Jökull - 01.01.2001, Qupperneq 5

Jökulsárlón at Breiðamerkursandur
Plate 2. The Jökulsárlón outlet river, Jökulsá, in 1998. The 108 m long bridge is located about 600 m from the
coastline. – Jökulsá á Breiðamerkursandi, 1998. Brúin er 106 m löng og liggur um 600 m innan við ströndina.
Photo:/Mynd: Helgi Björnsson.
from this point we estimate the variation in the veloc-
ity from the expression for longitudinal strain rate in
an ice shelf (Paterson, 1994, p. 290-293)

 

  (5)
where

is the height of the ice above the water level
and
  ! " for isothermal conditions. Thecontribution to deformation given by equation (5) is
almost negligible at the Breiðamerkurjökull ice shelf.
Surface velocities along a central flowline were
calculated and compared with measured values (Fig-
ure 7). The mapped surface slope of the glacier
was filtered by a triangle-filter using a baseline 10–
15 times the glacier thickness. The shape factor ( # )
was calculated from the geometry of the glacier cross
section. We assume that n = 3 and A = 1.5 $ &%(' )
Pa '* yr ' (as recommended for Vatnajökull by Guð-
finna Aðalgeirsdóttir, personal communication; see
also Aðalgeirsdóttir et al., 2000 and Björnsson et al.,
2001). The measured annual surface velocity (Fig-
ure 7) at a central flow line 1500 m above the calving
front was 415 m yr ' , + = 330 m, and sin , = 0.065.
Thus the model predicts that -/.102-3 = 170 m yr ' ,
so -43 = (415–170) m yr ' = 245 m yr ' . Water pres-
sure (576 ) is calculated along a flow line downglacier
using the model of Röthlisberger (1972) and ice load
(598;:=<   8 +>8 where +98 is the ice thickness). For598;:=<?0@596 =7.5 bar, A = 1.89 bar, B = 3 and C = 1.5
we obtain D = 2.0 m d ' bar ' FE G using equation (3).
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