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Seasonal Drainage Evolution (Dye Breakthrough Curve (Monitor: Dye conc…
Seasonal Drainage Evolution
Channelization
Increased water flux can initiate channelisation
Start melt season
No channels - no surface runoff
∴ no melt input to subglacial system
End melt season
Efficient channels present
Collapse/close over winter
--> ice deforms relatively quickly
Case Study Haut Glacier D'Arolla
Subglacial water pressures
Gordon et al (1998) & Hubbard et al (1995)
Investigation
nature of subglacial drainage
Dye tracing
Nienow et al 1988
Ice Motion
Mair et al (2004)
Dye Tracing
Inject fluorescent dye into moulin (subglacial)
Monitor emergence in proglacial streams
--> dyed water emerges at glacier snout
∴ Trace travel time of water
Monitor time since injection
Fluorometer
Measures fluorescence of water
∴ Accurately monitor dye in water down to p.p.b.
Dye = pink rhodamine
Dye Breakthrough Curve
Monitor: Dye conc emerging in proglacial stream & time since injection
Increases with time after injection
Peaks in conc after given time
Tails off with more time
Curve represents time taken for water to travel
through subglacial system
Inferring Drainage Nature
(Nienow, 1993)
Infer from:
Shape dye breakthrough curve
Time taken for dye to emerge
Efficient - peaked curve
Inefficient = dispersed curve w pronounced tail
Efficient drainage
Very peaked dye return curve
with dye emerging after short period i.e. 15 mins
e.g. Moulin connects straight to bed
--> low sinuosity conduit = hydraulically smooth
Rough channel
Higher roughness ∴ more eddies in flow
More dispersed curve
More pronounced tail
indicates local storage (retardation)
Anabranching conduits
Bifurcating conduits
Multiple peaks in return curve
Conduit with linked cavity
Main conduit + linked cavities
Water routed out of efficient channnel into linked cavities
Rapid initial rise
Main channel routes meltwater quickly
Second, dispersed peak
Dye emerges later from inefficient cavities
Inefficient Drainage
Storage - linked cavity / film / deformable basal till
Slow routing / pooling
∴ Delay - water released after storage