HELICAL CELL MOTIONS IN A SMALL ICE-COVERED MEANDER RIVER REACH

摘要

The investigation of the flow field with a pulse-coherent acoustic Doppler profiler has led to new high resolution observations of the secondary flow pattern occurring in a natural ice-covered meander reach. Surveys were conducted during two successive winter periods with different ice conditions. Massive frazil ice accumulation was present during one of the survey and its influence on the flow pattern could be assessed. Results show that the primary flow is clearly deflected towards the outer bend. Secondary flows are one order of magnitude less than the primary flow and they display two stacked counter-rotating helical cells pattern occurring at the entrance of the bend. This pattern is associated with the parabolic shape of the velocity profiles entering the bend. The pattern rapidly evolves downstream, reducing to one helical cell rotating in an opposite direction than what is observed in open channel flows. Flow mixing and morphological non-uniformity are potential factors governing the development of the helical cells throughout the bend. Our observations show that a similar coherent flow pattern rapidly forms downstream of a massive frazil ice obstruction in the bend. Frazil ice does not constrain the formation of helical flow pattern in river bends.