Modeling debris-covered glaciers: response to steady debris deposition

Abstract

Abstract. Debris-covered glaciers are common in rapidly eroding alpine landscapes. When thicker than a few centimeters, surface debris suppresses melt rates. If continuous debris cover is present, ablation rates can be significantly reduced leading to increases in glacier length. In order to quantify feedbacks in the debris-glacier-climate system, we developed a 2-D long-valley numerical glacier model that includes englacial and supraglacial debris advection. We ran 120 simulations on a linear bed profile in which a hypothetical steady state debris-free glacier responds to a step increase of surface debris deposition. Simulated glaciers advance to steady states in which ice accumulation equals ice ablation, and debris input equals debris loss from the glacier terminus. Our model and parameter selections can produce 2-fold increases in glacier length. Debris flux onto the glacier and the relationship between debris thickness and melt rate strongly control glacier length. Debris deposited near the equilibriumline altitude, where ice discharge is high, results in the greatest glacier extension when other debris-related variables are held constant. Debris deposited near the equilibrium-line altitude re-emerges high in the ablation zone and therefore impacts melt rate over a greater fraction of the glacier surface. Continuous debris cover reduces ice discharge gradients, ice thickness gradients, and velocity gradients relative to initial debris-free glaciers. Debris-forced glacier extension decreases the ratio of accumulation zone to total glacier area (AAR). Our simulations reproduce the "general trends" between debris cover, AARs, and glacier surface velocity patterns from modern debris-covered glaciers. We provide a quantitative, theoretical foundation to interpret the effect of debris cover on the moraine record, and to assess the effects of climate change on debris-covered glaciers.

Decadal Change in Supraglacial Debris Cover in Baspa Basin, Western Himalaya

“…However, response of debris-covered glaciers to changes in climate is not uniform. Varying trends of glacial retreat and mass loss in debris-covered glaciers have been reported across different regions of the Himalaya 5,7,11,13,20,24 . Therefore, it is essential to monitor the changes in SDC to better understand the glacier dynamics, such as changes in glacier extent, hydrology and mass balance due to climate change 2,7,18 .…”

“…This can result in glacier front stagnation and may lead to glacier fragmentation due to separation of the stagnant part from the main glacier body 7,21 . Thick debris cover can cause low accumulation-area ratios and influence mass balance 5,7,11,20 . Banerjee and Shankar 11 observed that specific mass balance is minimum at the terminus of a clean glacier, whereas it shifts upwards in case of debris-covered glaciers.…”

“…Weathering and erosion of these walls supply debris, which is transported down the glacier slope [1][2][3][4][5] . Hence, many of the glaciers in the Himalayan region are heavily debris-covered 1,2,[6][7][8][9] .…”

Modelling debris transport within glaciers by advection in a full-Stokes ice flow model

“…It also affects glacier dynamics and geometry such that stagnating, low-angled debris-covered ice can survive for longer at lower altitudes than neighbouring clean-ice glaciers (Benn et al, 2012;Anderson and Anderson, 2016). Thus the paleoclimatic signal represented by sediment deposits from a debris-covered glacier is not the same as one from a cleanice glacier.…”

“…To date, existing numerical models of debris-covered glaciers either restrict debris inputs to the ablation zone (Konrad and Humphrey, 2000;Menounos et al, 2013;Vacco et al, 2010), prescribe an englacial debris concentration (Bozhinskiy et al, 1986) or use empirical relationships to describe accumulation of debris on the glacier surface (Jouvet et al, 2011). Recent studies apply simplified treatment of englacial transport (Rowan et al, 2015;Anderson and Anderson, 2016), but as yet no model explicitly resolves fully 3-D (three-dimensional) time-evolving transport of debris within the ice flow field of the glacier body. This is a significant omission because, as surface debris mainly originates from localized debris inputs (rockfall or mixed avalanche events) in the accumulation zone, modelling englacial transport is crucial to predict the location and timing of surface emergence of debris, as well as its concentration and its spatial extent, all of which are required to constrain the nature of the developing debris cover and its resultant impact on glacier behaviour.…”