Numerical Analysis of Bank Roughness Impact on Sediment Dynamics in the Medjerda River, Tunisia

[Audio] Hello. I'm Hammami Saber, Phd student in University of Orléans. Today, I will discuss the numerical analysis of the impact of bank roughness on sediment dynamics in the Medjerda River, Tunisia. This study was conducted in collaboration with several institutions and is being presented as part of this conference..

[Audio] Here are the main points we will cover today: an introduction to set the context, a description of the study area, the modeling scheme, the methodology used, the results obtained, and finally, the conclusions..

[Audio] Vegetation plays a crucial role in fluvial sediment transport. It alters flow patterns, reduces erosion, stabilizes riverbanks, and can also increase flood risks and impact hydraulic structures. These challenges underline the importance of this study..

[Audio] To address the flooding problem in the Medjerda River, dredging works were carried out to clear vegetation obstructing the river sections. However, these works also increased sediment transport, contributing to the siltation of the Sidi Salem Dam and reducing its storage capacity..

[Audio] The main objective of this study is to assess the impact of bank roughness modification on sediment transport in the Medjerda River following dredging works. We used the Telemac-Sisyphe model on a regularly dredged section of 17.8 kilometers upstream of the Sidi Salem Dam. Four simulation scenarios were defined, keeping the riverbed smooth and gradually increasing the bank roughness. These scenarios were repeated for two constant discharges of 100 m³/s and 200 m³/s. The output data include water levels and bed evolution..

[Audio] The study area is located in the alluvial plain of the upper Medjerda Valley, 20 kilometers upstream of the Sidi Salem Dam. This section, 17.8 kilometers long, undergoes regular dredging works to mitigate flooding risks..

[Audio] The longitudinal profile analysis of the studied section of the Medjerda provides, highlighting a notable change in slope near the old bridge of Boussalem, where a slight increase from 0.24% to 0.36% is identified. Sandbars have also been observed near the Boussalem bridge..

[Audio] To model this area, we used a finite element mesh generated from a LIDAR survey. The grid consists of 54,279 nodes and 108,211 elements, with resolutions tailored to capture details of the minor bed and riverbanks..

[Audio] The sediment sampling was chosen based on the grain size analysis of sediment particles settled in the reservoir of the Sidi Salem Dam. Thus, the bed of the Medjerda is predominantly composed of fine, silty to clayey sediments..

[Audio] The configuration of this section contains two distinct parts: the first part, located at the riverbed, is entirely smooth with a roughness coefficient of 0.03 s/m⅓. The second part, situated on both sides of the banks, exhibits variable roughness depending on the pre-existing vegetation cover. Four simulation scenarios were established, keeping the riverbed smooth and gradually increasing the roughness of the second part. These four scenarios are replicated for two constant discharge values of 100 m³/s and 200 m³/s, chosen as open boundaries at the model inlet. The output data is assigned to height data corresponding to the rating curve of the last section..

[Audio] we kept the riverbed smooth and gradually increased the roughness on both banks. Values of 0.03 s/m⅓, 0.05 s/m⅓, 0.07 s/m⅓, and 0.09 s/m⅓ were respectively assigned to vegetated area..

[Audio] Let's go to the results part. Figure illustrates the velocity evolution along the longitudinal profile of the flow section in the first scenario, where the riverbed is considered completely smooth. This scenario reflects the immediate reality on the ground after the completion of dredging works..

[Audio] Figure also presents the calculated shear stress values for both discharges and at each section. Shear stress, representing the force per unit area acting parallel to the flow direction, influences the movement or retention of sediments on the riverbed. Despite the observed fluctuations due to different factors governing the flow, there is a clear increase in shear stress, with an average of 2.3 N/m² with the increase in discharge..

[Audio] The figures present a detailed analysis of the longitudinal evolution of the riverbed in response to variations in bank roughness under different scenarios and for the two distinct imposed flow rates. For 100m³/s flow, the maximum deposition decreases to values of 141cm, 60cm, 15cm, and 8cm, respectively, while the maximum erosion decreases to values of 28cm, 25cm, 22cm, and 11cm. For a flow rate of 200m³/s, the maximum deposition decreases to values of 150cm, 72cm, 42cm, and 23cm, respectively, and the maximum erosion decreases to values of 31cm, 28cm, 25cm, and 22cm,.

[Audio] Transversal evolution of the bed reveals the presence of two distinct regions. A deposition zone is observed on the smooth bed, while an erosion zone is noted along both banks, with a proportional decrease as roughness increases..

[Audio] In conclusion, According to these results, we can conclude that the greater the roughness of the banks, the less deformation there is in the riverbed. This is explained by the reduction in velocity and shear stress, which implies less sediment transport. Although dredging works combat flooding by facilitating flow transit, they lead to increased amounts of transported sediments that will deposit downstream in the dam. This reduces the exploitable water volume in the dam and accelerates its silting These findings suggest that the vegetation contributes to enhance the cohesion of the riverbed, minimizing the risks of erosion and excessive sediment transport. The combined effect of vegetation-induced roughness appears to play a crucial role in preserving bank stability, which can have significant implications for the management and preservation of river ecosystems.

[Audio] Thank you for your attention. Thank you for your attention.