Tag Archive for: 40%

Woodridge Village Detention Ponds Pass Another Test

Back on June 25, 2020, the Woodridge Village detention ponds passed their first modest test when they retained a 2.32-inch rain that fell in a little more than an hour. The mostly excavated, but not-quite-finished ponds eroded badly, but no one in Elm Grove or North Kingwood Forest flooded. Then came Beta.

Design Capacity of Detention Ponds

LJA Engineering designed the Woodridge Village detention ponds to hold only a 12-inch rain in 24 hours. And the night before the storm, forecasters predicted Beta could drop 12 inches in the Lake Houston Area. Beta had already dumped 15-inches just a few miles south. All this created high anxiety. But in the end, the Lake Houston Area received less rain.

Elm Grove resident Jeff Miller measured just 5.5 inches in his rain gage – well within the theoretical design capacity of the detention ponds. But that was almost exactly the amount that flooded Elm Grove on May 7, 2019.

Ponds 40% Short of Atlas-14 Requirements

On top of that coincidence, other factors contributed to the anxiety felt by residents. LJA did not design the ponds to meet new Atlas-14 rainfall requirements; they’re 40% short. Nor did LJA acknowledge floodplains or wetlands on the property when they calculated detention requirements. All of these factors contributed to flooding Elm Grove last year and called LJA’s ethics into question.

So it was a welcome relief when the people of Elm Grove rolled out of bed Wednesday morning to find they didn’t flood.

How Well Did Ponds Perform?

Here’s what the ponds looked like after about five to six inches of rain. All photos were taken shortly after Beta’s rain stopped on Wednesday, 9/23/2020.

Looking north at N3 Detention Pond along eastern border of Woodridge Village where it joins the east-west portion of Taylor Gully. Note pond is a little less than half full, not surprising for a rain that was a little less than 50% of the design capacity.
Likewise, the massive N2 detention pond on the western border was less than half full.
Looking SE across the empty N1 pond on the western border. It had already drained into N2.

Two Failures in N3 Pond

However, there were two failures, both in the N3 pond. Neither was mission critical.

Water could not get into N3 without overflowing the edge of the pond, causing erosion. Stormwater seems to want to collect here. This same area eroded badly in a previous storm.
In a second place along N3, erosion blew out the entire western wall.

Overflow Spillway Apparently Not Used

The overflow spillway at the county line between the concrete lined channel and the S2 detention pond was apparently not needed during Beta. There were no signs of erosion (see below) that were present after previous floods.

Rain was spread out enough that it appears water from Taylor Gully and N3 stayed in the concrete-lined channel rather than using the emergency overflow spillway that leads back into the S2 detention pond (right) and the twin culverts.

Nevertheless, despite recently planting grass along the banks of the ponds, Perry Homes still has a significant erosion problem. Note the color of the water in Taylor Gully at the top of the image above. The company is redepositing silt in the ditch, which HCFCD just cleaned out.

Living Under the Threat of Bad Planning

A big test of these ponds will be a 12-inch rain. If the ponds can successfully detain that much rain without flooding Elm Grove, we will know they at least function as planned…despite their 40% shortfall in capacity.

But the ultimate test will be when we get a larger rain. LJA Engineering and Perry Homes did NOT design them for that.

Unless Harris County and the City can piece together a deal to buy this property and build more detention, residents could flood again. The real disaster scenario here could be the purchase deal falling through. If Perry or some other builder develops this property, downstream residents will forever live under the threat of that 40% shortfall.

Posted by Bob Rehak on 9/24/2020 with thanks to Jeff Miller

1122 Days since Hurricane Harvey and 371 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.

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.