Debris-covered glaciers in the Himalaya play an important role in
the high-altitude water cycle. The thickness of the debris layer is a key
control of the melt rate of those glaciers, yet little is known about the
relative importance of the three potential sources of debris supply: the
rockwalls, the glacier bed and the lateral moraines. In this study, we
hypothesize that mass movement from the lateral moraines is a significant
debris supply to debris-covered glaciers, in particular when the glacier is
disconnected from the rockwall due to downwasting. To test this hypothesis,
eight high-resolution and accurate digital elevation models from the
lateral moraines of the debris-covered Lirung Glacier in Nepal are used.
These are created using structure from motion (SfM), based on images captured
using an unmanned aerial vehicle between May 2013 and April 2018. The
analysis shows that mass transport results in an elevation change on the
lateral moraines with an average rate of -0.31±0.26 m year-1 during
this period, partly related to sub-moraine ice melt. There is a higher
elevation change rate observed in the monsoon (-0.39±0.74 m year-1)
than in the dry season (-0.23±0.68 m year-1). The lower debris aprons
of the lateral moraines decrease in elevation at a faster rate during both
seasons, probably due to the melt of ice below. The surface lowering rates of
the upper gullied moraine, with no ice core below, translate into an annual
increase in debris thickness of 0.08 m year-1 along a narrow margin of the
glacier surface, with an observed absolute thickness of approximately 1 m,
reducing melt rates of underlying glacier ice. Further research should focus
on how large this negative feedback is in controlling melt and how debris is
redistributed on the glacier surface.
Introduction
Glaciers cover approximately 110 000 km2 in high-mountain Asia (HMA) and
as such constitute an important water storage of the region
. The glaciers are likely to melt rapidly
in the future, with projections ranging from a total mass loss of 36 % to
64 % in the coming century depending on the climate scenario
. While the entire HMA has experienced an overall
glacier mass and area loss in recent decades, the changes have been found to
be variable in space . Apart from
differences in climate, the presence of debris with variable thickness on the
glacier tongues plays an important role to explain this spatial heterogeneity
.
Although the debris-covered area only constitutes 11 % of the total
glacier area, 30 % of the ice mass below the equilibrium line altitude
(ELA) is covered with debris
. While a thin debris cover increases melt because
it decreases albedo, debris thickness above a critical thickness inhibits
melt . The surface of a debris-covered glacier is often
characterized by ice cliffs and supraglacial ponds, which result in high
local melt rates (;
; ;
). Knowledge about actual debris thickness is
limited to few field observations (;
; ) and attempts to derive it
from thermal bands of satellite imagery . Several studies suggest that
glaciers with prolonged periods of negative mass balance are associated with
an increase in debris cover . Glaciers that have a neutral or
positive mass balance, such as in the Karakoram in Pakistan, do not show any
positive trends in debris-covered area .
Not only is the debris-covered area variable, but the thickness also changes
over time. A recent study on the Baltoro Glacier in Pakistan suggested that
debris thickness changes at rates of multiple centimetres per year with a
maximum of 30 cm year-1 near the snout .
Schematization of debris supply processes towards the glacier
surface (A), showing basal erosion (a), rockwall
erosion (b) and sediment supply from lateral moraines (c).
Mass transport processes on a lateral moraine bordering the
glacier (B), showing debris flows (c1),
rockfall (c2) and slow slumping (partly caused by the melt of
sub-moraine ice) (c3).
The source of supraglacial debris has been studied for decades
. Three potential sources of supraglacial
debris can be distinguished, which can be divided into direct and indirect
sources. The direct sources are basal erosion and bordering rockwalls. The
indirect source is the lateral moraine, from which temporarily deposited
debris is remobilized and transported (Fig. A). The relative
contribution of these sources is variable but generally depends on the amount
and intensity of precipitation, glacier size, rockwall extent and erodibility
of the bedrock and sediments . In HMA, basal
erosion (Fig. Aa) is likely only of limited relative importance,
as the erosion processes on the steep rockwalls and lateral moraines are
highly active . Bordering rockwalls have been
observed to transport debris to the glacier via rockfall, rock avalanches and
landslides as primary transporting processes (Fig. Ab). These
periodic events can be triggered by processes such as extreme rainfall or
seismic events, and are known to increase due to glacier melt and downwasting
of the surface resulting in debuttressing and exposure of unsupported strata
.
Generally, long-term rockwall erosion rates are on the order of
1 mm year-1. However, the actual mass deposited on
the glacier surface is more difficult to determine and varies from glacier to
glacier, depending on its exposure to rockwalls and geologic conditions. The
reworking of these deposits on the glacier surface, as a result of local
hillslope geomorphology, surface melt, glacier flow and englacial processes,
may lead to small-scale variations in debris cover as found in the Himalaya.
In contrast to transport from the rockwalls, which deposits material on
specific locations, remobilization of lateral moraine debris can result in a
much more spatially uniform debris supply to the glacier
(Fig. Ac). Lateral moraines typically have a gullied upper part,
where debris flows are the main transport process
(Fig. Bc1). Most of these flows are released due to rainfall
saturation, but the melt of buried ice cores can contribute as well
. The oversteepened upper parts may also be susceptible
to rockfall (Fig. Bc2). Eroded gully material is
deposited near the glacier margin as coalescing debris cones, which, together
with rockfall and slumping, forms the lower debris apron of the moraine. The
material on these debris aprons is reworked by frost action, small
landslides, debris flows, snow avalanches and solifluction
(Fig. Bc3).
During downwasting, the glacier surface becomes lower than the moraine crest,
making lateral moraine slopes susceptible to mass transport processes. As
more and more of the moraine slopes become exposed, a larger amount of
moraine debris can be transported to the glacier surface and their relative
contribution to the sediment budget will increase. This is particularly the
case for stagnating glacier tongues, where the distal slope of the lateral
moraine has the opposite aspect of the adjacent valley slopes (Fig. ). Due to glacier retreat and downwasting, the valley wall is
disconnected from the actual tongue. Material deposited below the ELA will
furthermore remain at the glacier surface and add to the supraglacial debris
cover. Estimated rates of vertical lowering on the lateral moraines range
from 49 to 151 mm year-1 for the European Alps and 3 to
169 mm year-1 for Norwegian field sites . On glaciers in
the study area (Langtang Valley in Nepal), a supply rate of 0.4–31 mm year-1 was previously found. However, these values
are averages over a much longer time span (<550 years versus <247 years for
studies in Norway and <79 for the Alps; ) or include source
areas beyond the moraines . These rates are nonetheless
all much higher than rockwall erosion rates. All eroded material is deposited
on debris cones (often on top of the glacier ice), which are intensely
reworked throughout the study period.
Previous studies only reported glacier-averaged erosion rates, although it is
likely that the rates are spatially variable. We hypothesize that transport
of remobilized debris from lateral moraines can explain the thick and
continuous debris cover of tongues on downwasting debris-covered glaciers
that have been disconnected from headwalls, in HMA. To this
end, we use multi-annual, high-resolution orthomosaics and digital elevation
models (DEMs) acquired using an unmanned aerial vehicle (UAV) to quantify surface
lowering rates of the lateral moraines of a debris-covered glacier in Nepal.
We attempt to explain the spatiotemporal variability and sediment transport
processes using the terrain morphology and prevailing seasonal climates.
Finally, we assess how important erosion from lateral moraines is in the
formation of Himalayan debris-covered glaciers.
Study area
The research was conducted on Lirung Glacier (28.23∘ N,
85.56∘ E), which is located in the Langtang Valley in the Nepalese
Himalaya (Fig. b). The glacier has a southern aspect, steep
rockwalls near the mountain crest and an almost flat terminus. The lower part
of the glacier is covered entirely in debris (Fig. ) and has been
subject to strong downwasting in recent decades . The study area is located between 3900 and 4400 m a.m.s.l.
(Fig. c, orange lines). In this section, debris from the valley
rockwall cannot reach the glacier surface due to the opposite aspect of the
distal moraine slopes (Fig. ). The climate in the region is
dominated by the monsoon, with a wet season between June and September in
which 70 % of the annual precipitation falls. During the dry season
between November and May, considerably less precipitation reaches the area
and falls mostly as snow .
Study area including the setting (A), location of the study
area (B) and two more detailed overviews including the orthomosaic
and elevation data of the study area (C, D; from UAV flights in
October 2015). The glacier terminus and studied section of the moraine are
outlined on these maps. The black
squares in panel (C) indicate the location
of Figs. (a), a (b) and
b (c).
Cross sections over the glacier and moraine, with their
locations (a) and corresponding elevation profile (c). They
clearly indicate that the distal slope of the moraine has an opposite aspect
to the proximal slope, due to which there is no debris input from higher
slopes and rockwalls in the valley. In panel (b), the different
sections of the moraine are indicated, with the upper gullied
part (b1), the debris apron or coalescing debris cones (b2)
and the smooth section of the debris apron interpreted as gully washout
deposition (b2a).
Field images of Lirung Glacier from the upper part (a),
where the rockwalls are still connected to the glacier, along the middle
disconnected part (b) to the lower glacier tongue (c),
where the decoupling is clearly visible. Also note the mass movements from
the upper slopes (b) that do not reach the
glacier.
Data and methodsField data
Our observations of the lateral moraines of Lirung Glacier span 2013 to 2018,
consisting of images captured during multiple UAV flights with an optical
camera. A structure from motion (SfM) workflow is used to
derive three-dimensional (3-D) point clouds, which are georeferenced by marking measured ground control points (GCPs) and tie
points. These operations finally result in co-registered orthomosaics (0.1 m
resolution) and DEMs (0.2 m resolution; for further details regarding
measurements, processing and data quality, see ).
Details about the mapped part of the glacier for each UAV campaign are
provided in Table . The mutually overlapping area of all datasets
covers 1.5 km2 of the glacier tongue including the moraines. The
accuracy of all generated DEMs is tested by comparing the differences
between DEM and GCPs.
Date of acquisition (in yyyy/mm/dd format) and area
of the different datasets.
Precipitation data were available from the meteorological station at Kyanjing
Village (28.21∘ N, 85.57∘ E) about 1.3 km south of the glacier in 2013. As data
gaps are present between 2015 and 2018 for the Kyanjing station, the
meteorological station in Langshisha (28.20∘ N, 85.67∘ E) about 13 km
south-east of the glacier was used for the remainder of the period. Both
rainfall intensities and the cumulative rainfall between any two UAV time
slices were analysed.
Deriving change in elevation
The DEMs were used to calculate vertical elevation differences between time
steps. Two preprocessing steps were taken. First, vegetated areas were
selected using a maximum likelihood supervised classification applied to the
orthomosaic and masked out of the DEMs, to remove noise related to vegetation
growth and decay. Second, as the off-glacier and off-moraine terrain did not
show signs of sediment transport during the investigated time span, these
areas are assumed to be stable. Therefore, the DEMs were corrected for
elevation changes in off-moraine and off-glacier terrain. To avoid bias towards possible errors in
the DEM, we removed the outliers outside the 10–90th percentile range, while
making sure to retain large elevation change events related to debris flows
and rockfall (Table ). Finally, the glacial elevation change over
the entire period (2018–2013) is determined for a 20 m wide zone next to
the proximal slope of the moraine, which is an indication of the glacial melt
underneath the debris apron. When taking these corrections into account, the
difference in elevation between two time steps was assumed equal to the
amount of sediment transport.
Extreme values below and above which the data are removed from the
original dataset, as well as the mean of the whole dataset (μ).
The orthomosaics were analysed visually to examine patterns of erosion and
deposition, and compared against elevation differences. Furthermore, the
displacement, slope and roughness of the lateral moraines were derived from
the DEMs. We employed the COSI-Corr software for cross-correlation feature
tracking to calculate the displacement of debris on the lateral moraines
. As this software focuses more on
block movement than on individual clast displacement ,
correlating displacement with elevation change gives insight in slower slope
processes such as creep or slow slumping, which often occur on a scale large
enough to be detected by COSI-Corr. Fast events that travel beyond our chosen
maximum window size of 26.5 m are registered by the algorithm as noise and
hence do not bias our average velocities on the moraine. Slope maps are
created directly from the DEM. Following , the roughness
length z0 (m) was derived by
ln(z0)=0.65+1.37ln(σz),
with σz defined as the standard deviation of a 5×5 m window of
a detrended DEM. Although the window size greatly influences the roughness, a
window of 25 m2 is suitable for this approach . For
each window, a high roughness value indicates larger topographic variation,
such as boulders, while a small value indicates a more homogeneous surface
.
Moraine delineation
The DEM and orthomosaic, as well as their derivatives, were used to delineate
the lateral moraines and divide them into zones with comparable
characteristics. The moraine base is often characterized by a break in slope
(Figs. , b). Furthermore, a hillshade with a
hummocky appearance is an indicator for sub-debris ice , and
these areas are thus excluded from the moraine. Within the lateral moraine
two main zones were distinguished: an intensively gullied upper part
and a lower part that consists mainly of reworked debris,
usually in the form of coalescing debris cones (Figs. , , a), as also described by
. Although not distinguishable everywhere, a zone with
fine material was detected directly below the gullied upper part, accompanied
by a very low roughness (Figs. b, c). The
smoothness is also visible on the orthomosaic. We interpret this part of the
coalescing cones as the deposition zone of surface wash from the gullies
upslope.
Moraine delineation method. The crest is often well defined by the
orthomosaic (a) and slope (b). An intermediate zone with
gully surface wash deposition is clearly visible from the roughness
data (c). The moraine consists of an intensely gullied upper part
(right) and a lower debris apron consisting of coalescing debris cones
(left). See Fig. for location.
Runout model
To investigate the importance of the lateral moraines as a source for
supraglacial debris, we used a simple model to calculate how far moraine
material can travel onto the glacier. The model is based on the reach angle
principle. The reach angle is the angle between the origin and maximum reach
of a mass movement and has a range between 3 and 45∘. For debris flows, the reach angle is generally between
26 and 34∘. These variations are
mainly caused by differences in processes, but the volume of the mass wasting
is also important . Taking both process and volume into
account, minimum reach angles for rockfall, shallow slides and debris flows
are found to be 33∘ (with a volume of 100–1000 m3),
23∘ (800–2000 m3) and 22∘ (800–2000 m3),
respectively . Reach angles decrease for values beyond
these ranges and increase for smaller volumes.
The runout length RL (m) from the lower moraine boundary was calculated as
RL=ΔHtan(Rα)-Mw,
where ΔH is the difference in elevation between the start and
deposition location (m),
Rα (∘) is the reach angle (Fig. a), and
Mw is the planar moraine width (m). To derive the maximum runout
length, it was assumed that the start location of the mass transport is at
the moraine crest, though in reality they may start from anywhere inside the
gullied zone .
As the minimum reach angles were used and since we assume the moraine crest
as a starting point, the calculated runout length RL indicates the
furthest inward point on the glacier that debris can directly be transported
to. Due to a decrease in mass movement velocity after the abrupt slope change
on the glacier–moraine boundary, the amount of debris deposition is expected
to be highest close to the moraine and will decrease rapidly with distance.
To validate this estimated runout length, the actual runout length is
determined by detecting depositional features such as debris flow lobes and
rockfalls, and measuring their distance to the moraine edge.
Clast analysis
Model results were validated by performing a clast analysis, which is used to
distinguish between actively transported clasts and those that are mostly
affected by weathering and reworking in rapid mass movement events
and passively transported by the glacier. Moraine-derived
debris is assumed to have already been actively transported by the ice during
moraine formation, when the subglacial sediment was being deformed.
Therefore, it has a higher roundness than passively transported rockwall-derived debris,
which is expected to be more dominant in the centre of the tongue. If the
lateral moraine indeed is an important source of debris, clast roundness is
expected to decrease from the lateral moraine towards the glacier centre, as
the influence of the lateral moraine diminishes and material transported from
further upglacier becomes dominant. The clast analysis was conducted by
investigating 70 individual samples of debris. For each of the locations, on
average 46 (σ=25) clasts were analysed. The roundness is determined
based on the commonly used chart from , which results in
a percentage of clasts for each sample that are angular or very angular (RA
index). For 13 samples, the axis length of each clast was measured, allowing
us to determine the so-called C40, which is the 40th percentile ratio of
short to long axes (c/a) in a sample. Actively transported clasts are more
likely to have low C40 and RA values, in contrast to clasts that
experienced passive transport . It has to be taken into
account that both the C40 and RA indices decline downglacier
and that differences in lithology result in different index
values . However, the latter will be of minor importance as
the debris catchment is relatively small and homogeneous in lithology
.
Results and discussionObserved surface lowering rates
The mean elevation change rate of the non-vegetated moraine between May 2013
and April 2018 equals -0.31±0.26 m year-1 (Table ). Most
elevation change occurs on the lower moraine, consisting of extensively
reworked coalescing debris cones, at a rate of approximately -0.41±0.21 m year-1. The glacier downwasting near the debris apron occurred at a rate
of -0.60±0.45 m year-1, which indicates a deposition rate of +0.19 m year-1 on the apron, assuming a similar downwasting rate below the apron.
The upper gullied part on average has a surface lowering rate of -0.16±0.26 m year-1. Debris remobilization on other moraines that formed during
recent glaciations generally have lower elevation change rates than those
observed on the gullies in this study but also peaked at approximately -0.15 m year-1, with noting their rates
between -0.01 and -0.02 m year-1 to be minimum rates. The high gully
erosion rates found are indicative of rapid surface change on steep lateral
moraines above downwasted glacier tongues during deglaciation and
debuttressing. The vertical accuracy of the dataset is determined by
calculating DEM differences for off-glacier terrain for all datasets. The
total offset is -0.05±0.84 m over an area of 1.6 km2. Although the
offset varies from -0.04±0.70 to 0.12±0.66 m between the
different time steps, it is at any time smaller than the observed surface
lowering rates (Table ).
Seasonal elevation change values, furthermore divided into upper and
lower moraine. The zonal mean (μ) is reported, as well as the 1σ
standard deviation (σ). Precipitation is measured at Kyanjing station
in 2013 and Langshisha station in all others seasons. Elevation change values
are in m year-1, precipitation values in mm and mm h-1. The
significance column (sig) indicates the periods from which that specific
dataset statistically differs (p<0.05).
Off-glacier elevation differences on stable terrain. These
differences can be seen as the vertical accuracy of that specific dataset.
Values are in metres.
Dataset (yyyy/mm)μσ2013/05–2013/10 (wet)-0.040.442015/10–2016/04 (dry)-0.030.742016/04–2016/10 (wet)0.120.662016/10–2017/04 (dry)-0.020.692017/04–2017/10 (wet)-0.040.702017/10–2018/04 (dry)0.080.932013/05–2018/04 (total)-0.050.84Mass transport mechanisms and processes
Our data enable us to differentiate between different transport mechanisms,
which can be divided into three main categories: erosion due to running water
(entrainment and debris flows), larger mass movements (slumps and rockfall)
and slower downslope processes (for example, slow slumping). Despite the
stability of inter-gully arêtes on the upper moraine, the
gullied topography indicates the importance of flow erosion processes. Debris
flows and sediment loaded streams originate here mostly in the wet season,
when there is frequent rainfall, often with high intensity
(Table ). Relatively high rates of surface lowering can be found
just below the gullies, which might be related to the presence of easily
erodible surface wash deposits (Fig. a). The fact that these
higher rates can be found below more intensely gullied upper sections
supports this. Another possibility is that these higher rates are caused by a
steep scarp of fresh-looking till which commonly forms at the very base of
the eroding till cliff. This scarp may be caused by the separation of the
debris apron from the gullied upper section as a result of sub-apron glacial
downwasting. However, no such sharp step is visible on the orthophotos,
keeping the exact origin unclear.
Different types or mass transport processes. Fast mass movements as
slumps and rockfall occur on the moraine (a1, a2) as well as
water-driven movement as water flow and debris flows (c). Furthermore, the
slopes are slumping down slowly (b), which is clearly visible by the
alternation of positive (rocks moving in) and negative (rocks moving out)
elevation change values. See Fig. for location.
Further down, the debris apron the slope decreases and depositional features
are observed, mostly related to debris flows, as distinct levees along a
central flowpath and lobe-like features (Fig. c). Frequently
observed grain diameters of >40 cm on the debris apron furthermore indicate
the importance of debris flows over water flows , as
debris flows, in contrast to water alone, are able to transport boulders of
such a dimension. Nonetheless, many smaller channels show the importance of
water flows for further reworking the sediment. Together with debris flows
that cover the debris apron, water flow reworks the debris and transports
material onto the glacier. However, due to the large elevation loss on the
near moraine glacier, it is likely that there is net deposition on the debris
aprons, and only a limited amount of debris is transported further onto the
glacier. This is in line with the matching gully erosion rates (-0.16 m year-1) and approximate debris apron deposition rates (+0.19 m year-1).
The steep gullied upper slopes are also susceptible to larger mass movements,
such as the occurrence of slumps and rockfalls. These processes are enhanced
by oversteepening of the slope and cause locally high (>2 m) erosion rates
(Fig. a1). One large slump event was captured in our data
(Fig. a1–a2), deposited mostly on the debris apron, where its
loose material is susceptible to continued reworking. The debris apron of the
moraine is partly eroded by water flow but also moves downslope as a whole
as the glacier ice below melts (Fig. b). There is a horizontal
displacement towards the glacier of 0.93 m year-1, with a
90th percentile of 2.01 m year-1. This is in the range
of movement by solifluction and that of rock glaciers
. Despite the fact that the local
climate would favour solifluction , it is unlikely to be
the main transporting mechanism as velocities speed up in summer. The lateral
displacement is also higher than those typically related to creep of unfrozen
debris ; thus, it is most likely caused by slow slumping of
the debris apron. Although its contribution to surface lowering is unclear,
this process does result in a steady debris supply to the glacier.
Temporal patterns in surface lowering
Moraine surface lowering occurs throughout the year, and a significant
difference was observed between the wet (-0.39±0.74 m year-1) and
dry (-0.23±0.68 m year-1) seasons. Elevation change rates ranged from
-0.34±0.57 to -0.52±0.84 m year-1 in the wet season and -0.17±0.44 to -0.36±0.37 m year-1 in the dry season. This
difference is caused by heavier precipitation during the summer months (540–700 mm) compared to the winter months (117–145 mm) in addition to higher
glacial melt rates of -0.76±0.73 m year-1 and -0.57±0.45 m year-1, respectively. On the upper gullied moraine, erosion rates were
highly variable between the dry and wet seasons (-0.07±0.43 to -0.28±0.77 m year-1) but generally lower than rates on the debris apron
of the moraine (-0.25±1.0 to -0.60±0.82 m year-1). The
remarkably high elevation loss on the debris apron during the wet season is
mostly linked due to high glacier melt rates in the summer season. In
addition, more active mass transport processes were observed in the wet
seasons, contributing to the higher surface lowering rates. This also
indicates the importance of water-driven erosion and slumping (Table ).
The upper gullied moraine experiences less change throughout the dry season,
but its high seasonal differences indicate its sensitivity to precipitation
changes, which affect debris flow probabilities. During the dry season from
2016 to 2017, when surface lowering of the gullied part was especially high,
both the total precipitation as well as the intensity were considerably
higher. An increase of debris-flow-related landforms after the wet season
co-occurs with the larger negative elevation change that was observed on the
gullied upper part. Larger slumps and rockfall from this section were not
limited to a single season, as they are mostly triggered by a single rainfall
event rather than by continuous wetting. Surface wash deposits below the
gullies can also be seen throughout the wet and dry seasons, as is the case
for slow slumping of the debris apron. Nonetheless, the rate of movement was
much faster during the wet summer season, as a larger loading and a decrease
of shear strength may cause the moraine to slump faster as a result of buried
ice melt . Although the exact nature of this process is yet
unclear and needs further investigation, it is clear that this does
contribute substantially to the downslope movement of moraine debris.
Beyond precipitation, freeze–thaw cycles could also play a role in driving
erosion, with erosion increasing as moraine slopes warm up seasonally after
the dry winter season and diurnally through the rest of the year.
Towards a conceptual lateral moraine mass transport model
The steep intensely gullied upper part of the moraine had lower elevation
change rates (-0.16 m year-1) than the lower debris apron part (-0.41 m year-1) (Fig. ). However, with a glacial melt rate of -0.60±0.45 m year-1 there is net deposition on the debris apron. This is
in line with other studies that indicate that post-deglaciation the steep
upper slopes get rapidly deprived of their loose sediment
. This would suggest higher erosion rates on
the upper moraine part in the past, and this hypothesis is supported by the
large amount of deposited material below. Currently, erosion still occurs in
the gullies, albeit at a lower rate, and gully surface wash deposition just
below the gullies fills the gaps between the larger boulders and decreases
the surface roughness locally (Fig. c). These finer-grained
deposits are susceptible to reworking by water flow .
Downwasting of the glacier surface between May and October 2013
and surface flow direction
compared to elevation change on the moraines (a).
Insets (A1), (A2) and (A3) show hotspots of
surface lowering. Small black arrows show surface motion, either by active
debris transport or passive transport imposed by the glacier velocity. The
arrow size indicates exaggerated intensity of velocity, as velocities on the
moraines (mean: 0.89 m year-1) are far smaller than on the upper glacier
(mean: ∼6 m year-1). The long black arrow shows retreat of the
terminus between 2013 and 2018 (∼150 m). Blank areas are either
outside the study area, vegetated or outside the 10–90th percentile range.
Panel (b) shows the histograms of the elevation change on the upper
gullied section and lower debris apron over the entire studied period
(2018–2013).
Relation between elevation change and moraine velocity in the wet
and dry seasons. In the wet season, high velocities occur with higher surface
lowering rates. Debris apron displacement is likely caused by a slumping
movement due to sub-apron ice melt. No such relation exists in the dry
season.
Debris flows originating in the upper gullies both deposit and entrain
material on the coalescing debris cones that form the lower part of the
moraine. In addition, mass transport on this section of the moraine is caused
by a slower slumping process in the wet season, which moves down the moraine
as a block. During the wet summer, when surface displacement rates are on
average >1 m year-1, higher velocities coincide with a more negative
elevation change, possibly related to the melt of ice underneath (Fig. ).
In the dry winter season, displacement rates are lower
(<1 m year-1) and do not show a correlation with elevation change, indicating a
smaller importance of slow slumping processes. This is remarkable considering
the still relatively high melt rates (-0.57±0.45 m year-1). The
slumping might be enhanced in close proximity of areas of high glacier ice
mass loss, as the ice thins vertically but also recedes laterally (Fig. ). These slump deposits may be identified on the moving glacier
as positive surface elevation change due to the dispersal of the deposited
slump sediments (Fig. b). The gullied upper moraine did not
show any horizontal displacement over both seasons, which indicates the
absence of sub-moraine ice in these parts and the absence of a slumping
process. As these displacements can be linked directly to a slumping process,
they can easily be used to indicate areas of active mass transport on lateral
moraines.
It is difficult to quantify the importance of fast processes such as debris
flows, rockfall of individual large boulders and landslides on the moraine.
However, the intensely gullied upper part prevents large flows from occurring.
Though rockfalls and a single slump are observed, they are infrequent and
occur at a small scale (Fig. ). As a result, large infrequent
events are of minor importance for the lateral moraine-derived sediment
budget on this glacier.
The importance of frost action for supplying detachable sediments can also
not be directly derived from the DEM differencing. However, with surface
temperatures well below freezing level for a long time in winter, as
evidenced by field measurements, the formation of ice crystals is likely.
These may detach sediments that can easily be removed afterwards. No debris
flows or landslides were originated on this part; hence, it is most likely
that mass transport on the lower part was triggered by water and debris flows
that originated in the upper gullies. The coalescing debris cones that make
up the lower part of the lateral moraine along this glacier are constantly
reworked, as debris flows, water flow and slow slumping transport material to
the glacier. However, net deposition takes place from the gullies above, in
contradiction to what the elevation change rates suggest. The continuous
glacial melt also constantly increases the sediment accommodation space,
resulting in less debris to reach the glacier surface.
Debris distribution onto the glacier
The main processes (debris flow, shallow slides, rockfall) described above
were included in the model to calculate on-glacier debris deposition.
Conceptual diagram of model approach (a), the modelled
runout lengths (b) and the modelled variability in runout
lengths (c).
Using a specific reach angle for each process, the maximum runout length on
the glacier is 39 m for rockfall, 111 m for debris flow and 122 m for
shallow slides (Fig. b). The runout length is not equal along the
glacier, as a result of differences in moraine elevation and the hummocky
glacier surface (Fig. c). The observed runout length is manually
derived from the imagery and has a maximum of 51 m, which suggests rockfall
as the most important process, closest to this value. As rockfalls were not
observed to be the most important process on the moraine, this difference
also indicates that debris flows and small slumps possibly occurred with
smaller runout lengths than modelled. There are two possible explanations for
this. First, many mass movements might have a smaller volume than the
800–2000 m3 range used in the calculation, which reduces the runout
length . Second, the rough surface on the
glacier obstructs the runout path and decreases runout length substantially
. Using a smaller volume (<800 m3) and
an obstructed path, the reach angle of debris flows and shallow slides
decreases to 30∘, which results in a maximum runout length of 56 m,
much closer to the observed length of 51 m (Fig. b). Nonetheless,
both the calculated and measured runout lengths indicate that debris from the
moraine cannot directly reach the centre of the glaciated surface, which is
approximately 200 m from the moraine. Rough calculations of required reach
angles for moraine-derived debris to reach the centreline on other glaciers
in the catchment, based on the mean moraine prominence ,
show that this is likely true there as well. For the largest glacier,
Langtang, the reach angle becomes 6∘, while for the second largest,
Langshisha, it is 18∘. Due to differences in valley shape and moraine
size, very different rates of relative coverage by moraine-derived debris are
however possible. Due to an increasing elevation range between the moraine
crest and glacier tongue, the runout length has increased over time. The
debris currently found at the glacier's centreline has either moved there by
secondary processes such as glacier movement and on-glacier sliding, or
originated from other sources, e.g. rockwalls or basal
debris, (re-)emerging at the glacier surface towards the tongue
. This result also indicates that the
proportion of moraine-derived supraglacial debris increases as the glacier
downwastes. The formation of a fully lateral moraine-derived debris cover is
more likely in the case of upstream medial moraine formation by confluencing
glaciers, from which secondary dispersal mechanisms may form a supraglacial
debris cover . Furthermore, advancing glaciers will have
little moraine-derived input due to a lack of extensive lateral moraine
slopes.
Angularity samples taken on the glacier surface; observed and
modelled runout lengths are also shown. The inset shows angularity values for
samples beyond the modelled runout length (“glacier”), between the runout
length and the base of the moraine (“runout”) and on the moraine
(“moraine”). In the top right corner, marked in blue, the glacier is still
connected to rockwalls by way of avalanching.
Clast analysis
The locations of the clast samples are shown in Fig. . The
C40 index is relatively low (0.2 to 0.48), while the rounded and very
rounded fractions of the investigated sample are less than 6 % on average,
indicative of angular clasts. Looking at the angular and very angular
fractions, the RA index provides a stronger indication of transport processes
(Fig. ). The RA index on the moraine is on average just above
30 %, suggesting a dominance in more rounded samples. This is due to their
previous transport path along the glacier bed before they were deposited
along the moraine. Values are decreasing downglacier, corresponding to
findings in , indicating that englacially transported and
moraine-derived debris becomes dominant. Higher RA indices on the moraine are
found only where the moraine is still connected to the rockwall (Fig. ). Clast samples in the centre of the glacier have a much higher
RA index, >50 %, as they have been dominantly sourced directly from rockwalls
further upglacier and less frequently emerged from englacial pathways.
Clast samples from between the moraine and the modelled runout length have an
RA index higher than 50 %, closer to the RA index of the samples in the
centre of the glacier. This may indicate that the modelled runout length is
an upper maximum and debris from the moraines rarely reaches this far, which
is in line with the observed runout length being shorter and the limited
debris supply due to deposition on the debris apron.
Dominant processes at lateral moraines and consequences for the
glacier terminus of a debris-covered glacier. Mass transport from lateral
moraines (a1) as well as debris transport from head walls and
englacial transport (a2) are shown. Slumping as a consequence of
debuttressing at the terminus (a3) brings more material into the
glacier forefield (a4), as the tongue retreats in time (t). As a
consequence, the moraine crest slumps (b2) and the moraine becomes
shallower compared to upper parts where transport is mainly to rockfall and
debris flows (b1).
Consequences for debris thickness, terminus retreat and glacier downwasting
Debris-covered tongues are schematized with a convex–concave upglacier
thickness pattern, causing lower tongues to be covered in thick debris that
causes debris-covered tongues to stagnate . This can be explained with debris accumulating
continuously at the snout at higher rates than it can be evacuated
(Fig. a2). Moraine-derived debris can not be directly deposited
over the complete width of the tongue (Fig. a1), although
secondary processes may be capable to further distribute it across the
glacier. However, to get a first-order estimate of its potential relative
contribution to the thick cover expected on the snout, surface lowering rates
from the moraines were compared to the overall debris thickness of the
glacier. These net rates of -0.16 m year-1 on the gullied upper part
and +0.19 m year-1 on the debris apron suggest that no debris
reaches the glacier surface, but only a relocation of moraine material takes
place. However, we do see depositional features beyond the current debris
apron on the glacier. In addition, one could argue that the glacial melt
rates underneath the debris apron are slightly lower than those on the
glacier due to the thick debris layer on top , which
would decrease the net deposition rate on the apron and would at maximum
result in no net deposition or erosion at the debris apron. This suggests
that the maximum debris supply to the glacier is only related to the gully
erosion rate of -0.16 m year-1, which occurs over an area of
0.095 km2. If it is assumed that this amount of debris is distributed
equally over the entire glacier surface within the UAV domain
(0.33 km2), this results in an annual increase in debris thickness of
0.05 m, although this is unlikely given the observed runout lengths. If the
debris is only deposited within the area constrained by the maximum runout
distance (0.19 km2), this implies an annual increase in debris
thickness of 0.08 m. Observed debris thickness for this glacier is in the
range of 0.11–2.3 m (μ=0.84 m)
and 0.4–1.6 m closer to the moraine
measured around the on-glacier weather station.
Furthermore, on the lower part of the Baltoro Glacier in the Karakoram,
similar debris accumulation rates have been found, ranging between 0.05 and
0.30 m year-1, suggesting that debris-covered tongues get buried
rapidly .
The clear concave arcuate terminus appearance of retreating
debris-covered glaciers (DCGs) as opposed to the generally convex terminus of clean
glaciers however does suggest extensive moraine sediment supply, along with
internal ablation due to drainage conduits. As the tongue retreats, the
debuttressing of the glacier causes the moraine to slump and fill the space
available with moraine material (Fig. a3 and b2), which might
cause the higher debris supply here. At Lirung Glacier, the terminus
retreated at a rate of 30 m year-1 at the centreline and much slower
at the margins, as here the moraine-derived debris quickly covered the ice,
resulting in the moraine crest to slump and become shallower
(Fig. b2), and melt rates to decrease.
It is also remarkable that the near-moraine glacier downwasting rate (-0.6 m year-1) is much lower than previously found downwasting rates over the
entire glacier (<-1.3 and -2.18 m year-1;
;
). This indicates a possibly thicker debris cover near the
moraines suppressing downwasting rates, which is also in line with the lack
of ponds and ice cliffs close to the moraine and
supports the debris supply rates presented in this paper. Considering these
processes, our results indicate that lateral moraine mass transport can play
an important role in debris supply to the margins of a downwasted glacier
tongue with steep lateral moraines, where it offsets the downwasting of the
ice with deposition of debris. As the glacier downwastes, more moraine
surface is susceptible to erosion, while more space becomes available for
debris apron formation, continuously inhibiting debris to reach the glacier
surface. The lower the downwasting rate, the higher the possible surplus of
debris that can possibly move beyond the apron and closer to the centre of
the tongue, resulting in a thickening debris cover and making it a negative
feedback effect. This is important to take into account in energy balance
models , which often use a uniform debris
cover derived solely from rockwalls and do not take deposition and
remobilization of lateral moraine debris into account. Lateral moraine
debris supply is also found to be important for the form of terminus retreat.
Conclusion
In this study, a time series of 5 years of UAV data is used to investigate the importance of lateral moraine mass transport to a debris-covered tongue and the following key conclusions are drawn:
The surface lowering on the lateral moraines is high at an average rate of 0.31 m year-1, also attributed partially to the melt of sub-moraine ice. This
translates to a maximum increase of debris thickness of 0.08 m year-1 in a narrow runout zone of approximately 50 m next to the moraine.
As the downwasting rate decreases, this rate of thickening will likely increase, resulting in a negative feedback with increasing debris thickness and
reduced melt rates. If supplied far enough upstream, this additional debris may then be redistributed on the glacier surface.
There is a strong seasonality in lateral moraine mass transport and the rates are higher in the wet summer season, which indicates the combination of
water-driven processes and glacier melt as key mechanisms.
The steep upper part of the moraine is intensely gullied and transports debris quickly to the depositional debris apron. Here, the more gentle slopes
are reworked by water flow, debris flow entrainment and slow slumping.
Rockfall and landslides occur occasionally and influence the vertical elevation change pattern; they are, however, of minor importance in the overall balance.
Runout distance modelling shows that it is unlikely that these processes are responsible for the distribution of the eroded material on the
glacier since the maximum distance is small (∼56 m). This is supported by the clast analysis, which shows angularity to increase
rapidly from the moraine (30 % being angular or very angular) towards the glacier centre (>55 % being angular or very angular), in addition to
reduced melt rates near the glacier margin.
Considering these results and revisiting our initial hypothesis, debris supply from lateral moraines alone can not explain the thick and
continuous debris cover of tongues on debris-covered glaciers. While a considerable amount of debris from the moraines reaches the surface,
it can only explain thickening on the margins of the tongue.
Further research is needed that incorporates glacier dynamics and lateral drag with the moraines and its implications for possible debris
transport on the glacier surface. In addition, methods to quantify rockwall erosion rates as well as subglacial erosion on debris-covered
glaciers need to be developed to understand the full sediment balance of a debris-covered glacier tongue. Better estimations of sub-debris
apron glacier melt would also greatly improve the debris supply estimations. Using different future scenarios of glacier recession, the
long-term development of lateral moraines and their changing contribution to an increasing debris cover could also be investigated in a dedicated modelling study.
Data availability
The elevation difference datasets from which the values in
this paper are derived can be found at the following DOI:
10.5281/zenodo.2682541.
Author contributions
JFS, PDAK, ESM and WWI developed the research goal. TvW, JFS and
PDAK performed the primary data analysis and model development. TvW, JFS and WWI
wrote the paper. PDAK and ESM helped with interpretation of results.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The input from Tjalling de Haas on mass transport processes is greatly
appreciated.
Financial support
This research has been supported by the European Research
Council (ERC) under the European Union's Horizon 2020 research and innovation
programme (grant agreement no. 676819) and by the research programme VIDI with
project number 016.161.308 financed by the Netherlands Organisation for
Scientific Research (NWO).
Review statement
This paper was edited by Niels Hovius and reviewed by Simon Pendleton, Martin Kirkbride, Martin Truffer, and one anonymous referee.
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