Persistence of Episodic Extreme Events: Sustained Colluvial Contributions of Fine Sediment to Vermont Rivers Post-Irene

Abstract:

Tropical Storm Irene generated devastating floods in New England in 2011, causing more than $500 million of damage. In intervening years, many geomorphic signs of disturbance have attenuated, suggesting that impacts may be ephemeral. Yet persistent impact continues: channel-proximal landslide scars linger as point sources of fine sediment 3 yrs post-Irene. We evaluate the legacy of this major disturbance while also testing conceptual models of hillslope-channel connectivity and subsequent downstream sediment routing. We measure sustained landslide erosion by comparing DEMs generated by a Terrestrial Laser Scanner and trace sediment mobility using in-channel measurements of embeddedness, sediment concentration, and fallout radionuclide activity. We augmented detailed temporal sampling of an 850 m² landslide along a 2^nd-order stream with a spatially robust summer 2014 field campaign, scanning an additional 12 landslides. The initially sampled landslide eroded 250 m³ of sediment between fall 2013 and May 2014, averaging 0.3 m of erosion with nearly all erosion occurring during a two-week spring snowmelt. Landslide sediments had high measured ⁷Be activity (t_1/2=53.4 d), caused by subaerial exposure; sediment collected downstream of the landslide had higher ⁷Be activity than that collected upstream, suggesting landslide provenance. Channel sediment upstream of the landslide had remained in the channel long enough for ⁷Be to decay below detectable activity. Embeddedness, a measure of fine sediment on a channel bed, was higher downstream of the landslide than upstream. Remote sensing reveals >50 similar landslides within the White River alone, and hundreds more in Vermont. Thus, landslide scar inputs may continue to influence the regional fine sediment budget. Ongoing successive scans in multiple watersheds show erosion continues in summer, an observation corroborated by elevated suspended sediment concentrations downstream of landslides after rain events. Summertime erosion has generally been low, but one extreme storm triggered >4000 m³ of erosion on a 3500 m² landslide along the 5^th-order Williams River, averaging 1.3 m erosion across the landslide. Understanding the loci of affected reaches and the magnitude of the continued effect is critical in assessing the long-term legacy of extreme events.