--  Photo by UMass Amherst

This  photo shows how the Connecticut River spewed sediment into Long Island Sound on Sept. 2, 2011, showing the widespread erosion caused by Tropical Storm Irene.

Via ecoRI News (ecori.org)

AMHERST, Mass.

Lake sediments reveal that erosion from Tropical Storm Irene flooding in 2011 caused the most severe erosion of the historic record, according to a new study.

The recent study reveals that increasing soil moisture is raising flooding, erosion and landslide risks in New England, and found that erosion risks have multiplied four times as a result of climate change.

Led by University of Massachusetts Amherst geologist Brian Yellen, a team of scientists has been using sediment deposits in New England lakes to evaluate erosive destruction of historic floods. When floodwaters reach the quiet conditions of a lake, they drop their sediment and leave a layer at the lake bottom that can be used to reconstruct the erosive conditions of the causal flood, according to the researchers.

In a new paper in the journal Earth Surface Processes and Landforms, Yellen and his team show that the 2011 flood from Tropical Storm Irene in Massachusetts and Vermont caused historically unprecedented erosion in the form of landslides and rivers that jumped banks and destroyed most everything in their paths.

“When considering river floods here in hilly New England, the greatest risk we face is from fast, steep rivers undermining our structures — not from broad areas of inundation, like in the flat Midwest,” Yellen said.

Using chemical clues in the sediment layers, Yellen and his team showed that Irene flooding was uniquely capable of eroding ancient glacial till, riverbank material that has remained unmoved for the past 15-20 thousand years. Previous storms with greater precipitation totals and peak river flows weren’t as erosive, according to Yellen.

So what made Irene so erosive and destructive? It turns out that the 2011 tropical storm arrived on the tail of an anomalously wet period. In Vermont, total precipitation for the month immediately before Irene fell in the 95th percentile.

Increased soil moisture weakened banks and allowed for massive erosion, despite river flows that had been exceeded in the instrumental record.

Most alarming, the study found that rainfall statistics are shifting as a result of climate change. Based on trends in rainfall records, monthly preceding rainfall for Irene was four times more likely in 2011 than in 1911, according to the researchers.

“The jury is out on whether we will see more hurricanes as a result of climate change,” Yellen said. “But what is known with a good deal of confidence is that our region will continue to become wetter. This increasing baseline moisture primes the system for maximum destruction when big rain events occur.”