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Mapping Glacial Lake Outburst Flood Landforms for Enhanced GEOGRAZ 68 - 2021 SCHWERPUNKT ADAM EMMER ON THE AUTHOR Adam Emmer is physical geographer Mapping glacial lake and member of the re- search group Cascade with specialization in outburst flood landforms high mountain geomor- phology and natural hazard science. In his for enhanced understanding research, he focuses on hazardous conse- of their occurrence quences of retrea Sudden release of water retained in a glacial lake can produce a glacial lake outburst flood (GLOF), which is among the most effective geomorphic agents in mountain regions during the time of glacier ice loss. Extreme GLOFs imprint in relief with specific landforms (e.g. failed moraine dams, outwash fans) which can be used to document their occurrence in space and time. This contribution introduces typical landforms associated with GLOFs and outlines their utilization ting in reconstructing timing and magnitude for enhanced hazard identification and glaciers, mainly lake outburst floods. assessment. 1 Introduction: GLOFs in a range of triggers including landslide into areas as well as agricultural land (Haeber- changing world the lake, extreme precipitation and snow- li et al. 2016). Concerns about increasing melt or earthquake (Clague and O’Con- risk of GLOFs are driven by two factors: (i) Glacial lakes are typical features of re- nor 2015). GLOFs are rare, low frequency, increasing population pressure and settle- cently deglaciated high mountain regions high magnitude events with extreme char- ment expansion in mountain regions (espe- across the world (Shugar et al. 2020). acteristics (peak discharge, flood volume), cially in South America and Central Asia); While glacial lakes contribute to human with thousands of fatalities and tremen- and (ii) increasing number of glacial lakes well-being by providing goods and ser- dous material damages claimed globally and volume of stored water. While urbani- vices (e.g. fishing, hydropower generation, (Carrivick and Tweed 2016). zation and landscape planning can be (at tourism), they can also represent a threat if Hand in hand with retreating glaciers, new least to some extent) managed and so can retained water is released suddenly. Such a glacial lakes form and grow upstream sett- reflect these potential risks, the occurrence release is called glacial lake outburst flood led piedmont areas, raising the question of GLOFs is largely stochastic. (GLOF) and may occur as a result of lake of GLOF risk to societies and assets such As such, understanding the occurrence dam overtopping or failure due to a wide as hydropower plants and bridges, mining of GLOFs is an everlasting need in effec- tive hazard and risk identification, assess- ment and mitigation. Since the occurrence of potential future GLOFs is complicated to predict in space, time and magnitude, our efforts often rely and build on the analysis of past events and extrapolation of observed trends into the future as well as knowledge transfer in between regions. While GLOF inventories in some of the re- gions are fairly complete and reliable with centuries long records in documentary data sources (e.g. European Alps, Iceland), some others suffer from missing, incom- plete and / or questionable records (Em- mer et al. 2016; Fig. 1). Strikingly, GLOF records were documented to be largely in- complete in the regions where documenta- ry data are not available, such as the Third Fig. 1: Eleven hotspots of GLOF occurrence which account for a total of >1,300 GLOFs globally (modified and Pole region (Hindu Kush, Himalaya) or updated from Emmer, 2018). Note that only documented events are considered in this figure and real numbers are likely higher in regions with incomplete records (see the text). extensive regions of the South American Andes (Peru, Bolivia, Chile). 26 Tab. 1: Gegenüberstellung der unterschiedlichen Paradigmen im Naturschutz (eigene Darstellung, nach Pimbert und Pretty 1995, Phillips 2003, Mose und Weixlbaumer 2006). SCHWERPUNKT Fig. 2: Visual appearance of breached moraine ON THE AUTHOR dam and outwash fan of the 1951 GLOF from Lake Artesoncocha (Cordillera Blanca, Peru). (A) shows upstream view through breached moraine dam with outwash fan deposition (OF) located in the forefield; pre-GLOF dam crest is roughly indicated with orange dashed line, arrow indicates breach depth and dotted line indicates typical trapezoidal cross-profile of the breach. Steep moraine slopes exposed during the breach (ES) are also visible. (B) shows almost vertical inner slopes (IS) of breached moraine dam; note no grain sorting and chaotical placement of large boulders forming the dam; material reworked by landslides (L) after the breach is visible; (C) shows aerial view 3 years before the GLOF, i.e. in 1948 (image: archive of the Autoridad Nacional del Agua, Huaraz, Peru; acquisition date: 2nd September 1948); pre-GLOF images from pre- satellite period are quite rare; (D) shows 2020 satellite image of the same area as displayed in (C); note decreased lake size as a result of partial drainage during GLOF (image: Maxar Technologies, Google Earth Digital Globe, acquisition date: 3rd August 2020). Field images taken by the author in 2014. Ground control points (GCPs) from (A) and (B) are displayed on (C) and (D). 2 Employing geomorphology in interpretation of field and remote sensing is decreased (longitudinal profile gets less GLOF studies images and geomorphological mapping. steep). Some of these landforms are iden- The use of these methods in the context of tical to those produced by “classical” hy- Since landscapes and landforms are pro- understanding GLOF occurrence in space dro-meteorologically-induced floods (e.g. minent archives of palaeo-geographic in- and time for enhanced GLOF hazard undercut river banks, in-stream deposition formation, understanding them and their identification and assessment is outlined. bars, etc.) and debris flow depositions, but dynamics can help us better understand some are only associated with GLOFs. processes and physical conditions during 3 Landforms associated with The most distinct landforms associat- their formation and evolution. Geomor- GLOFs ed with GLOFs are those created during phology – a study of landforms and pro- the outbursts of extraordinary large Late cesses of their formation – is a vital tool in GLOFs were documented to rank Pleistocene ice-dammed lakes, with de- these efforts. For instance, moraines are among the most effective geomorphic cline of extensive ice sheets. Thousands traditionally used to reconstruct and date agents in high mountain regions when cubic kilometers of meltwater were recon- past glacier extents, river terraces help us glaciers are retreating (Cook et al. 2018). structed to be released during individual to understand landscape polygenesis, i.e., GLOFs are characterized by enormous episodes, producing the most powerful interactions between tectonic uplift and erosion and transport potential and sedi- floods in Earth’s history with reconstruc- exogenic processes, and the extent of de- ment yield and can also transform into ted peak discharges overcoming 107 m3/s positions of pyroclastic material give us an flow-type movements, for instance hy- in the most extreme cases (O’Connor insight into the past volcanic activity and per-concentrated flow or debris-flow. and Costa 2004). These mega-floods left frequency-magnitude relationships of the- Landforms associated with glacial lake behind continental-scale geomorphic se processes. outburst floods can be classified into two imprints and formed characteristic land- Analogically to these examples, specific basic groups: (i) erosional landforms, and scapes in North America and Siberia. For landforms can indicate past occurrence of (ii) depositional landforms. Clearly, ero- example, the Late Pleistocene outburst GLOFs, infer about their hydrodynam- sion dominates in the sections of the val- from ice-dammed palaeo-lake Missoula in ic characteristics and magnitudes (Costa ley where the stream power is high (for North America shaped entire landscapes and O’Connor 1995). In this contribution, instance where the longitudinal profile along the Columbia and Snake Rivers in some of the typical landforms associated gets steeper) while deposition generally contemporary Idaho, Washington and with GLOFs are presented, building on the occurs in the sections where stream power Oregon states. 27 GEOGRAZ 68 - 2021 SCHWERPUNKT Fig. 3: A conspicuous outwash fan associated with repeated lake outburst / debris flow activity on the east-facing slopes of the main massif of the Cordillera Huayhuash, Peru. While the fresh scars and depositions are clearly identifiable, the attribution to the outburst of upstream located Lake Chaclan or rainfall-induced remobilization of moraine material with only marginal role of the lake is ambiguous (see also Bat’ka et al., 2020). Recent GLOFs (i.e. those which oc- (Fig. 3), maybe up to kilometers long, sev- channel is buried by GLOF depositions curred since the end of the Little Ice Age) eral hundred meters wide, and is mainly and a new channel is formed nearby. Ex- have comparably smaller magnitudes (with composed of the material eroded from the treme, up to several tens of meters deep peak discharges rarely exceeding 104 m3/s) dam (i.e. moraine material previously de- erosion down to the bedrock bottom of the and spatially limited geomorphic imprints. posited by a glacier). While moraine ma- valley floor has been documented during However, they are still capable of produc- terial is generally poorly sorted with clay, major events. GLOFs were
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