DOWNLOAD: Voodoo Hydrology: Andy Reese on ‘Pitfalls of Urban Hydrology Methods & What You Need to Know’



Note to Reader:

in the July/August 2006 issue of Stormwater Magazine, Andy Reese offered his perspective on urban hydrologic practice. His article is about understanding some of the basics of methods drainage designers have taken for granted for years. In this regard, the article serves as an especially relevant Primer on contemporary practices.

The article is standing the test of time. In February 2013, Andy Reese delivered a webinar for Forester University titled “Voodoo Hydrology – Pitfalls of Urban Hydrology Methods & What You Need to Know’”. This was the catalyst for posting a downloadable document on about the 2006 article. The relevance of the document is that it provides context for a pragmatic and holistic approach that has been evolving in British Columbia over the past decade.

To download a report-style PDF copy of the 2013 story complete with the 2006 article by Andy Reese, click here.



Voodoo: Magical practice considered to be a form of black magic, but also considered a religion to some

Andy Reese is a leading American water resources engineer and popular writer, speaker, and co-author of the best-selling textbook Municipal Stormwater Management. He is known for his mantra: Stormwater – Back to the Future.

“Andy Reese’s commentary on Voodoo Hydrology resonates. His choice of metaphor captures the obvious disconnect between preciseness and accuracy. Too often practitioners lose sight of the distinction. Andy’s metaphor also highlights why it is important for drainage practitioners to ask the right questions,” comments Kim Stephens, Executive Director of the Partnership for Water Sustainability in British Columbia.

In 2002, Andy Reese and Kim Stephens were co-authors of Rainwater/Stormwater Management: Build a vision, create a legacy is the “Tenth Paradigm”.

In the absence of flow data…the “Big Assumption”

“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, ultimately striking a compromise between accuracy and data availability, and resulting in an answer that is close to correct,” explains Andy Reese.

“All uses of rainfall instead of flow data make the ‘Big Assumption’. We know that there is only one 10-year peak flow for our site. If we were able to ask some Omniscient Being,she would be able to tell us the current one-in-10-year peak flow to 10 significant digits. There is only one. Our problem is trying to estimate it.”

“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,” concludes Andy Reese.


To Learn More:

To download a report-style copy of the 2013 story complete with the 2006 article by Andy Reese, click on Voodoo Hydrology: Andy Reese on “Pitfalls of Urban Hydrology Methods and What You Need to Know”.

Provocative Articles by Andy Reese!

Do you know where you really are in the shifting paradigms of stormwater management? – Andy Reese examines how our ideas about stormwater have changed since the 1800s. He insightfully looks back at why we pursued stormwater management in ways which unknowingly – at the time – foreclosed opportunities for more sustainable, livable communities. (July-August 2001)

Surface Water Management in the United States: “We have one chance to develop things right,” says Andy Reese –  “The cost of trying and failing may be high. But the cost of doing nothing is higher still. We have one chance to develop things right—to provide for safe and attractive neighborhoods, ecological balance, and clean water. If we mess it up, it will take decades and millions of dollars to fix it later.” (September 2007)

Volume-Based Hydrology explained by Andy Reese – “We are now facing another sea change in thinking that is reaching ’pandemic’ proportions. Recent discussion by stormwater opinion leaders is now pointing to a convergence on what we will call volume-based hydrology (VBH) and movement away from the peak-flow-based version.” (September 2009)

Green Infrastructure and Storm Depth Retention Criteria explained by Andy Reese – “Volume-based criteria can only be rightly developed by understanding the long-term flux of water volume; and the only way to accurately do this is through well-constructed  continuous simulation modeling that accounts for drying processes between storms.” (July-August 2010).