Sediment mobilization by rain-driven landslides in a subtropical moist, mix-use, and predominantly metamorphic setting

Abstract

Sediment mobilization by rain-driven landslides tends to be responsible for the relatively high sediment yields of high-standing islands within tropical cyclone corridors where sediments represent a threat to water reservoirs and marine environments such as coral reefs. An opportunity to quantify sediment releases and delivery to streams by shallow landslides emerged as a consequence of the precipitation associated with Hurricane María in 2017 within the predominantly metamorphic Río Loco watershed in western Puerto Rico. Aerial photo interpretation and high-resolution digital elevation models representing conditions before and after the hurricane were used to map and quantify the volume of individual landslide scars. The 642 landslides mapped were estimated to have mobilized 40,640–78,530 Megagrams (Mg) of sediment for an overall denudation of 1.6–3.0 mm, and with ~30 % of the sediment being delivered to the stream network. Susceptibility to landslides was controlled by slope, lithological-soil substrate, land cover, and distance to roads. Landslide mobilization during this single rainstorm equaled 10–35 years of sediment production by surface erosion on croplands and 0.8 to 2.3 years of watershed-scale sediment delivery. However, area-normalized mobilization for Río Loco (1870–3620 Mg km−2) was lower than that on the adjacent Río Yauco-Lago Lucchetti watershed (5260–9960 Mg km−2), even though the two watersheds share similar topographic relief and had similar precipitation amounts during Hurricane María. The difference between the two areas is related to the dominance of serpentinite rocks within Río Loco (69 % of the watershed), a rock type that weathers into soils that are not apt for the type of coffee and mixed-crop cultivation that is typical of the central-western highlands of Puerto Rico. As a consequence, portions of the Río Loco watershed underlain by serpentinite are predominantly forested and have a relatively low road density (~7 km km−2) in comparison to actively-cultivated areas that are underlain by intrusive rocks within Río Loco (22 km km−2) and Río Yauco-Lago Lucchetti (~21 km km−2). Given the importance of proximity to roads in inducing landsliding, the lower road densities in Río Loco led to relatively lower watershed-scale sediment mobilization rates than in Río Yauco-Lago Lucchetti during Hurricane María. The results highlight the diversity of geomorphic response to the same rainstorm, in this case driven by lithological substrate controls on past and present land use and road building.