Debris Flow Dynamics in a Coastal Arid Catchment: Rainfall Thresholds and Numerical Simulation During the 2023 Yaku Event, Peru

Abstract

Debris flows pose a significant hazard in mountainous and coastal arid regions, yet their rapid post-event assessment remains challenging. In March 2023, the Yaku event generated extreme rainfall across Peru’s Pacific watershed, triggering a hyperconcentrated flow in the Cusipata catchment, Lima department, Peru. This study presents an integrated analysis combining morphometric classification, hydrological modelling, and numerical simulation to characterise the hyperconcentrated flow hazard and support risk assessment. The catchment was classified as debris flood-dominated using three independent morphometric approaches, consistent with a hyperconcentrated flow regime inferred from field-measured sediment concentration. A local rainfall intensity–duration threshold was derived using a method based on Linear Regression-True Skill Statistic, providing a basis for early warning systems and validated operationally during the 2026 rainy season. The catchment hydrological response was simulated using event-based modelling, and the resulting hydrograph was used as input to RAMMS debris flow simulations with a Voellmy friction model. Two scenarios were evaluated: the Yaku event using the pre-event topography, and a post-Yaku scenario reflecting damaged retention infrastructure. Hazard maps indicate a significantly increased flow extent and intensity in the post-Yaku scenario due to compromised protective structures. These results were applied to support a formal risk assessment and demonstrate the value of integrating drone-derived topography, locally calibrated thresholds, and physically based modelling for post-disaster hazard characterisation in arid Andean catchments.