Turbulent Viscosity and Flow Resistance in Tidal Estuaries

Abstract

Field hydrometric studies at the estuaries of the White Sea basin yielded data on some hydrodynamic features of reverse tidal currents. Among the mouth areas of tidal rivers studied in 2015–2022, the most interesting results were obtained at the mesotidal Kyanda estuary, flowing into Onega Bay, and at the macrotidal Syomzha estuary, flowing into Mezen estuary. The essence of the method used in the field studies is synchronic measurements of water flow by acoustic Doppler profilers and water levels by autonomous barometric recorders in two cross-sections, located at different distances from the river mouth, during an entire semidiurnal tidal cycle. The results of these measurements were used to evaluate the terms of Saint-Venant equation of motion and the roughness coefficients. It was found that in the tidal rivers, the flow resistance varies considerably during a tidal cycle. In periods of quasi-steady water flow in both directions during flood and ebb, the values of the Darcy–Weisbach friction factor are 0.04–0.07, as is typical for rivers with similar morphological channel pattern and characteristics. However, in several cases, in periods close to slack water, the friction factor took negative values. A possible explanation of this phenomenon is a negative turbulent viscosity, which manifests itself in some phases of the tidal cycle, when the energy of eddy formations can be transferred to the translational motion of the water mass.