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Volume 12, issue 2 | Copyright
Ocean Sci., 12, 417-432, 2016
https://doi.org/10.5194/os-12-417-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 10 Mar 2016

Research article | 10 Mar 2016

River bulge evolution and dynamics in a non-tidal sea – Daugava River plume in the Gulf of Riga, Baltic Sea

Edith Soosaar, Ilja Maljutenko, Rivo Uiboupin, Maris Skudra, and Urmas Raudsepp Edith Soosaar et al.
  • Marine Systems Institute at Tallinn University of Technology, Tallinn, Estonia

Abstract. Satellite remote sensing imagery and numerical modelling were used for the study of river bulge evolution and dynamics in a non-tidal sea, the Gulf of Riga (GoR) in the Baltic Sea. Total suspended matter (TSM) images showed a clearly formed anti-cyclonically rotating river bulge from Daugava River discharge during the studied low wind period. In about 7–8 days the bulge grew up to 20km in diameter, before being diluted. A high-resolution (horizontal grid step of 125m) General Estuarine Transport Model (GETM) was used for detailed description of the development of the river plume in the southern GoR over the period when satellite images were acquired. In the model simulation, the bulge growth rate was estimated as rb ∼ t0.5 ± 0.04 (R2 = 0.90). Both the model simulation and the satellite images showed that river water was mainly contained in the bulge and there were numerous intrusions at the outer perimeter of the bulge. We performed numerical sensitivity tests with actual bathymetry and measured river runoff without wind forcing (1) having an initial three-dimensional density distribution, and (2) using initially a homogeneous ambient density field. In the first case, the anti-cyclonic bulge did not develop within the course of the model simulation and the coastal current was kept offshore due to ambient density-driven circulation. In the second case, the river plume developed steadily into an anti-cyclonically recirculating bulge, with rb ∼ t0.28 ± 0.01 (R2 = 0.98), and a coastal current. Additional simulations with constant cross-shore and alongshore winds showed a significant effect of the wind in the evolution of the river bulge, even if the wind speed was moderate (3–4ms−1). While previous studies conclude that the mid-field bulge region is governed by a balance between centrifugal, Coriolis and pressure gradient terms, our study showed that geostrophic balance is valid for the entire mid-field of the bulge, except during the 1–1.5 rotation period at the beginning of the bulge formation. In addition, while there is discharge into the homogenous GoR in the case of a high inflow Rossby number, the river inflow might split into two jets, with strong mixing zone in-between, in the plume near-field region.

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Remote sensing imagery and numerical model study of river bulge evolution and dynamics in a non-tidal sea showed an anti-cyclonically rotating bulge during the studied low wind period in the Gulf of Riga. In about 7–8 days the bulge grew up to 20 km in diameter, before being diluted. Both model and satellite images showed river water mainly contained in the bulge. The study shows significant effects of the wind in the evolution of the river bulge, even if the wind speed was moderate (3–4 m s−1).
Remote sensing imagery and numerical model study of river bulge evolution and dynamics in a...
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