3D modeling of coastal hydrodynamics becomes important when dealing with stratified flows or when wind blows over shallow water basins. Wind driven currents are created by the wind exerting stress on the sea surface. They enhance vertical mixing and are often responsible for water quality of shallow water bodies. Figure 1 shows the July 17, 2000 Landsat image of Chabahar and Pozm bays with emphasis on band 2 color over the water. The yellow color in this figure is likely an indication of suspended matter near the water surface and represents a snap shot of a bay-wide circulation created by winds and tides. Winds at the time of imagery were typical of the winds in monsoon season and blew mostly from SSE to SE directions. In Figure 1, there seems to be an outgoing flow towards SE direction in response to the above wind field. Outside of the bay, the suspended plume is transported to the east.
Figure 1 Landsat Image of Chabahar Area with Emphasis on Color Band 2
Three-dimensional numerical modeling is required to determine the full spatial picture of such a circulation and its sensitivity to environmental factors such as tides and winds.
MISED 3D Model
The Baird in-house 3D numerical model MISED was used in this study. MISED is a three-dimensional finite element model to simulate tidal flow, temperature, salinity, sediment transport, morphology and water quality in rivers, estuaries, and coastal and open sea areas. The model utilizes a new numerical method that is efficient and unconditionally stable and allows using much large time steps. It can be applied to simulate tidal circulation in large areas, wind driven current, stratified flow, sediment erosion and deposition, thermal plume and effluent dispersion.
The bathymetry of the model domain was developed using recent surveys of the entire bay completed by Iranian National Cartographic Center (NCC) and Darya-Tarsim for the present project, as well as existing charts of the Oman Sea for the area outside of the bay. The data were merged together in GIS and were all referenced to the datum at SB Port. The offshore boundary was set at 50 m depth far enough from the bay entrance to minimize boundary effects on the project area. The entire model domain is shown in Figure 2.
Figure 2 Entire Model Domain Grid Cells Overlaid on Bathymetry
MISED simulations were completed for the months of February and March 2007 and the results were compared with the measurements. Figure 3 is an example snapshot of surface velocity vectors during rising tide on February 20. Figure 4 shows snapshot of surface velocity vectors during lowering (ebb) tide on February 17. Winds were insignificant in both cases and tidal currents are predominant. Tidal currents outside Chabahar Bay during this falling tide event were from west to east. There is no clear corresponding tidal current direction during the rising tide event.
Figure 5 shows comparison of predicted and measured velocity vectors (quiver plots) during the neap tide from February 10 to 14, 2007 at AW1. The sensor was located at the east side of the entrance to the bay near the SB Port breakwater. In this figure the top panel shows variation of water level as measured by the pressure sensor of AW1. The water depth ranged between 11 and 13 m in this period. The second panel shows the wind velocity vectors measured on the top of the restaurant building in SB Port. The remaining 3 panels show comparison of measured (red) and calculated (black) velocity vectors near the surface, at the mid depth and near the bottom. A very good agreement in both magnitude and direction of velocities is observed. Wind-driven currents have clearly enhanced the outgoing ebb tide current in the morning of February 11 when the wind blew from NW to SE.
Figure 3 Example of Calculated Surface Velocity Vectors at Flood (rising) Tide with Offshore Open Boundary Condition
Figure 4 Example of Calculated Surface Velocity Vectors at Ebb (lowering) Tide with Offshore Open Boundary Condition
Figure 5 Velocity Comparison Quiver Plots at AW1 for February 10 to 15, 2007
A Lagrangian Particle tracking Model (LPTM) was employed to simulate the fate of buoyant particles in Chabahar Bay under wind and tidal actions. Simulations of particle movements were completed using the MISED results for the periods of 10 to 25 of February and 5 to 25 of March, 2007. Particles were released at the water surface at all grid points over the entire calculation domain. For the month of February, particle tracking was completed with MISED results with and without winds (tides only) as input driving force.
Figure 6 shows the final distribution of the released particles under the action of tides, while Figure 7 presents the corresponding results when both winds and tides are considered. From Figure 6 there seems to be a net flux of particles under the tides outside of the bay over the simulation period. The direction of the net movement is from west to east. Inside of the bay, however, the particles do not show any considerable redistribution compared to their original positions. Figure 12, on the other hand, shows the final distribution of particles when winds (and wind-driven currents) are also taken into account. Winds in general have worked to push the particles out of the bay over the simulation period. The particle distribution pattern in Figure 12 is very similar to the pattern observed in the satellite image of Figure 1.
It was noted that winds were mostly from SE direction between Feb 19 and 26. when SE winds last long enough, suspended buoyant particles will be transported away from the NW Lagoon and central bay areas to outside of the bay.
Particle tracking results completed for the month of March with winds and tides are shown in Figure 8. There is a net flux of particles outside of the bay over the simulation period. The direction of the net movement is different from the previous month and is from east to west. This indicates monthly variation of tides in Chabahar area. During March 2007, winds were equally from southwest and southeast directions and worked to push the particles towards the west and out of the bay over the simulation period.
Figure 6 Final Distribution of Particles Under Tides (Feb 2007)
Figure 7 Final Distribution of Particles under Tides and Wind-Driven Currents (Feb 2007)
Figure 8 Final Distribution of Particles under Tides and Wind-Driven Currents (March 2007)
MISED 3D hydrodynamic numerical modeling was completed for Chabahar Bay for February and March 2007 and the results were compared with the measurements at several locations with satisfactory agreement. Hydrodynamics of Chabahar Bay was found to be generally complicated and dominated by tidal and wind-driven currents. For the present simulations, uniform winds were applied over the entire calculation domain. More accurate simulation of wind-driven currents requires application of a spatially variable wind field over the bay.
Particle tracking simulations showed that wind-driven currents are responsible for carrying suspended material out of the bay. Winds at Chabahar are mostly from SW to SE directions. Strong winds from these directions create near-bottom currents inside the bay that tend to carry suspended particles from NW and NE sides of the bay towards central bay area and from there gradually to outside of the bay. The combination of winds and tides, therefore, have a very important flushing function for water quality of Chabahar Bay.