Published on: 3.2.2020

Information produced by Finnish Environment Institute

Frazil ice causes winter floods

Talvinen jokimaisema, hyydetulva muodostumassa jokeen.

In winter, a river may be struck by a frazil ice flood, a peculiar phenomenon which may arise quickly if the conditions are right.

Frazil ice floods are particularly likely to occur if early winter brings a period of very low temperatures. The river does not yet have an ice cover, and its waters may cool down to a sub-zero temperature. Small ice crystals are formed in this supercooled water, which stick together and gradually float to the surface.

On the surface, the ice crystals form frazil ice floes or slush, which may accumulate under an area with ice cover and create a dam. In the eddies of the flowing water, the ice crystals may also adhere to the channel bottom and start building anchor ice. Frazil ice dams are usually found under the ice cover of a stream pool below rapids, whereas anchor ice at the bottom usually accumulates around stones in rapids and bridge structures, for instance. Both may block the channel and create a local flood.

Frazil ice dams do not occur in all rivers, and not every winter. It takes suitable weather conditions, and shallow rapids with a flow rate strong enough to cause eddies in the water and prevent the formation of an ice cover.

While predicting frazil ice floods takes experience, it helps to know that frazil ice dams usually occur in the same sections of rapids year after year.

Image: © Olli Karjalainen, Vastavalo