evaluating the effects of ice momentum on jam thickness and profile. The numeri-
cal model used for this purpose is a significant advance on prior models in so far
that it includes ice momentum and directly couples ice and water motion.
LABORATORY EXPERIMENTS
Introduction
As indicated in the last section, the literature on ice jams contains no studies
describing how jam shoving and thickening occur, or generally evaluating the
importance of ice momentum in jam development. The laboratory experiments
conducted here provide the first diagnostic information demonstrating the impor-
tance of ice momentum.
All prior studies, certainly those that do not include ice motion, treat shoving
and thickening as an instantaneous process. When the forces exerted in the down-
stream direction on a jam reach the level of the passive pressure resistance of the
jam, prior formulations let the jam simply thicken. No mention is made of the time
required for thickening to take place, or where the ice mass required for the thick-
ening originates. Equilibrium thickness theory (e.g., see Uzuner and Kennedy 1976)
carries with it many assumptions, including steady, uniform flow and a stationary
ice cover. Certainly, when a jam fails, it violates the latter assumption, which in
turn violates the steady and uniform flow condition, because ice movement influ-
ences water flow. Once an ice jam comes into motion, the shear stress on the under-
side of the jam is reduced, because it is a function of the difference between water
and ice velocities. Furthermore, the principal assumption used for describing the
compressive stress state of ice jams diminishes in validity once a jam fails. The
Mohr-Coulomb theory has been used with great success in describing the com-
pressive strength of granular materials, such as ice rubble in a jam, under various
states of stress. Once failure begins, however, the material undergoes changes in
stress levels that are not well handled using this theory. As well as thickness and
velocity changes, other jam characteristics, such as porosity or even ice-piece size
or shape, may change.
To model shoving and thickening, the principal effect of ice momentum, it is
necessary to know how the process occurs. Though numerous ice jams and their
failures have been observed for a wide variety of situations in the field, observa-
tions are typically limited to the surface of the cover from the perspective of the
shoreline. Even when jam failure and reformation are observed from the air, prac-
tical limitations (i.e., altitude and sight distance) render the observations reach-
averaged at best. The highly unsteady nature of most breakup jams reduces oppor-
tunities for direct measurements of jam properties. Only in the rare incidence where
a jam formed and refroze in place, following a reduction in water flow and a return
of lower air temperatures, might this be done. While these few cases may provide
useful data on jam thickness profiles, other items of interest, such as ice velocity
and local water discharge at the time of jamming, remain unknown. A final note
concerning jam observations is that, while the date of breakup ice runs and jam-
ming in the northern U.S. might average 10 March (near equal amounts of daylight
and darkness daily), about 80% of ice runs and jams take place during darkness.
To qualitatively examine the importance of ice momentum on jam processes, a
laboratory study was undertaken to simulate the shoving and thickening of a fail-
ing ice cover. Of particular interest are the timing and mechanics of the process.
20
Previous Page
