27
DESIGN OF ICE BOOMS
Q Q
50, 500
V=
=
=
= 2.3 ft3 / s
A LD (883) (25.2)
From Table 4, 50,500 ft3/s is a reasonable winter discharge.
Periods of higher discharge are possible, however. Under these
conditions, Froude number and water velocity may be too high
for successful ice retention by the boom. During the high dis-
charge periods, measures such as raising the pool height or up-
stream flow control may be necessary.
An alternative method would be to select a maximum expected
winter discharge, find the water depth using the weir equation,
then check if the FR ≤ 0.08 and V ≤ 2.3 ft/s criteria are satisfied. If
channel cross-section geometry data are available, numerical mod-
els such as HEC-2 (U.S. Army Corps of Engineers 1990) are
helpful in determining hydraulic conditions over a range of dis-
charges at potential ice boom locations.
For this example, the following values of various parameters
are assumed: channel bed roughness nb = 0.02, ice roughness ni =
0.03, brash ice cover thickness ti = 0.75 ft and porosity e = 0.5.
The average width of the upstream ice cover contributing to the
load on the boom is assumed to be 900 ft.
Under-ice depth: yt = D 0.92 ti = 25.2 (0.92) (0.75) = 24.5 ft
For wide rectangular channels, under-ice hydraulic radius (Rice )
can be approximated as half the under-ice depth:
Rice = 0.5 yt = (0.5) (24.5) = 12.2 ft
The composite roughness for the ice covered channel (nc) can be
calculated using the Belokon-Sabaneev formula:
(
)
2/3
(0.03)3 / 2 + 0.023 / 2
2/3
ni3 / 2 + nb / 2
3
nc =
= 0.025
2
2
(2.3)2 (0.025)2
V 2 nc
2
Sf =
=
= 0.000053
2.22 Rice (2.22) (12.2)
4/3
4/3
To find the shear stress on the underside of the ice cover, the
average channel depth above the isoline of maximum velocity
(yice ) is needed:
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