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Mechanisms leading to floods
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Water that flows in a river is controlled by two main forces opposing each other: gravity and friction.[1][2] The friction results from the shearing action the river bed and the shorelines exert on water flow. In addition, where rivers freeze in the winter, the boundary between water and the ice cover becomes another source of resistance to flow - a consequent flow reduction induces higher water levels upstream, known as backwater'. [3][4] The roughness of that yearly interface determines its resistance to flow. It is smaller in the case of a smooth interface and is higher when the ice cover forms from the juxtaposition of ice fragments where a jam occurs. If the pressure is such that it can compress the ice cover, a thick accumulation of fragments can extend at depth, thereby restricting flow. The higher the roughness, the higher the upstream water levels.
Ice jams on rivers usually occur in the springtime as the river ice begins to break up, but may also occur in early winter during freeze-up. The break-up process is described in three phases: pre-break-up, break-up and final drive.[5] Pre-break-up usually begins with increased springtime river flow, water level, and temperatures fracturing the river ice and separating it from the shore. Changes in river height from dam releases may also affect the pre-break-up. During the break-up, the ice in areas of rapids is carried downstream as an ice floe and may jam on still frozen sections of ice on calm water or against structures in the river such as the Honeymoon Bridge, destroyed in 1938 by an ice jam. Smaller jams may dislodge, flow downstream and form a larger jam. During the final drive, a large jam will dislodge and take out the remaining jams, clearing the river of ice in a matter of hours. Ice jams usually occur in spring, but they can happen as winter sets in when the downstream part becomes frozen first. Freeze-up jams may be larger because the ice is stronger and temperatures are continuing to cool unlike a spring break-up when the environment is warming, but are less likely to suddenly release water.[6]
Three types of natural ice jams can occur:[7]
- a surface jam, a single layer of ice in a floe on calm water;
- a narrow-channel or wide-channel jam; and
- a hanging jam, the accumulation of river ice at slow current areas which only occur during freeze-up.
Ice jams also occur at sharp bends in the river, at human-constructed objects such as bridge piers, and at confluences.[6]
- ^ Principles of river hydraulics. New York, NY: Springer Berlin Heidelberg. 2017. ISBN 978-3-319-68099-6.
- ^ Radecki-Pawlik, Artur; Pagliara, Stefano; Hradecký, Jan; Hendrickson, Erik, eds. (2018). Open channel hydraulics, river hydraulic structures and fluvial geomorphology: for engineers, geomorphologists and physical geographers. Boca Raton, FL London New York: CRC Press, Taylor & Francis Group. ISBN 978-1-4987-3082-2.
- ^ Ashton, George D. (2004). River Lake Ice Engineering (Second Printing ed.). Littleton(Colo.): Water Resources Publications. ISBN 978-0-918334-59-6.
- ^ Beltaos Spyros; Beltaos, Spyros (1995). River ice jams. Highland Ranch (Colo.): Water Resources Publications. ISBN 978-0-918334-87-9.
- ^ Dingman, S. Lawrence (2009). Fluvial Hydraulics. Oxford University Press. p. 104. ISBN 978-0-19-803856-6.
- ^ a b White, Kathleen D.; Kay, Roger L.; (U.S.), Cold Regions Research and Engineering Laboratory (1996). Ice Jam Flooding and Mitigation: Lower Platte River Basin, Nebraska. DIANE Publishing. ISBN 978-1-4289-1388-2.
- ^ Beltaos, S. (1995). River Ice Jams. Highlands Ranch, Colorado: Water Resources Publication. ISBN 978-0-918334-87-9.