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Below The Surface

Tuesday, September 13, 2011

Goodnight Irene ... When Will We See the Likes of You Again?

Hurricane Irene had tremendous effects throughout our region, especially in areas South of the Basin. Lives were lost, property destroyed, bridges and roads removed outright. Irene was a natural event that we will not forget. But when might we see the likes of her again?

Whether the flooding caused by Irene is a 100-year or 500-year flood will not be determined for some time, and the determination may vary from one river system to another. Labeling floods by return period, that is, saying that a flood is a 50- or 100-year flood is done by hydrologists. If Irene produced a 100-year flood event in the Winooski River, then we can be assured that we're done with floods that big for 100 years, right? Not quite. This is where scientific language doesn't do us any favors, especially when it gets used without explanation.

The beginning of this podcast has a good explanation that helps clarify things a bit:
USGS Multimedia Gallery: Two 500-Year Floods Within 15 Years?

To quote Bob Holmes, United State Geological Survey's Flood Coordinator, "Essentially, a 500 year flood is just that quantity of water that has the 1 in 500 chance in happening in any one year. Another way to say it would be, there's a .2% chance of a flood of this magnitude occurring in any one year."

So it's not likely that we'll see large floods next year, but it is possible. The process of assigning flood return intervals uses historical data from stream gauges that measure how much water is flowing, or discharge, at any given time. Hydrologists use the data from the largest discharges recorded over time to generate the probabilities of similar large discharges happening in the future. But probabilities, or odds, are tricky things. Just because the odds may tell us that something is unlikely to happen doesn't mean it will not.

As for flood events, probabilities are determined directly from past history and assume that the conditions that influence river discharge are not changing. This is rarely the case, and thus hydrologists update their predictions with new data from time to time. What conditions affect events? In short, anything that affects how much precipitation falls on the land that drains into waterways and/or affects the rate at which water moves from the land into streams and rivers. Under current climate change predictions, the northeast will experience more extreme weather events (more water in the form of snow and rain) in the future. Our Basin, and most watersheds world-wide are experiencing increasing development of land for human needs which in-turn means higher rates of run-off from roads, parking lots, buildings, and agricultural fields. Our steep mountainous landscape and its underlying geology also affect the rate at which water moves. Given these patterns, we can expect that what we now consider to be a 100-year flood event will become more common and the future's 100-year flood event will be larger.

How we deal with the influences of our own activities on the damage caused by floods largely depends upon how communities manage their landscapes. On a town level, adopting planning and zoning regulations that incorporate these concepts (in the form of FEMA flood maps) to limit development in low lying areas can be successful. In urban areas, support of stormwater management regulations can offer a one-two punch, reducing the rate at which water runs off of impervious surfaces and reducing the pollution into our waterways from the chemicals that might wash off those surfaces. Like any natural disaster, it takes time to recover from the tremendous loses. Part of the recovery is looking for opportunities to change how we use and manage the landscape, and implementing changes before the next extreme weather event comes along.

Photo credit: Vehicle chaos, Hancock, VT by Lars Gange & Mansfield Heliflight

For more info, check out:
Vermont Dept. of Environmental Conservation's River Management Program

New York Dept. of Environmental Conservation Floodplain Management

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