Prediction of Debris-Flow and Flood Characteristics Caused by Potential Outburst of the Imja Glacial Lake in Nepal

Abstract

Debris flows and floods caused by outburst of glacial lakes have frequently occurred in the Himalaya of South Asia. The frequency of these disasters has been increasing tremendously due to global climate change. It is thus necessary to investigate debris-flow and flood characteristics due to outburst of glacial lakes in order to manage the hazards and risk in the areas. In this study, the characteristics of debris flows and floods caused by potential outburst flood from the Imja glacial lake in Nepal due to moraine dam failure by water overtopping were analyzed using developed numerical model. The numerical model was validated with the flume experimental cases. The relationship between peak outburst discharges with the volumes of lake water for experimental conditions was also analyzed. The erosion and deposition processes along the river valley were also considered in the analyses of potential outburst floods from the Imja glacial lake. Sensitivity analyses of the parameters of river bed sediment were also conducted. The predicted results of debris-flow and flood characteristics such as outburst discharge, flood arrival time, sediment concentrations, peak flow velocity and maximum flow depth can be useful for effective preventive measures, river basin management and infrastructure planning in the river basin.

Modeling the Glacial Lake Outburst Flood Process Chain in the Nepal Himalaya: Reassessing Imja Tsho's Hazard

“…Results indicate that Imja Tsho presents little hazard from an avalanche-induced GLOF to downstream communities for the next three decades, if current trends of lake expansion continue. This seems to validate the conclusions of some earlier nondynamic model studies regarding the terminal moraine as a buffer (Fujita et al, 2009;Watanabe et al, 2009) various GLOF mechanisms (Bajracharya, 2010;Somos-Valenzuela et al, 2015;Shrestha and Nakagawa, 2016), results from this study highlight the importance of modeling the entire process chain to determine if moraine failure is likely to occur.…”

“…However, some data is available from localized GLOF modeling studies at sites both within and outside the region, which makes it possible to approximate moraine properties based on field observations. Samples from Tsho Rolpa (27.87º N, 86.47º E), a glacial (Shrestha and Nakagawa, 2014), confirming estimations from earlier field surveys at Imja Tsho (ICIMOD, 2011;Shrestha and Nakagawa, 2016).…”

“…Previous work has considered the possibility of self-destructive 25 moraine failure through piping, seepage, and subsequent erosion (Shrestha et al, 2013;Shrestha and Nakagawa, 2016;Somos-Valenzuela et al, 2015), which historically is the second most common cause of GLOFs in the Himalayas (Emmer and Cochachin, 2013). The melting of buried ice within the moraine could trigger self-destructive failure, although the results of this study indicate surface erosion from an overtopping wave will not likely reach the ice core and accelerate melting.…”

Modeling the glacial lake outburst flood process chain in the Nepal Himalaya: reassessing Imja Tsho's hazard

“…However, some data are available from localized GLOF modeling studies at sites both within and outside the region, which makes it possible to approximate moraine properties based on field observations. Samples from Tsho Rolpa (27.87 • N, 86.47 • E), a glacial lake 45 km from Imja Tsho, indicate an internal friction angle of 35 • for wetted sediment (Shrestha and Nakagawa, 2014), confirming estimations from earlier field surveys at Imja Tsho (ICIMOD, 2011;Shrestha and Nakagawa, 2016). Outside of Nepal, moraine material at Imja Tsho also bears a strong resemblance to that of Ventisquero Negro, Argentina (see Worni et al, 2012), with maximum slopes around 80 • , similar grain size distributions (d 10 ≈ 1 mm, d 50 = 15-20 mm), and a noncohesive, unconsolidated mix of boulders, sand, and gravel, such that failure angles would likely be similar between the two sites.…”

“…Bajracharya et al (2007) similarly assumed dam breaching due to a melting of the moraine's ice core as well as a decrease in the width of the moraine. Shrestha and Nakagawa (2016) modeled inundation scenarios for an overtopping event, but did not model wave processes in the lake or the overtopping wave causing the moraine erosion. Furthermore, hazard assessments of Imja Tsho that are not based on numerical or experimental modeling (i.e., those based on remote sensing and in situ surveys) have had mixed results, with some indicating high hazard (Kattelmann, 2003;ICIMOD, 2011;Somos-Valenzuela et al, 2015), low hazard (Hambrey et al, 2008;Fujita et al, 2009;Watanabe et al, 2009), or a moderate hazard at the present and high hazard in the future .…”