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 DeLeon, 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 use. 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 uses 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 Current Reality

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 11-acre tract within Kings Mill.

Calculations on page 9 of Construction Plans Part 2 reference 2023 City of Houston Regulations for detention volume.

Nor do plans provide a narrative explaining how all the pieces of this jigsaw puzzle fit together.

The City of Houston stormwater people have not approved the construction plans; only the wastewater people have.

Worse, as Kings Mill built out, the new section does not always correspond with old drainage analyses. For instance, the Enclave is 11 acres of residential. But the revised October 2012 drainage analysis that was (referenced in the construction plans) indicates the same area is 15.6 acres of commercial.

Does the size discrepancy skew baseline calculations? They may have given themselves a credit for roughly 50% more pre-existing detention volume than they deserved.

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.
• 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.

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