Aufeis[pronunciation?] (German for "ice on top") is a sheet-like mass of layered ice that forms from successive flows of ground water during freezing temperatures. This form of ice is also called overflow, icings, or the Russian term, naled. The term was first used in 1859 by A. T. von Middendorff following his observations of the phenomenon in northern Siberia.
Aufeis accumulates during winter along stream and river valleys in arctic and subarctic environments. It forms by upwelling of river water behind ice dams, or by ground-water discharge. The latter mechanism prevails in high-gradient alpine streams as they freeze solid. Ground-water discharge is blocked by ice, perturbing the steady-state condition and causing a small incremental rise in the local water table until discharge occurs along the bank and over the top of the previously formed ice. Successive ice layers can lead to aufeis accumulations that are several meters thick. Aufeis typically melts out during summer and will often form in the same place year after year.
Sheets of aufeis may block stream channels and cause their flood plains to widen as spring floodwaters are forced to flow around the ice. Research on aufeis has to a large extent been motivated by the variety of engineering problems the ice sheets can cause (e.g. blocking drainages and causing flooding of roads). Culverts and pipelines can actually help to block flow and lead to the development of more extensive aufeis.
Aufeis can present an extreme danger to recreational boaters even during summer months, who can find themselves trapped between walls of ice or pulled under aufeis by the current of the river. Breaking dams of aufeis can also cause flash floods downriver. Proper scouting and precautions when choosing campsites can minimize these risks.
In late 2011, Mongolia planned to test the use and storage of artificial naleds as a way of cooling Ulan Bator in the hot Mongolian summer, and reducing the use of energy-intensive air conditioning.
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