65% Impervious Cover? Get Out the Ruler!
In January of this year, when they kicked off construction at the Preserve at Woodridge, I posted about the Montgomery County development claiming 65% impervious cover. It’s time to get out the ruler. See pictures below taken on 10/22/22.
I felt at the time that the 65% claim didn’t pass eyeball test. I feel even more strongly about that now.
The walkway going left to right through the bottom of the frame will be people’s front yards. Not quite southern mansions. But there’s plenty of room for a daffodil and a fire hose.
How Impervious Cover Can Contribute to Flooding
The higher the percentage of impervious cover, the less stormwater soaks into the ground. It runs off faster. And without sufficient detention pond capacity, flood peaks build higher.
That’s why I’m so concerned about the accuracy of the 65% estimate. The capacity of their detention pond was configured based on one third grass.
The lack of green space upstream is a growing issue downstream. Our drainage systems never anticipated this kind of density.
And don’t forget, this development also based its drainage calculations on pre-Atlas 14 rainfall rates.
Growth of Impervious Cover
USGS says one third of Harris County is now impervious cover. With more developments like this, the southern part of Montgomery County could one day surpass Harris County!
In December, the New York Times published a story about a company called Descartes Labs, which had trained computers to scan satellite images to detect changes in impervious cover. Descartes found that Texas had 9 of the top 20 counties in the U.S. when ranked by the growth of impervious cover.
To put 65% impervious cover in perspective (assuming the developer’s estimate is accurate), nationwide only about 4.4% of the land in the U.S. has more than 40%. And usually only shopping malls and high-density apartment complexes have more than 65%.
Current drainage capacity rarely anticipates development like this. That’s why so much of Houston’s drainage infrastructure struggles to function properly in heavy rains. It’s also why in 2010, the City of Houston instituted a drainage fee based on the percentage of impervious cover. The purpose: to raise money to repair/upgrade antiquated drainage systems taxed by overdevelopment and to encourage developers to leave more green space.
Close inspection of this site shows that the developer did leave leave one row of pet-friendly trees along the northern side.
Posted by Bob Rehak on 10/22/22
1880 Days since Hurricane 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.
