B023-01
Variable Physical Drivers of Near-Surface Turbulence in a Regulated River
Tuesday, 8 December 2020: 07:00
Virtual
Sofya Guseva1, Mika Aurela2, Alicia Cortés3, Rigel Kivi2, Eliisa Selina Lotsari4, Sally MacIntyre5, Ivan Mammarella6, Anne Ojala7, Victor Stepanenko8, Petteri Uotila9, Aki Vähä7, Timo Vesala6, Marcus Wallin10 and Andreas Lorke11, (1)University Koblenz-Landau, Landau, Germany, (2)Finnish Meteorological Institute, Helsinki, Finland, (3)University of California, Santa Barbara, Santa Barbara, United States, (4)University of Eastern Finland, Joensuu, Finland, (5)Univ California Santa Barbara, Marine Science Institute, Santa Barbara, CA, United States, (6)University of Helsinki, Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, Helsinki, Finland, (7)University of Helsinki, Helsinki, Finland, (8)Moscow State University, Moscow, Russia, (9)INAR/Physics, University of Helsinki, Helsinki, Finland, (10)Uppsala University, Uppsala, Sweden, (11)University of Koblenz-Landau, Landau, Germany
Abstract:
Rivers and streams are an important source of the greenhouse gases CO
2 and CH
4 to the atmosphere [Raymond et al., 2013]. Besides dissolved gas concentration, a major controlling parameter of the gas exchange across the air-water interface is the transfer velocity (
k), which is mainly driven by near-surface turbulence in the water. The commonly used surface renewal and film model describe the dependence of the transfer velocity on the dissipation rate of turbulent kinetic energy near the water surface [Lamont & Scott, 1970]. While in lakes and reservoirs near-surface turbulence is mainly driven by atmospheric forcing, including wind shear, convective cooling, and surface wave-breaking, in shallow rivers and streams it is mainly generated by bottom friction. Knowledge about the main physical drivers and the resulting spatial and temporal variability of the gas transfer velocity in large rivers is rather limited. Conceptual models suggested a transition from the dominance of the flow velocity for rivers with smaller channels to wind control for estuaries and large rivers [Alin et al., 2011].
In this study, we aimed to identify the key drivers of near-surface turbulence using a comprehensive dataset from a field campaign conducted in a large regulated river (Kitinen River, 67.3665º N, 26.6230º E) in Finland. Continuous measurements of flow velocity, water temperature, and meteorological forcing were conducted throughout the ice-free season (June – Sep. 2018). We compared observed dissipation rates against commonly applied scaling relations in terms of bulk parameters (wind speed and flow velocity) and a one-dimensional numerical turbulence model (k-ε). Our results revealed the comparable contribution of wind and bed shear to near-surface turbulence during the season. Predicted dissipation rates using the scaling relations had a good agreement with observations (Figure 1). Based on our findings, we provide a mechanistic concept for the relative importance of wind forcing and river flow for near-surface turbulence as a function of flow depth. One-dimensional k-ε model may be considered as a universal tool for quantification of vertical transport of water constituents from bottom to surface.
Caption to Figure 1. Predicted dissipation rates of turbulent kinetic energy ε versus observed values εADV at 0.4 m water depth.
