atlas 14 rainfall probabilities

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.