Flood Plain-Impact

By Yuan Ma and Loren Savini, State House correspondent

Superstorm Sandy wreaked $28 million worth of havoc in Massachusetts a year ago October, cutting power to 400,000 customers and forcing evacuations in low-lying areas from Dartmouth to Plum Island.

It could have been much worse.

According to Preparing for the Rising Tide, a study by the Boston Harbor Association, if the Sandy had hit Boston five and a half hours later – at high tide rather than low tide – floodwaters could have floated boats in the Back Bay. The Charles River would have overflowed into Cambridge’s Harvard Square. More than 6 percent of Boston would have been under water.

Massachusetts coastal and river areas can’t hope to escape such destruction forever. A variety of studies by government and private organizations warn that climate change will intensify storms and raise sea levels, increasing the threat to coastal and riverfront areas.

The reports echo data that has led both commercial and government insurers to boost rates significantly over the past year.

The Boston Harbor Association study says over 30 percent of Boston could be flooded if the sea level rises by two and half feet. It predicts that Boston’s sea level will rise two feet by the year 2050 and six feet by the year 2100.

Logan International Airport, which is constructed on reclaimed wetlands, could be affected by late-century flooding, according to the Confronting Climate Change in the U.S. Northeast by the Northeast Climate Impacts Assessment Synthesis Team, a collaboration of the Union of Concerned Scientists and more than 50 independent experts.

The Massachusetts Executive Office of Energy and Environmental Affairs’ Climate Change Adaptation Report, a study ordered in 2008 by the state Legislature, predicts that by the year 2100 such landmarks such as Commonwealth Avenue, Newbury Street, Old South Church and Copley Square would be inundated in so-called 100-year events.

A 100-year event is defined as a storm that has a one-in-a-100 chance of occurring each year.

But such odds are getting shorter with the increase in the number and intensity of storms. The 1991 “Perfect Storm” – once rated a 1,000-year event – is now considered a one in 200 to 500-year event.

The state report predicts that by 2050, Boston could experience the current 100-year flood every two to three years on average.

Destruction wouldn’t be confined to immediate storm damage. The state report warns the constant pounding of angrier seas will bring more coastal erosion; saltwater could encroach on underground aquifers, making some freshwater supplies undrinkable.

Climate change will also affect the state’s riverfront communities. Spots on major rivers face a greater threat of flooding, such as the Charles River at Waltham, the Indian Head River at Hanover, the Taunton River near Bridgewater and the Segreganset River near Dighton. According to the U.S. Geological Survey all these location hit their highest flood levels during the 2010 spring rains.

The state report warns that by 2100, the current 100-year riverine flood is expected to occur every one to two years.