Jökull - 01.01.2001, Side 4

Helgi Björnsson et al.
Plate 1. The front of Breiðamerkurjökull calving into Jökulsárlón in 1998. – Sporður Breiðamerkurjökuls við
Jökulsárlón, 1998. Photo:/Mynd: Helgi Björnsson.
Deformation velocity and basal sliding
Measurements of the glacier surface velocity ( 
) in
1997 to 1999 were used to tune theoretical models of
deformation and basal sliding. The surface velocity of
the glacier is defined as the sum of basal sliding (  )
and deformation velocity (  )

  

    "! (2)
where  is the ice thickness,  and  are parameters
from the flow law of ice,  $#&% (' *),+- denotes
the shear stress at the bed, where - is the slope of the
glacier surface, % = 900 kg m .0/ is the density of ice,' = 9.81 m s .01 is the gravitational acceleration, and #
is a shape factor (see Paterson, 1994, p. 268).
Basal sliding is calculated as
 32 45"6  ! (3)
where 6 87 :9 7; is the effective basal pressure,7 is ice overburden and 7&; water pressure at the bed,
and 2 , < and = are constants. The effective water pres-
sure at the calving front was estimated as
6 >% ?' 9 %@; ' ; ! (4)
where % is the density of ice and %A; the density of wa-
ter,  ice thickness and ; water depth at the calving
front measured from the lake level.
Theoretical models to estimate basal sliding at the
terminus are very sensitive to the value of the effective
pressure ( 6 ), which approaches zero at the calving
front. Equations 2–4 were used to calculate the ve-
locities down to a point approximately 1500 m above
the calving front where a singularity in  causes the
calculated velocity to approach infinity. Downglacier
4 JÖKULL No. 50