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Voodoo Hydrology has been an industry staple ever since Andy Reese published his article. “Urban hydrology is stormwater management’s dirty little secret. It has been estimated that one out of every three design plan submittals has significant errors in hydrology estimates but only one in twenty is caught.” says Andy Reese. “The truth is: urban hydrology—including newer Green Infrastructure sizing approaches and even detailed modeling approaches—as commonly practiced, is an inexact science at best. We can just now make the same mistakes at near light speed! “

“Years ago I was privileged to travel around the US with EPA putting on seminars,” stated Andy Reese in 2011. “Three off-the-cuff words have probably have had the biggest impact in influencing land design of any sort of regulatory program that ever was, and perhaps that ever will be. Those three words were maximum, extent and practicable. Back then, none of those words were capitalized. They were just a made-up term. But MEP is now taking on green infrastructure overtones, sustainability overtones, LID overtones, and on and on.”

Andy Reese coined the term Voodoo Hydrology in 2006. “As a stormwater community, we have for years relied upon common urban stormwater hydrologic design methodologies and trusted their results. But, should we? We must understand that urban hydrology, including newer Green Infrastructure sizing approaches, as commonly practiced, is an inexact science where we are simply trying to get close to the right answer. We are dealing with probabilities and risk, a changing land-use environment, and many real-world factors that can alter the answer. The applications we may encounter can vary radically,” stated Andy Reese.

“All uses of rainfall instead of flow data make the ‘Big Assumption’. This is a problem, because there are an infinite number of combinations of all the variables within the watershed we have to estimate to try to arrive at that one peak flow. So we must make simplifying assumptions about everything that affects stormwater volume and that moderates its flow rate. Our hope is that, through some miraculous application of the Central Limit Theorem and a bit of engineering judgment (a.k.a. luck on steroids), we can arrive at an unbiased estimate of the peak flow or outflow hydrograph,” stated Andy Reese.

For years practitioners have relied upon common urban stormwater hydrologic design methodologies and trusted their results. But, should they? In the Voodoo Hydrology webinar series, Andy Reese exposes the black box of urban hydrology. In this webinar, Andy (with his normal humor) “lays bare” the popular urban stormwater methodologies, as well as their key elements, assumptions, most common misuses, and proper application. “Urban hydrology, as commonly practiced, is an inexact science at best. We rely on engineering judgment and guesswork,” says Andy Reese.

“It’s what we think is true and right about a certain subject. It’s the grid through which we put all information and input about a subject. In fact, it’s everything we think is true about something …..Whether our paradigm is, in fact, true and effective is not the point. We believe it is. And we only reluctantly change our ways and agree with someone else’s paradigm. Stormwater management is full of cantankerous people with strongly-held opinions. The good news is that who can prove you are wrong,” said Andy Rees.

“Urban hydrology, as commonly practiced, is an inexact science at best. If we were omniscient, we could do an exact job of urban hydrology. Instead, we rely on engineering judgment and guesswork. Perhaps, if we make enough estimates of enough factors, the errors in estimation, high and low, will average out to the right answer. This is where voodoo really comes in handy. The good news is that, as Dr. Debo says, ‘Who can prove you are wrong?’ Well, the Omniscient Being can, but is probably busy elsewhere,” wrote Andy Reese.