Mobility characteristics of debris slides and flows triggered by Hurricane Maria in Puerto Rico
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Erin K. Bessette-Kirton I Jeffrey A. Coe I William H. Schulz I Corina Cerovski-Darriau I Mason M. Einbund
Mobility characteristics of debris slides and flows triggered by Hurricane Maria in Puerto Rico
Abstract Mobility is an important element of landslide hazard and risk assessments yet has been seldom studied for shallow landslides and debris flows in tropical environments. In September 2017, Hurricane Maria triggered > 70,000 landslides across Puerto Rico. Using aerial imagery and a lidar digital elevation model (DEM), we mapped and characterized the mobility of debris slides and flows in four different geologic materials: (1) mudstone, siltstone, and sandstone; (2) submarine basalt and chert; (3) marine volcaniclastics; and (4) granodiorite. We used the ratio of landslide-fall height (H) to travel length (L), H/L, to assess the mobility of landslides in each material. Additionally, we differentiated between landslides with single and multiple source areas and landslides that either did or did not enter drainages. Overall, extreme rainfall contributed to the mobility of landslides during Hurricane Maria, and our results showed that the mobility of debris slides and flows in Puerto Rico increased linearly as a function of the number of source areas that coalesced. Additionally, landslides that entered drainages were more mobile than those that did not. We found that landslides in soils developed on marine volcaniclastics were the most mobile and landslides in soils on submarine basalt and chert were the least mobile. While landslides were generally small (< 100 m2) and displayed a wide range of H/L values (0.1–2), coalescence increased the mobility of landslides that transitioned to debris flows. The high but variable mobility of landslides that occurred during Hurricane Maria and the associated hazards highlight the importance of characterizing and understanding the factors influencing landslide mobility in Puerto Rico and other tropical environments. Keywords Landslides . Debris flows . Landslide mobility . Travel distance . H/L . Puerto Rico . Hurricane Maria Introduction Landslide hazards are largely controlled by the size and location of initiation and the total distance traveled (mobility) after initiation. Landslide mobility is often evaluated by the ratio of landslide-fall height (H) to travel length (L), H/L (e.g., Heim 1932; Corominas 1996), where low values of H/L indicate high mobility and high H/L values indicate low mobility. High mobility landslides that travel long distances are often more hazardous and can pose a greater risk to people or infrastructure downslope (e.g., Corominas 1996; Evans et al. 2007; Iverson et al. 2015; Coe et al. 2016; Kean et al. 2019). Landslide mobility can be controlled by intrinsic factors such as material and pore-pressure characteristics (e.g., Terzaghi 1950; Sassa 1985; Iverson 1997; Davies et al. 1999; Iverson et al. 2015; Manzanal et al. 2016) or source area volume (e.g., Heim 1932; Corominas 1996; Legros 2002). Extrinsic factors such as volume growth through the
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