Collaborative Research: RAPID: The fate of landslide-derived sediment following tropical cyclones: a case study of Hurricane Fiona in Puerto Rico

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From September 18-20, 2022 Hurricane Fiona delivered more than 64 cm of rainfall to the island of Puerto Rico. This excessive precipitation caused mass wasting and flooding across the island, similar to the effects of Hurricane María in 2017. Both events represent extraordinary opportunities to study the impact of tropical cyclones on the evolution of a high relief, tropical landscape. This project will support two deployments of scientists and equipment to collect time-sensitive and perishable data from two catchments in southern Puerto Rico strongly impacted by mass wasting after the hurricane. Using these catchments as a model study area the investigators will: 1. survey the topography of landslide deposits, scars, and post-landslide river aggradation using drone-based structure-from-motion photogrammetry; 2. collect samples of river sediment downstream of landslide areas for grain size analysis and cosmogenic nuclide tracing of landslide-derived sediment, 3. monitor and record the movement of still-unstable landslides and actively aggrading river channels with time-lapse cameras. The project will be collaborative with co-investigators based in Puerto Rico and coordinated in concert with federal and local scientists working in hazards assessment and prevention. Field work deployments will engage at least 3 graduate and 3 undergraduate students.For more than sixty years, geologists have debated the contribution of large, yet infrequent storm events to the long-term evolution of eroding landscapes. For large storms like tropical cycles, the immediate geomorphic impact to tropical landscapes may be obvious: mass wasting and flooding. However, the extended response of tropical landscapes to the storm event is often more complex, and measurement of landscape response requires repeated observations across a catchment area, beginning immediately after the storm event. This project supports collection of time-sensitive and perishable data to explore three hypotheses concerning the response of a tropical landscape to landslides and flooding caused by a tropical cyclone. Specifically, the investigators will address the questions: 1. Where (and why) do storm events destabilize hillslopes? by surveying landslide scars and deposits in-person, monitoring still-unstable hillslopes with time-lapse cameras, and mapping landslides regionally with remotely-sensed imagery; 2. How do river networks adjust to an influx of landslide-derived sediment? by measuring channel aggradation/erosion patterns with repeat drone-based topographic surveys and direct tracking of coarse sediment transport using time-lapse images of channels downstream of landslide areas; and 3. What is the residence time of landslide-derived sediment in a tropical landscape? by repeated measurement of cosmogenic radionuclide concentrations in river sediment to detect changes in the flux of landslide-derived sediment over the following year.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.