LIMIT EQUILIBRIUM OF ICE JAMS

The concept of a unique condition for the stability of ice jams in rivers at breakup, which may occur at a specific site for a given water discharge, was first proposed from theoretical considerations. With similar considerations, it was also shown that the stability also depends on the thickness of ice jams but that under certain conditions no jam can ever be stable at a given site. However, recent publications on river ice engineering, rarely mentioned this concept, although it is still extensively used in practice to solve many flood problems caused by ice jams. In this paper, we reexamine the physics of ice jams and show why a unique limit equilibrium stage for ice jams exists. Laboratory and field data are also used to illustrate this major characteristic of ice jams. The limit stability criterion, the critical jam depth, is better defined and a very efficient method is proposed to determine the sites of ice jamming in rivers and the maximum discharge that can occur before jams shove downstream to clear the ice. Typical river singularities giving rise to major jam formation are discussed.