Flow hydraulics and geomorphic effects of glacial‐lake outburst floods in the Mount Everest region, Nepal

Abstract

Glacial-lake outburst floods (GLOFs) on 3 September 1977 and 4 August 1985 dramatically modified channels and valleys in the Mount Everest region of Nepal by eroding, transporting, and depositing large quantities of sediment for tens of kilometres along the flood routes. The GLOF discharges were 7 to 60 times greater than normal floods derived from snowmelt runoff, glacier meltwater, and monsoonal precipitation (referred to as seasonal high flow floods, SHFFs). Specific stream power values ranged from as low as 1900 W m 2 in wide, low-gradient valley segments to as high as 51 700 W m 2 in narrow, high-gradient valley segments bounded by bedrock. Along the upper 16 km of the GLOF routes, the reach-averaged specific stream power of the GLOFs was 3Ð2 to 8Ð0 times greater than the reach-averaged specific stream power of the SHFFs. The greatest geomorphic change occurred along the upper 10 to 16 km of the GLOF routes, where the ratio between the GLOF specific stream power and the SHFF specific stream power was the greatest, there was an abundant supply of sediment, and channel/valley boundaries consisted primarily of unconsolidated sediment. Below 11 to 16 km from the source area, the geomorphic effects of the GLOFs were reduced because of the lower specific stream power ratio between the GLOFs and SHFFs, more resistant bedrock flow boundaries, reduced sediment supply, and the occurrence of past GLOFs.

Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya

“…These results reflect the dissipation of flow energy with lateral flow expansion and travel distance and are characteristic of the sedimentology of outburst flood deposits observed both in the Himalaya (Cenderelli and Wohl, 2003) and further afield (Kershaw et al, 2005;Carrivick, 2007aCarrivick, , 2007bBreien et al, 2008).…”

“…Comparison with other documented moraine-dam failures Geomorphological reworking of valley-floors downstream of moraine-dammed lakes by GLOFs has been widely documented (Clague and Evans, 2000;Richardson and Reynolds, 2000;Cenderelli and Wohl, 2003;Korup and Tweed, 2007;Osti and Egashira, 2009;Worni et al, 2012). In many instances, distinct alluvial debris-fans similar to that observed beneath the main breach at Chukhung are located immediately downstream of moraine (Vuichard and Zimmerman, 1987;Evans and Clague, 1994;Kershaw et al, 2005) and landslide dambreaches (Dunning et al, 2006).…”

“…Downstream, reaches variously undergo erosion and deposition both during and following GLOF passage. The morphodynamic regime is primarily dependent on the longitudinal distribution of channel expansion and contraction (Clague and Evans, 2000;Cenderelli and Wohl, 2003;Kershaw et al, 2005).…”

“…Given a triggering event, moraine-breaching may result in a sudden and rapid release of significant volumes of water and sediment (Lliboutry et al, 1977;Vuichard and Zimmerman, 1987;Costa and Schuster, 1988;Evans and Clague, 1994;Walder and Driedger, 1994;Watanabe and Rothacher, 1996;Clague and Evans, 2000;Cenderelli and Wohl, 2001;Kattelmann, 2003;Xin et al, 2008;Osti et al, 2013;Westoby et al, 2014). Such Glacial Lake Outburst Floods (GLOFs) represent a highmagnitude and low-frequency catastrophic phenomenon, and are capable of performing significant geomorphological reworking of moraine, in-channel and overbank environments (Costa and Schuster, 1988;Cenderelli and Wohl, 2003;Kershaw et al, 2005;Breien et al, 2008;Worni et al, 2012). GLOFs may also have devastating impacts on downstream settlements and infrastructure, and result in significant loss of life (Reynolds, 1998;Richardson and Reynolds, 2000).…”

GPR-Derived Sedimentary Architecture and Stratigraphy of Outburst Flood Sedimentation Within a Bedrock Valley System, Hraundalur, Iceland

“…Jö kulhlaups and lahars erode both unconsolidated sediments (e.g., Russell and Marren, 1999;Gomez et al, 2002) and bedrock (e.g., Tó masson, 2002;Carrivick et al, 2004a). Subsequent transport and deposition of a vast amount of sediment (e.g., 10 8 tons; Bjö rnsson, 2002) can produce a suite of diverse and sometimes distinctive landforms (Haeberli et al, 1989;Desloges and Church, 1992;Cenderelli and Wohl, 2003).…”