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.
Making landfall on August 27th, 2011, Hurricane Irene generated unprecedented flooding throughout the northeastern US. Its effects were especially pronounced in eastern and north-central Vermont where the intense precipitation generated significant damage to homes, roads, and other infrastructure with estimated regional damages approximating one billion dollars, making it the 10th costliest disaster in U.S. history. Erosion related damages included more than 500 miles of road closures due to more than 2,000 damaged roadways and 200 bridges needing repair or replacement, including multiple iconic 19th-century covered bridges. Besides its impacts on infrastructure, Irene-induced flooding spawned record-setting flood heights and discharges contributing to significant but spatially variable geomorphic adjustments. Floods of the size of Irene provide a rare opportunity to understand the responses and/or resiliency of natural systems to disturbance. Taking advantage of Irene’s rapid path through Vermont, this research project documented the magnitude, type, and spatial variability of geomorphic change to the extreme precipitation and flooding generated by Hurricane Irene. The major goals of our research were to document the specific effects of Hurricane Irene and to determine if it is possible to develop models and tools to better predict the location of major geomorphic change. Predicting the spatial and temporal variations in channel erosion due to extreme floods presents a long-standing challenge in geomorphology. Previous research has demonstrated marked variability in the specific responses of watersheds to profound disturbances. In particular, the geomorphic signature of extreme floods can vary widely depending upon local controls such as regional geology or the spatial variability of energy expenditure. To better identify loci of erosion, we develop two methodologies that allow for rapid, regional-scale assessments to identify stream reaches susceptible to channel widening. The first proposes that channel widening occurs when unit stream power (a measure of energy availability) exceeds a critical threshold (300 W/m2). The second was motivated by the observation that widening often occurs at channel bends. We introduce a new metric, the bend stress parameter, which is proportional to the centripetal force exerted on the curved concave bank. Through incorporation of these two thresholds, this study offers a significant improvement in predicting the location and type of geomorphic response, or more precisely, lack of response. Together the metrics represent a simplified and rapid method for delineating reaches where widening during future extreme floods may or may not occur. In the case of the White River, these metrics successfully identified more than 98 percent of significant width changes, while excluding 26 percent of the reach as having little susceptibility to channel widening. In the case of the much steeper Saxtons River, the intensity of the flooding was so great that nearly the entire reach (99 percent) was susceptible to widening and virtually all of the significant changes in width occurred along reaches where at least one threshold is exceeded. Because the susceptibility of a channel reach to widening during a flood depends upon factors other than unit stream power and specific bend power, our metrics alone cannot perfectly identify all the locations where widening will occur. But the combination of bend stress and unit stream power helps identify the fundamental geomorphic processes generating change, ultimately providing a better toolkit for watershed managers to ascertain critical areas to protect or manage. Besides its contribution to policy makers and floodplain managers, this research contributes theoretically to earth system scientists and physical geographers. There has been a longstanding debate amongst geomorphologists concerning the geomorphic criteria for characterizing an extreme flood as "catastrophic". From a probabilistic perspective, extreme floods are rare occurrences, but not all extreme floods generate spectacular geomorphic impacts. Previous research shifted the focus away from flood discharge and recurrence intervals as the appropriate metrics to that of flow energy, yet flow dynamics do not singularly predict or explain the type and range of geomorphic responses to large floods. Current geomorphic thinking posits that flow duration of energy expenditure above a critical value needs to be incorporated, along with stream power, to better explain the driving forces of geomorphic change, yet this has rarely been tested. Results from our research highlight the combined role of flow duration and energy expenditure in controlling the type, location, and magnitude of geomorphic change. We show that floods with rapid rise rates and minimal flow duration can indeed generate significant geomorphic effects such as intensified landsliding, robust overbank deposition, and transportation of large boulders. Our results indicate that flow duration is indeed an important variable: a rapid time to peak discharge can contribute to profound sedimentological impacts, but the short flood duration may have other limited geomorphic expression. Through the development of our predictive model and elucidation on the geomorphic role of short duration events, our results should provide an important regional template for local and federal agencies to help guide flood preparation strategies.
