OPTIMIZING THE MATHEMATICAL FORMULATION FACTORS FOR SIMULATING THE MORPHOLOGICAL CHANGES IN THE NILE RIVER, USING DELFT3D NUMERICAL MODEL
Creators: name: Ahmed, Sherif
Publisher: name: figshare
Year: 2024
Subjects: classificationCode: 400513
subject: Water resources engineering

description: <b>Abstract</b><br>The Nile River, a crucial source of fresh water in Egypt, provides approximately 55.5 billion cubic meters annually. This water is essential for various activities, including drinking, irrigation, power production, and inland navigation. These activities, in turn, influence the river's flow characteristics and often necessitate alterations to the riverbank geometry, thereby impacting the river's morphological changes. Conversely, the local changes in the riverbed level and the overall morphological changes can affect the sustainability and operational efficiency of these activities.Given the above, it becomes crucial to simulate and predict the morphological changes in the river to ensure the sustainability and efficiency of the activities that rely on the river. In recent times, numerous numerical models have emerged to simulate river hydrodynamics and sediment transport, thereby predicting morphological changes. For instance, models like Delft3D provide a range of sediment transport formulae for this purpose.<br>The development of sediment transport formulae is a complex process, relying on empirical data from laboratory and/or field measurements. However, it's important to note that no single formula can be universally applicable for predicting sediment transport in all rivers. Each formula is unique, with a multiplication factor that fine-tunes its sediment transport rates in a specific regime.In this research, we have undertaken a comprehensive investigation of five sediment transport formulae to assess their reliability in simulating morphological changes in the Nile River. The five formulae under scrutiny are Meyer-Peter-Muller (1948), Engelund-Hansen (1967), Van Rijn (1984), Van Rijn (1993), and Wilcock–Crowe (2003). Each formula was calibrated to optimize its performance in predicting morphological changes.<br>The Nile River downstream of the High Aswan Dam (HAD) is divided into four reaches with different flow conditions. However, the sediment transport process for the four reaches can be described mathematically by one formula, as these reaches are not disconnected and share similar sediment characteristics. Accordingly, two case studies in two different reaches (the second and fourth) have been used to investigate the most reliable formula for describing the sediment transport process.Also, river hydrograph schematization has been investigated in two different forms. Each representative form consisted of 12 flow conditions for the 12 months of the year. First, the 12 flow conditions were obtained from the flow duration curve resulting from historical data and then arranged in the form of the actual river hydrograph. In the other form, the 12 flow conditions represented the monthly average flow calculated from measured data throughout the simulation. For both forms, the corresponding water levels were calculated based on the historical relationship between the water level and the river flow in each study area.<br>The representation of sediment characteristics has been investigated in different forms offered in Delft3D, such as uniform mean sediment diameter (D50) with Piecewise log-uniform distribution, D50 and geometric standard deviation (Sg) with lognormal distribution, or D50 and D90 with lognormal distribution.The model results have been analyzed by the percentage of the rightly predicted morphological changes, representing the model's accuracy in predicting erosion and sedimentation areas. In addition, the Brier Skill Score (BSS) was used to show the accuracy of the prediction of riverbed changes.<br>The comparison between different model formulations has been conducted using an evaluation index calculated as the average percentage of the rightly predicted morphological changes and the BSS. The comparison showed that the formula of Meyer-Peter-Muller (1948) got the highest evaluation index and was considered the most reliable formula for both case studies.