Ice jam
From HydroWiki
Overview:
An ice jam is initiated when the ice on a partially or totally frozen stream or river breaks apart and flows downstream. The ice debris can then encounter channel constrictions such as shallow areas, debris piles, and bridges or other structures where the ice blocks can accumulate. The accumulation of ice debris dams the river and water levels can quickly rise behind the dam and overflow the banks causing flooding. The pressure of water against the ice dam can also cause damage to structures such as bridges. When the ice dam fails the water stored behind can be released suddenly as a flash flood causing extensive damage downstream of the dam site. This flash flood can be especially destructive due to the ice flow that is carried with the flood wave.
Formation:
Ice jams can occur either with initial freeze-up as ice begins to form on the water surface or, more commonly, as established ice breaks up. The latter scenario can be initiated by a rise in river stage and/or melting of the ice pack. The most significant ice jams occur following an abnormally cold winter with low river flows, which leads to significant ice build-up, followed by a warm spring or winter thaw, which leads to rapid snowmelt and, when combined with heavy rainfall, leads to rapid rises in river levels thus breaking up the ice cover.
Control:
Certain measures can be taken to reduce the formation of ice jam build-ups as well as to break jams apart before they become a threat. Ice control structures can include seasonal booms and fixed structures and are typically designed to initiate ice formation upstream of problem areas and collect floating ice early in the winter. This reduces formation and collection of ice downstream in problem areas and allows it to move more freely during high flows thus reducing ice-jams. For more information see the US Army Corps of Engineers publication “Design of breakup ice control structures” by Tuthuill and Lever. If ice jams do build up in high impact locations it may be necessary to break the jam apart or open spillway channels. This can be achieved through the use of explosives and other demolition devices as well as cutting and mechanical removal of ice.
Historical Events: More than 12,000 ice jam events have been recorded in the USACE CRREL database causing untold losses in money and human lives. Some memorable events have occurred on the Yukon River in Alaska in 1992, along the Lehmi River in Idaho in 1984, and in Oil City, PA on many occasions.
References:
http://ks.water.usgs.gov/Kansas/pubs/fact-sheets/fs.024-00.html
http://www.crrel.usace.army.mil/icejams/index.htm
http://www.crrel.usace.army.mil/ierd/icejam/icejam.htm
http://www.usatoday.com/weather/wicejam.htm
http://aprfc.arh.noaa.gov/resources/rivwatch/rwp_printer.pdf
