Modelling tidal processes in the Persian Gulf : With a view on Renewable Energy

Abstract

The main semidiurnal (M2 and S2) and diurnal (K1 and O1) tidal constituents were simulated in the Persian Gulf (denoted PG). The topography of the PG was discretized on a spherical grid with a resolution of 30 seconds in both latitude and longitude. It included coastal areas prone to flooding. The model permitted flooding of drying banks up to 5 meters above mean sea level. At the open boundary, it was forced by 13 harmonic constituents extracted from a global tidal model. The Model results agreed well with tide gauge observations. Co-tidal charts and flow extremes were presented for each tidal constituent. The co-tidal charts showed two amphidromic points for semidiurnal and one for diurnal tidal constituents. Maximum tidal amplitudes were obtained in the north-western part of the PG, where coastal flooding prevails in wide areas. Strong tidal currents occurred in the PG but their locations are driven by different, dominant tidal constituents. Maximum velocities were found in shallow regions. Particularly high amplitudes of elevations and high velocities of currents were found in the canal between Qeshm Island and the mainland. Tidal rectification around Qeshm Island influenced hydrodynamics across the PG, as far as the coast of Saudi Arabia and the northern part of the PG. The results provided a good estimate of the annual kinetic energy output for the PG on the Iranian side, for the entrance of Musa estuary and Qeshm canal. Geological studies in the PG have revealed the existence of sub-seabed salt-domes. Suitable, high-pass filtering of the PG seabed topography revealed the signature of these domes on the seabed, i.e. numerous hills and valleys with amplitudes of several tens of meters and radii from a few up to tens of kilometers. It was suspected that the 'shark skin' of the PG seabed may affect the tidal residual flow. The interaction of tidal dynamics and these obstacles was investigated in a non-linear hydrodynamic numerical tidal model of the PG. First, the model was used to characterize flow patterns of residual currents generated by a tidal wave passing over symmetric, elongated and tilted obstacles. Thereafter, it was applied to the entire PG. The model was forced at its open boundary by the four dominant tidal constituents residing in the PG. Each tidal constituent was separately simulated. Tidal residual currents in the PG, as depicted by Lagrangian trajectories, revealed a stationary, eddy-rich flow. Each eddy can be matched with a topographic obstacle. This confirmed that the tidal residual flow field is strongly influenced by the nonlinear interaction of the tidal wave with the bottom relief which, in turn, is deformed by salt-domes beneath the seabed. Different areas of maximum residual current velocities were identified for each type of tidal constituent. Two main cyclonic gyres and several adjacent gyres rotating in opposite directions and a strong coastal current in the northern PG were identified. The non-linear hydrodynamic numerical tidal model was applied to investigate the tidal resonance in the PG. The model was used to characterize the amplified response of the basin to different forcing periods. It was forced by waves with the same amplitude but different periods ranging from 3.5 to 35.5 h with increments of 0.5 hours at its open boundary to the Oman Sea. Each wave was simulated separately. The results revealed that, in three areas, tidal elevations were significantly larger than the forcing amplitude, indicating resonance. These areas were the northern PG (maximum Amplification Factor (AF) = 1.61), the Strait of Hormuz (maximum AF = 3.00), and the southern PG (maximum AF = 1.33). The amplification factor (AF) is the ratio of local amplitudes vs. forcing amplitude. Diurnal tides were resonant in both the northern and southern PG. Semidiurnal constituents were amplified in the northern Gulf as well as in the Strait of Hormuz. A simulated sea level rise due to climate change of 1 meter increased the AF off the Arabian coast for semidiurnal tides from 0.72 to 1.21. In the Qeshm Canal in the Strait of Hormuz, AF increased from 1.36 to 1.57 for semidiurnal and from 0.73 to 1.1 for diurnal tides. The results provide a good estimate of the annual potential energy output for the PG on the Iranian side, for the Musa estuary and Qeshm canal. The tidal barotropic residual circulation in the Musa estuary was described by applying a non-linear, 3-d hydrodynamic numerical model. The main point was to consider the effect of evaporation on density-driven circulation and interactions with tidal currents. The Musa estuary is located in the north-west of the PG. The climate in this region is arid, resulting in an excess of evaporation over precipitation. There is no river runoff in this estuary. The results suggested that the structure of the residual circulation and stratification depended on the evaporation rate and, of course, the difference in tidal amplitude, i.e. neap and spring tide. As a result of evaporation, the residual circulation was vertically stratified, with a dense, saline, near-bottom flow toward the PG and relatively fresh inflow at the surface. But, in scenarios using tides, residual circulation occurred in the opposite direction. The results revealed a high salinity outflow and a lower salinity inflow but a stable salinity across the whole region. There was a horizontal gradient in tidally-averaged salinity with the salinity increasing towards the head of the Musa estuary, which is a salt producing, coastal system.