ASSESSING THE GEOMORPHIC IMPACT OF HURRICANE IRENE FLOODING ON UNREGULATED GRAVEL-BED RIVERS IN NEW ENGLAND

The geomorphic response to extreme floods resulting from hurricane induced precipitation has been well documented in the literature, however, few studies focus specifically on using reach-scale and/or regional controls to develop quantitative predictive tools for assessing the geomorphic responses to watershed disturbances. This study combines pre- (summer 2011) and post Hurricane Irene field survey data from several watersheds in New England with pre- and post-flood high resolution aerial imagery to assess the geomorphic response to this extreme flood. Additionally, in-stream and overbank sediment samples were collected at several locations for fallout radionuclide analysis (²¹⁰Pb_ex (t_1/2 22.3 yrs) and ⁷Be (t_1/2) to assess sediment sourcing. The floods of August 28-30, 2011, exceeded the largest measured discharge at USGS gages in several watersheds in New England. In Central Vermont where precipitation in locations exceeded the 200-year precipitation event, peak discharges were observed to approach and in some locations exceed the estimated 500-year recurrence interval event. This study uses this “natural” experiment to enhance the prediction of the magnitude and type of geomorphic adjustments that is associated with large floods. Immediate post-Irene field-data suggest that the geomorphic response to large floods is highly variable on the watershed scale, including channel avulsion, bank erosion, overbank deposition, and formation and modification of gravel bars. Preliminary GIS analysis shows that changes in Stream Gradient Index (SGI) seem to indicate locations where a significant geomorphic response can be expected, assuming that the channel has the ability to adjust. Furthermore, SGI along with field data, stream power, sediment flux and watershed structure will be incorporated into a GIS to develop a quantitative framework for predicting the geomorphic effects of large floods. Hurricane Irene was a reminder of the enormous social implications of large floods, and the results of this study will help guide state and federal agencies in future flood mitigation efforts.