PORTRAYING RUNOUT AND INUNDATION FROM HURRICANE-INDUCED LANDSLIDES IN PUERTO RICO

Landslides can be destructive and potentially lethal in places where human infrastructure is present. In 2017, Hurricane Maria triggered more than 70,000 landslides in Puerto Rico; most landslides originated as shallow failures on steep slopes. After initiation, these landslides mobilized to varying degrees – some slides only traveled partway downslope, whereas others reached drainages and mobilized into long traveled debris flows. These moderate to high mobility landslides were likely to block roads, thereby disrupting access to medical services and electricity. As demonstrated by Hurricane Maria, forecasting the potential landslide runout zones is critical; however, many landslide-susceptibility maps do not identify areas vulnerable to landslide runout. To address this need, our methodology integrates user-provided potential landslide source zones with two methods to identify areas susceptible to landslide runout.

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.