ESTIMATING DISCHARGE OF FLOODING OF THE CASTLETON RIVER, VERMONT, RESULTING FROM TROPICAL STORM IRENE

On August 28, 2011, Tropical Storm Irene dumped up to eleven inches of rain in less than 24 hours as it passed over Vermont. This intense rainfall resulted in catastrophic flooding, which damaged or destroyed over 500 miles of roads, more than 200 bridges, and countless houses and other structures. Streams rapidly entered flood stage, with many recording their largest discharge ever. There are 58 USGS stream gages in Vermont, but these are placed only along the larger streams. Determining flood discharge on ungaged streams must therefore be done indirectly.

The Castleton River in southwestern Vermont has its headwaters northwest of the town of Proctor, and flows approximately 23 miles to Fair Haven, where it empties into the Poultney River (which then flows into Lake Champlain). Stream velocity and channel profile measurements of the Castleton River in the town of Castleton have been taken over the past twenty years; the most recent measurements (10/2/09) are typical, with an average flow velocity of 0.97 m/s and a calculated discharge of 2.8 m³/s. During Tropical Storm Irene, it was impossible to reach the river to take any direct measurements of stream velocity. Eleven days later, stream debris caught in trees and fences and other flood indicators were used to determine the flood’s cross-sectional area. The peak flood discharge was then estimated using three different methods for calculating flood flow velocity: (1) using the Manning equation, (2) comparing the flood cross-sectional area with the profile from 2009, determining the scaling factor, and applying it to the 2009 discharge to estimate the flood discharge, and (3) comparing stream gage data from the Poultney River in Fair Haven to determine the scaling factor for Irene discharge compared to 10/2/09 discharge, and then applying that scaling factor to the Castleton River to estimate flood discharge. The resulting Irene flood discharge estimates range from about 200 m³/s to over 300 m³/s. Fortunately, the Castleton River has an ample floodplain, which greatly minimized property damage.

Project co-authors: Geology 3110 (Hydrogeology) class, Fall 2011.