Geology Reference
In-Depth Information
on the ice surface and the interaction between individual
ice floes in response to internal forces within and between
them. At this scale, the deformed ice represents hazard-
ous conditions for both marine navigation and offshore
structure. Rough ice along marine navigational routes
delays marine traffic, while mechanical loading of
deformed ice threatens offshore structures. Medium‐
scale deformations are defined by a spatial scale that
extends a few tens of kilometers. They are usually mani-
fested in the forms of heavy and extensive ridging as well
as cracks and leads in the ice sheet. At this scale, the
deformation is driven by mesoscale weather/or oceanic
forces. Large‐scale deformations with characteristic
dimensions in the order of hundred to several hundred
kilometers are caused by large circulation systems, par-
ticularly in the Arctic. The circulation controls conver-
gence and divergence of the regional ice cover and creates
shear zones. A key synoptic weather pattern that causes
such circulation and deformation is polar or subpolar
gyre. All forms of deformation that exist at the medium
scale can also be active at the large scale.
Hutchings et al
.
[2011] investigated the temporal and spatial scaling
deformation of sea ice (from 10 to 140 km) using a set
of drifting buoys that were deployed in arrays in the
Beaufort Sea. They indicated that deformation is con-
trolled by a balance between external forces and internal
dissipation. The array design allowed linking weather
events to deformation scaling behavior and changes in
coherence of deformation events between large and small
spatial scales. Details on common forms of deformation
in connection to the three scales are presented next.
Rafting and pressure ridging are the most common
forms of ice compression at small and medium deforma-
tion scales. They contribute to the increase of ice thick-
ness. They occur when two ice sheets are pushed against
each other (Figure 2.41). As a rule of thumb, if the sheets
are thin rafting is more likely to occur and if they are
thick, a pressure ridge will form. Details on favorable
conditions for rafting and ridging will be presented later.
Shear ridging, on the other hand, occurs when a floating
ice sheet moves along the edge of stationary ice (e.g., fast
ice). The study of the mechanics and the energy to create
these forms is important because the energy expended in
ice deformation determines the large‐scale strength of
pack ice [
Weeks and Kovacs
, 1970;
Hopkins et al.
, 1999;
Tuhkuri and Lensu
, 2002]. The thickness and strength of
rafted ice and pressure ridges have to be taken into
account in the design of Arctic vessels and offshore struc-
tures [
Bailey et al
., 2010].
Typical thin ice thickness that deform into rafting is a
few centimeters to a few tens of centimeters although
rafting may be found with thicker ice when relatively
small floes collide. In rafting, a moving thin ice sheet
overrides another sheet when they collide and continues
riding under compression force and against an increasing
frictional force between the rafted segments. It eventually
stops when the frictional force between sheets arrests
motion or causes buckling in the ice sheet.
Hopkins
[1999]
suggested that the frictional force is proportional to the
product of the difference between the unit weight and the
buoyancy of either sheet multiplied by the length of the
overlapping segment between the two sheets. The rafting
mechanism is well documented in literature that addresses
ice jams in rivers and lakes [
Michel
, 1978].
Rafting has received relatively less attention in ice
mechanics studies compared to ridging (particularly
pressure ridging). This is because it occurs mostly in thin
ice, which does not exert significant mechanical loading.
As such, it has been of less concern to engineers. However,
rafting plays an important role in increasing ice thickness
in the Antarctic pack ice. It is the dominant dynamic
mechanism for floes of thicknesses between 40 and 60 cm
(more information is available from the Antarctic Sea Ice
Processes and Climate program).
Rafting is manifested in two forms: (1) simple rafting
whereby one sheet simply overrides the other (Figures 2.9
and 2.42a) and (2) finger rafting whereby two sheets inter-
lace when they meet; pushing over and under each other
Rafting
Compression
Pressure ridge
Shear
Shear ridge
Figure 2.41
Cracking of ice covers by compression and shear forces (left); rafting and pressure ridging are
compression‐driven while shear ridging is shear‐driven (right).
