, so it doesn’t have time to soak in. Concentrated rainfall turns into flash flooding. The Las Vegas Wash funnels a 1,879 square mile watershed toward a metro area of 2.29 million people.

From Flood Hazards and Flood Risk in Nevada’s Watersheds

I was almost killed by a flash flood in Tucson once. While hiking along a stream bed with friends in the desert, we saw rain in the distant mountains. They immediately suggested moving to higher ground. Minutes later, a wall of water 6-8 feet high came boiling down that stream bed!

Third, we build to different rainfall standards. Las Vegas averages 4.18 inches of rain each year. Harris County averages 51.84 inches.

In Harris County, new building codes and flood-mitigation standards currently use the 24-hour, 100-year amount shown in the Atlas-14 table below – 17.3 inches.

atlas 14 rainfall probabilities
Atlas-14 rainfall probabilities for northern Harris County now form the basis for building codes and flood-mitigation projects.

In other words, we build things to withstand more rain in a day than Las Vegas receives in FOUR YEARS.

That takes some talent. Especially when surrounding areas send ever-increasing amounts of floodwater downstream because of lack of comparable controls upstream. And that could be why flood control districts around the country try to recruit talented HCFCD employees.

Despite our occasional frustrations, we should never forget: They stand between us and disaster.

Posted by Bob Rehak on 8/13/2022

1810 Days since Hurricane Harvey

Despite Heavy Weekend Rains, Most Area Channels and Streams Stayed Within Banks

Despite heavy weekend rains, with a few exceptions, streams and channels stayed within their banks. There are several possible explanations.

  • Soil was dry before the rains.
  • Rainfall came in two waves separated by several hours, allowing the first peak to start working its way through the system before the second hit.
  • The amount of rainfall was within the designed capacity of most channels.
  • The heaviest storms occurred under relatively narrow bands of training supercells.
  • Harris County Flood Control has been actively working on channels!

Rainfall Map of Heavy Weekend Rains

In the image below, note how much higher the rainfall totals are near the red line compared to areas farther away. Most upstream areas received less than an inch or two, limiting the amount that traveled downstream.

Red line indicated path of supercells that tracked across the center of the county last weekend. Note how highest rainfall totals parallel line.

Heavy But Not Harvey

If you were under one of those supercells, you probably received 5-8 inches of rain between Saturday afternoon and Sunday morning – a little more than a 12-hour time span. Consulting NOAA’s Atlas-14 Rainfall Probability table for this area, you can see that those totals correspond to 2- to 10-year storms. Heavy! But not Harvey!

atlas 14 rainfall probabilities
NOAA’s Atlas-14 rainfall probabilities for the Lake Houston Area

Storms Tracked Perpendicular to Most Watersheds

In Harris County most watersheds track from NW to SE. But the storms tracked perpendicular to that. That limited the amount of water dumped in most watersheds. It might have been very different had the storms tracked parallel with bayous.

Here was the channel status report from Harris County Flood Control on Sunday shortly after noon. It shows that virtually all channels were well within their banks. Only the gage at Luce Bayou and SH321 in Liberty County indicated flooding was a possibility near Lake Houston (warning triangle in upper right).

Despite receiving the highest rainfall total in the area (8.56 inches)…

…Luce Bayou never did come out of its banks at that location. See below. As of today, Luce is falling.

Halls Bayou near 45 briefly came out of its banks, but no structures were reported flooded. Same for Greens Bayou at 59. Water briefly got up to the feeder road there.

Brickhouse Gully, White Oak and Buffalo Bayous were also briefly in danger of coming out of banks in places, but receded quickly according to a HCFCD source. They were all back in banks before I could get there with a camera.

Photos of Area Streams and Bayous

At the East Fork and FM1485, I found a high water caution sign on the road Sunday afternoon. But again, the river was well within its banks. The closest it came to flooding was 2 feet from the top of bank three hours before I took this photo.

Here’s how some other local streams and channels fared in the heavy weekend rains.

A tributary channel of Bens Branch between Woodridge Forest and Northpark Drive next to Kingwood Park High School. That cleared area is the new Preserve at Woodridge that will offer 660 SF homes.
Bens Branch looking E (downstream toward Woodland Hills Drive. CVS on Northpark Drive (left). This was the highest part of the highest stream I found. Notice how it’s almost coming out on the left.
St. Martha’s School Parking lot flooded again a little farther downstream on Bens Branch.
Looking west at Bens Branch toward West Lake Houston Parkway. Note debris line on the left bank in the sun.
The debris line in Taylor Gully shows water never got more than halfway up the bank. Looking upstream from the Maple Bend bridge.
Kingwood Diversion Ditch north of Walnut Lane in distance just hours after tornado ripped through area.
Confluence of East Fork San Jacinto (right) and Caney Creek (left). Note docks still above water on right.

No Reports of Flooded Structures in Harris County

As of 8 PM Monday, Harris County Flood Control had not received any reports of structures flooding from the heavy weekend rains.

Storms of this magnitude are common in Houston, but not for January. Jeff Lindner, Harris County’s meteorologist remembered two in the last decade.

“We had comparable totals on 1-9-2012 in the Brays Bayou watershed (6.6 inches peak in 12 hours). On 1-18-2017, we also had several 4-7 inch gage readings on Brays and 7.0 inches in 12 hours on Lower White Oak Bayou.”

For now, most Harris County residents can chalk this one up in the “close-call” column. But let’s remember that people in Plum Grove DID flood. And pray for the tornado victims in Humble, Kingwood and Forest Cove.

Posted by Bob Rehak on 1/10/2021

1595 Days since Hurricane Harvey

Commissioners Vote Tuesday on Contract for Woodridge Village Detention Pond Excavation

Tuesday, 7.20.21, Harris County Commissioners will vote on a contract with Sprint Sand & Clay for excavation of a Woodridge Village detention basin. Item #21-3394 on the agenda is only for $1000, but it gives the contractor the right to enter the site and begin removing up to 500,000 cubic yards of dirt (at no cost to HCFCD) which it can then sell.

Backup provided to commissioners states that “This benefits the District because excavation and removal is always the highest cost of any stormwater detention basin that is constructed.”

Details of Proposed Contract

Here is the full text of the proposed agreement. Highlights include:

  • Amount of excavation TBD – somewhere between 20,000 and 500,000 cubic yards, depending on plans that HCFCD will deliver to the contractor based on the outcome on an engineering study currently underway.
  • The contractor must properly dispose of the spoils, which it is allowed to sell to make its money on the contract.
  • Contractor is liable for any materials that are disposed of improperly, i.e., within Base Flood Elevation or the 500-year flood plain and must identify all disposal locations.
  • Time allowed: 3 years.
  • Termination of contract possible if contractor fails to excavate a minimum average of 5,000 cubic yards every month.
  • Contractor responsible for environmental mitigation if necessary, excluding wetlands.
  • The contractor must provide an approved Stormwater Pollution Prevention Plan and abide by it.

The contract outline contains the map above but does not specify the exact size, depth or location of the proposed work within the outlined area – just that it will occur in Montgomery County. Engineers will supply additional details at a later date.

Making up for the 60% Solution

Assuming commissioners approve this, it is good news for the people who live who live in Elm Grove and North Kingwood Forest – indeed, for everyone who lives along Taylor Gully. The detention ponds installed by Perry Homes before they sold the land to Harris County were based on old rainfall statistics and will only hold about 60% of a new 100-year rain defined in Atlas-14.

Looking SE across Woodridge Village toward Elm Grove and North Kingwood Forest, areas where hundreds of homes flooded badly in 2019 – twice. Photo taken May 26, 2021.

Sprint Sand and Clay is a regular contractor for HCFCD. Currently, the company is excavating the massive Cutten Detention Basin near 290, Beltway 8 and Cutten Road.

Posted by Bob Rehak on 7/20/21

1421 Days since Hurricane Harvey

Valley Ranch Med Plaza and Shipping Complexes Planned

The Valley Ranch area in Porter near US 59 and SH 99 is developing rapidly these days. Signorelli Company calls Montgomery County the 18th fastest growing county in the US.

Medical Plaza Site Cleared

The developer has just cleared a 200+ acre site for a medical plaza in this area. Signorelli’s website says, “Envisioned as the ‘place of wellness’ for the region, the Medical District is a visionary mixed-use concept blending healthcare with restaurants, specialty services, hotels, and high-density residential, providing a broad range of health care services, from primary physicians to acute care and every specialist in between.”

Looking SW across US 59 in foreground. Photo taken on 5/3/21 after three inches of rain on April 30 and May 1. This area drains into the White Oak Creek Watershed.
Detention ponds, both in this picture and the developer’s website seem to be planned for the area back from the freeway. Photo taken 5/3/2021.

The Montgomery County Engineer’s office says it does not yet have construction, drainage plans, or an H&H analysis specifically for the medical plaza property. In response to my FOIA Request, the engineer’s office said, “This is all we have on file at this time.” Their drainage mitigation study they sent me was produced in 2014, long before Atlas 14. That means its runoff calculations are likely 40% short of the current standard. The study also does not isolate this portion of the overall development.

The study concludes, the entire development will have “no impact to adjacent properties” because of the timing of the runoff. Last year, the Montgomery County Engineer tried to get the MoCo Commissions Court to ban hydrograph-timing studies because of their limitations.

It’s unclear at this time whether Signorelli is planning to update its drainage mitigation study and incorporate Atlas-14 standards into its medical plaza drainage.

Amazon Distribution Complex Across Freeway

Right across the freeway from the Valley Ranch medical plaza, Amazon is building a distribution complex.

Looking NNW toward 59 and 99. Right across the freeway from the Signorelli development, a transportation hub is reportedly being built for Amazon. According to Community Impact, the company hopes to open the delivery center this year. Photo taken 5/3/2021.

That green area that snakes its way across 99 and then 59 from the top left to upper right is White Oak Creek. White Oak runs southeast through Porter then joins Taylor Gully and Mills Branch south of the Triple PG Sand Mine near Woodstream Forest. Ultimately, it joins Caney Creek near Dunnam Place and then the East Fork of the San Jacinto. See below.

From Harris County Flood Education Mapping Tool.

The Amazon facility did not require a H&H analysis because of its size.

Page 8 of Amazon Construction Plans shows site is covered with wetlands. Site borders floodway and part is in White Oak’s floodplain.

The Amazon site has a floodway/floodplain permit. For a high-res, printable version of the site plan above, click here.

Look out below!

Posted by Bob Rehak on May 10, 2021

1350 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.

Interim Guidelines for Atlas-14 Implementation Until New Flood Maps Released

While reviewing the MAAPnext website today, I came across this 1-page PDF that outlines major changes to Harris County’s Policy Criteria & Procedure Manual (PCPM). It describes changes – based on new Atlas-14 Rainfall Statistics – that engineers and developers must follow when designing and constructing flood-control features as part of any development within Harris County.

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’

Harris County works with surrounding counties and municipalities to upgrade and harmonize their floodplain regs. However, the effort has not yet yielded much fruit.

Surrounding counties, such as Liberty and Montgomery, have not yet mirrored these restrictions. In fact, those counties still use their comparative lack of regulation as a competitive tool to attract new development. That, of course, makes it doubly difficult for residents of Harris County. They must not only contend with their own runoff, they must contend with their neighbors’.

Posted by Bob Rehak on 11/30/2020

1189 Days since Hurricane Harvey

MoCo Development on Ben’s Branch Understates Current Detention Pond Requirement by 30%

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.

Brooklyn Trails
Most of Brooklyn Trails is still vacant...
…but time is running out to do something. High density homes are going up quickly.

The developer’s engineering company (A&S Enginners, Inc. at 10377 Stella Link in Houston) submitted its drainage analysis for approval on December 15, 2018, just days before new MoCo regulations went into effect on January 1, 2019. They would have required more detention capacity. And that would have meant fewer salable lots.

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.

Atlas 14 Never Apparently Discussed

I filed a Freedom of Information Act request for the drainage analysis and correspondence relating thereto. The documents show that the subject of Atlas-14 apparently never arose as Montgomery County reviewed the plans.

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:

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.

Why do engineers whose first responsibility is protect the safety of the public do stuff like this! Because MoCo allowed it. And because increasing the size of the detention pond would likely have reduced the number of salable lots.

This is the same game that LJA Engineering played when it calculated detention requirements for Woodridge Village. Then hundreds of homes in Elm Grove flooded twice with sheet flow from Woodridge Village. Harris County Flood Control and the City of Houston have been mired in negotiations with Perry Homes for most of this year trying to buy the land. They want to put a regional floodwater detention facility on it to prevent further floods.

Potential Adverse Impacts

In my opinion, this drainage scheme could harm people downstream, adjoining property owners, and even homeowners within Brooklyn Trails.

Time to Fix is Running Out

Everyone who lives or works near Ben’s Branch should be concerned.

Camcorp the developer plans to put 414 homes with average size of .12 acres on this property. Such high density development will accelerate runoff.

To make matters worse, it’s unclear whether all the detention ponds downstream in Woodridge Forest are functional.

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.

Perry Detention Ponds Pass First Modest Test, But Eroded Badly

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

You can always find up to the minute weather forecasts at this National Weather Service page.

To check on rising rivers and major streams, visit the Harris County Flood Warning System, and click on channels and channel status simultaneously. To see further upstream, click on All Gages. That will show you the status of gages operated by the SJRA in Montgomery County.

Posted by Bob Rehak on 6/25/2020

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.