The impacts of near-bed flow characteristics on sediment transport under varying ice-cover conditions of a seasonally frozen river

Abstract

<p>The global climatic change has been projected to affect hydrology, ice-covered flow period and river morphology, including changed sediment transport conditions, in northern high-latitude regions. For understanding the impacts of expected shortening of frozen period on sediment transport, it is needed to understand the present sediment transport conditions in high-latitude northern rivers with annually occurring ice cover. Thus, the aims of this paper are 1) to define the impacts of ice-cover on near-bed flow characteristics during hydrologically varying years, and 2) to analyze the impacts of these mid-winter flow-characteristics on sediment transport potential. The analyzes are based on Acoustic Doppler Current Profiler (ADCP: 2016–21) and Acoustic Doppler Velocimeter (ADV: 2020-21) measurements performed in mid-winter ice-covered conditions of sandy and small (circa 20 m wide) Pulmanki River, Northern Finnish Lapland.</p><p> </p><p>Despite the ice-covered river conditions, there is sediment transport throughout the year. The critical shear velocities of mid-winter conditions were exceeded each winter (2016-21), bedload transport occurred according to bedload measurements, and calculated bed capture rates (Gaeuman and Jacobson [2006] method) indicate transport. Three different situations occurred regarding the sediment transport and near-bed velocity conditions: 1) high measured mid-winter discharges indicate high velocities throughout the meander bend; 2) low measured mid-winter discharges inidate low near-bed velocities throughout the meander bend; 3) winters having intermediate discharges indicate near-bed velocities and sediment transport potential being higher at the upstream inlet and apex sections of the meander bend, but clearly lower downstream of the apex. The confinement by the river ice cover, i.e. bottom-fast ice, explains the velocity variation. The near-bed velocities were the highest at the upstream inlet section of a symmetrical meander bend, where the measurement cross-sections were narrower and shallower. The velocities were the lowest downstream of the apex, where the channel changed from relatively narrow to wider and deeper.</p>