Strudel (ice)

A strudel /ˈstruːdəl/ (plural: strudel) is a vertical hole in sea ice through which downward jet-like, buoyancy-driven drainage of flood water is thought to occur. This feature is less than a few tens of meters in size and typically occurs within 30 km from a river mouth, in the sea ice expanse that is fastened to the coastline (known as fast ice). Once the water that flooded the ice has completely drained off the ice surface, strudel become recognizable by a radial pattern of feeder channels that lead to the hole. They are elongated and irregularly spaced, with the larger ones up to several kilometers apart. Their distribution tends to be controlled by weak areas in the ice — in places, they line up along fractures or refrozen extensional cracks. The ice sheet where they occur may be 2 m in thickness, at water depths (below the ice) in the order of a few meters.

The term strudel is German, and designates a whirlpool, in reference to the water vortex that forms above these features during drainage. It has been suggested that this vortex could present a hazard for investigators wishing to study this phenomenon in the field, and that this would explain, at least partly, why little is known about strudel.

The formation of strudel is related to the break up of a frozen river during the melting season, where this river runs into a sea ice expanse. At that time, fresh water flows onto the fast ice with a progression rate of about 2–3 m/s, extending up to a few tens of kilometers away from the river mouth. Water depths above the ice surface may be up to a few meters. A strudel forms as a result of water carving its way through the ice sheet. Drainage is initiated through small openings or cracks in the ice resulting from the weight of the freshwater. Some are reportedly enlarged seal-breathing holes. Within a few days, the water drains off the ice. Drainage is driven by the buoyancy of the ice, not by the density difference between freshwater and sea water. The pressure difference responsible for that flow is a function of the ice thickness and the difference in unit weight between the ice and the water.

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