IRENE-GENERATED HILLSLOPE FAILURES IN NEW ENGLAND: WIDESPREAD, PERSISTENT SOURCES OF SEDIMENT TO STREAMS

A combination of regional analysis, terrestrial lidar scans, and suspended sediment collection has shown that failures initiated or reactivated during Tropical Storm Irene are prevalent and persistent point sources of sediment to adjacent streams. Hillslope failures are distributed widely throughout the region: a Google Earth imagery analysis showed 1010 hillslope failures occurred in a 12,000 km² mapped area comprising central and southern Vermont, and northern Massachusetts. Many of the failures were unprecedented in recent time; however, 48% of these failures reactivated pre-existing scars. Repeat terrestrial lidar differencing at 17 failures between April and October, 2014 shows that scars shed sediment episodically during the year, particularly during high flow events and spring snowmelt. A bankfull flood on the Williams River caused reactivation of two previously scanned failures, which together eroded >4000 m³ of sediment, all of which was deposited into the river. Elevated suspended sediment concentrations downstream of these sites show that sediment eroded from failure scars represents an important component of the stream sediment budget. Suspended sediment fluxes ranged between 150 and 500 kg/s downstream of landslides, 1-2 orders of magnitude higher than upstream fluxes. As a result of sediment influx from active slope failures, pre- and post-Irene suspended sediment rating curves show higher suspended sediment fluxes post-Irene. Many failures persist from the beginning of the Google Earth record in the early 1990s, suggesting that reactivated failures are loci of persistent sediment input to adjacent streams. If new failures evolve similarly to those present before Irene, significant erosion at these sites will persist for decades.