Glacial lake outburst floods (GLOFs)

As glaciers recede and downwaste in response to climate change, the occurrence and expansion of glacial lakes is increasing (e.g. Carrivick and Tweed, 2013; Nie et al., 2013). When dammed by unstable or ice-cored moraines, these lakes can breach catastrophically, releasing a large volume of water called a GLOF. Rock and ice avalanches are considered the most important GLOF trigger mechanisms in mountain environments, causing dam erosion by overtopping and the formation of seiche waves (Benn et al., 2012; Westoby et al., 2014). GLOFs can be hazardous to downstream communities and infrastructure (e.g. hydropower installations), hence early identification of potentially hazardous lakes are essential to implement remedial works or implement early warning strategies. This is a pertinent issue in Himalayan countries such as Bhutan where many expanding glacial lakes exist and where land pressure in the foothills leads to increased vulnerability to hazardous events.

Large volumes of sediment are eroded and deposited in the fluvial environment during a GLOF event. An example is clearly visible following the 1994 GLOF event in the Lunana region of Bhutan. The red polygon (centre bottom of the image) in the animation below shows the large-scale landscape change following a GLOF event.

Hazard assessments:

Examples of the range of factors that often contribute to a GLOF hazard assessment are outlined in Table 1. Comprehensive assessments follow a multi-stage approach that considers both the source and downstream environment, and the dam-breach trigger mechanism. However, assessments are often constrained by data availability in Himalayan countries, where site-specific information such as moraine dam evaluation is often unavailable. The hazard assessment process lacks standardisation and often carries large uncertainties. Assessments can take a qualitative approach, involving subjective interpretation from the researcher (e.g. Huggel et al., 2004a); a semi-quantitative approach, where a hazard scoring system is assigned subjectively (e.g. Bolch et al., 2011); or a quantitative approach, where subjective elements in a scoring system are replaced with critical threshold  values (e.g. McKillop and Clague, 2007; Emmer and Vilímek, 2013).

Table 1. Considerations for a glacial lake hazard assessment including examples from selected studies. Adapted from Huggel et al. (2002) and Quincey et al. (2005).

Worni et al. (2014) and Westoby et al. (2014) provide comprehensive reviews of the modelling process and uncertainties of the GLOF process chain. These concern the trigger mechanism initiating the dam breach, the dam characteristics that determine its stability, and the application of a hydrodynamic model.

Downstream GLOF assessments:

To guide the application of physically based hydrodynamic models or where sufficient data for their implementation is unavailable, or for broad regional-scale assessments, GIS-based first-pass assessment are often implemented (e.g. Figure 1; Huggel et al., 2003; Huggel et al., 2004; Mergili and Schneider, 2011). A model called the MC-LCP was recently published by Watson et al. (2015) (Figure 2). GIS-based assessments lack a physical basis in flood propagation, but can nevertheless provide an indication of the likely GLOF flow path and inundation probability in the case of the MC-LCP. First-pass assessments are especially important where only coarse resolution digital elevation models are available.

Figure 1. Application of the GIS-based MSF flow routing model to a catchment in Bhutan.

Figure 2. Application of the MC-LCP to a Himalayan catchment. The model is run iteratively, producing an inundation frequency output.

Detailed flood assessments can be carried out using fine-resolution elevation models and 2D flood models. The example below uses drone-derived topography and a 2D flood model (HEC-RAS).


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