The effects of ice cover on flow characteristics in meandering rivers
are still not completely understood. Here, we quantify the effects of
ice cover on flow velocity, the vertical and spatial flow distribution,
and helical flow structure. Comparison with open-channel low flow
conditions is performed. An acoustic doppler current profiler (ADCP) is
used to measure flow from up to three meander bends, depending on the
year, in a small sandy meandering subarctic river (Pulmanki River)
during two consecutive ice-covered winters (2014 and 2015).

Under
ice, flow velocities and discharges were predominantly slower than
during the preceding autumn open-channel conditions. Velocity
distribution was almost opposite to theoretical expectations. Under ice,
velocities reduced when entering deeper water downstream of the apex in
each meander bend. When entering the next bend, velocities increased
again together with the shallower depths. The surface velocities were
predominantly greater than bottom/riverbed velocities during
open-channel flow. The situation was the opposite in ice-covered
conditions, and the maximum velocities occurred in the middle layers of
the water columns. High-velocity core (HVC) locations varied under ice
between consecutive cross-sections. Whereas in ice-free conditions the
HVC was located next to the inner bank at the upstream cross-sections,
the HVC moved towards the outer bank around the apex and again followed
the thalweg in the downstream cross-sections. Two stacked
counter-rotating helical flow cells occurred under ice around the apex
of symmetric and asymmetric bends: next to the outer bank, top- and
bottom-layer flows were towards the opposite direction to the middle
layer flow. In the following winter, no clear counter-rotating helical
flow cells occurred due to the shallower depths and frictional
disturbance by the ice cover. Most probably the flow depth was a
limiting factor for the ice-covered helical flow circulation, similarly,
the shallow depths hinder secondary flow in open-channel conditions.