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 points out some of the methodologies that can be used to obtain any number of “correct” answers. This is the spirit of the article in being provocative, as contrasted with a technical compilation of current and acceptable state of the practice.
To download a report-style copy of this web 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”.
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.
The article is standing the test of time. It is as relevant in 2013 as when it was published in 2006. Extracts from Voodoo Hydrology are presented below to whet the appetite of the reader to learn more. These extracts also provide context for a pragmatic and holistic approach that has been evolving in British Columbia over the past decade.
Voodoo: Magical practice considered to be a form of black magic, but also considered a religion to some
“My old friend Dr. Tom Debo, late of Georgia Tech and co-author of our textbook Municipal Stormwater Management, is fond of saying, ‘I love urban hydrology. They can never prove you are wrong, only inconsistent.’ 
As a principal author of state and local design manuals and as a past university teacher in hydrology, I have been repeatedly reminded of the black box (maybe black magic?) nature of urban stormwater hydrologic design, and the often minimal level of understanding of many designers who are sizing and placing infrastructure within urban neighborhoods and other developments every day,” wrote Andy Reese to set the context in the opening paragraph of his 2006 article.
“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.”
In the absence of flow data…the “Big Assumption”
“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.”
“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.”
“In a nutshell, perhaps the largest of these assumptions is that the 10-year rainfall depth, duration, and storm distribution we pick will produce the 10-year runoff. 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.”
“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.”
Andy Reese looks into the Future
“The day is fast approaching when we all will be able to model whole systems using continuous simulation models hard-wired to our local area, use drag-and-drop design approaches, practice ‘what if’ analysis on the fly, perform effective reviews, and let the computer do the crunching while we do the thinking.”
“That will be a day to both appreciate and fear. Techno-color, gamer-controlled, distributed-parameter, real-time, drag-and-drop, three-dimensional, Web-served, GIS-based, satellite-derived, NEXRAD-based, mega-bit, multi-screen voodoo processed at the speed of light can still be just voodoo hydrology, after all,” concluded Andy Reese in 2006.
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)
The View from British Columbia
“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.
“Circa 2000, the pressing need to re-invent urban hydrology had become apparent in British Columbia. Some of us had recognized that technical approaches, rainfall-runoff modelling tools and engineering practices that were commonplace elsewhere in Canada and the United States would not achieve our salmon-centric watershed protection goals.”
In 2002, Andy Reese and Kim Stephens were co-authors of Rainwater/Stormwater Management: Build a vision, create a legacy is the “Tenth Paradigm”. “We can begin to think about the Tenth Paradigm as one involving making decisions aimed at achieving healthy urban watersheds. Having a Target Condition provides direction for the long-term processes of change. An Action Plan then provides a ‘road map’ for getting there over time,” they wrote.
A Science-Based Road Map
“In 1996, the landmark findings of Richard Horner and Chris May (Washington State University) opened our eyes and our minds. Their work led us to look at rainfall differently in British Columbia. Horner and May’s seminal paper identified the four factors limiting stream health in order of priority. A key message was this: get the hydrology right and water quality typically takes care of itself. Horner and May provided us with a science-based road map for integrated rainwater management,” summarizes Kim Stephens.
“The insights resulting from an understanding of the four factors led the Province of British Columbia to develop the Water Balance Methodology 
and initiate a paradigm-shift in the way rainwater is managed. The methodology is the technical foundation for Stormwater Planning: A Guidebook for British Columbia, released in 2002. British Columbia was the first provincial or state government in North America to promote the Water Balance Methodology,” reports Peter Law, Chair of the inter-governmental Guidebook Steering Committee (2000-2002).
Looking At Rainfall Differently in British Columbia
“The volume-based approach that is being implemented in British Columbia picks up the baton that Dr. Ray Linsley started more than ageneration ago. As a professor of Civil Engineering at Stanford University, Linsley pioneered the development of continuous hydrologic simulation as the foundation for water balance management. Ray Linsley was responsible for the well-known HSPF Model,” wrote Dr. Thomas Debo in an article that he co-authored with Kim Stephens in 2003.
The article was titled Re-Inventing Urban Hydrology: Going Back to Basics to Develop New Tools. Dr. Debo is Professor Emeritus at the Georgia Institute of Technology. (Note: Thomas Debo was a former colleague and friend of Dr. Linsley)
“Ray Linsley fought a difficult war to replace the established procedures that had been used for many years, and that continue to be used in most urban hydrologic analyses throughout North America and in other locations around the world. He believed that continuous simulation was the only hydrology that should be used for most design and analysis applications.”
“Looking at rainfall differently resulted in development of the Water Balance Model in 2003 as an extension of the Guidebook; and, more recently, the Water Balance Model Express for Landowners. These web-based tools 
enable the modeller to carry out scenario comparisons. The value of comparisons is in forcing modellers to think about the implications of their assumptions. At the end of the day, it is not the preciseness of the model output that is important. What matters most is that stream health will benefit from actions implemented on the ground,” adds Richard Boase, Co-Chair of the Water Balance Model Partnership.
Watershed-Based Planning: What’s the Goal?
“Andy Reese, along with Paul Crabtree (leader of the Rainwater-in-Context Initiative), strive to improve our development and engineering practice by providing thought provoking articles and insight into common practice. 
Both suggest the need to examine hydrology (and hence rainwater management) from a watershed view and to seek more accurate answers than those often derived by using standard approaches that provide consistent answers to regulatory questions,” observes Jim Dumont, Engineering Applications Authority for the Water Balance Model Partnership.
“In British Columbia we are taking the watershed approach to address the concerns surrounding the need to protect our small headwater streams from urban impacts. We are going forward in our endeavors to provide the answers to how the streams might be impacted and then to describe how we can avoid those impacts. Only be examining the stream can we provide the necessary direction to the planners and designers of our urban environment.”
To Learn More:
In its November-December 2010 issue, Stormwater magazine launched the Green Infrastructure & Community Design Series. Articles in the series have been contributed by members of the Rainwater-in-Context Initiative.
The most recent article in the series was co-authored by Kim Stephens and Jim Dumont, Canadian contributors to the Rainwater-in-Context Initiative. Published in November 2011, the article elaborates on how science-based understanding has informed the process for moving from awareness to action in British Columbia. To download a copy, click on “Rainwater Management in a Watershed Context – What’s the Goal?”.
“This latest article by Kim Stephens and Jim Dumont makes important comparisons between stormwater management in the US and Canada. Although both are moving toward greater use of green infrastructure, the 
differences in approach are significant. I believe it’s important to consider the context in which decisions about water quality are made, and practitioners in the US can learn a great deal from BC’s approach,” concluded Janice Kasperson, Editor of Stormwater Magazine.
