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Woodridge Village Detention Calculations Off by More Than 40% According to New Standard

Developers in Montgomery County try to avoid building detention ponds by beating the peak. They also have attempted to minimize the amount of detention ponds they must build by beating the clock.

Woodridge Plans Approved One Month Before NOAA Updates Flood Data

A year to the day after the peak of Hurricane Harvey on August 28, LJA Engineering submitted a hydrology report to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a 100-year storm that dropped 12.17 inches of rain in 24 hours.

From Page 2.1 of LJA Hydrology Report Addendum, 8/28/2018 (page 32 of pdf.)

Two weeks earlier, USGS had issued its report on peak streamflows and high water marks for Hurricane Harvey.

At this point, the world knew that flood maps would soon change radically. But the LJA report contains no mention of Harvey, USGS, or NOAA’s new Atlas 14 data. And in fact…

Less than one month after the LJA Engineering hydrology report, on September 27, NOAA issued new rainfall frequency values for Texas. Called Atlas 14, the NOAA analysis established significantly higher rainfall frequency values for this part of Texas.

New updated NOAA Atlas 14 data shows that a hundred-year rain for the Lake Houston area is now defined as 17.3 inches in 24 hours, up from 12.17 inches by the old standards.

NOAA redefined the amount of rainfall it takes to qualify as a 100-year or 1000-year event. They defined the new 100-year rain as 17.3 inches in 24-hours – a 42% increase. That means that to meet new 100-year standards, Perry would have had to increase its detention capacity by 42%. Instead of 271 acre feet, it would have needed 385.

Using Atlas 14 would have reduced the number of salable lots and the economic projections for the development to a substantial degree.

The one flood map in the 59-page LJA Engineering hydrology report shows flood plains magically stopping at the county line.

The one flood map that the LJA hydrology report does include (page 51 of PDF and above) shows flood zones stopping at the county line (the black diagonal) and the boundary of the Perry property (the maroon-bordered polygons). Pretty odd for a site partially covered by wetlands!

National Wetlands Inventory Map shows both sections of Woodridge Village contain wetlands.

Woodridge Plans Approved Even Before LJA Submitted Hydrology Report

Now here’s where it gets even more interesting. City of Houston approved the detention plans on 8/12/18 – two weeks BEFORE the LJA hydrology report on 8/28/18 and only a month BEFORE NOAA released the new Atlas 14 data. Hmmmm! Think they were in a hurry to get these approved? (Note: The approval date for MoCo is unreadable).

Signature block for City of Houston from Woodridge Village detention plans.

Perry Homes played a game of beat the clock and was winning … until May 7, 2019.

Future Flood Risk Remains Even with Planned Detention Ponds

Until now, I have been posting about the lack of detention ponds. Closer analysis reveals that this is only part of the problem. Even if Perry builds the remainder of the detention ponds as planned, they will be insufficient to meet the new NOAA standards and will pose a flood risk to people downstream.

After contributing to two floods in Elm Grove and North Kingwood Forest, the engineers and owners of Woodridge Village surely must realize how dangerous trying to Beat the Clock was.

Forty-two percent of a 100-year flood as defined by the new Atlas-14 data will overflow the banks of the detention ponds and add to the load on Taylor Gully or go into the streets of Elm Grove and North Kingwood Forest.

Facing west. This panoramic drone image by Chris Betz takes in most of the Woodridge Village constructions site. Note the ponding water 3.5 days after a two-inch rain.

This image taken Friday night at Sunset shows how impervious the Woodridge soil is. Water is still ponding three and a half days after a two-inch rain (October 28, 2019).

I wonder if the LJA engineers calculated the runoff coefficient accurately. Given some of the other problems in this report, perhaps an engineer would care to comment on their calculations.

Posted by Bob Rehak on 11/2/2019

795 Days after Hurricane Harvey and 45 after Imelda

The thoughts expressed in this post represent my opinions on matters of public policy and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the great State of Texas.