Tag Archive for: flood routing

Two New MoCo Developments Will Total Almost 3,400 Acres, Have No Detention Ponds

Two new developments in Montgomery County, Audubon in Magnolia and Country Colony in Porter, will have no detention ponds. The two developments total almost 3,400 acres. During heavy rainfalls, they will dump their floodwater directly into local streams.

How Do They Get Away With That?

Sixteen months ago, on the second anniversary of Harvey, Montgomery County commissioners voted to leave open a loophole that lets developers avoid detention pond requirements. They said they would revisit the issue after the San Jacinto River Basin Study was completed. Even though partners shared study results with the public last August, MoCo commissioners have still not revisited the loophole called a “flood routing study,” also known as hydrograph timing.

The commissioners expressed concern in their 2019 meeting about placing economic hardships on developers. Residents complained about the economic hardships caused by flooding. The developers won.

Theory and Problems with Flood Routing Studies

The idea behind flood routing is simple. If you can show you can get your floodwater to the river faster than a flood’s peak arrives, theoretically, you’re not adding to the peak. Therefore, theoretically, you’re not making flooding worse.

However, engineers and hydrologists point out several flaws with this “beat the peak” theory.

  • Flood-routing studies don’t consider the cumulative effects of other developments.
  • They are almost always based on outdated hydrologic models.
  • They assume “ideal” storm conditions.

“If you start with a brand new hydrologic model,” said one county engineer, “the modeling a developer does could theoretically be accurate. But his/her runoff changes the model. That runoff rarely gets incorporated into the model that the next developer uses.” 

And, of course, if everybody rushes floodwater to a river during a flood, that’s the exact opposite of what you want. Holding water back in detention and retention ponds is the best way to reduce flooding.

The two new developments in MoCo exploit this routing-study loophole to avoid the cost of building detention ponds.

Audubon in Magnolia

The first, called Audubon Magnolia will contain 5,000 homes at buildout. It occupies 3,300 acres that drain into Mill Creek, Spring Creek and then the West Fork San Jacinto.

Here is the entire 186-page Drainage Impact Analysis for section one. It also shows project plans and location.

From Audubon’s drainage impact analysis. Note school being placed at edge of floodplain…before new floodplain maps are redrawn based on Atlas-14.

Section 3.1 of the Impact Analysis includes a description of the flood routing study and concludes, “Therefore, the increased flows are able to exit into Mill Creek before the flow from the bulk of the upper drainage basin arrives at the mouth of the stream.”

Correspondence with Montgomery County officials at the front of the document shows their concerns: use of pre-Atlas 14 data; impact on wetlands; building in floodplain; roughness co-efficients used to model speed of water over various terrains; and impact on water surface elevations.

Clearly, the floodplain administrator had major concerns about use of pre-Atlas 14 rainfall data. However, the developer was grandfathered based on the date of the original permit application.

Montgomery County’s new Atlas 14 standards increased the total for a 24-hour, hundred-year rain by 4 inches. The Flood Plain administrator encouraged the developer to model the higher rainfall totals and build to higher elevations. Why? To ensure the likelihood of compliance for those homes in the future.

These documents, dated 2019, are the latest available from Montgomery County. The county engineer’s office indicated that no detention ponds are being planned by the developer or demanded by the County.

Country Colony in Porter

The second development, Country Colony, lies just north of the Harris County/Montgomery County line at the end of West Lake Houston Parkway. Country Colony occupies approximately 80 acres immediately west of the Triple PG sand mine.

Note county line at southern edge of development. Also note how some of the lots are actually in the floodplain of White Oak Creek.
Country Colony. Taken December 7, 2020, looking east toward Triple PG Sand Mine in background.

No detention ponds here either! A big heads up to the people downstream in Walden Woods, Woodstream Forest and beyond.

Posted by Bob Rehak on 12/12/2020

1201 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.

Mark Your Calendar: Important Meetings Related to Flooding

This month, you have three opportunities to attend meetings that could help reduce flooding in the Lake Houston area.

TWDB Visits Tomball to Solicit Input on State’s First Flood Plan

On Friday, August 9, the Texas Water Development Board (TWDB) will hold a meeting in Tomball. Purpose: to solicit public opinion on rules and guidelines for Texas’ first statewide flood plan. Here’s more information about the event and the TWDB. The TWDB is the state’s water agency. It’s primary mission is developing and maintaining lakes and reservoirs that support economic growth. This year, the legislature put them into the flood mitigation business, too. They’re looking for the best ways to spend $800 million on flood mitigation from the state’s Rainy Day Fund. TWDB will hold the event at:

  • Beckendorf Conference Center at Lone Star College–Tomball
  • 30555 Tomball Pkwy. 
  • Tomball, TX 77375
  • 9:30-11:30 a.m.
  • Friday, August 9

Sign up for more information about these meetings and other flood information at the TWDB’s website. You can also contact the TWDB at (512) 463-8725 or flood@twdb.texas.gov.

TWDB Makes Repeat Appearance at Houston City Hall

If you can’t make the event in Tomball, you have another chance. TWDB will repeat the event at:

  • Houston City Hall
  • Council Chamber, 2nd Floor
  • 901 Bagby Street
  • Houston, TX 77002
  • 9:30 to 11:30
  • Friday, August 23

Montgomery County Will Hear Testimony on Closing Detention Loophole

On Tuesday, August 27th, Montgomery County Commissioners will consider a motion to close a loophole that allows developers to avoid building detention ponds. Expect developers to testify against closing the “flood routing study” loophole. You can testify FOR closing it, however.

The meeting starts at 9:30. Montgomery County has special sign-up procedures for citizens who wish to testify; make sure you sign up beforehand. Check the agenda beforehand to plan your time. You can also register your opinion with county commissioners via phone or email.

You don’t have to be a Montgomery County resident to testify. As downstream residents of the Lake Houston Area, you may be affected by this more than Montgomery County residents are.

Posted by Bob Rehak on 8/7/2019

708 Days since Hurricane Harvey

MoCo Will Consider Requiring More Detention for New Developments in August 27 Meeting

Montgomery County commissioners will consider changing flood mitigation requirements for new developments at their regular August 27 meeting. Commissioners will hear public testimony and consider approving a revision to the Montgomery County Drainage Criteria Manual. The change would close a loophole that allows developers to substitute “flood routing studies” for detention ponds in new Montgomery County developments. 

How Developers Use Flood Routing Studies

Flood routing studies calculate when runoff from a new development will hit a river during a major rain event. If results show that the runoff will reach the river before the crest of a flood, developers may not need to build detention ponds. The idea: it’s not adding to the peak, so why run up costs needlessly?

Why Flood Routing Studies are Inadequate

In principle, that sounds good. However, routing studies almost always contain flawed assumptions according to Jeff Johnson, Montgomery County’s Engineer.

First, they don’t consider the cumulative effects of other developments. Second, they are almost always based on outdated hydrologic models. And third, they assume “ideal” storm conditions.

“If you start with a brand new hydrologic model,” says Johnson, “the modeling a developer does could theoretically be accurate.” But his/her runoff changes the model. That runoff rarely gets incorporated into the model that the next developer uses. “So the next developer is dealing with outdated assumptions,” says Johnson. Same way with the third and fourth developers, etc. They all keep going back to the original model, even though they know it has been changed by previous developments. Said another way, additional runoff is not added to the model on which subsequent developers base their calculations. So they all show no consequences when the cumulative effects can be large.

Another problem. They all base calculations on ideal assumptions. Johnson estimated that only a small percentage of storms conformed with ideal conditions. For one example, calculations are valid only if rain stops before the flood reaches its peak.

Shortage of Detention Leads to Downstream Flooding

As a result, there’s not enough detention upstream to protect downstream residents during a major storm.

Many developers like the flawed assumptions behind the routing studies. They justify building less detention, which costs developers time and money. And with less detention, they can develop and sell more lots per acre. So they reduce costs and increase income.

But when that happens, somebody downstream pays the price. “They’re not being responsible,” said Johnson. “This is a public safety issue.”

One flood expert that I interviewed for this article said, “Only good things come from more detention.”

City of Houston Public Works Director Agrees

As if to punctuate Johnson’s point, shortly after my interview with him, I attended a talk by City of Houston Publics Work Director Carol Haddock. Haddock emphasized that flooding today largely stems from problems inherited from legacy infrastructure. “We’re living with infrastructure developed before we knew what we now know about flooding,” said Haddock.

Haddock argued for both higher drainage and detention capacity. They will help accommodate future floods and future development – while protecting people and property downstream, she argued.

Projected MoCo Growth Underscores Need to Close Development Loophole

Getting drainage and detention right is crucial, not just for families downstream in northern Harris County, but also for families in Montgomery County itself. The New Caney ISD (NCISD) is projected to grow substantially in the next few years. The NCISD just completed a demographic update from Population and Survey Analysts (PASA). (Caution: 58 meg download.) Page 6 of the study shows that the District expects to grow by more than 19,000 housing units in the next 10 years. That’s almost as large as Kingwood. And it doesn’t even include commercial space.

A graphic from a Caldwell Brokerage brochure shows some of the major current and planned developments in the area between the Woodlands and Kingwood with the number of homes.

In the previous 5 years, the NCISD had the second highest percent change in school district enrollment in the region at a whopping 30.3%. Only Alvin had a higher increase at 31.6%.

PASA graphic comparing 5-year growth rates in area school district enrollments.

PASA predicts the new commercial area near 45 and 99 will have as much square footage as downtown Austin. And, further upstream, Conroe was the fastest growing City in America in 2017.

Fortunately, the new San Jacinto River Basin Survey will update hydrologic models. But with projected growth like this, they will become outdated as soon as they are complete. All the more reason to move away from the flood routing paradigm of development and require more on-site detention. ASAP.

Register Your Opinion

Expect developers to testify against closing the “flood routing study” loophole. You can testify for closing it, however. Montgomery County Commissioners will hear public testimony at their regular meeting on August 27th. The meeting starts at 9:30. Montgomery County has special sign-up procedures for citizens who wish to testify; make sure you sign up beforehand. Check the agenda beforehand to plan your time. You can also register your opinion with county commissioners via phone or email.

Posted by Bob Rehak on 7/31/2019

701 Days since Hurricane Harvey