Ice jams occur when the ice that is drifting down-current in a river comes to a stop, for instance, at a river bend, when it contacts the river bed in a shallow area, or against bridge piers. Doing so increases the resistance to flow, thereby inducing an increase in water level upstream of the jam (referred to as backwater). Ice jams are thus a main cause for flooding during the winter. In addition, when the jam is released, depending on the conditions under which this happens, the amount of water that was retained behind the jam can also lead to flooding downstream of where the jam occurred. Ice jam floods are generally less predictable and can also be faster than open-water floods.
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👉 Ice jam in the context of Ice floe
An ice floe (/floʊ/) is a segment of floating ice defined as a flat piece at least 20 metres (66 ft) across at its widest point, and up to more than 10 kilometres (6.2 miles) across. Drift ice is a floating field of sea ice composed of several ice floes. They may cause ice jams on freshwater rivers, and in the open ocean may damage the hulls of ships.
Ice floes pose significant dangers due to their instability, unpredictability, and susceptibility to environmental forces. Unlike thick, grounded ice, floes are buoyant and mobile, drifting with ocean currents and winds at variable speeds. This movement can rapidly separate a floe from the shoreline or from other floes, trapping individuals or wildlife with no means of return. Structurally, ice floes are often riddled with hidden fractures and varying thickness, making them prone to sudden breakage or collapse under weight. Additionally, temperature fluctuations can weaken their integrity, while tidal shifts and wave action can cause tilting or rolling, creating crushing forces or ejecting occupants into frigid, hypothermia-inducing waters. For vessels, ice floes present navigational hazards as collisions with even modest floes can damage hulls or jam propellers, especially in poorly reinforced ships.
In this Dossier
Ice jam in the context of Flash floods
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice and snow. Flash floods may also occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from regular floods by having a timescale of fewer than six hours between rainfall and the onset of flooding.
Flash floods are a significant hazard, causing more fatalities in the U.S. in an average year than lightning, tornadoes, or hurricanes. They can also deposit large quantities of sediments on floodplains and destroy vegetation cover not adapted to frequent flood conditions.
Ice jam in the context of Groyne
A groyne (in the U.S. groin) is a rigid aquatic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment. There is also often cross-shore movement which if longer than the groyne will limit its effectiveness. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation.
All of a groyne may be underwater, in which case it is a submerged groyne. They are often used in tandem with seawalls and other coastal engineering features. Groynes, however, may cause a shoreline to be perceived as unnatural. Groynes are generally straight but could be of various plan view shapes, permeable or impermeable, built from various materials such as wood, sand, stone rubble, or gabion, etc.