a. Unlimited ice supply.
b. Ice supply limited by Caribou
dam (hi = 28 in.).
Figure 15. Effects of an ICS at
Haley Island (Q = 2000 ft3/s).
the water level is initially reduced by about 2 ft,
average of approximately 3 ft and as much as 6
but at about station 630 it goes back to the level
without the ICS.
structure at station 423. In the case of limited ice
The effects of an ICS at station 656 in the
supply (Fig. 16b), the Haley Island structure
vicinity of Haley Island are shown in Figure 15
would again lead to increased upstream water
for the two cases of unlimited ice supply and ice
levels, since it would create a local jam by hold-
supply limited by the Caribou dam. The figures
ing the ice between the structure and the Cari-
show no discernible effect of the ICS on the
bou dam that would otherwise have passed
water level below station 500, and a maximum
drop of about 3 ft in the water level at the ICS
We again emphasize that in developing Fig-
location. For the case of unlimited ice supply,
ures 13 through 16 we assumed that the jams
the water level upstream of the ICS goes back to
would be fully developed, a conservative as-
the full jam value at about station 740. On the
sumption, and that ice volumes would corre-
other hand, if the Caribou dam indeed limits the
spond to a before-breakup ice cover thickness of
downstream ice supply, the ICS at Haley Island
28 in. Depending on the actual thickness of ice
leads to an increase in upstream water level (Fig.
formed in a given year and, especially, the ex-
15b) because it retains all the ice between Haley
tent of ice decay before breakup, the actual ice
Island and the dam.
volumes, ice jam length and resulting water lev-
Finally, the effects of installing ICSs both at
els may be less.
station 423 and at Haley Island are shown in
Figure 16a for unlimited ice supply and in Fig-
ure 16b for ice supply limited by the Caribou
dam. The combined effect of both structures is to
reduce the water levels in the reach from about
In spite of the very mild ice breakup condi-
station 330 to the Haley Island structure by an
tions in April 1992, our 199192 winter field ob-