Unbreakable ice pieces
vertical distortion is acceptable for many hydrau-
Ice piece accumulations
lic modeling situations, it must be used with great
Breakable sheets
caution in situations involving ice failure pro-
Hydraulic modeling and ice-load modeling may
duced by vertical forces and ice loads. Ashton
involve any of these types of model ice materials.
(1986) and Ettema et al. (1992) show, however,
Commonly, though, hydraulic modeling involves
that vertically distorted models adequately repli-
unbreakable sheets and ice pieces. Ice-load model-
cate ice jams, provided the angle of internal resis-
ing usually involves breakable sheets and ice piec-
tance of the model ice is the same as at full scale.
es that may or may not be breakable. Exceptions
In modeling ice-cover breakup or the impact of
exist.
an ice cover with a structure, some researchers
have only considered the strength properties to
govern, with the actual thickness of the cover be-
Unbreakable sheets
Unbreakable sheets are used to simulate solid
ing secondary (Ashton 1986). Michel (1975) sug-
ice boundaries, such as a floating ice cover or large
gests using double distortion in which the ice
ice masses. The primary dynamic similitude con-
thickness scale is different from the vertical length
cerns are buoyancy and frictional resistance.
scale. Model distortion may extend the limits of
Often, provided the model sheet floats, strict repli-
current model ice materials in replicating proto-
cation of ice buoyancy is relaxed when modeling
type ice strengths. Other hydraulic variables be-
flow in an ice-covered channel. Sometimes the
sides ice strength that are important in the break-
wetting performance of the material is important.
up process include stage, water velocity, and the
The sheets can be formed from plastic, wood, or
shape of the inflow hydrograph. Geometric dis-
Styrofoam or be of composite construction (e.g., a
tortion to achieve adequate strength characteris-
ballasted floating box). They must float on the wa-
tics may cause mismodeling of other important
ter surface and move in accordance with the
variables.
Froude number criterion (for replication of inertial
Besides vertical distortion, other forms of dis-
and gravitational forces). Some sheets, such as thin
tortion may be used to design a model that repli-
polystyrene or plastic, may be flexible. To replicate
cates the process of primary interest. Among
flow resistance, additional materials, such as plas-
them is time distortion, which becomes impor-
tic bubble-packing, metal or plastic mesh, horse-
tant when the modeling situation contains a pro-
hair pads, filter cloth, or
cess that proceeds at a
particulate material, may
rate independent of
have to be attached to the
the time scale given in
sheet. Figure 1, for exam-
eq 2. Fracture devel-
ple, shows a model-ice
opment, ice growth,
panel of composite con-
and the downstream
struction, formed from
migration of wave-
Styrofoam sheets. One
like accumulations of
side is smooth while the
ice pieces (analogous
other has a layer of ex-
to alluvial bedform
truded plastic mesh to
movement or snow-
increase roughness. The
bank drift) are exam-
panel was used in a
ples of these process-
study of flow in an ice-
es. Model results need
careful interpretation Figure 1. Rigid ice panel showing extruded plastic mesh. covered channel.
in those situations.
Unbreakable ice pieces
Ice-piece modeling falls into two categories: the
MODEL ICE MATERIALS
movement of individual ice pieces, and the aggre-
gation and strength behavior of ice-piece accumu-
The material selected for use as model ice must
lations. Examples of the former category, which is
conform with the purpose and principal simili-
by far the simpler of the two, are ice-mass drift, ice-
tude criteria guiding operation and interpretation
floe fields, and frazil-floc transport. Examples of
of the model. Thus, model ice materials can be
the latter category are ice-jams, fields of ice rubble,
grouped and discussed as follows:
and ice ridges.
Unbreakable sheets
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