Topographic controls on landslide mobility: Modeling hurricane-induced landslide runout and debris-flow inundation in Puerto Rico
Abstract. In 2017, Hurricane Maria triggered more than 70,000 landslides in Puerto Rico. After initiation, these predominantly shallow landslides mobilized to varying degrees – some landslides only traveled partway downslope, whereas others reached drainages and mobilized into long-traveled debris flows that could severely impact roads and infrastructure. Thus, forecasting potential landslide runout and inundation zones is critical for estimating landslide and debris flow hazards. Here we conduct an in-depth topographic analysis of landslide-affected areas from nine study areas and apply a linked modeling technique to estimate locations susceptible to varying degrees of landslide runout in Lares, Utuado and Naranjito municipalities.
We find that longer runout length is observed on high-relief escarpments, although highly mobile long-runout debris flows also occurred in lower-relief dissected uplands. These topographic differences indicate that landslides initiating under similar conditions and possessing equal potential to mobilize as debris flows may not travel the same distances or affect the same areal extent. Our modeling approach allows the local topography to automatically control the implementation of two runout methods: 1) H/L runout zones are assigned directly downslope of landslide source zones, and 2) debris-flow inundation zones are estimated in the presence of a channel network. Debris-flow volumes are calculated as a function of area-integrated growth factors, estimated as a function of the upstream areas susceptible to shallow landslides. Applying our empirical modeling scheme over an area of 560 km2 , our results highlight the efficacy of our methods for assessment of the potential for landslide runout and debris-flow inundation over diverse terrains with varied susceptibility.
