Modelling the variability of hydrological parameters of the Sasyk reservoir impoundment under various options of its operation

Abstract

This paper highlights the results of adaptation and verification of the Delft3D Flexible Mesh numerical model under the conditions of the Sasyk reservoir. The objective of this work is to evaluate the expected spatio-temporal variability of water salinity in the Sasyk reservoir after completion of the Sasyk renaturalization project by means of establishing a constant water exchange with the sea through the artificial channel.
 The Sasyk Lagoon was separated from the sea and transformed into a freshwater reservoir in the late 1970s. However, due to the poor water quality in the Sasyk reservoir in the modern period, the solution of the problem of transforming the reservoir to its original coastal conditions, i.e. the renaturalization, has acquired particular importance.
 Model results, obtained under conditions of 2019, indicate the possibility of using a hydrodynamic model to evaluate the effectiveness and possible consequences of various scenario-based decisions to be implemented for the management of the hydrological regime of the reservoir under various options of its operation.
 The model runs were used to evaluate the flushing time of the Sasyk Lagoon and the pattern of the salinity fluctuations in the reservoir after the restoration of the artificial channel in the sand bar with a width of 100 m and a depth of 1.5 m and the establishment of the constant “sea-lagoon” interconnection. Applying the hydrometeorological conditions of 2019, a scenario-based modelling for two consecutive years was performed. For the first year of the simulation, a variant of water management under the absence of freshwater inflow from the Danube and the presence of sea water inflow throughout the year was considered. The model results at the end of the first year of the simulation were set as the initial conditions for the second year, and the rest of the external forcing remained unchanged. Additionally, the variant considering the Danube discharge into the lagoon during May-July for the second year of simulation was investigated.
 Based on the simulation results, it was found that in the case of a constant water exchange with the sea through the connecting “sea-lagoon” channel (with the abovementioned morphometric characteristics) and under the absence of the Danube freshwater inflow throughout the year: 1) the flushing time of the lagoon will be 1 year and 5 months; 2) stabilization of water salinity in the lagoon will not occur, i.e. the salinization of the lagoon water takes place in the long-term perspective; 3) the time period from the moment of initiating the water exchange with the sea, during which the lagoon reaches the water salinity limit of 7-8 ‰, that is critical for the existence of freshwater species of flora and fauna, is expected to amount to 4 months for the southern part and to 5.5 months for the northern part of the lagoon; after which the formation of the marine ecosystem of the reservoir will begin.
 It is shown that the stabilization of water salinity in the lagoon in the second year of the simulation can be achieved under conditions of ensuring the Danubian freshwater discharge in the period of significantly higher water level in the Danube River, compared to the lagoon water level (May-July 2019).