To what extent does ice momentum affect jam thickness? If jams move upon
failure, the force levels acting on the jam inevitably change. What are these
changes and how do they affect the forces acting on the jam?
What are the effects of the interaction of the water and ice motion? The water
shear stress on the jam underside is one of the principal forces on the jam.
When a jam fails and moves, however, this force is reduced, which interac-
tively reduces the resistance to water flow. As water and ice motion are inter-
related, what are the consequences, for jam thickness prediction, of uncou-
pling their influences as is currently done in existing formulations?
The inherent unsteadiness of jam formation and failure is obvious from field
observations. As an example, observations of a freezeup jam on the Salmon River
near Salmon, Idaho, made from a small bridge about 1 km downstream of the lead-
ing edge (head) of the jam, are presented here. The jam was approximately 25 km
long, and, owing to mild weather (3C), the leading edge had been stationary
over the night. Water levels were slightly more than bankfull at the bridge and
thick frazil accumulations filled the channel.
The water level began rising, first noticeably at the treeline along the bank, then
in mid-channel, as the water began to flood the surface of the jam (Fig. 1). The ice in
the channel appeared to rise slightly, and shear cracks could be seen forming about
10 m out from the banks. The water levels continued to rise and the ice, groaning,
began to slowly move downstream en masse (Fig. 2). In a matter of seconds, the
entire channel section of the jam for about 1 km on either side of the bridge
(between the shear cracks) was moving downstream (Fig. 3). As the ice moved, the
water level fell, until the center portion of the channel was clear of ice upstream to
where the leading edge had previously been located. Once the ice had passed,
water levels dropped by approximately 0.5 m, exposing shear walls of ice along the
Figure 1. Ice jam at first signs of failure; view is looking downstream.