Estuarine hydrodynamics in arid climates – a case study of the Persian Gulf and its small estuaries

Abstract

Unlike positive estuaries, low-inflow estuaries such as inverse and salt-plug estuaries have received less scientific attention. This thesis aims to investigate the hydrodynamic aspects of these remarkable estuarine systems by example of the Persian Gulf (PG), the Mond River Estuary (MRE), and an idealized funnel-shaped estuary using observations and numerical modeling. The PG region, characterized by an arid climate, represents an extraordinary environment for studying estuarine hydrodynamics. Classified as an inverse estuary due to higher evaporation than precipitation and river discharges, the PG provides a valuable case study for understanding the dynamics of low-inflow estuaries. The system is studied with a particular focus on the influence of tidal forcing on the exchange flows and salinity. Numerical experiments indicate that tidal forcing significantly increases the annual inflow-outflow rates at the Strait of Hormuz (SOH) to approximately ±0.54 Sv, which is more than double previous estimates. Tides enhance PG salinity through (i) intense landward (entering the PG) salt flux by tidal advection, and (ii) strong tidal mixing in the PG, especially at the SOH. On the northwestern and southern shallow banks of the PG, high salinity values imposed by tides coincide with peak values of the tidal Froude number (FrT) and the formation of amphidromic points in these coastal areas. The MRE, located on the northern coast of the PG, exhibits a salt-plug estuarine system during warm months, representing another type of low-inflow estuaries. In fact, the MRE transitions from a positive estuary in wet months to inverse and salt-plug estuaries during dry months. Also, the MRE exhibits a highly unsteady estuarine state following heavy rains. The unsteady state is characterized by the formation of a relatively freshwater lens between two high-salinity water masses: one from the PG and another from the hypersaline runoff passing saline soils upstream of the estuary (freshwater lens estuary). The Péclet Number (Pe), calculated from observed salt flux terms, indicates a predominance of strong advective transport imposed by hypersaline runoff over diffusive transport throughout the MRE. Additionally, numerical modeling of the MRE indicates that a sea-level rise of 20 cm can result in an increase in salinity within the freshwater lens and positive estuaries, in contrast to a salinity decrease in inverse and salt-plug estuaries. Furthermore, a three-dimensional hydrodynamic model is employed to compare various scenarios under different tidal conditions in an idealized funnel-shaped salt-plug estuary. The analysis reveals that when log(FrT) > 3, tidal forcing overcomes the density gradient, reinforcing the salinity maximum zone with a landward displacement. In this idealized estuary, the model demonstrates the development of longitudinal flows in two cells of positive and inverse estuarine circulations, detached by a salinity maximum zone. Meanwhile, two cells of positive estuaries are separated by a mixing maximum zone in the idealized freshwater lens estuary. The results of these studies provide valuable foundations for understanding biogeochemical processes and sediment dynamics in low-inflow estuarine systems, which can support sustainable development considering water quality and climate change impacts worldwide.