ИСТИНА

The Saint Venant equations offer the mathematical base for the most 1D software aimed for non-steady simulation in various environments including tidal estuaries. It is usually accepted that a flow resistance under unsteady regime is approximately equal to a resistance of steady flow in a channel of the same geometry and roughness, and a friction slope could be expressed like a geometric slope in Chezy or Darcy–Weisbach equations. Variations of the resistance in unsteady flows are studied deficiently, they are assumed to be related with changing structure of turbulent currents, flow pulsation effects, streamwise energy gradients, and transformation of velocity vertical profiles. In order to investigate flow resistance and other hydrodynamic features of the reverse currents during the tidal cycle detailed field measurements were undertaken in small estuaries of the White Sea. The spring tide ranges from 2 m at mouths of the Kyanda and Tamitsa to 9 m at the Syomzha estuary and the tidal flow significantly exceed the river runoff at low water season. The method of field research was based on the simultaneous registration of water level by barometric loggers and discharge measurements by ADCP in cross-sections located in different distance from the mouth. The obtained data allowed the direct calculation of the momentum equation terms and evaluation of their changing during the tidal cycle. It was discovered that the hydraulic resistance varies in wide range and under some conditions even drop below the zero just after the slack water. This phenomenon could be linked with the nature of eddy viscosity and the mechanism of energy dissipation in turbulent flows as the case of reversed energy transfer from turbulent pulsations to the mean flow. The reported study was funded by RFBR, project number 19-35-90032.