EP 1110-2-11
30 Nov 94
flooding depend on the flow conditions, the available ice supply from the upstream reaches of the river, and the strength
and size of the ice pieces.
e. Midwinter thaw periods marked by flow increases may cause a minor breakup jam. As cold weather
resumes, the river flow subsides to normal winter level and the jammed ice drops with the water level. The jam may
become grounded as well as consolidated or frozen in place. During normal spring breakup, this location is likely to be
the site of a severe jam.
f. Combination jams. Combination jams involve both freezeup and breakup jams. For example, a small
freezeup jam forms in a location that causes no immediate damage. Before the thaw, the jam may provide a collecting
point for fragmented ice that floats downstream. On the other hand, it could break up at the same time as the remainder
of the river. Since the jam is usually much thicker than sheet ice, it significantly increases the volume of ice available to
jam downstream.
g. In some rivers, frazil ice does not cause freezeup jams; instead, it deposits beneath sheet ice in reaches of
slow water velocities. These frazil ice deposits, called hanging dams, are many times thicker than the surrounding sheet
ice growth and will tend to break up more slowly than thinner ice. Such a frazil deposit could also provide an initiation
point for a later breakup jam, as well as increase the volume of ice available to jam downstream.
3-3. Causes of Ice Jams
a. River geometries, weather characteristics, and floodplain land-use practices contribute to the ice jam flooding
threat at a particular location.
b. Ice jams initiate at a location in the river where the ice transport capacity or ice conveyance of the river is
exceeded by the ice transported to that location by the river's flow. The most common location is in an area where the
river slope changes from relatively steep to mild. Since gravity is the driving force for an ice run, when the ice reaches
the milder slope it loses its impetus and can stall or arch across the river and initiate an ice jam. Water levels in reser-
voirs often affect the locations of ice jams upstream as a result of a change in water slope where reservoir water backs up
into the river. Islands, sandbars, and gravel deposits often form at a change in water slope for the same reasons that ice
tends to slow and stop. Because such deposits form in areas propitious to ice jamming, they are often mistakenly identi-
fied as the cause of ice jams. While these deposits may affect the river hydraulics enough to cause or exacerbate an ice
jam, the presence of gravel deposits is usually an indication that the transport capacity of the river is reduced for both ice
and sediment. Ice jams located near gravel deposits should be carefully studied to determine whether the gravel deposit
is the cause of the jam or a symptom of the actual cause.
c. Ice jams also commonly form where a tributary stream enters a larger river, lake, or reservoir. Smaller rivers
normally respond to increased runoff more quickly than larger rivers, and their ice covers may break up sooner as a result
of more rapid increases in water stage. Ice covers on smaller rivers will typically break up and run until the broken ice
reaches the strong, intact ice cover on the larger river or lake, where the slope is generally milder. The ice run stalls at
the confluence, forming a jam and backing up water and ice on the tributary stream.
d. River bends are also frequently cited as ice jam instigators. While river bends may contribute to jamming by
forcing the moving ice to change its direction and by causing the ice to impact the outer shoreline, water slope is often a
factor in these jams as well (Wuebben, Gagnon, and Deck 1992, Urroz and Ettema 1994).
e. Obstructions to ice movement can cause ice jams, for example closely spaced bridge or dam piers. In high
runoff situations, a partially submerged bridge superstructure obstructs ice movement and may initiate a jam. In smaller
rivers trees along the bank sometimes fall across the river causing an ice jam.
f. Some structural or operational changes in reservoir regulation may lead to ice jams. For example, changes in
hydropower operations can inadvertently cause ice jam flooding. Sudden releases of water such as those characteristic of
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