Rainfall-Runoff Modeling in Rocky Headwater Catchments for the Prediction of Debris Flow Occurrence

In the Dolomites, steep rocky cliffs are marked by numerous narrow gullies. When high-intensity short-duration precipitation occurs, these gullies concentrate and direct surface runoff to the screes at the foot of rock cliffs. Surface runoff mixes with loose sediments, creating a solid-liquid surge that, as it moves downhill, increases its volume entraining debris material and transforms into a granular debris flow. Given the ongoing challenge of modeling the relationship between intense rainfall, surface runoff, and debris flow initiation, we take advantage of data from three monitoring stations operating in distinct debris flow active catchments in our study area to make progress. These stations, strategically positioned close to debris flows initiation zones, record videos and different types of flow-stage data, helping us pinpoint the timing and form of incoming discharge hydrographs. Over a 15-year period of observation, we collected a comprehensive data set on runoff and mass movement in these catchments, offering valuable insights into their hydrological behavior and the initiation of granular debris flows. To compute infiltration excess runoff generation, we refined an already existing hydrological model and calibrated it using discharge measured at one of the monitoring stations. Testing this updated model against observations from two other larger debris flow sites showed that it can reproduce the initial phases of a debris flow, when sediment concentration rapidly rises. These findings suggest that a well-tuned hydrological model can predict the discharge from intense, short rainfall events that typically trigger debris flows, as well as the early stages of these phenomena.