Towards a Better Understanding of Uncommon Glacier Outburst Floods
OPEN ACCESS
Author / Producer
Date
2024
Publication Type
Monograph
ETH Bibliography
yes
Citations
Altmetric
OPEN ACCESS
Data
Rights / License
Abstract
Glacial lakes can form at various locations relative to glaciers when the resulting meltwater is naturally dammed by ice, moraine, or bedrock. When the lake dam fails, or is overtopped by the lake water, the lake drainage can lead to an outburst flood, so called glacial lake outburst floods (GLOFs). If the outburst f lood stems from a reservoir of water within the glacier, we call these events water pocket outburst floods (WPOFs). GLOFs and WPOFs pose significant hazards to downstream infrastructures and communities, and can cause severe geomorphic changes. Understanding GLOFs and WPOFs is crucial for modelling and thus predicting their impacts. However, current modeling of glacial outburst floods is hindered by significant uncertainties due to the lack of in situ field measurements during drainage events, as well as limited data on the reservoirs themselves in the case of water pockets. This thesis aims at improving both the understanding of glacier outburst floods, in particular stemming from ice-dammed marginal lakes and water pockets, and the interpretation of ground penetrating radar (GPR) signals for detecting englacial water pocket in alpine glaciers. The thesis is organized into three chapters.
The first chapter presents extensive field measurements from an ice-dammed lake drainage into a supraglacial channel at Glacier de la Plaine Morte (Switzerland). We studied the hydraulic and thermodynamic parameters that control the channel geometry and the channel stream flow during the lake drainage. In particular, we calculated the Darcy-Weisbach friction factor which characterizes the channel flow resistance, and the Nusselt number which controls the channel melt rate and thus the lake outflow discharge. These two parameters are used in modeling studies of ice-dammed lake drainage. Our results show that the Darcy-Weisbach friction factor varies from 0.17 to 0.48 during the drainage and should be considered as a stochastic variable in modelling studies. We found that the Nusselt number differs significantly depending on which method for calculation is used, and propose new empirical coefficients to calculate the Nusselt number based on our observations. We suggest considering these empirical coefficients as stochastic variables as well. We recommend future research to develop a unified modeling framework for ice-dammed lake drainage into supraglacial channels based on comprehensive field data. This framework could integrate various independent field measurements, including those presented in this thesis, to allow a robust comparison of recent lake drainage models.
The second chapter characterizes the spatial and temporal distribution of WPOFs in the Swiss Alps and proposes mechanism for their formation and rupture. Through an updated and extended inventory of 91 WPOFs from 37 glaciers since 1699, we analyzed the glacio-geomorphic and meteorological drivers for 32 of these events since 1961. Based on our inventory, we suggest that smaller-scale topographic and glacio-geomorphic variables control the WPOF occurrence, rather than glacier-wide glacio-geomorphic variables. We show that high temperature within few days prior to the WPOF and strong precipitations during the day of the outburst coincide with most of the events. This suggest that rapid (within few days) water accumulation from melting and rainfall may have triggered most WPOFs reported in our inventory. Based on our inventory and a literature review, we propose four main mechanisms for water pockets formation: temporary blockage of subglacial channels, hydraulic barriers, water-filled crevasses, and thermal barriers (i.e. formation of water pockets at the transition of cold and temperate ice). We encourage more field-based research aiming at detecting and monitoring water pockets to advance understanding of WPOFs.
In the case of englacial features detection, such as water pockets for example, such field-based efforts can make use of ground penetrating radar. The third chapter thus explores the limitation in detecting englacial bodies in temperate glacier by using GPR. We combined field measurements and numerical modeling at 25MHz, revealing that englacial water inclusions cause strong scattering and attenuation effects in GPR signals. We demonstrated that even small liquid water content (e.g., 0.2%) with decimeter-scale water inclusions can obscure bedrock and subglacial channels reflections. These findings highlight the challenges in interpreting GPR signals of englacial water bodies in temperate ice, and also in interpreting the glacier thermal regime (cold or temperate) based on the presence or not of scatterers. Indeed, we show that temperate ice can appears free of scatterers for low liquid water content.
Overall, this thesis i) better constrains from field-based observations the hydraulic and thermodynamic parameters used in ice-dammed lake drainage modelling, ii) provides a better understanding of water pocket outburst floods, and iii) improves the interpretation of detected englacial features in ice using GPR.
Permanent link
Publication status
published
External links
Editor
Book title
Journal / series
Volume
277
Pages / Article No.
Publisher
Eigenverlag der Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW), ETH Zürich
Event
Edition / version
Methods
Software
Geographic location
Date collected
Date created
Subject
Organisational unit
03820 - Boes, Robert / Boes, Robert
09599 - Farinotti, Daniel / Farinotti, Daniel
Notes
Funding
Related publications and datasets
Is variant form of: https://doi.org/10.3929/ethz-b-000711790