LANDSLIDES MODULATE TRANSIENT ADJUSTMENT TO CATASTROPHIC FLOODING

Catastrophic floods cause immediate and transient changes in fluvial systems, but their rarity hinders real-time investigation. Tropical Storm Irene, the flood of record for much of New England, offers an important opportunity to quantify both during- and post-event effects of a >100 yr event. We seek to answer 1) how does sediment input during large events contribute to the long-term budget; 2) in the years following a flood, do channel-adjacent landslide scars continue to be a major source of sediment to streams; and 3) how does abundant available sediment influence post-event recovery? To address these questions, we mapped nearly 1,000 landslides generated in a 14,000 km²study area during Tropical Storm Irene, and have conducted intensive follow-up investigation on 20 representative scars beginning in 2013. Aggregate during-event input from these features was equivalent to 0.13 mm of erosion across the study area. Conservatively assuming 50% export to mainstem rivers, this sediment contribution exceeds the average regional annual sediment flux by 30 – 50 times.

We quantify subsequent landslide change by comparing repeat terrestrial lidar models. The landslides—generated in glaciolacustrine sediments, outwash, and till—continue to erode rapidly (average erosion depth: 1 – 80 cm/yr). Some eroded sediment is stored at landslide toes, contributing to decreased scar slope observed at 100% of landslides since 2013. However, small floods easily mobilize this sediment, and additional sediment is eroded during rain events and the snowmelt period. As a result, suspended sediment concentrations are elevated regionally by an order of magnitude relative to pre-Irene values. Continued landslide change has locally rerouted channels, and abundant available sediment has caused streams to reconfigure in proximal reaches. To accommodate changes in sediment caliber (D50 changes of +/- 320%) and flux, downstream geometry has adjusted rapidly through thalweg migration, bar creation, and knickpoint propagation. Sediment input from some landslides has slowed, but most remain unvegetated and active. Five years after Irene, we have observed no attenuation in suspended sediment concentrations or channel reconfiguration rate, suggesting that transient adjustment initiated during Irene may persist for a decade or more.