Performance of Models for Flash Flood Warning and Hazard Assessment: The 2015 Kali Gandaki Landslide Dam Breach in Nepal

Abstract

Lessons learned from the tsunami disaster caused by the 2011 Great East Japan Earthquake and improvements in JMA's tsunami warning system Abstract A huge tsunami generated by the 2011 Great East Japan Earthquake (also known as the 2011 off the Pacific coast of Tohoku Earthquake) that struck at 14:46 JST (UTC+9) on March 11, 2011, hit a huge stretch of the Pacific coast of Japan and caused severe damage over an area extending from the Tohoku district to the Kanto district. In the aftermath of the disaster, the Japan Meteorological Agency (JMA) investigated the content and expressions of the tsunami warning bulletins released as well as the timing of their issuance in relation to this occurrence at that time. The results were used to support consideration of how tsunami warnings could be improved in their role as a type of disaster preparedness information to protect life. The investigation revealed several problems, including the underestimation of earthquake magnitude promptly determined after the quake which in turn caused the underestimation of forecast tsunami heights. Another issue was the inappropriate announcement of tsunami observations; all the observed tsunami heights were announced even while waves were still small, which caused some people to believe evacuation was unnecessary for what they thought was a minor tsunami.JMA's tsunami warnings must be timely, easy to understand and useful for organizations related to disaster prevention. To overcome the problems found in the investigation and to better meet these requirements, JMA improved the approach used in its tsunami warning system and enhanced the content and expressions of bulletins so that warnings urge people to evacuate as appropriate. As part of such improvements, JMA also remains active in its awareness-raising efforts as a key area in the appropriate usage of warnings and successful evacuation.Based on these activities, JMA introduced a new tsunami warning system on March 7, 2013.

A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal

“…Numerical GLOF modeling A simulation of the GLOF was performed using the Basic Simulation Environment for Computation of Environmental Flow and Natural Hazard Simulation (BASEMENT) model, an openaccess numerical model based on the 2-D shallow water equations (Vetsch et al 2017). BASEMENT's inclusion of sediment transport makes it particularly suitable for GLOF simulations, since it can simulate erosion, scouring, and debris flow in addition to water flow (Worni et al 2014); moreover, it is a 2-D model, which is superior to the geometric and 1-D models that have been used for this type of application (Bricker et al 2017). In addition, characteristics of the overtopping wave were validated with the Heller-Hager model, which combines analytical and empirical equations to study wave generation and propagation resulting from mass movement into a reservoir (Heller et al 2009).…”

Bumps in river profiles: uncertainty assessment and smoothing using quantile regression techniques

“…More detailed error analyses, however, reveal the spatial variability of height errors in DEMs (Gorokhovich and Voustianiouk, 2006;Scherler et al, 2008). Global accuracy statistics are thus often insufficient for information about the uncertainties of DEMs (Carlisle, 2005) and may lead to grave underestimation of the uncertainties in DEM-based modeling results (Canters et al, 2002;Hancock et al, 2006;Nardi et al, 2008).…”

“…Hydrodynamic simulations in mountainous environments frequently use globally available DEMs, such as the SRTM or ASTER GDEM (Jarihani et al, 2015;Watson et al, 2015), but erroneous river profiles can introduce numerical instabilities (Paiva et al, 2011) and roughness, thus severely limiting the reliability of flood assessment, hazard zonation, and risk management (Watson et al, 2015). In a recent study on flash flood warning and hazard assessment in the Nepal Himalayas, a preliminary version of the CRS algorithm provided an important preprocessing step to improve the accuracy of estimating flow depth, flow speed, and flood wave arrival times (Bricker et al, 2017).…”

Hydraulic Parameters for Sediment Transport and Prediction of Suspended Sediment for Kali Gandaki River Basin, Himalaya, Nepal

“…The failure of the mountain slope of Kali Gandaki catchment in 1988, 1989, and 1998 was due to an evolved rock avalanche and caused the damming of the Kali Gandaki River [2]. The shockwaves after the massive 7.8 M w Gorkha earthquake, Nepal on 25 April 2015 and its aftershocks on 23 May 2015 created cracks in the weathered rocks and weakened the mountain slopes of this catchment, which brought rocks, debris, and mud down into the river [41,42]. The river was blocked about 56 km upstream from the hydropower dam by a landslide on 24 May 2015 for 15 h [41] (Figure 9a,b).…”

“…The downstream fluvial discharge after the blockage was almost zero and a flash flood occurred after an outburst of the natural landslide dam (Figure 9c,d). Extreme flooding during the monsoon period due to high rainfall and a flash flood (Figure 9b), generated by the overtopping of landslide dams [42], was responsible for the noticeable transport of large boulders in the river bed of Kali Gandaki River. The combination of fluid stress, localized scouring, and undermining of the stream banks may cause small near vertical displacements of large boulders [43].…”