Glacier Hydrology. Where does the Water Go? Why study water flow? Why study (cont.) Water routes (1/3) Water routes (2/3) Vatnafræði

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1 Why study ater flo? Glacer Hydrology Vatnafræð Þröstur Þorstensson Glacer rvers provde much of the ater for: Agrculture (Canadan prares, central Asa) Hydro-electrcal poer producton (Iceland, France, Stzerland, Noray). Glacer run-off has unusual features such as: large durnal varatons maxmum flo occurs durng summer (hen many rvers are dry) storage of ater Why study (cont.) In the Alps and Noray, sub-glacal tunnels drlled to get ater for hydroelectrc plants. Need to kno ater flo at the bed. Sudden dranage of glacer-dammed lakes or of ater stored thn glacers, jökulhlaup Assocated mudflos are also a rsk. Becomes more mportant as ctes and roads move closer and closer to out-flo planes and glacers. Water at bed affects sldng; tll deformaton, sldng, surges Where does the Water Go? Water at the surface of a glacer, that ether fell as ran or melts there, can choose beteen several dfferent routes before reachng the ocean. Belo s a short summary of some of these routes Water routes (1/3) Water routes (2/3) 1. Water n the accumulaton area can percolate through the sno and frn don to the ce layer. 2. Water can flo on the surface, even all the ay to the glacer termnus. 3.a A fracton flos through ens that form at the ntersecton of grans (lqud due to hgher surface tenson) 3.b Most of the ater drans to the bed through moulns. 4.a Water at the bed can flo n tunnels that ncse the ce, so-called R-tunnels. Eskers (malarásar) commonly form n those. 4.b Water at the bed can flo n tunnels that ncse the bed, so-called N-tunnels. 4.c Water at the bed can flo as a thn sheet of ater (W-sheet). Unlkely to be stable. Throstur Thorstensson (ThrosturTh@h.s) 1

2 Water routes (3/3) 4.d Water at the bed can flo as groundater n sedments under the ce. 4.e Water at the bed can flo n cavtes, hch could be lnked. 4.f Water at the bed can flo n a system of broad and shallo canals, at the top of sedments. Water n Glacers Water can also accumulate n sub-glacal lakes. Sub-glacal lakes sometmes dran n catastrophc ay as glacer outburst floods, or jökulhlaup. 5. The ater that emerges at the snout flos to the sea as a glacer rver. Water pathays - schematc Surface ater Wens Surface ater Mouln Some ater flos n channels on the surface, lke rvers, and off to the sdes or the termnus Much dsappears through cracks and vertcal passages, called mouln s. Tunnel Mouln Mouln Form here a stream flos nto a crevasse If crevasse flled th ater e have P gh gh P Mouln do most often form n areas of extenson Throstur Thorstensson (ThrosturTh@h.s) 2

3 Mouln (II) Water system at the BED Melt-ater on the surface of Greenland ce sheet. Photo: Brathate Drect measurement dffcult Durng summer, glacer acts as a reservor (especally early summer) Durng nter mouln s etc close In sprng there s a delay for reopenng Water pressure potental The drecton of ater flo n and under glacers s controlled by the ater pressure potental f, f 0 p z f 0 f p gz s a constant s ater pressure s elevaton Water pressure potental (II) The ater flo s perpendcular to the surfaces of constant f. If e assume, p g( z z) that s ce overburden pressure, e get f f0 gzs ( ) gz s Breðamerkur jökull Breda (II) Surface elevaton and bed topography. Bed goes from 300 m to 1807 m, and surface elevaton ranges from sea level to 2012 m. Pressure potental and drecton of flo f (perpendcular to surfaces of constant p-potental). Throstur Thorstensson (ThrosturTh@h.s) 3

4 Steady flo n a tunnel Tunnels Sze of a condut determned by: Meltng, due to vscous dsspaton and frcton aganst the alls. And f ater above 0 C the release of latent heat Closure by overburden pressure (f greater than ater pressure) Response tme s slo, order of a eek meltng Water pressure Ice deformaton Water flux Closure rate Closure rate due to ce deformaton gven by c dr P ra dt n P p p, n p gz Melt n steady state In steady state the sze of the tunnel must reman constant, that mples that ce melted per unt length M s, dr M 2 r dt P M 2 SA, S r n n 2 TUNNELS Tunnels Q vs p Large tunnels gro snce for ncreasng flux Q the ater pressure p drops. Large tunnels capture ater from small tunnels. Throstur Thorstensson (ThrosturTh@h.s) 4

5 Water at the bed Water at the bed (I) Channels cut upard nto the ce Röthlsberger -, or R-channels Channels n the bedrock Nye-, or N-channels Canals on soft bed, broad and shallo If Q p, so stay small and seperate Cavtes Fnd length by consderng ho long t takes a partcle to travel from A to B. Water at the bed (II) Water at the bed (III) Cavtes Closure and melt rate same as for tunnels Assumng that the length l s much greater than the heght r, then the length s r tv, c m t h l u Cavtes, length of t t v h ru l c m Channels Pcture of a tunnel: R-channel Throstur Thorstensson (ThrosturTh@h.s) 5

6 G. Favre Sss flm team Throstur Thorstensson 6

7 Throstur Thorstensson 7