Glaciology L3: Glacier hydrology and the proglacial area

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What are supraglacial lakes? When do they occur and to what extent?
Form in summer when 1,000s of lakes of snowmelt fill depressions in the surface. e.g. Greenland. Increasing number and size.
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Over what timespan do supraglacial lakes form and disappear?
Form over days but can drain quickly through cracks in the ice down to the glacier bed
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Why is glacier hydrology important?
Glacier meltwater represents a negative mass balance. Has a major role in glacier sliding- can increase velocity of flow. Responsible for picking up and transporting debris.
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What area of the Earth is glaciated and how much water is held in this store?
15 million km2 (10% land area) holds 75% of the freshwater on Earth (75 m of global sea level equivalent)
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Give some examples of dependence upon the provision of water from glaciers?
Central Asia and Canadian prairies- agriculture; France, Switzerland, Norway- Hydropower
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Why might glaciers represent a risk to human populations?
Hazard- sudden drainage events- flooding- risk of increasing in frequency e.g. from meltwater lakes bursting banks- concern in areas such as Iceland, Peru, Alaska, New Zealand.
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How is water generated from glaciers?
Melting by heat sources- Sun, basal friction between ice and bed (from sliding), geothermal heating. 3 locations: Supraglacial (surface), Englacial (internal); subglacial (at the bed)
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What is the most important source of meltwater? How does this source of water fluctuate over time?
Surface melting. Seasonal and diurnal fluctuations
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What sources of water may also add the meltwater?
Rainfall (if warm enough); valley side streams
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Describe the characteristics of supraglacial channels.
Usually < few meters wide. Usually smooth sided- encourages high velocity. Often exploit existing weaknesses e.g. crevasses/cracks. May be straight or meander.
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How is supraglacial water transported to the glacier bed?
Channels- often interrupted by vertical shafts which divert water into the glacier. Shafts can be crevasses or moulins (cylindrical vertical tunnels derived from crevasses)
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What happens to the water after entering moulins?
Transported into intersecting englacial tunnels= tunnels that descend from the base of moulins at angles of 0-45 degrees.
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What effects the size of englacial tunnels?
Result of ice pressure (weight of the overlying ice) to close tunnels & presence of meltwater- frictional heat= generated as water flows through the ice. Tunnels grow as discharge increases.
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Describe the primary and secondary permeability of a glacier.
Primary- drainage occurring between ice crystals. Secondary- larger scale drainage routes e.g. crevasses, moulins linking the surface and englacial or subglacial environments
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How does subglacial drainage occur?
2 mechanisms: Channelised (discrete)- water is carried through a network of channels. Distributed- water is spread over much (or all) of a bed.
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What are the three types of channels (discrete drainage) that can transport subglacial meltwater?
R- Rothlisberger (ice); N-Nye (rock); Tunnel (sediment)
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When do R (Rothlisberger) channels form and where?
Tend to exist at low pressure- form when a flow of water melts walls of the tunnel. Can evacuate water v. quickly but could be closed by ice creep.
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How do R channels differ to englacial tunnels?
Similar but R channels have a base of bedrock
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Why are N (Nye) channels more rare? Where can they be observed?
Tunnels eroded into bedrock- likely to occur where bedrock is easily eroded e.g. limestone. Needs focused erosion along same route over time. Observed when glaciers retreat- incised river channels are often found across the newly deglaciated terrain
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Describe tunnel channels.
Incised branching channels cut into subglacial sediments. The shape/size of channel bottoms is controlled by erosion of the sediment with the shape and size of the channel tops controlled by the balance between melting and pressure from overlying ice
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What are the theories explaining distributed subglacial drainage ?
Thin film- sheet flow- idea that where ice is close to the pressurised melting point and it overlies rock- thin water film forms; linked cavity system- enlarges over time during melt season
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Why is the theory of sheet flow not widely accepted?
Films couldn't be more than a few mm thick before breaking up to form channels in depressions/filling cavities behind bed obstacles- water isn't going to be evenly distributed across whole bed-few beds are smooth. limited capacity for holding water.
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Describe the linked cavity system for distributed subglacial drainage
Beginning of melt season- increasing water vol- water collects in cavities. Cavities form in the lee of bedrock bumps. Cavities are not well linked- throttling effect- more water in than leaving high pressure.
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How does the rapidness of meltwater flow affect the presence of linked cavities?
If meltwater (Q) gradually increases- linked cavity system gradually enlarges to accomodate. If Q is large enough- melting of cavity roof > closure by deformation- cavities may evolve into tunnels (R). If Q is v. quick- glacier may be lifted off bed
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How might drainage systems evolve over the course of a year?
Winter- If any supraglacial water-slow flows through snow & firn. R-channels+englacial channels= likely to close. Some cavities may remain open if sliding is maintained. Spring- poorly connected cavities begin to fill. Dependent on "Q"- evolution
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What happens if Q increases too quickly in the Spring to be accommodated by cavity growth?
Basal water system can't evacuate water quickly enough- dramatic water pressure= speed-up events or "Spring events". Until meltwater pressure subsides or network becomes more efficient.
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Despite the various transportation methods what happens to meltwater eventually?
Water emerges from a glacier through the glacier snout and is carried away from the glacier margins as proglacial discharge
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What features might meltwater form as it leaves a glacier snout?
Proglacial stream/ proglacial lake (mark edge of glacier)
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When do proglacial lakes form?
A glaciers retreat they form between the ice front and a topographic barrier (morraine ridge)
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Why do proglacial lakes represent a potential hazard?
If morraine ridges fail- huge outburst of meltwater- substantial flooding of nearby areas- risk to nearby settlements
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How does meltwater usually leave proglacial lakes?
Usually water drains away slowly across glacier forefield via proglacial streams
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What affects the size of proglacial streams?
Water delivery from glaciers. There may be annual and daily cycles or random events related to weather. Affects the energy balance in a glacier and causes fluctuation of meltwater delivered
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What is the energy balance inside a glacier?
The surplus or deficit of energy available for melting after ice has been raised to melting point. Surface melting increases with higher temp/rainfall. Melting @ bed from friction/geothermal heat= more constant over time)
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To what extent does meltwater differ between winter and summer?
Output can vary as much as 100x. Most melting= mid-day summer
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When are braided proglacial drainage systems formed. Can you provide an example?
In areas of high energy turbulent water with a high sediment supply which promotes formation of alternating bars/islands of sediment- highly changeable. e.g. Southern margins of Vatnajokull
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In the long-term what in theory might happen to proglacial areas?
If over time- volume of ice and therefore meltwater decreases and less water drains from glaciers- braiding is likely to become less extensive and the proglacial area may become stabilised if vegetation becomes established etc.
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What ultimately happens to all proglacial water?
Leaves the drainage basin via rivers and is transported to the sea
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Card 2

Front

Over what timespan do supraglacial lakes form and disappear?

Back

Form over days but can drain quickly through cracks in the ice down to the glacier bed

Card 3

Front

Why is glacier hydrology important?

Back

Preview of the front of card 3

Card 4

Front

What area of the Earth is glaciated and how much water is held in this store?

Back

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Card 5

Front

Give some examples of dependence upon the provision of water from glaciers?

Back

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