Storrs - The March 2010 floods in southeastern Connecticut and Rhode Island are part of a new pattern of extreme rainfall events in New England that is likely to continue as the region becomes warmer and wetter with climate change.
So said David Vallee, lead hydrologist at the National Weather Service's Northeast River Forecast Center in Taunton, Mass., during the keynote address at the Connecticut Conference on Natural Resources at the University of Connecticut on Monday.
"I believe we are now seeing the impact of a warming climate in the intensity of storms and the blocking up of multiple weather events in succession," Vallee said to an audience of about 270 gathered in the Student Union.
Tropical plumes are carrying slow-moving "atmospheric rivers" of rainfall to the Northeast with historic frequency and intensity, he said, as more moisture is carried over land as average temperatures rise.
"We are just not cold anymore," he said, showing a graph of rising average winter termperatures over the last two decades.
He traced the beginning of the pattern of annual floods in New England to twin flooding events in October 2005 that caused deaths and dam failures in southeastern New Hampshire and eastern Massachusetts. Next came the May 2006 floods in the Merrimack Valley in Massachusetts, then the April 2007 floods in New Hampshire and Maine, and the 2008 deluge in northern Maine and Quebec.
"This was an area with a very infrequent flood history," Vallee said, showing photos of a town in Quebec under water. "Then in 2010, it hit my hometown."
On the projector were photos of floodwaters lapping the roof of the Warwick, R.I., mall and another scene of a nearby flooded downtown showing the Catholic church where Vallee was confirmed as a boy. This was followed the next year by record flooding in the Lake Champlain region of New York state and Vermont, only to be repeated in the fall of 2012 when Hurricane Irene brought destructive floods to other sections of the Green Mountain state.
Small watersheds of about 75 to 230 square miles are more vulnerable compared to large watersheds, he said, because of fewer large flood-control structures and development patterns that have sent runoff to rivers more quickly.
"We are extremely vulnerable before the spring green-up and just after the fall leaf-out," he said.
The increased intensity of flooding, Vallee said, has widespread implications for land-use and infrastructure decisions. Many existing culverts, bridges and dams were built based on what are now outdated assumptions, he said. Flood plains where development is discouraged may have to be expanded.
"What we built for in the '60s may be grossly undersized because we are wetter," he said.