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The marginal lake Grænalón

  
  Location

Lake Information

Statistics

Flood Dates

Risk of Damages

Flow Characteristics

References

Tabulated version

 
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The glacier margin at lake Grænalón, looking south into water flow direction.
                       Photo: Sigurdur Thorarinsson, 6. Aug. 1962

 
     
     
     
     
     

One of the best known marginal lakes in Iceland is Grænalón. Its area has been  up to 18 km2 and floods have reached peak discharges of 5000 m3/s. The lake area varies between years, but floods occur frequently with typical peak discharges of 2000 m3/s. The last jökulhlaup occurred in late September 2001.
 

 

     
 

Location of lake Grænalón

  
 

The lake Grænalón is situated west of the outlet glacier Skeidarájökull and dammed by part of it. Before the 1950´s the lake was occasionally so large that it reached the ice of the small outlet glacier that today lies about 2 km east of Grænalón.

During most of the last century the glacier was very thick and the highest possible lake level did not cause the ice to lift, hence there were no floods. Instead there was a steady outflow out of the lake over a ridge (ca. 650 m a.s.l) and into river Núpsá.

As the glacier became thinner the water volume became sufficient to lift part of the outlet glacier, drain beneath and trigger a flood. The largest ones occurred in 1935 and 1939 with numerous icebergs reaching the sander several km south of Skeidarájökull.

The glacier continued to thin and after 1943 floods appeared more often. These floods had lower volumes and peak discharges, the latter beiing  reached in 2 days instead of 3-5. Since 1943 the lake level has never reached the natural outflow over the ridge into Núpsá again and it certainly will not in the near future.

 

  
 

Geographical coordinates

64°11´ N          17°09´ W

Name of outlet glacier

Skeidarájökull

Lake type

Lateral

Type of dam (e.g. ice, moraine)

ice dam
 
 

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Lake Information on Grænalón
         
 

Lake level elevation
(m a.s.l.)

560 – 580

Depth (m)

200 (1939)
150 (1989)

Area (km2)

18 (1939)
10 (1989)

 

  

 

 

 

 

Lake Grænalón has often varied in size. This map of  lake Grænalón was drawn from a digital map (derived from aerial photographs taken on 10 Sept. 1986), the AMS-map from 1946 and a surveyed section from 1935 (when the lake drained comletely).

 

A map of lake Grænalón. Björnsson and Pálsson, Jökull 39     
Click on the figure to see it at full size.

  

            
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Grænalón:  Statistics and Risk of Damages
 
 

Flood interval

1 per year to
1 per several years

Flood Volume (106 m3)

100 – 200

Peak discharge (m3/s)

1500 – 3000

Duration (days)

4 – 7

Time of peak
discharge (day)

2 – 3

Lake level change (m)

17 – 23

 

Downstream topography

 x

Sediment /
Debris Content

low

Rockfall
into the lake

no

Failure mechanism

hydraulic, subglacial drainage

Mortality

one man drowned
in 1898

Damages

road, bridges
 
 

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Flood Dates (Grænalón)

  
 
Year Date River Flow Discharge Comments
(1785)   Núpsvötn   fertile land was destroyed
1898 End of November Súla, (Núpsvötn) took 5-6 days to reach peak one man drowned
(1913)   Núpsvötn    
1935 5 Sept - 12 Sept Súla, (Núpsvötn) peak discharge:
 12 Sept		 lake volume ca. 1.5 km3,
the lake emptied
1939 23 July - 31 July Súla, (Núpsvötn), Blautakvísl 4,000 - 5,000 m3/s large icebergs were carried downstream, fertile land was destroyed
1943       Glacier much thinner, waterlevel lower, no more discharge to Núpsá (ever). Jökulhlaups not as large.
1946        
1949        
1951 15 Oct - 22 Oct Súla   First time that water breaks through beneath the very eastern end of glacier tongue, lake level change: 22m
: typical total Vol. 0.3 km3, often one jökulhlaup per year
1973 6 Aug - 9 Aug   peak discharge 7 Aug
2,000 m3/s		 total volume: 0.2 km3
:        
1977        
1981 10 July Súla 750 m3/s jökulhlaup flow irregular, 
nonsteady as in 1977
1983 16 Aug - 18 Aug Súla 2200 m3/s seemed to be steady outflow after the
jökulhlaup for 2 months
1984 15 Aug - 16 Aug      
1986 29 Aug - 31 Sept Súla and Gígukvísl 2,000 m3/s
and 800 m3/s 		dikes were damaged
1987 10 Aug - 11 Aug      
1988 20 Aug - 21 Aug      
1989  1 Aug -  2 Aug      
1990 11 July - 12 July   peak discharge 12 July  
1991 16 Aug, 15 Sept, 30 Sept      
1992 3 Oct - 6. Oct      
1995 15 Jan - 22 Jan      
2001 23 Sept - 31 Sept      
 
     

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Flow Characteristics (Grænalón)
 

Length of floodpath

18 km to
glacier margin

Flow path
(e.g, subglacial, supraglacial)

marginal subglacial and over land

∆ h
(lake surface to
flood plain, m)

558 – 110 = 448

Average grade

1.4 %

Flood river

Súla, Blautakvísl
(Núpsvötn)

 

 The jökulhlaup of 1986. The bridge of the ringroad above river Súla, about 25 km south of the lake.


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References (Grænalón)
 

Maps

I. Geod. Surv. (1939)               AMS (1945)             DMA (1982-1993)



Articles

Áskelson, Jóhannes. 1936. On the last eruptions in Vatnajökull. Societas Sci. Isl. XVIII, Reykjavík, 68 pp

Björnsson, Helgi and Pálsson, Finnur. 1989.  Rúmmál Grænalóns og breytingar á stærð og tíðni jökulhlaupa. Jökull 39, 90 - 95

Kjartansson, Gudmundur. 1938. Um nokkur jökulhlaup í Harðangri og á Íslandi. Náttúrufræðingurinn 8, 21-33

Rist, Sigurjón. 1970. Annáll um jökulhlaup. Jökull 20, 89 - 90

Rist, Sigurjón. 1973. Jökulhlaupaanáll 1971, 1972 og 1973. Jökull 23, 55 - 60

Thorarinsson, Sigurdur. 1939. The ice-dammed lakes of Iceland, with particular references to their values as indicators of glacier oscillations. Geografiska Annaler 21, 216 - 242

Thorarinsson, Sigurður. 1974. Vötnin stríð. Saga Skeiðarárhlaupa og Grímsvatnagosa. Menningasjóður, Reykjavík, 254 pp

THE  MAIN  GLACIORISK  REFERENCE  PAGE
 

Aerial Images

Iceland Land Survey (Landmælingar Íslands)
Photographs

Helgi Björnsson        Oddur Sigurðsson        Sigurdur Thorarinsson

 

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