Almost everyone in New England can recall certain major weather events such as the last hurricane that hit the area, the worst tornado to have affected the region, or certainly this winter, with the biggest snowstorm in recent years. But can you recall the last flash flood that affected the region, or the most widespread flash flood episode?
Flash flooding is the number one weather related killer in the United States and we in New England are by no means immune to their occurrence. A flash flood is a type of flood event that occurs quickly, usually within 6 hours of a rain event. Flash floods can also result from dam breaks or even an ice jam on a river or stream during the winter months. More often than not, they are usually isolated events, affecting only one or a few small streams in a particular area and often do not receive the widespread notoriety of larger weather events.
When a flash flood occurs, the water in the affected small stream or waterway rises very rapidly inundating the surrounding area. Within minutes, what was once a harmlessly looking stream can become a raging rapid capable of rolling boulders, tearing out trees, and destroying buildings and bridges. Time is of the essence.
Flash floods can occur just about anytime of year in the region. Prolonged cold spells during the winter can cause a build up of river ice. Once this ice breaks apart and moves downstream, it can become jammed against a bridge resulting in flash flooding upstream of the jam.
During the spring and summer months, slow moving thunderstorms are usually the catalyst. These thunderstorms are capable of producing several inches of rain in just one hour, resulting in flash flooding.
Living in New England, we can never ignore the threat of an approaching hurricane or tropical storm. Many new englanders tend to associate hurricanes with just widespread coastal damage due to the storm surge and high winds. Several tropical storms and hurricanes during the past 70 years have caused just as much, if not more damage inland from flash flooding.
Your National Weather Service provides flash flood watches and warnings to alert the public to the threat of flash flooding. A flash flood watch means that conditions are such that flash flooding could occur. Keep an eye on rivers and streams and pay close attention to the latest statements and forecasts. Above all, be ready to head to high ground and know alternate ways to get there should roads become washed out.
A flash flood warning means that flooding is imminent or occurring. Immediate action is needed to protect your life. Get out of low lying flood prone areas as quickly as you can.
As for the most recent flash flood? One occurred March 13th 1997 in Alton, New Hampshire. A dam on the Merrymeeting river failed on a dry, sunny day. It caused one death, destroyed a 300 to 500 foot section of roadway, and destroyed several downstream homes and one highway bridge.
As for the most widespread flash flood event? it occurred in August of 1955 from tropical storms connie and diane. These two tropical storms struck New England 5 days apart. Together, they produced 10 to 25 inches of rain which caused 90 deaths and millions of dollars worth of damage along many rivers and streams.
Southern New England is unique because it is affected by such a variety of weather systems throughout the year. Anyone living along rivers and streams in the area must realize that flash flood events can occur anytime, anywhere. Your state of readiness is what will save your life in the midst of a flash flood event.