PSU Professors Research Floodwater from Tropical Storm Irene

October 19th, 2011 by blyndes

Plymouth, N.H. – While Tropical Storm Irene was barreling through New Hampshire in late August, two Plymouth State faculty members realized the unusual weather event was an opportunity to learn more about the science of flooding.  Hydrologist Mark Green and geologist Lisa Doner, both with PSU’s Center for the Environment, sprang into action, working
nearly non-stop from Saturday, August 27, through Thursday, September 1, to collect numerous water samples from the Pemigewasset River watershed in the Plymouth/Campton, NH area.

Rainfall from Tropical Storm Irene created this flood water covering Route 175-A in Holderness, N.H. Photo courtesy Phillip Haskell

What was so important about the storm from a scientific point-of-view?  When rain falls on the landscape it can cause elements such as carbon, phosphorus, and aluminum to be released and flushed through the system and into the rivers and streams. A significant amount of rainfall, such as that from Tropical Storm Irene, can cause a large release of elements and nutrients moving through a watershed. Capturing such a large release at one time is unique, and the data from the storm’s water samples can help evaluate the potential implications for forest health after such large flushing events. The fact that the Pemigewasset watershed is largely unregulated by dams and other impoundments results in a chance to better understand the response to the storm in a more natural setting. The team gathered water samples during the day and night.

“We selected sites to gather samples from based on anticipated flow of water, proximity to river, and accessibility. The samples were taken from the river banks and it was difficult at times, especially at night” noted Doner. In some cases, Doner and Green had to get permission from emergency personnel to access the river. The samples were analyzed in PSU’s Center for the Environment’s Environmental Research and Sediment laboratories, and at the analytical laboratory of the U.S. Forest Service – Northern Research Station, to determine the nutrient and element concentrations in the samples, particle-size distributions and isotopic composition, which will help explain the sources of the floodwaters throughout the event.

Green has analyzed data from the collected samples and data from other sources such as US Geological Survey river gauges, rain gauges and soil moisture data from the US Forest Service’s Hubbard Brook Experimental Forest. His analysis indicates that Irene produced anywhere from four to ten inches of rain throughout the Pemigewasset River watershed, resulting in the largest flood ever recorded in August and ninth-largest flood during any month in the 107-year history of the Pemigewasset River station at Plymouth. Green noted that “the amount of flooding was caused not just by a lot of rain, but also be the fact that the surface layers of the soil were wet from storms that occurred in the two weeks before Irene, so the soil was not able to absorb much of the rain from Irene.”

With regards to water quality, Green estimates that at a minimum, 26 tons of dissolved nitrogen and 41 tons of dissolved aluminum were moved out of the watershed by the flood.

“Flushing the soils like this can have implications for the region’s forest since some of these elements are nutrients and others may inhibit forest growth,” said Green. “Over time, we will be able to better understand how large storms can influence the region long after the flood waters recede. We hope this information about the interrelationships between forests, soils, and water systems can be used in forest management.”

Doner has recruited an undergraduate student from PSU’s Environmental Science and Policy program to process dozens of samples for analysis. These results will provide useful and important insight into the role of large summer floods in flood plain health and stability. The Irene flood, for instance, is the fourth largest tropical storm flood on record for Plymouth, and the largest since 1959.

“Although events of this scale are relatively rare, climate forecasts indicate that they may become much more common,” said Doner. “Irene has offered us a terrific opportunity to understand
how large flood impact our rivers today, with existing levels of watershed and floodplain development.”

For more information about this release, contact Bruce Lyndes, PSU News Services Manager, (603) 535-2775.