Unsteady Ice Jam Processes
JON E. ZUFELT AND ROBERT ETTEMA
INTRODUCTION
Background
Ice jams cause massive damage annually throughout the world's northern tem-
perate regions. In the U.S. alone, the annual damages from ice jams average 5
million (USACE 1994). These include property losses, emergency assistance, flood
insurance, and increased operation and maintenance costs, replacement of infra-
structure, and loss of hydropower revenues. Most damages are caused by high
water levels associated with ice jams, though some are from the direct impact of
moving ice during ice runs.
The formation and evolution of ice jams comprise a series of inherently unsteady
processes, in which moving ice is brought to rest in accumulations that shove and
thicken in accordance with changing forces exerted by water flow, accumulation
weight, and bank roughness. These processes are even more unsteady when a jam
collapses, plows downstream, and possibly reforms. Prior formulations for ice jams
treat them as stationary, steady-state ice accumulations that are subject to invariant
flow conditions. This study, however, presents the first formulation for and exami-
nation of the fully coupled dynamic nature of the unsteady processes associated
with jam formation.
Need for research
In efforts to protect life and property from the damages attendant to ice jams
and related flooding, prior models were developed to predict water-level changes
caused by ice jams. Those models treat the evolution of ice jam thickness (shoving
and thickening) as quasi-steady, with jam thickness spontaneously adjusting to a
new equilibrium value in concert with water flow changes. Steady-flow models,
such as HEC-2 modified with ice cover option, simply provide the steady water
levels that would exist with a uniformly thick jam already in place. The long-stand-
ing assumption used is of an ice jam of equilibrium thickness, floating in static
force equilibrium just on the verge of stability or failure. Other models simulate the
unsteadiness of the water flow using the conservation of mass and momentum
equations for the water, but solve in an uncoupled manner for thickness between
time steps, again by the static force balance.
There currently are no formulations that describe the coupled interaction of the
water and ice movement and their effects on flow depth and ice thickness. Also, no
information exists on how jams evolve, fail, and thicken. In that regard, the follow-
ing important groups of questions need to be addressed:
How do ice jams evolve? Present formulations allow for instantaneous changes
in the jam thickness attributable to changes in the forces acting on the jam. No
account is currently made for the impact forces generated by moving ice. Once
a jam fails, ice is mobilized and travels downstream, often at high speed
(Henderson and Gerard 1981). Do jams move and then thicken upon failure,
thicken as they fail, or thicken and result in a progressive downstream-mov-
ing failure?
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