Why Are We Sizing Urban Stormwater Designs for Failure?

A recent article from The Atlantic titled, “Houston’s Flood Is a Design Problem” highlights an increasingly relevant question raised by stormwater engineers today – Why are we still designing for failure?

Maybe the flooding we’re experiencing has nothing to do with functional, maintenance, or structural failures—or even a lack of funding in some cases. What if the flooding we’re experiencing has more to do with the fact we’re engineering our urban stormwater designs for failure?

But, as we know, the impacts of flooding in cities and developed areas are far more constant than those produced by Harvey; so let’s take a step back to consider why our urban stormwater designs are still failing in “typical” rainfall conditions.

Check out our Stormwater focused upcoming Webinars!

Urban Stormwater Designs Sized for a 10-Year Storm
Say you’re designing a culvert for a ten-year storm. How many ten-year storms do you think the county you’re designing it for will experience in ten years? One? Not even close!

Because the kinds of rainfall we typically face are thunderstorms that naturally switch on and off targeting localized areas, any county or large city area may experience 20, 30, or even 40 ten-year storms in one year because every point of land within a county has to, on average, experience a ten-year storm once every 10 years.

So, maybe our urban stormwater designs are failing because of our design standard and urban hydrology predictions. If we’re sizing our urban stormwater designs to be overtopped once every 10 years, we’re designing for a 10% failure rate for every structure, every year. Or, if you think about it another way, when we size our urban stormwater designs for a 10-year storm, we’re planning and calculating a local tolerable failure rate.

In what other field of science or engineering do we do that? We don’t design water and wastewater treatment plants for failure. We don’t design heating and cooling for failure. So, why do we do it in the stormwater profession?

Why Urban Stormwater Designs are Sized for Failure
Many of the shortcomings of urban stormwater designs stem from issues related to cost and appearance. Greater safety and a conservative design approach to sizing urban stormwater facilities increases cost exponentially and degrades appearance as well. If we size urban stormwater designs for a 100-year storm, we could have a grand canyon out in front of every site that uses a ditch system.

However, with new green infrastructure retrofits and the evolution of stormwater and continuous simulation, we can design for flooding. So...why are we not?

Should We Demand More?
At what point should we demand more from our hydrologic predictions and urban stormwater designs? After all, in high risk areas like Houston, the uncertainty in probability is overruled by the certainty of the high cost of failure. Having a stronger understanding of the contributing factors that can lead to urban stormwater design failures will lead to better, more resilient systems.

Interested in learning more about the common pitfalls and proper application of urban hydrology?
Listen in as Andy Reese, P.E., LEED AP, “lays bare” urban stormwater methodologies in his on-demand webinar
Voodoo Hydrology – The Pitfalls of Urban Hydrology Methods