| The study describes the influences of floating level ice-covers on flow and bedload transport in dune-bed channels, and proposes a semi-empirical method for calculating flow resistance in such channels. Heretofore, cover influence was poorly understood, with the consequence that engineers lacked a satisfactory method for calculating resistance in ice-covered alluvial channels.; Two series of laboratory experiments were performed to determine the influence of floating covers. The experiments were designed and interpreted in accordance with a set of key parameters established by means of dimensional analysis of ice-covered alluvial flow variables. The parameters, supported by results from the experiments presented here and in prior studies, enabled development of the semi-empirical calculation procedure mentioned above. In one series, the bed was loose and could deform in relation to the flow. In the other, the bed was fixed to facilitate detailed measurements of the flow velocity field over dunes without and with a cover. The loose-bed experiments provide data on the ultimate influence of ice-cover on flow depth, bulk flow velocity, bed roughness, dune geometry, and bedload transport rate; the fixed-bed experiments reveal the initial flow response to ice-cover presence. The principal findings are that ice-cover increases flow depth, decreases bulk flow velocity, significantly decreases bedload transport rate and dune steepness, and increases dune length. These changes are accomplished not only through a reduction of bed shear stress, but also a reduction of Reynolds stress and turbulent kinetic energy over the dune-bed. Sediment transport relations developed from open-water data are applicable to ice-covered dune-bed flows, provided they are supplied with actual bed shear stress.; The measurements of velocity field over a dune showed that cover presence and increasing roughness redistribute the flow over a vertical section, diminishing flow speed near the cover, but, by deflecting more flow to the bed, maintain near-bed velocities during the initial stages of bed adjustment to ice-cover presence. |
