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.
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.
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:
- 12.17 inches = Rate specified in 1989 MoCo Drainage Criteria Manual and used by A&S as basis for calculations (see table at top of page 10 with giant X through it.)
- 16.10 inches = Rate specified in 2019 Drainage Criterial Manual (see bottom of same page)
- 17.3 inches = Rate for 100-year/24-hour rain for Lake Houston Area by NOAA (see below)
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.
- Downstream because when a true 100-year rain comes along, more water will likely go into Ben’s Branch faster than before the wetlands which used to criss-cross this area were destroyed
- Neighbors because overflow from the detention pond will be focused on the property to the east
- Within because 22 homesites were raised just enough to get them out of the calculated 100-year floodplain
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.