N.H. Flood Maps Outdated, FEMA-Sponsored Study Finds

July 3, 2008

Federal maps used by most New Hampshire communities to determine the scope of once-per-century floods are sometimes decades out of date.

A study recommends updating them to steer development away from risky areas.

“If you have a house you think is five feet higher than the flood plain, in fact, it might be in the flood plain,” said Brent McCarthy, a project manager for Watershed Concepts. The private consulting firm participated in the federal study.

But officials say updating the information used to create Federal Emergency Management Agency maps would be expensive and time-consuming.

“I get a lot of calls from communities that talk about the age of the maps, and we all wish they were updated, but there’s a cost factor involved,” said Jennifer Gilbert, the state coordinator for flood plain management.

FEMA spent $330,000 on the six-month study to respond to questions from the state about the back-to-back spring floods in the state in the past two years. They followed floods near Keene in 2005. The report is expected to be filed next month. The study was done by a team led by the URS Corp., an international engineering firm under contract with FEMA.

McCarthy said a key message of the study is to keep development out of the flood plain to mitigate damage from floods. He said studies by the U.S. Geological Survey after recent New Hampshire floods indicated the height of floodwaters was often different than predicted by FEMA maps because rivers had changed. The differences were as much as five feet.

Gilbert said FEMA is modernizing its maps, including in Hillsborough and Merrimack counties next year, by digitizing existing maps and laying them over satellite photos. Gilbert said the modernization generally does not involve gathering new data on the ground.

The team that did the FEMA study concluded that global warming and shifting climate patterns probably will make floods more common and possibly larger than those New Hampshire experienced in May 2006 and April 2007.

The study recommends getting sediment and debris out of rivers. One potential hurdle will be obtaining environmental permits needed when dredging affects fish habitats.

The study also recommends adopting a version of Vermont’s Fluvial Erosion Hazard Mapping methodology, which takes into account erosion damage as well as damage from flooding.


Information from: The Telegraph, http://www.nashuatelegraph.com


Information from: The Telegraph, http://www.nashuatelegraph.com

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