COLD REGIONS TECHNICAL DIGEST NO. 96-1
10
Wind drag on
Wind shear is not the predominant force in most river situa-
ice cover
tions, since outside forces are generally not felt by a boom beyond
three to five river widths upstream of the structure. However, for
ice control structures on large lakes, such as the Lake Erie ice
boom, wind on the ice cover can be the most important driving
force.
For rivers, the wind force per unit width of boom can be esti-
mated by
fa = Cd ρair U 2 5B
(3)
where Cd = the drag coefficient of air flowing over an ice cover,
in the range 1.7 2.2 103
ρair = mass density of air (2.5 103 slugs/ft3 at 32F, 1.3
kg/m3 at 0C)
U = wind velocity 33-ft (10 m) above the ice surface.
To estimate the extent of the ice area contributing to the wind load
on a wide lake ice boom, refer to Figure 3.
Gravity force
The downstream weight component of an ice cover results
from water surface slope, Sf. Since slope is usually mild at sites
where ice retention is possible (on the order of 0.001 to 0.0001),
the gravity component is relatively small. This force can become
significant if the ice cover is extremely thick, however. For rivers,
the gravity force acting on a unit width of boom is given by:
fg = (1 - e)γ ice ti Sf 5B
(4)
where e = the porosity of ice cover, which is on the order of 0.5
for conditions of maximum loading (newly formed
freezeup jam, or accumulation of loose brash ice)
γice = specific weight of ice (57.2 lb/ ft3, 9000 N/m3)
ti = ice thickness
Drag on boom units
Because the boom units are relatively small, compared to the
cross section of the river, and the size of the ice cover, the drag
force on these units is generally negligible. In many ice boom
applications, the bottom of the ice cover is roughly even with the
bottom of the boom unit, rendering water drag on the boom unit
insignificant. If needed, the water drag force on the boom unit,
per unit river width, can be approximated from:
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