FLUVIAL GEOMORPHIC IMPACTS AND SUSTAINED RECOVERY FROM TROPICAL STORM IRENE

Using the geomorphic signature of the 2011 Tropical Storm Irene flooding across an array of watersheds in New England, we have used pre- and post-channel transects and in-channel sediment measurements to monitor the response and recovery of rivers impacted by Irene. Topographic resurveys of 17 transects reveals that two-thirds of the transects initially increased in cross-sectional area by as much as 50%, with an average increase in area of 10%. Decreases in channel area were less-frequent and less extreme, with a maximum decrease in area of about 10% and an average decrease of about 5%. However, at almost all sites there has been little significant subsequent change in cross-sectional area since the initial change during Irene, indicating little channel recovery to pre-Irene conditions over three years of monitoring. Repeat pebble-counts along 20 transects indicate no significant change in grain size immediately post-Irene, but instead indicate a rapid decrease in median grain size (to about 40% of its pre-Irene size) during the year after Irene, followed by a more gradual decrease in the subsequent two years. The finer size fraction (D16) has decreased at a more continuous rate over the last three years and is currently about 30% of its pre-Irene size. In contrast there has been little change in the size of the coarse size fraction (D84), either immediately post-Irene or during the three year post-Irene period. The data demonstrate a complex response and recovery of channel morphology to Irene and the persistence of Irene’s impacts. Channel-bed sediment has become progressively finer, likely due to the continued input of fine sediment from eroding slopes that failed during or immediately after Irene. However, this input of fine sediment has had little impact on channel geometry, which changed during Irene but that has generally shown little recovery since. This observation suggests an episodic character to the sediment budget of New England rivers where slopes destabilized during extreme events provide a chronic source of fine sediment to the channel. The fate of this fine sediment is less clear as we did not observe a coarsening of the channel bed during Irene that would have indicated the flushing of fine sediment from earlier extreme events.