Evaluating the contribution of avalanching to the mass balance of Himalayan glaciers

Abstract

Avalanching is a prominent source of accumulation on glaciers that have high and steep valley-walls surrounding their accumulation zones. These glaciers are typically characterised by an extensive supraglacial debris cover and a low accumulation area ratio. Despite an abundance of such glaciers in the rugged landscapes of the High Himalaya, attempts to quantify the net avalanche contribution to mass balance and its long-term variation are almost missing. We first discuss diagnostic criteria to identify strongly avalanche-fed glaciers. Second, we develop an approximate method to quantify the magnitude of the avalanche accumulation exploiting its expected control on the dynamics of these glaciers. The procedure is based on a simplified flowline model description of the glacier concerned and utilises the known glaciological mass-balance, velocity and surface-elevation profiles of the glacier. We apply the method to three Himalayan glaciers and show that the data on the recent dynamics of these glaciers are consistent with a dominant contribution of avalanches to the total accumulation. As a control experiment, we also simulate another Himalayan glacier where no significant avalanche contribution is expected, and reproduce the recent changes in that glacier without any additional avalanche contribution.

Estimation of the total sub-debris ablation from point-scale ablation data on a debris-covered glacier

“…Frequent avalanches and rockfalls efficiently transport the debris onto the glacier. These avalanches, in fact, contribute to the majority of accumulation in this glacier (Laha and others, 2017). The slope of the glacier in the debris-covered part is relatively gentle.…”

“…As discussed before, Satopanth is not an ideal glacier for mass-balance studies (Kaser and others, 2003) at least on three counts: 1) The presence of extensive supraglacial debris, 2) very strong avalanche contribution to mass balance, and 3) inaccessibility of the upper ablation zone and accumulation zone. These problems are quite typical in the Himalayan debris-covered glaciers (Laha and others, 2017), often making it difficult to estimate the corresponding net balance accurately (Azam and others, 2018). For Satopanth Glacier, the long-term mean of avalanche contribution to mass balance has been estimated (Laha and others, 2017).…”

“…These problems are quite typical in the Himalayan debris-covered glaciers (Laha and others, 2017), often making it difficult to estimate the corresponding net balance accurately (Azam and others, 2018). For Satopanth Glacier, the long-term mean of avalanche contribution to mass balance has been estimated (Laha and others, 2017). However, its inter-annual variation, which is likely to be significant, are not known.…”

“…4) was averaged over the total melt season, using the area-elevation distribution of the clean-ice region. We extrapolated the cleanice melt data, with a cut-off of 150 cm/yr in net accumulation (Laha and others, 2017). The net specific balance for the whole glacier was obtained by averaging the melt estimates for debris-covered and debrisfree parts, weighted by the corresponding area fractions.…”

“…100m surface elevation contours are plotted with thin blue lines, with thicker lines highlighting 4000, 4500, 5000, 5500, and 6000 m contour lines. (Reynolds, 2000;Sakai and others, 2000;Miles and others, 1993), vertical and horizontal variation of the thermal properties of debris (Nicholson and Benn, 2013;Rowan and others, 2018), random short-scale spatial variation of debris thickness (Mihalcea and others, 2006;Zhang and others, 2011; Mertes, 2017; Rounce and others, 2018), and the accumulation contribution from avalanches (Laha and others, 2017) need to be quantified for accurate surface mass-balance estimates on any typical debris-covered Himalayan glacier. The standard glaciological mass-balance measurement protocol (Kaser and others, 2003) may not be designed to handle some of the above issues.…”