FlexSemFlexSemAarhus University department of Ecoscience


A hydrodynamic model that computes a hydrostatic solution to the Navier-Stokes equations for the area around Unguja, Zanzibar. The computational mesh has 10,143 elements, 32 vertical layers and a total of 70,814 computational cells. The horizontal resolution varies from 250 meter around Stonetown, 500 m in the shallow coastal water aroung Unguja and up to 3 km near the open boundaries. The vertical resolution varies from 3 meter in the upper layers to 100 meters in the deepest layers.

A bathymetry for the model was constructed by combining a 10 meter resolved satellite based bathymetry of the area provided by DHI GRASS and the .25 degree resolved GEBCO 2019 bathymetry.

On the open boundaries the model is forced with temperature, salinity, water level and velocities obtained from the Copernicus Marine Environment Monitoring Service (CMEMS). Tide components obtained from the OCETide model was added to the 24 hours CMEMS means of water level and velocities. The model includes a surface radiation model which is forced by atmospheric wind velocities, air temperature, dew point temperature and cloud cover and a k-epsilon turbulence model.

To the right: computational mesh and the model bathymetry in color code.

Below: Particles has been inserted into the surface of the model and particle trajectories have been simulated by Lagrangian particle movement. The particle tracks illustrates how the tidal surface currents runs around Unguja and comes together in the shallow area around Stonetown.

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  • An artificial tracer has been added to the model in the surface at the port of Stonetown. The animation shows how the tracer is transported by currents and diluted in the open sea.
    Tracer animation

  • Surface velocities in the area around Stonetown. The colors show bathymetery.
    Velocity animation 1

  • Surface velocities. Colors show the water level. Notice how the tidal currents are changing in the shallow area between Unguja and the main land, whereas the large scale current on the deeper ocean on the east side of Unguja area more constant on this time scale.
    Velocity animation 2