Friday, April 2, 2010


In my 40+ years as an engineer, I’ve encountered a lot of people who try to tell me my business. As a consequence, I’ve seen a lot of misinformation being spread around by people who don’t have engineering degrees or licenses.

Sembler Company has proposed building a MEGA shopping mall in little Okatie, SC. The size of the mall has been widely reported as 1.5 MILLION square feet and 1.6 MILLION square feet. What’s 100,000 sq. ft. between friends? The area of the Sembler site has been reported as 280 acres and 282 acres. What’s 2 acres between friends?

The impervious area of the site has been reported to be in excess of 200 acres, and as high as 100% impervious. For a civil engineer, these numbers are not only incredibly high, they’re just plain INCREDIBLE. I’ll explain.

In runoff calculations the runoff coefficient is a measure of the imperviousness of a surface. A 0.3 runoff coefficient means that 30% of the rain striking that surface will run off. Runoff coefficients for some NATURAL surfaces are greater than 0.5. However, runoff coefficients are always less than 1.0. Not ALL rain striking even a hard surface runs off. In engineering calculations there is no 100% impervious.

I came to find out that Sembler had submitted some PRELIMINARY runoff calculations with their PRELIMINARY proposal:

Basin #1 – 107.81 acres, about 70% impervious
Basin #2 – 114.38 acres, about 70% impervious
Basin #3 – 101.90 acres, about 70% impervious
Basin #4 – 14.01 acres, about 90% impervious

When added together the total area = 338.1 acres, NOT 280 or 282. It seems that Sembler included approximately 56 acres of housing along with their mall proposal.

Engineers use “equivalent impervious acres” in their drainage calculations. For example, a 100-acre site with a UNIFORM 0.5 runoff coefficient (or 50% impervious) would contain 50 “equivalent” impervious acres. However, that's NOT to say that there are 50 other acres on the site with a runoff coefficient of zero, or TOTALLY pervious. The 50 "equivalent" impervious acres are NOT an actual area; they are an "equivalent", or virtual, area.

This above example is a GROSS oversimplification. The “equivalent” acres for the 338-acre Sembler site from their PRELIMINARY calculations are:

Basin #1 – 75.5 acres impervious
Basin #2 – 80.1 acres impervious
Basin #3 – 71.3 acres impervious
Basin #4 – 12.6 acres impervious

When added together, the total “equivalent” impervious area is 239.5 acres, for the 338 acres.

To a trained eye, it’s apparent that these are ROUGH numbers. Basin #4, given its small area and “about 90% impervious” is probably mostly hard surfaces, e.g., roofs or pavements. The fact that Basins 1, 2, & 3 are all “about 70% impervious” tells me that these are ROUGH calculations. For these three basins to all have the SAME aggregate imperviousness of 70%, the ratio of hard surfaces to natural surfaces would have to be EXACTLY the SAME for all three areas. Obviously (to a trained eye), that CANNOT be the case.

It’s helpful to know what you’re talking about before you start talking (or defending) numbers.


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