be built up in multiple layers by succeeding flood-
The ice cover in the Eagle River within ERF con-
ing tides.
tinually moves up and down because of the tidal
The extent and thickness of the existing snow
fluctuations. The surface of the ice sheet along the
cover also play a role in the rate of ice buildup due
banks and extending into the channel was very
to tidal flooding. When snow is present, the tidal
smooth because of the constant flux of water from
water moves laterally through the snow cover and
successive flooding. Ridges of broken ice, 24 m
wicks upward several centimeters into the over-
wide, in the centerline of the channel extend over
lying snow pack, either partially or totally satu-
a major portion of the river reach. Wide hinge
rating the snow, depending on its thickness. Water
cracks were evident along both shorelines at low
under the snow that has only partially saturated the
tide. Ice chunks and flows up to several meters in
snow cover can remain unfrozen for a consider-
diameter were scattered along either riverbank and
able length of time due to the insulating proper-
extended a short distance from the channel. Dur-
ties of the overlying snow. The saturated snow,
ing the high tides, parts of the ice cover detached
when it freezes, produces a characteristic bubbly
from the bank support, broke into small floes and
or white "snow ice" that is less dense than the
floated onto the Flats for a short distance, depend-
clear congelation ice. This frozen saturated snow
ing on the terrain topography and water depth.
produces a thicker ice layer than would be pro-
duced if no snow cover had been present. Most
Impact area description
high tides do not flood the entire flats. Rather, tidal
The impact area for the test was located on the
flood water spreads out from the heads of tidal
east side of the Eagle River, about 500 m west of
distributary channels as lobes or splays of water that
the EOD gravel pad, and covered an area of ap-
proximately 700 700 m (Fig. 1). Prior to the artil-
saturate the snow cover, freeze and build up a layer
of ice several centimeters thick over a limited area.
lery tests, we characterized the site by measuring
These lobes of superimposed ice are then slightly
ice thicknesses and snow depths. Six ice cores were
higher than the surrounding non-flooded areas.
obtained using a hand-held 7.62-cm-diameter SIPRE
Flooding water from the next high tide will then
core barrel. Ice thicknesses varied from 30 to 60
be displaced slightly, building up a lobe of ice ad-
cm, with a minimum of 25 cm and a maximum of
jacent to the previous ice lobes. Over time, much
40 cm of frozen sediment below the ice. The snow
of the area of ERF can be covered by an ice sheet
depth ranged from 15 to 20 cm within the test im-
built up from successive multiple lobes of ice. An
pact area, with an estimated snow density of 0.3
g cm3.
occasional extreme tide may flood the entire area,
adding an additional ice layer.
During the 1990-91 winter, sparsely vegetated
Previous research on cratering
Little information is available in the literature
mudflat areas that are normally subaerially ex-
on the cratering and demolition effects of artillery
posed in the summer had 3060 cm of superim-
fire on ice. A few 105- and 155-mm projectiles were
posed ice by March. The ice thickness in the ponds
fired onto the Imjin River in 1977 to determine their
ranged from 40 to 70 cm, with the ice surface of
effectiveness in breaking floating ice covers; they
the ponds as much as 20 cm above the normal sum-
were not effective. Several authors have looked at
mer water surface elevation due to the superim-
the effects of explosions in ice and snow (Living-
posed ice. In the mudflat areas where a superim-
ston 1960, Mellor 1965), in frozen ground (Living-
posed ice sheet had formed, frozen sediments were
ston 1956, 1959, Mellor and Sellmann 1970) and in
found under the ice sheet, and in some cases the
and under floating ice sheets (Mellor 1982, 1986a,
sediments were frozen greater than 40 cm. The
1986b). Mellor (1986a) summarized the guidelines
brackish water from the tidal flooding did not ap-
for blasting on ice sheets and gave estimates for
pear to significantly alter the ice growth rate rela-
the sizes of craters that will form, depending on
tive to freshwater behavior. The salinity gradient
the weight of the explosive charge and its position
in ice samples ranged from brackish (20 ppt) near
in the ice sheet.
the river channel to fresh (<2 ppt) in the pond near
The traditional analysis for determining the ap-
the east edge of ERF. Petrographic and chemical
parent scaled radius Ra and the scaled depth Da of
analysis of the ice and sediment cores taken dur-
craters formed by explosions uses cube-root scaling
ing the 1990-91 field season indicate a complex
(Mellor 1986a) to remove the effect of charge size
buildup of ice from tidal flooding and some fresh-
(all linear dimensions are divided by the cube root
water runoff (Taylor et al. 1994).
3
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