Interactions of Tides, Storm Surge, and River Flow in the Microtidal Neretva River Estuary

Estuaries and tidal rivers are highly dynamic transitional zones where marine and riverine processes interact, creating complex hydrodynamic environments. These regions are influenced by natural phenomena such as tidal oscillations, storm surges, and river flow, as well as human activities like water management, hydropower operations, flood protection, and navigation. Effective management of these environments relies on understanding and predicting their hydrodynamic behavior, particularly under extreme conditions such as flooding or abrupt water level changes.

This study examines the microtidal Neretva River estuary in Croatia to investigate the interactions between tides, storm surges, and river discharge, and their impacts on water level variability. A modified non-stationary harmonic analysis, based on the NS_Tide model, was developed specifically for microtidal conditions. This model incorporates storm surge and river discharge, improving the predictive accuracy of water levels along the estuary, from tide-dominated downstream sections to discharge-influenced upstream areas. The new version of NS_Tide also allows for a more detailed decomposition of total water levels and tide-surge-river interactions.

The results reveal that river discharge is the primary factor influencing water levels at most stations, while the impact of storm surge decreases upstream. Tide-river interactions were observed throughout the study area, whereas tide-surge interactions had minimal effects. The analysis showed that high-frequency discharge fluctuations caused by hydropower operations amplify the S₁ tidal constituent in upstream river sections. These fluctuations also modulate the amplitudes of other tidal constituents in estuarine and tidal river regions, highlighting the complex influence of human activities on tidal dynamics.

The proposed non-stationary harmonic model proved highly effective for the microtidal Neretva River, capturing the complex interactions between tidal and non-tidal forces under various conditions. Its adaptability to local conditions suggests it could also be applied to mesotidal and macrotidal systems, offering a practical tool for managing estuaries and tidal rivers across diverse environments.