Investigating the Effect of Changing the Topography of Arvandroud River on the Flow Pattern and Water Level Fluctuation

Arvandroud is the most important and only navigable river in Iran. It is where the river flows into the sea and the flow velocity is drastically reduced, causing the deposition of suspended sediments and reducing the depth of the confluence of the river and the sea. This decrease in depth at the entrance of the Arvandroud is such that it has restricted the entry of high-capacity vessels, and its dredging seems necessary to supply water to vessels on this international waterway. In the present study, the effect of dredging the Arvand estuary and also constructing an earthen pier in the Arvandkenar position on the physical parameters of flow velocity and water level fluctuation has been simulated.

For simulation, Mike 21.3's three-dimensional hydrodynamic model was used in two dimensions. This model performs based on the irregular triangular meshing approach and the main computing platform of the model is the HD module, which is based on the numerical solution of 2 and 3-dimensional Navier-Stokes average equations for the non-compressible state, taking into account Bozinsky and hydrostatic pressure. Equations of continuity, momentum, temperature, salinity and density are also considered. In the initial settings, the initial conditions of the model, consisting of temperature and salinity, are uniform for the whole field at 25 °C and 32 psu, respectively. Initially, the water level and flow velocity in the whole basin were considered zero. Due to the small width of the river, the wind effect has been ignored. The model has a sea boundary downstream and a river boundary upstream. Temperature and salinity at the sea border were applied by nesting method from the mother model of the Persian Gulf to the daughter model. The time series of water level fluctuation at the sea border was constructed from the four main components of the tide (M2, S2, O1, K1) related to the SHATT AL ARAB OUTER BAR and applied to the model. At the upper boundary of the river, the temperature and salinity were constant, and 26 ˚C and 2.39 psu were adjusted, respectively. At this limit, the water level fluctuation was adjusted so that the net discharge of the river was 600 m3/s.

In the validation stage, it was found that the result of the root mean square error test for water level fluctuation in Faw was 0.16 m, indicating good accuracy of the model results compared to field measurements. The results showed that if the Arvand mouth is dredged to reach a depth of 8 m and a width of approximately 300 m, the water level after dropping will drop to a maximum of 10 cm near the mouth, and the flow velocity in the dredged area will decrease. In addition, if 33% of the river section in Arvandkenar region is blocked, the difference in water level on both sides of the gorge will be up to 13 cm, and the maximum tidal velocity in the middle of the gorge will reach 0.57 m/s from 0.87 m/s in normal condition.

Dredging of the Arvandroud estuary reduces the flow rate in the dredging canal and this in turn increases sedimentation in the dredging canal. However, reducing the width of the river can compensate for this slowdown and prevent sedimentation. These two methods should be used in combination to reduce the maintenance costs of the channel.