Exploring extreme rainfall impacts on flow and turbidity dynamics in a steep, pristine and tropical volcanic catchment

Tropical volcanic landscapes are important because of the short timescales (< years) over which they transform. Sediment sources, availability and transport can be highly dynamic, but our understanding of these is limited by a lack of data in these complex environments, especially with regards to extreme events. To investigate the responses to extreme rainfall events in particular, we conducted extensive monitoring in a pristine tropical rainforest catchment (3.2 km2), located in the Volcanic Cordillera of Tilarán, Costa Rica. We established high temporal resolution hydro-meteorological and turbidity monitoring from June 2015 to July 2016. This included a record convective rainfall event in August 2015 which resulted in an estimated >50 yr return period flood event. We also surveyed hillslope soils, landslides, and sediments of the river network, to characterize sediments before and after the extreme event. Our results suggested that rainfall events activated surface flow pathways with associated mobilization of material. However, erosion processes were mostly linked to finer material (sand, silt) properties of the soils that developed on more highly weathered bedrock. The single extreme event (return period >50 years) had an overriding impact on the general sediment dynamics. Recovery in the form of fine material transport and associated hysteresis took only about three months. We conclude that the combined use of high-temporal resolution monitoring with spatially distributed surveys provided new insights for the initial assessment into the fluvial geomorphology and transport dynamics of steep, volcanic headwater catchments in the humid tropics with potential to establish more complete time scales of land-forming processes. This work can build the foundation for more complete monitoring using radioisotopes as a tool to fingerprint the sediment origin and composition.