Woodridge Village N2 Detention Pond

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

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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.