Wednesday, January 17, 2018

Column: The Tampa Bay region shouldn't rely on a misguided sense of security with the notion of a 100-year flood

With all eyes on Hurricane Irma, we already have Hurricane Harvey and its impacts on Houston as sobering reminders of the vulnerability of the Tampa Bay area to hurricanes and tropical storms. Let's be clear: Even if Irma spares us its worst effects this time, our turn will come.

We need to ask ourselves difficult questions: Are we prepared? Are there investments in infrastructure that should be made now to make our area safer? Are we making good decisions about where we place people and property in relation to future damaging or even deadly flooding?

For planning purposes, local governments and homeowners have relied on maps showing expected 100-year and 500-year flood levels — representing the statistical likelihood of epically damaging floods. Harvey showed what many scientists have known for a long time: Such maps are misleading and often incorrect. Here's why.

To many people, a term like "100-year flood" implies that there is some predictability to flooding, or at least some statistical knowledge of the frequency of big floods. If you've just been hit with the 100-year flood, perhaps it is safe to rebuild in the same place because you now have another 99 years before the next one. It doesn't work that way.

Scientists and government officials actually have no idea of the frequency of future flooding. A flood record based only on information from the last 100 years is too short to provide reliable statistics on "100-year" floods. In addition, since the climate is changing, the frequency of future flooding will change as well.

It is important to distinguish between flooding from high rainfall, flooding from streams and rivers swollen by high rainfall at some distance upstream, and coastal flooding from hurricane storm surge. Evacuation routes could differ depending on flood type. However, Harvey demonstrated a "new normal" — cities like Houston or Tampa can get hit by all three at the same time.

The atmosphere's ability to deliver massive amounts of water to urban areas in a several-day period has dramatically increased in the last few decades. Our models need to take this into account.

Here are some ways we can improve things and make our region more flood-resilient:

• IMPROVE THE MAPS. Lets develop some commonsense flood prediction maps. A good place to start is Harvey — we need to plan for the possibility of 50 inches of rain over several days hitting our urban areas at about the same time as a 15- or 20-foot storm surge. Elevation above sea level and proximity to the coast (the main factors in current maps, which focus on storm surge) are important but don't tell the whole story when it comes to high rainfall events. We can't predict when the next Harvey will hit, but it's unlikely to take 100 years.

Let's use modern technology to develop more flexible and updatable maps — ones, for example, that consider the flood risk for different types of structures and different types of events. The flood risks are quite different, for example, for non-elevated single-family homes, elevated homes, and larger buildings where the first two or three floors are used for parking.

• REDUCE THE HARDSCAPE. Cities by definition have lots of "hardscape," impermeable areas that limit the ability of rainwater to infiltrate into the ground, exacerbating flooding. Property tax incentives are a simple way to encourage improvements. An example: Bioswales (swathes of green space that allow rain and flood water to percolate into the ground) can be placed in and around parking lots and other large expanses of asphalt or concrete, greatly reducing the rapid runoff that overwhelms local drainage systems during high rainfall.

• ELEVATE CRITICAL INFRASTRUCTURE. Institutions such as hospitals and certain commercial companies need to be reminded to elevate pumps, backup power and other critical infrastructure well above likely future flood levels.

The images of exploding trucks and toxic gas plumes at the Arkema chemical plant near Houston showed that even after Hurricane Katrina in 2005 and the tsunami that crippled the Fukushima nuclear plant in Japan on 2011, there are still folks who need to be reminded that backup power systems don't work underwater. Municipal zoning laws have a clear role to play.

• RETREAT FROM FLOOD-PRONE AREAS. This will be the most challenging part of our adaptation to global warming and rising sea level. Studies have shown that some properties in Houston, New Orleans and several Florida cities have repeatedly flooded in the last two decades and been rebuilt in the same spot, to the same specifications. As a society, we need to make it easier for flood-prone areas to adapt, and perhaps to revert to green space, without individuals and small businesses bearing all of the costs.

Harvey (or something like it) is coming our way. Let's be better prepared than Houston.

Timothy Dixon is a professor of geophysics at the University of South Florida. Parts of this article are based on his recent book "Curbing Catastrophe," published by Cambridge University Press.

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