Enclave Detention Basin Will Overflow into Kingwood Evacuation Route

5/30/25 – The Northpark Enclave detention basin may be undersized and the developer plans to dump overflow stormwater during extreme events into the only all-weather evacuation route for 78,000 people. That’s according to construction documents and drainage plans obtained from Montgomery County under the Freedom of Information Act.

The last thing you need during a mass evacuation is street flooding.

EHRA, the developer’s engineering company, told Ralph De Leon, project manager for the Northpark expansion project, that all drainage for the new subdivision would be routed south to the Kings Mill Stormwater Detention Basin and then via an outfall channel to the Kingwood Diversion Ditch near the County Line. But the construction drawing above indicates otherwise.

Technically a part of Kings Mill, this 11-acre strip bordering Northpark is just now being developed for single-family residential. Most of Kings Mill was built in phases between 2004 and 2018. And therein lies a huge problem.

Old Plans Use Antiquated Rainfall Statistics

Some of the drainage analyses/plans for the Enclave detention basin date back to 2002 – almost 25 years ago.

Back then, assumptions about “probable maximum rainfall” were very different from today’s. Montgomery County defined a 100-year rainfall then as 12.1 inches in 24 hours.

However, today, MoCo defines a 100-year/24-hour rainfall as 16.1 inches – a 33% increase. But for this development’s location, NOAA defines one as 17.1 inches – a 41% increase. Why the difference?

MoCo adopted Atlas 14 rainfall statistics in 2019. But NOAA fine-tunes its statistics for individual locations. And MoCo regulations use Conroe’s statistics for the entire county. Rainfall decreases as you go farther inland and Conroe is 40 miles north of this location. But that’s not all.

Confusing Documents Don’t Match

It’s not clear how engineers have updated the old drainage analyses in the new plans. MoCo did not provide an updated drainage analysis in response to my FOIA request.

Yet within the construction plans, it appears that EHRA may have tried to mitigate for higher rainfall standards in the development of this tract within Kings Mill.

Calculations on page 9 of Construction Plans Part 2 reference 2023 City of Houston Regulations for detention volume. They show the size (line 3 below) as 11 acres.

Plans do not provide a narrative explaining how all the pieces of this jigsaw puzzle fit together. And trying to track the pieces back through documents dating back almost 20 years is confusing.

For instance, the same plot that’s 11 acres in 2025 was listed as 15.6 acres in the October 2012 drainage analysis.

At the very least, it’s safe to say that unexplained differences like these make one question the quality and consistency of calculations in the plans.

55% Impervious Cover?

And they never do explain how they can put 100 homes on 10 acres (minus one acre for the detention basin) and get only 55% impervious cover. Experience suggests that more realistic estimates for that much density would range from 65% to 85% depending on the size of homes and garages.

For More Information

Montgomery County Engineering provided:

• Plans for Construction of Drainage Facilities for Kings Mill Outfall Channel and Detention Lake, approved by Montgomery County on May 24, 2007. City of Houston approvals date back to 2002, but stormwater detention requirements increased after Hurricane Harvey in 2017.
• Drainage and Detention Analysis for Kings Mill, 2/2012 with a letter of no objection issued by Montgomery County Engineering on 3/12/2012
• Drainage and Detention Analysis for Kings Mill Subdivision and Commercial Development along Northpark Drive, Revised October 2012
• Construction Plans for Northpark Enclave Part 1, April 14, 2025 (32 Megs), references Oct. 2012 drainage study
• Construction Plans for Northpark Enclave Part 2, April 14, 2025 (32 Megs)

There Should Be A Law

Only two things became clear after struggling to understand these documents:

• There should be a statute of limitations on “grandfathering” permits based on when someone first applied for them.
• Someone should have required a new drainage analysis for this project.

More news to follow.

Posted by Bob Rehak on 5/30/25

2831 Days since 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.