PORTRAYING RUNOUT AND INUNDATION FROM HURRICANE-INDUCED LANDSLIDES IN PUERTO RICO
We build on USGS research combining soil depth and slope-stability modeling (TRIGRS) used to identify areas susceptible to shallow landslides. The areas susceptible to shallow landslides provide initial source zones for our two methods to identify potential runout and inundation zones. For moderate mobility slides, we define runout zones by height/length from the source. For channelized debris flows (a minority of landslides), we identify inundation zones using empirical volume-area relationships in concert with empirical debris-flow growth factors. Our growth factors integrate growth over a drainage network and are defined as a function of upstream contributing landslide source areas. This approach determines the spatial distribution and volumes of potential debris flows. We apply these methods in three municipalities that had high landslide density from Hurricane Maria: Utuado, Lares, and Naranjito, covering a total area of 560 km2. The resulting maps provide a preliminary assessment of areas susceptible to landslide runout from mobile landslides. Our USGS software package, Grfin (growth+flow+inundation) Tools (under development), integrates these methods and enables runout assessment over large regions without the computational effort required by physics-based models.