NH43D-03
Gorkha earthquake-induced landslides and dammed lakes: Evolution and outburst modeling

Thursday, 17 December 2015: 14:10
309 (Moscone South)
Daniel H Shugar1, Wouter Immerzeel2, Niko Wanders3, Jeffrey S Kargel4, Gregory J Leonard4, Umesh K Haritashya5 and Brian D Collins6, (1)University of Washington Tacoma Campus, Tacoma, WA, United States, (2)Utrecht University, Geosciences, Utrecht, Netherlands, (3)Princeton University, Civil & Environmental Engineering, Princeton, NJ, United States, (4)University of Arizona, Tucson, AZ, United States, (5)University of Dayton, Dayton, OH, United States, (6)USGS Western Regional Offices Menlo Park, Menlo Park, CA, United States
Abstract:
On 25 April 2015, the Gorkha Earthquake (Mw 7.8) struck Nepal, generating thousands of landslides in Nepal, Tibet (China), and India. While the majority of these hazards were triggered co-seismically, many are considered secondary effects occurring during the weeks following the main shock, based on high-resolution WorldView satellite imagery. Here we report on a series of shallow, post-seismic landslides into the upper Marsyangdi River in the Annapurna region of the central Nepal Himalayas. These landslides constricted and blocked the river, causing impoundments that presented acute flood risks to communities downstream. On April 27, two days following the main shock, ~4.7 x 104 m3 of water was impounded behind a series of small constrictions. By May 28, the total volume of impounded water had increased to ~6.4 x 105 m3. The downstream flood risk was especially significant in the event of a domino-like cascade of dam breaches. We examine the timing, distribution and evolution of the landslide-dammed lakes and quantify the risk of inundation-scenarios to downstream communities with a hydrological model. The model uses a fully kinematic wave simulation at a 30 m-spatial and 2 sec-temporal resolution to resolve the height, timing and volume of a possible outburst flood wave. Our modeling shows that a rapid dam burst involving only the lowest, largest lake would increase water levels at the nearest village of Lower Pisang ~2 km downstream by >7m in a matter of minutes. Approximately 70 km downstream, the flood wave would be mostly attenuated, raising water levels only tens of centimeters. Fortunately, at the time of writing, no flood had occurred.