Surface Sediment Stratigraphy from Hurricane María – Event Driven Deposits vs Background Sedimentation

Reservoirs are the main freshwater storage infrastructure for agriculture, human consumption, and hydroelectric power generation across the globe. Yet, its long-term sustainable use is threatened by sedimentation, which can be exacerbated by soil surface erosion and landslides within its watershed. In wet tropical steep terrains like Puerto Rico (PR), extreme precipitation from tropical cyclones often serves as the mechanism for inducing landslides, increasing the amount of sediment available for transport. However, the relative role of these extreme events in landscape denudation and resulting reservoir infilling patterns remains unclear, largely due to lack of geomorphic observations from extreme events. Hurricane Maria (HM) passed over PR on September 20-21, 2017 and triggered over 70,000 landslides (Hughes et al., 2019), providing a rare opportunity to analyze resultant sedimentation within the island‘s reservoirs and compare specific sediment loads associated with extreme events relative to that associated with longer-term sediment yield. We combined measurements of dry bulk density, organic content (LOI), grain size, x-ray fluorescence (XRF)-derived elemental abundances, and x-radiographic imagery. We identified three high-energy event deposits, characterized by low-organic, fining upward sand sequences with sharp lower contacts and organic-rich mud caps (Figure 1). Elemental composition of these event deposits shows high Ca and Sr concentrations relative to the finer grained sediments they are embedded in. We interpret these event deposits as a result of three peak discharge pulses (up to 36,000 ft³/s) during HM as observed in 15 min discharge data from an upstream stream gage. Based on the HM deposit thickness and mean bulk density (837.5 kg/m³), we estimated a volume between 390,000-750,000 Mm³. This represents 1.5 to 3 years of annual infilling rates based on repeated bathymetry surveys (Soler-López, 2012). We applied a power-law regression model using USGS daily mean discharge and suspended sediment concentration data from 1995-2005, and estimated between 900,000-970,000 Mg of sediment were delivered during HM, which is 4.3 to 4.7 years of annual sedimentation from bathymetric surveys. The Caonillas reservoir experiences periodic fine sediment accumulation punctured by episodic higher energy event-driven deposition and multiple sediment discharge pulses within a single storm. This study suggests that extreme precipitation from tropical cyclones plays a significant role in sediment yield, and consequently in reservoir storage loss.