60% impervious cover?

66% Impervious Cover? Really?

Because the Laurel Springs RV Resort was grandfathered under old drainage regulations, it got away with building a detention pond that was half the size required by current regulations.

But assuming the developers were just shrewd businessmen who legally and successfully exploited the system, did they follow the rest of the rules? Let’s look at two other things.

  • The percentage of impervious cover on the site
  • How the number of parking spots increased 25% without the impervious cover increasing.
Laurel Springs RV Resort as of 10/22/22. The contractor still has one more “pour” to complete the concrete in the far upper right of the image.

Were Impervious Cover Calculations Correct?

Detention-pond volume calculations begin with impervious cover (i.e., land covered by concrete plus the entire detention pond area). See below.

66% impervious cover
Laurel Springs RV Resort Detention Pond calculations from approved permit plans.

The total site covers 20.032 acres. The proposed impervious portion, they claim, covers 13.349 acres. That works out to 66.6%. So one third of the site should be grass, trees and other vegetation. But since the entire 5-acre detention pond counts as impervious, mathematically, the remainder of the site can have no more than about 60% concrete and still comply with the percentage they promised.

But just eyeballing that trapezoidal area in the photo above, it seems much more than 60% is covered with concrete.

If my eyeball assessment is correct, then the detention pond is even more undersized than I initially thought because the percentage of impervious cover has increased and with it the amount of runoff.

I wish the developer would show us the basis for those calculations.

Plans Show Increase in Density With No Increase In Impervious Cover

The developer’s permit allows 182 RV spaces, but the plans show 226 – about a 24% increase. However, the impervious cover shown on the plans before and after the permit approval did not change. That could also affect detention pond capacity requirements. And explain why the percentage of concrete appears higher than they claim.

Why Underestimate Impervious Cover?

Why would a developer underestimate the amount of impervious cover? Two reasons:

  1. It would make the detention pond smaller and thus allow the remaining property to produce more income.
  2. By claiming they’re providing more detention than required, they can get a discount on their drainage fees. See page 10.

I’m not alleging they did anything illegal. I’m just saying that much more than 60% of that trapezoid in the photo above appears to be concrete and I sure would like to see how they arrived at their figures. I requested the drainage analysis twice and never got it. That should tell you something.

Posted by Bob Rehak on 10/23/22

1881 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.