RAPID: Geomorphic Response and Recovery to Hurricane Irene Floods: Characterizing Reach-Scale and Regional Controls on Fluvial Adjustments

Dartmouth College, Hanover NH

Investigators

Abstract

This rapid-response research project will examine the geomorphic signature of the Hurricane Irene flooding in late August 2011 in central New England to document the regional geomorphic response across an array of watersheds in central Vermont and test the covariant ways that channels responded to Hurricane Irene flooding and the way channels may potentially recover to establish pre-flood equilibrium conditions. The floods of August 28-30, 2011, exceeded the largest measured discharge at longstanding USGS gages on innumerable small (up to 50 km-square in area) to large (up to 1,400 km-square) watersheds across New England, but especially in east-central Vermont, where peak discharges in places approached and even exceeded estimated 500-yr recurrence interval events. The investigators will build on an intense 2011 summer (pre-storm) field campaign in the flood-affected region that focused on differentiating reach-scale adjustments of the critical Shields number to inputs of sediment at tributaries in alluvial, bedrock, and mixed-channel reaches in regulated and unregulated watersheds. The data from these locations provide a strong baseline dataset to use in determining how extant boundary conditions in combination with flood power, sediment flux, and watershed structure (including valley confinement and tributary inputs) limit or enhance stream channel and floodplain responses to large floods. These field data will be augmented with detailed image analysis and analyses employing geographic information systems. The investigators also will examine significant overbank deposits with distinct sediment packages. They will use the fallout radionuclides lead-210 (with a half-life of 22.3 years) and beryllium-7 (with a half-life of 53.4 days) as indicators of sediment sourcing and test the hypothesis that overbank sediments with significant radionuclide activities represent sediment delivered from hillslopes connected to adjacent channels while overbank sediments lacking radionuclide activities come from eroded streambanks (with these being "dead" sediment lacking fallout activity). This project will enhance basic understanding of the magnitude and nature of geomorphic adjustments to large floods. The project will capture geomorphic controls on channel responses across an array of alluvial settings and evaluate the pattern and isotopic signature of overbank deposits to potentially reveal the relative sourcing of sediment (hillslope vs. channel banks). Results from this project will help guide state and federal agencies in future flood mitigation efforts and will provide valuable new insights for understanding natural processes that will impact on river-restoration efforts.