AMHERST, Mass. - Two days before Hurricane Irene hit western New England last August and dumped torrential rains far up the Connecticut River watershed, University of Massachusetts Amherst geoscientist Jon Woodruff hastily launched a canoe to take baseline sediment sample cores under ponds and coves in the lower river’s floodplain, from Northampton’s Oxbow Lake south to Long Island Sound.
He thus set in motion a timely study of the effects of a rare autumn flood event, one that may happen only once every 50 years. Woodruff recently won a one-year, approximately $21,000 "rapid response" research grant from the National Science Foundation (NSF) for himself and several geosciences students to characterize floodplain deposition from Hurricane Irene along the lower Connecticut River. Documenting sediments will help them gain insight into the significance of such rare events in building the landscape of a "low-gradient" river system.
Woodruff, a sedimentologist whose research interests include fluvial and coastal processes, clarifies, "In the lower river, say south of Hartford, Irene really wasn’t that rare in terms of freshwater discharge, having maybe a seven-year return frequency. However, in terms of sediment load this event seems to be much rarer due to extreme flooding in the upper watershed."
The last hurricane to bring a greater intensity autumn flood came with the one-two punch of hurricanes Connie and Diane in 1955, he adds. "An intense event at the end of a dry period, one that hits areas that are rarely flooded, acts like a broom sweeping a dirty floor. It can send an amazing amount of material down the river."
The recent grant recognizes the need to act quickly, because one of the modern techniques for analyzing sediment involves measuring the decay of a radioactive ion, Beryllium-7, in mud cores, which is deposited from rainfall and decays completely within a few months. Geosciences graduate student Laura Kratz is expected to present preliminary results of the project at a Geological Society of American Northeast regional meeting in March 2012.
The lower Connecticut River’s floodplain is internationally recognized as having some of the best-preserved wetlands in North America, Woodruff points out. This includes tidal ponds and coves that are critical habitat for birds and fish, and heavily used by humans for recreation. "One of the main points of the study is to evaluate how important extreme events like Irene are in filling up these environments with sediment, and to learn if this sediment is fundamentally different than the type of material that is trapped at these locations during more routine flooding in the annual spring snow melt."
He and his undergraduate sedimentology students estimated the volume of sediment per event by measuring the thickness of flood-related sediment across several cores and estimating its overall mass as part of the semester-long project. "The amount we were able to accomplish in such a short time speaks to their hard work and dedication."
Woodruff also calls technicians at the U.S. Geological Survey "the unsung heroes" of this Irene-related work because they made "valiant" efforts to keep river gauges functioning to document flows. They also collected key water samples used to estimate how much sediment was in suspension during the record-setting event.
In addition to assessing the importance of an autumn event like Irene, the study will allow the UMass Amherst geoscientist and his students to evaluate whether such a rare event’s "signature" can be identified in prehistoric sedimentary records. "We want to know if cores can help us to interpret much older sediments collected at these locations. We may be able to reconstruct the long-term history of this floodplain, evaluate how much of the floodplain is built by such events and to obtain better estimates for the return frequency for these types of floods."
Another aspect of the project will be to evaluate contaminant levels in deposits related to Hurricane Irene and compare them to deeper sediment core samples obtained earlier. This is part of a collaboration between Woodruff and Anna Martini of Amherst College to assess mercury and other heavy metals, supported in part by UMass Amherst faculty research funds and a grant from the Massachusetts Water Resources Center.
As Woodruff explains, most east coast rivers have a legacy of contamination left over from heavy industrial activity along the river from the mid-1850s to the 1970s, which are now stored in floodplain sediments. Sampling sediments after Irene offers a chance to assess the role extreme events play in the storage and/or remobilization of such material.