Whom to Blame for a 'Manageable' Hazard ? --- Part 2

In this post, let's continue to look at the existing and emerging methods to model the stage of a GLOF,  the reason why the advanced models are so far rarely seen in glacial hazard literature, and what are the main challenge for the modellers and research groups to achieve their expected goal of hazard assessment and prediction.

Picture: Rumbak Valley in the catchment of Hemis National Park of  Himalayas, Rumbak is one of the numerous villages in the Ladakh Range region of north-west India. 
Credit:  Zhu Jing (writer), travelling in 2014

Review of available methods 

Westoby et al., 2014 very well summarises and presents a number of approaches to model different stage or episode of a GLOF, which can be categorised as

• A Trigger Mechanism 
• A Dam Breach 
• An outburst downstream passage

The simulation of a dam breach initiation and development largely rely on the empirical or analytical formulation. The Empirical models are simple and direct approach exhibiting the peak discharge and time to peak by testing the case studies of historical GLOF or observed failure. The analytical or semi-physical models are employed to analyze the process to reach the max with the assumption that time taken to reach the maximum is known, they are slightly more advanced than empirical ones. The most advanced method is fully physical numerical dam-breach modelling, which applies geotechnical, flow hydraulics and sediment-erosion theory, to identify difference moraine breaching type. Following the dam breach simulation, the last step in GLOF modelling framework is to set up a simulation to identify outburst floods routing via smoothed particle hydrodynamics or GIS-based Digital Elevation Model (DEM). 

The Challenge 

GLOF is NOT a water-only problem but rather an incredible complexity involving a large volume of sediment, debris and even vegetation changing the flow viscosity and mobility, i.e. non-Newtonian fluid dynamics. The ideal advanced GLOF modelling requires the inclusion of numerically-sophisticated codes and data, associated with highly specific and complex computing system establishment. Hence the high-quality data acquisition and computational cost burden remain as the biggest challenges for both glacial hazard professionals and government officials. Another well-known challenge is the global warming trend, which makes it even unlikely easy to predict flood inundation, magnitude, when and where the future burst will form and make trouble for us.  

Despite the challenges, we are making progress with the advanced models and calling for international joint work and experience sharing. Below are some good examples of outburst flood modelling work:

1. ''Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya'' (Westoby et al., 2014): ''A numerical dam-breach model, executed within a probabilistic framework, was used to simulate moraine breaching of Chukhung Glacier ''

A two-dimensional perspective of the final hill-shaded, Structure-from-Motion-derived digital terrain model of the Chukhung moraine dam complex and floodplain. These data were used to aid geometric characterisation of the moraine dam, to reconstruct the bathymetry of Chukhung Tsho (inset), and to serve as the topographic domain for hydrodynamic GLOF simulation.

2. ''Analysis and dynamic modelling of a moraine failure and glacier lake outburst flood at Ventisquero Negro, Patagonian Andes (Argentina)'' (Worni et al., 2012) : ''A field-based reconstruction of the Ventisquero Negro moraine failure (Patagonian Andes, Argentina in May 2009 ) provided valuable input data for the simulation of the breaching event and allowed calibration and validation of the dynamic erosion-based dam breach mode BASEMENT model'' 

The lake bathymetry, the topography of the moraine and the surrounding terrain is the model domain, represented by a mesh of triangular cells. The water surface elevation and external water source are the model’s initial conditions.

3. ''Coupled fluid dynamics-sediment transport modelling of a Crater Lake break-out lahar: Mt. Ruapehu, New Zealand''(Carrivick et al., 2010): ''A fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from Mt. Ruapehu, New Zealand..... A key advantage of our modelling is the ability to consider transient phenomena including incremental erosion and deposition and hence gross geomorphic work''

Maximum bed erosion modelled (A) agrees well with that observed (B) although B highlights additional sediment input from valley sides. Changes in bed shear stress from 10 to 20 min (C) and 30 to 40 min (D) indicate substantial frontal wave effects. Maximum modelled deposition (E) is perhaps different to that observed (F) due to F only considering the net change.