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 Second, they are almost always based on outdated hydrologic models.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 “So the next developer is dealing with outdated assumptions,” says Johnson.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