Basrah province southern Iraq is a tidal stream and it represents the main feeder of water to Basrah province. The river is characterized by having many branches during its course from upstream to downstream. The main aim of this study is to model a 2D hydrodynamic model for Shatt Al-Arab River with seven branches inside Basrah city which are Jubyla, Muftya, Robat, Khandek, Ashar, Al-Khora, and Saraji in addition to Karmat Ali river by the Hydrologic Engineering Centre's River Analysis System (HEC-RAS) software, where most of the previous studies relied on the consideration of the river as one-dimensional, i.e., neglecting the transverse or vertical directions. Accurate input data like Digital Elevation Models (DEMs) were provided and enhanced with the help of Geographic Information Systems (GIS) as well as a data for the year 2014 was used as boundary conditions to develop the hydrodynamic model. The discharge values at the Qurnah station were extracted from the results of a previous one-dimensional HEC-RAS mathematical model. To prove the efficiency of the model, mean absolute error (MAE), root mean squared error (RMSE), and Nash-Sutcliffe efficiency (NSE) were implemented to check the convergence between observed and simulated data through calibration and validation processes. The result of NSE was 0.836, and this indicates an acceptable convergence between the simulated and observed values. Also, the methods of RMSE and MAE support that convergence to zero. It is worth noting here that the NSE method was more sensitive to the change in the resulting values and therefore it can be recommended to use this method in hydrodynamic models. The hydrodynamic model can provide important information for models of sediment transport or water quality. Therefore, it is essential to have a good understanding of the hydrodynamic processes in a water system, before embarking on studies of sediment transport, or water quality.

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The hydrodynamic model is essential for building a water quality model for rivers, lakes, estuaries, and other water systems. Most model software, such as HEC-RAS, can perform a complex hydrodynamic surface water body and limitations to represent water quality for the corresponding area. In contrast, other models, like WASP, can simulate a wide range of contaminants in a multidimensional geometry of rivers, estuaries, lakes, and reservoirs. Still, it requires flow information from separate hydrodynamic models. This article aims to develop a comprehensive water quality model of the Shatt Al Arab River south of Iraq by linking HEC-RAS with WASP. A variety of software techniques has sequentially been used. This software includes GIS for DEM modification, HEC-RAS for the hydrodynamic model, Python code with PyCharm to run the external coupler, WASP software for advective and dispersive contaminant transport, and finally, WRDB software for full calibration process and results display. The results showed successful transportation of flow information had been achieved. Moreover, the article described an effective calibration process by plotting comparison graphs and statistical summaries to make the appropriate decision. Another goal of this work is to collect the equations and associated reaction rates of source/sink kinetic for eutrophication’s state variables.