Key aspects of seawater intrusion in the Dniester River during storm surges

Introduction. This article explores the potential for saline water from the Dniester Estuary to travel upstream into the mouth of the Dniester River, a critical area where the “Dniester” station supplies potable water to Odesa City and where the intake point of the Lower Dniester Irrigation System is located. The study is urgent due to the risk that saline water poses to the quality of drinking and irrigation water at these intake points. The primary objective of this research was to utilize hydrodynamic modeling to determine the specific hydro-meteorological conditions under which saline, transformed sea water from the estuary could penetrate the mouth of the Dniester River. Material and methods. To address this task, a simplified version (without considering the thermal factor) of the numerical 3-D non-stationary hydrodynamic model MECCA (Model for Estuarine and Coastal Circulation Assessment), supplemented with a block for the transport of a conservative tracer, was used. The input data for the model included observations of wind condition variability and corresponding sea level fluctuations at the marine boundary of the Dniester Estuary, as well as observations of water level fluctuations in the mouth section of the Dniester River. Results and Discussion. The findings of the study identified two key conditions necessary for such an event: (1) a significant reduction in the average daily river discharge to below 100 m³/sec, and (2) the occurrence of a strong surge wind from the south or southeast at speeds exceeding 15 m/sec, sustained for several consecutive days. Under these conditions, transformed sea water from the estuary can travel upstream through the right arm of the river, the Glybokyi Turunchuk. From there, it reaches the point where the Dniester River divides into two arms. The saline water is then drawn into the left arm, the Dniester, and eventually returns to the estuary. This process represents a significant threat to the region’s freshwater resources, especially during periods of low river discharge and adverse wind conditions. The penetration of brackish estuarine waters into the Dniester River’s mouth branches, namely the Dniester and Hlybokyi Turunchuk branches, occurs over some time of 12 to 24 hours. It has been concluded that due to the presence of two mouth branches through which the Dniester River flows into the Dniester Estuary, considering their location in the northeastern part of the estuary and the characteristics of water level rises in the estuary during wind surges, the penetration of estuarine waters with increased salinity (up to 7 ‰) into the main channel of the Dniester River (above the point where the river splits into two mouth branches) is unlikely, even when river discharge falls below 100 m³/s. Conclusion. The case study presented in the article can serve as a valuable reference for experts tasked with designing hydro-engineering solutions. One proposed solution is the construction of a second estuarine canal (branch) to prevent the intrusion of transformed sea water into the river mouth, thereby safeguarding the quality of drinking and irrigation water in the region.