Sediment Mobilization by Hurricane‐Driven Shallow Landsliding in a Wet Subtropical Watershed

Abstract

AbstractThe role of individual tropical cyclones in mobilizing sediment by shallow landsliding has been studied widely in islands of the Pacific but hardly within the Insular Caribbean. An opportunity to conduct such a study materialized in 2017 when Hurricane María provoked over 70,000 landslides on the island of Puerto Rico. Through aerial photo interpretation and high‐resolution digital elevation models, this study provides an estimate of the net mobilization and delivery of sediment by shallow landslides in a wet subtropical and actively cultivated 43.8 km2 watershed. Landslide mobilization and delivery are contextualized here in terms of other active sources of sediment and annual sediment delivery. Maximum hourly and 24‐h average rain intensities within the study watershed during Hurricane María were 68 and 10.2 mm h−1, respectively, and these represent intensities with local recurrence intervals of less than 10 years. However, these rain intensities were sufficient to trigger 2,318 landslides that mobilized 230,510–436,330 Megagrams of sediment, an amount that is much greater than surface erosion contributions and represents the equivalent of 0.5–3.6 years of annual watershed‐scale sediment delivery. The high susceptibility to landsliding in this region of Puerto Rico is linked to the abundance of roads (∼21 km km−2), particularly those crisscrossing terrain with slopes ranging from 30° to 60°. Only 15% of these roads are within actively cultivated areas. The remaining roads represent the support infrastructure of abandoned coffee farms that still appear to be inducing a legacy of much geomorphological and watershed management relevance.