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“So what is the nub of the issue? In standard practice, only surface runoff is considered, and this has led to degraded streams. The other pathways by which rainfall reaches streams are ignored,” explained Jim Dumont. “If communities are to truly benefit from use of nature’s assets to provide vital community infrastructure services, then we must change the engineering standard-of practice to one that is state-of-the-art and reflects real-world hydrology.”

James Careless had an assignment to look into stormwater modelling tools (for projecting flow and other patterns); both to determine the most common tools used, and some of the most innovative approaches that are coming into use. His research into BC’s water balance approach led him to switch gears from an examination of modelling tools to learning what ‘establishing watershed objectives for stormwater management’ means in practice.

“The Water Balance Methodology is based upon watershed and stream function and operation. Understanding how precipitation makes its way to the stream allow us to assess how a watershed and stream operates and to analytically demonstrate impacts of development and the effectiveness of any mitigation works,” states Jim Dumont. “The Methodology provides solutions with verifiable results and where mitigation systems optimized for cost and function.”

“Research has demonstrated the water quality and channel protection benefits of GI; however, the effect of reducing the severity of flood events has not been investigated at a watershed level,” said Dr. Dan Medina. “The EPA posed this question – would the combined effect of thousands of rain gardens designed to capture relatively small volumes of runoff lead to a significant reduction in flood risk? The answer to this question is YES.”

“All uses of rainfall instead of flow data make the ‘Big Assumption’,” states Andy Reese. “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.”

“The Stormwater Guidebook for British Columbia, published in 2002, offers direction and guidance on how to do stormwater management planning, design principles, and objectives,” wrote Jim Dumont. “We must be driven to investigate the problems and issues that stimulated preparation of the Guidebook. In doing so, we will be able to advance the science and engineering practice in a manner intended by the Guidebook.”

For years practitioners have relied upon common urban stormwater hydrologic design methodologies and trusted their results. But, should they? Join returning speaker Andy Reese as he 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.

“Good practice of the art is dependent on understanding the theory of the science; not the reverse. Become fundamentally sound in the theory, and your ability to accept and adjust for uncertainty in boundary conditions will reflect good judgement. Hydrology remains one of the few opportunities in technology where one’s diagnostic capability is put to the test of both theory and experience,” stated Robert L Smith in 1990.

“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,” says Andy Reese.

“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,” writes Andy Reese.