We need to stave off any more developments along our bayous. Our new reality is bleak enough as it is.
"That analysis by the National Oceanic and Atmospheric Administration, which looked at rainfall data stretching back decades, up to and including Harvey, shows the amount of rain that defines a "100-year storm" -- one that has a 1 percent chance of occurring in any given year -- has risen by 3 to 5 inches in Harris County since the last estimates were put in place in 2002.
Instead of expecting 12 to 14 inches in a day during a 100-year storm, the preliminary data show the county should expect 15 to 18 inches."
For most area watersheds, the current rate is 13.2" or 13.4" in 24-hours. Harris County requires that developers hold 55% of that stormwater runoff. Clearly 55% of 18" would be more water detained.
Unless the City of Houston rewrites their Infrastructure Design Manual's Chapter 9 on Stormwater, Houston won't require more water to be held in detention. That's because Houston requires 0.5 acre-feet per acre of increased impervious cover and is not related to the 100-year rainfall rate. That's equivalent to 45% and would be reduced to 33% if the new rainfall rate will be 18".
Worse still, the City "grandfathers" concrete already on the ground. If concrete has been on the ground anytime in the last millenium, even if it's been green for the last 20 years, a developer doesn't need to mitigate for stormwater runoff.
To be fair, former Mayor Parker "fixed" grandfathering. During her term Steve Costello's firm came up with new requirements requiring a small amount of detention. For example, if a 10-acre property was ever completely covered in concrete, the developer is required to install detention of 0.4 acre-feet, or 4%. Of course, the developer can still elevate the same property a foot and a half, displacing 15 acre-feet of water, then install the 0.4 acre-feet and repave the whole thing. Seems reasonable, huh?