Name: Numerical Analysis of Bank Roughness Impact on Sediment Dynamics in the Medjerda River, Tunisia
Uploaded: 2024-12-03
Duration: 9 min 21 s
Description: 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] 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.. 

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* National Institute of Agronomy of Tunisia ** University of Orléans, INSA-CVL, PRISME

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

3rd International Conference on Recent Academic Studies December 03-04, 2024 : Konya, Turkey

[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.. 

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1.Simulations scenarios 2.Roughness variation

[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.. 

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Vegetation plays a crucial role in fluvial sediment transport, it : slow down water velocity alter flow patterns reduce bed shear stress  enhance local sediment deposition  stabilize banks reduce erosion input into the river

alter the morphology of watercourses by reducing the wadi beds’ transit capacity,

affect the operation of hydraulic structures (Bridge)

[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.. 

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To solve Flooding problem, in Medjerda river (Tunisia) dredging works were carried out, in order to minimize the damage that has occurred.
 The purpose of these works was the elimination of vegetation, mainly Tamaris, which obstructed the sections.

Dredging works encourage particle transport

Also, increase sediment deposition in Sidi Salem dams at 6 million m³/year (Tunisia's largest downstream dam), reduces the dam's 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.. 

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The main objective of this study is to assess the impact of roughness modification on the sediment transport process in the Medjerda, the longest perennial river in Tunisia, following dredging works carried out over the last decade as part of flood protection measures for the Boussalem city.
  We employed the Telemac Sisyphe model along a section regularly subjected to dredging, with a length of 17.8 kilometers upstream of the sidi salem dam. 
 Given that the configuration of Medjerda section is divided into two parts: the first, entirely smooth river bed; the second part, both banks, exhibits variable roughness depending on the pre-existing vegetation cover.
 Four simulation scenarios were set up, keeping the riverbed smooth and gradually varying the roughness of the second part from smooth to rough, in order to analyse on bed evolution and  shear stress

[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.. 

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— Study area
—Medjerda river
Cross section
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A stretch of the Medjerda River passing through the city of Boussalem, located between the coordinates 36° 36' 40" north, 8° 58' 11" east, within the alluvial plain of the upper Medjerda valley, has been selected to illustrate the aforementioned impacts, where the Medjerda River undergoes regular dredging works to combat flooding. The total length of this section is 17.8 km. The study area is situated 20 km upstream from the Sidi Salem Dam,

[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.. 

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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.

2019
2007
15.000
_0120'oo•
5000
125
115
105
10.000
Distance (m)

[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.. 

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The considered area covers an equal surface of 40.71 km² with a river length of 17.8 kilometers. 
 We created an unstructured surface mesh, based on the finite element method. generated from a LIDAR survey (mesh size : 5 meters minor bed and 50m for the rest of the domain)
 The grid, illustrated in the figure, consists of 54,279 nodes and 108,211 elements.

[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.. 

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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.. 

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The configuration of this section contains two distinct parts:  the first part, located at the riverbed with a width less than 10 meters, 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.. 

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Roughness increases in tandem with vegetation development. To assess the influence of vegetation on particle transport, 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.

BOTTOM FRICTION
Major vegetated bed
Minor bed
Control section

€2
0.09
c 0.07
0.05
00.03
o
20
40
60
80
—n = 0.03 s/m"ä
—n = 0.05
—n = 0.07
—n = 0.09s/ml'2
100
Cross distance (m)

[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.. 

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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.

Velocity (m/s) (200m³/s) 234.716101606 494.90505663900001 739.48382200100002 1409.1922138479999 1693.115476425 1914.7404695360001 2125.0521584550002 2407.0387292449991 2587.7069817539991 2817.959372201999 3025.7949047429979 3292.8933232149971 3516.8492323909973 3712.7170788729973 3905.9251691979971 3999.7112308634969 4164.4517199294969 4428.7984460174966 4520.1074450272963 4675.3322630022967 4871.0509680882969 4989.3449799042955 5241.7829895672958 5322.5319692852954 5521.672265684294 5740.3061441612936 5902.8374729822926 6042.1037338132928 6212.4814949212914 6473.8606901352905 6763.3315821482902 6885.8772741952898 7025.2344332402899 7472.5754642522888 7669.6189689112889 7723.4052099553892 7868.1198348603884 7979.1042437873884 8237.5487985283871 8481.6880252413866 8519.245809631986 8556.5060071846856 8589.6206729970854 8732.8900775890852 8947.1954912470846 9126.3572633380845 9316.2135254040841 9514.3136103610832 9856.6216307770828 10084.241110130082 10447.100849478082 10823.60178345108 11043.017117823079 11300.067122044078 11595.239897671077 11792.381898515076 12142.815723785076 12521.672413562075 12937.990765573075 13300.244168646075 13765.646186938075 14119.842238211075 14475.578412242076 14937.591529999074 15319.240919994072 15538.947783904072 15895.128966224072 16259.522338999072 16547.142633564072 16833.937269603073 17127.803472066073 17385.398209380073 17484.812255614172 17555 17666 17750 1.4595644000000001 1.8373786999999999 1.8646132 1.6735092 1.4104487000000001 1.3699444999999999 0.41543159000000002 1.5829648999999999 1.3925604 1.0882365000000001 1.5490093 1.4910493 1.3041392999999999 1.0636128 1.1262265 1.3035232000000001 1.2485877999999999 1.1343529000000001 1.3232874999999999 1.1742808 1.1477386999999999 0.88462138000000001 0.93235725000000003 0.92616635999999997 1.2685706999999999 0.82590770999999996 1.0082816999999999 0.74906552000000004 1.0073833000000001 1.1042901000000001 0.86213660000000003 0.97612584000000002 1.0316057999999999 1.0564016000000001 1.1078416 1.1780600999999999 1.2662952999999999 1.1655966 1.0838239000000001 1.0386671000000001 1.0437495000000001 1.0766597 1.1870558 1.0822689999999999 1.0350815 1.4452069999999999 1.450037 0.97424005999999996 1.1301998 1.306546 0.9267512 1.1153181000000001 0.58455628000000004 0.89212440999999998 1.0078491999999999 1.1068115000000001 0.95937925999999996 0.59476054 1.2599423999999999 1.1954024999999999 0.96413678000000003 0.40098071000000002 0.91411816999999995 0.85447276000000005 0.98315936000000004 1.2830629 1.4970102000000001 1.3484335999999999 1.4204281999999999 1.3421407000000001 1.0667586 1.2325280999999999 1.4172222999999999 1.4825515 1.3396455 1.1352899000000001 Velocity (m/s) (100m³/s) 234.716101606 494.90505663900001 739.48382200100002 1409.1922138479999 1693.115476425 1914.7404695360001 2125.0521584550002 2407.0387292449991 2587.7069817539991 2817.959372201999 3025.7949047429979 3292.8933232149971 3516.8492323909973 3712.7170788729973 3905.9251691979971 3999.7112308634969 4164.4517199294969 4428.7984460174966 4520.1074450272963 4675.3322630022967 4871.0509680882969 4989.3449799042955 5241.7829895672958 5322.5319692852954 5521.672265684294 5740.3061441612936 5902.8374729822926 6042.1037338132928 6212.4814949212914 6473.8606901352905 6763.3315821482902 6885.8772741952898 7025.2344332402899 7472.5754642522888 7669.6189689112889 7723.4052099553892 7868.1198348603884 7979.1042437873884 8237.5487985283871 8481.6880252413866 8519.245809631986 8556.5060071846856 8589.6206729970854 8732.8900775890852 8947.1954912470846 9126.3572633380845 9316.2135254040841 9514.3136103610832 9856.6216307770828 10084.241110130082 10447.100849478082 10823.60178345108 11043.017117823079 11300.067122044078 11595.239897671077 11792.381898515076 12142.815723785076 12521.672413562075 12937.990765573075 13300.244168646075 13765.646186938075 14119.842238211075 14475.578412242076 14937.591529999074 15319.240919994072 15538.947783904072 15895.128966224072 16259.522338999072 16547.142633564072 16833.937269603073 17127.803472066073 17385.398209380073 17484.812255614172 17555 17666 17750 1.6235698000000001 1.4975204 1.4779154999999999 1.4180448000000001 1.0861014 0.88009214000000002 0.64494532000000004 1.1114069 0.99963009000000003 0.83508879000000003 1.2968978 1.1272036999999999 1.0091772000000001 0.79523832000000005 0.84852755000000002 0.96587115999999995 0.96183543999999999 0.81974338999999996 1.0008762 0.85474925999999996 0.87846780000000002 0.69106173999999998 0.70209127999999998 0.68724883000000003 1.0874959 0.65530657999999997 0.82985222000000003 0.59951275999999998 0.72650051000000004 0.8766796 0.69106065999999999 0.83132284999999995 0.86522931000000003 0.89110725999999996 0.94663870000000006 1.0410276999999999 1.0710223999999999 0.92650712000000002 0.88991182999999996 0.8177219 0.84500372000000001 0.87664461000000005 0.98666852999999999 0.87542909000000002 0.74812239000000003 1.1582193000000001 1.2720522999999999 0.85897678 0.87602639000000004 0.89781928 0.86021822999999997 0.92422526999999999 0.50636612999999997 0.70700699 0.80181800999999997 0.87081509999999995 0.78085452 0.54167712000000001 0.91177284999999997 0.98729425999999998 0.74336111999999999 0.37906957000000002 0.72331917000000001 0.70856308999999995 0.70927744999999998 1.1738405000000001 1.1411804000000001 1.1198311000000001 1.3042450999999999 1.0705017999999999 0.80847442000000003 1.0574967 1.1145612 1.2950345999999999 1.1373055999999999 0.95590942999999995 Bed shear stress (N/m²) (200m³/s) 234.716101606 494.90505663900001 739.48382200100002 1409.1922138479999 1693.115476425 1914.7404695360001 2125.0521584550002 2407.0387292449991 2587.7069817539991 2817.959372201999 3025.7949047429979 3292.8933232149971 3516.8492323909973 3712.7170788729973 3905.9251691979971 3999.7112308634969 4164.4517199294969 4428.7984460174966 4520.1074450272963 4675.3322630022967 4871.0509680882969 4989.3449799042955 5241.7829895672958 5322.5319692852954 5521.672265684294 5740.3061441612936 5902.8374729822926 6042.1037338132928 6212.4814949212914 6473.8606901352905 6763.3315821482902 6885.8772741952898 7025.2344332402899 7472.5754642522888 7669.6189689112889 7723.4052099553892 7868.1198348603884 7979.1042437873884 8237.5487985283871 8481.6880252413866 8519.245809631986 8556.5060071846856 8589.6206729970854 8732.8900775890852 8947.1954912470846 9126.3572633380845 9316.2135254040841 9514.3136103610832 9856.6216307770828 10084.241110130082 10447.100849478082 10823.60178345108 11043.017117823079 11300.067122044078 11595.239897671077 11792.381898515076 12142.815723785076 12521.672413562075 12937.990765573075 13300.244168646075 13765.646186938075 14119.842238211075 14475.578412242076 14937.591529999074 15319.240919994072 15538.947783904072 15895.128966224072 16259.522338999072 16547.142633564072 16833.937269603073 17127.803472066073 17385.398209380073 17484.812255614172 17555 17666 17750 12.176728000000001 19.173145000000002 19.464741 15.77224 10.502981999999999 10.016709000000001 0.87054628000000001 13.520238000000001 10.267657 6.1536635999999998 13.382353999999999 12.330551 8.9027385999999993 5.6701826999999998 6.5072793999999998 8.8153658000000004 8.0764589000000004 6.5602932000000003 9.0438966999999995 6.9580107 6.7106309 3.8401320000000001 4.3504538999999998 4.2944484000000003 8.4738349999999993 3.4191810999999999 5.1498131999999996 2.8031956999999998 5.0648489000000003 6.1142807000000001 3.6505036 4.8307905 5.4959626000000004 5.7792367999999996 6.4323068000000001 7.4374093999999999 8.9409351000000008 7.1404098999999999 6.3194919000000001 5.6207308999999999 5.6551080000000002 6.2846751000000003 7.6514730000000002 6.2167139000000002 5.6584710999999999 11.466989999999999 11.280108999999999 4.9178161999999999 6.7986012000000002 9.2157687999999993 4.6210389000000003 6.4788031999999998 1.8316051 4.2071218000000004 5.4242229000000002 6.6764530999999998 4.8591528000000004 1.8897139000000001 8.5602149999999995 7.2476716000000003 4.6998410000000002 0.80933427999999996 4.3486729000000004 3.7626102000000001 5.1690388 9.0979548000000001 12.385026999999999 10.527683 10.696974000000001 9.3267088000000005 6.0663156999999996 8.4137087000000008 10.894142 11.970374 9.5069827999999994 6.9845122999999996 Bed shear stress (N/m²) (100m³/s) 234.716101606 494.90505663900001 739.48382200100002 1409.1922138479999 1693.115476425 1914.7404695360001 2125.0521584550002 2407.0387292449991 2587.7069817539991 2817.959372201999 3025.7949047429979 3292.8933232149971 3516.8492323909973 3712.7170788729973 3905.9251691979971 3999.7112308634969 4164.4517199294969 4428.7984460174966 4520.1074450272963 4675.3322630022967 4871.0509680882969 4989.3449799042955 5241.7829895672958 5322.5319692852954 5521.672265684294 5740.3061441612936 5902.8374729822926 6042.1037338132928 6212.4814949212914 6473.8606901352905 6763.3315821482902 6885.8772741952898 7025.2344332402899 7472.5754642522888 7669.6189689112889 7723.4052099553892 7868.1198348603884 7979.1042437873884 8237.5487985283871 8481.6880252413866 8519.245809631986 8556.5060071846856 8589.6206729970854 8732.8900775890852 8947.1954912470846 9126.3572633380845 9316.2135254040841 9514.3136103610832 9856.6216307770828 10084.241110130082 10447.100849478082 10823.60178345108 11043.017117823079 11300.067122044078 11595.239897671077 11792.381898515076 12142.815723785076 12521.672413562075 12937.990765573075 13300.244168646075 13765.646186938075 14119.842238211075 14475.578412242076 14937.591529999074 15319.240919994072 15538.947783904072 15895.128966224072 16259.522338999072 16547.142633564072 16833.937269603073 17127.803472066073 17385.398209380073 17484.812255614172 17555 17666 17750 15.231088 13.042569 13.087844 11.90888 6.6053614999999999 4.2614302999999998 2.2925624999999998 7.0485096 5.5974335999999996 3.8576503 10.540368000000001 7.4523400999999998 5.7100223999999997 3.3678012000000002 3.9206723999999999 5.143014 5.1102271000000004 3.6722592999999999 5.5755686999999998 3.9511888000000002 4.2458501000000002 2.5452189000000001 2.6966177999999998 2.5761018 6.6774592000000004 2.3636352999999999 3.8050888 1.9880827999999999 2.8314474000000001 4.0988173000000003 2.5391724 3.8335995999999999 4.2427096000000004 4.5060320000000003 5.1627855 6.4296521999999996 7.3269677 4.9994125 4.8065157000000003 3.8042676000000002 4.0760069000000003 4.6907759000000002 5.8922008999999997 4.5262380000000002 3.2406647 8.1796188000000001 10.079221 4.2253641999999996 4.497725 4.5629191000000002 4.4550866999999998 4.9816637000000004 1.5521098 2.9559755000000001 3.8285415 4.6071925 3.6148454999999999 1.7538091 4.8897686 5.4705900999999999 3.0623521999999999 0.7994194 3.0000540999999998 2.8599508 2.9500522999999998 8.2520112999999995 7.9132594999999997 7.9896798000000002 10.029313999999999 6.7572483999999999 3.8311335999999998 7.0274849000000001 7.523015 10.046726 7.5000910999999997 5.4246749999999997

[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.. 

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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,. 

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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.

16000 14000 12000 10000
8000
Distance (m)
n = 0.03 s/m%
—n = 0.05 s/m%
1.5
1.3
1.1
0.9
0.7 E
0.5
0.3 >
0.1
-0.1
-0.3
-0.5
2000
— — — -n = 0.09 s/m%

160m
140m
n = 0.03 s/m%
120m
1.5
1.3
1.1
0.9
0.7
0.5
- n 0.05 s/m%
Distance (m)
-03
05
2000
n = O.09s/m%

[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.. 

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the 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.

Bed Evolution in a Cross-Section as a Function of Roughness Change for Q = 100 m³/s (a) and Q = 200 m³/s (b)

0.8
0.6
0.4
— 0.2
-0.2
-0.4
-0.6
20
60
80
—n = 0.05 s/m'ß
—n = 0.07
—n = 0.09 s/m'0
100
Cross distance (m)

[image] —n = W03s•m"
n = 009
w •0.2
Cross distance (m)

[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. 

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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. 

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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.

avatar

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.

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.

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.

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.

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.

To model this area, we used a finite element mesh generated from a lidar survey. The grid consists of 54279 nodes and 108211 elements, with resolutions tailored to capture details of the minor bed and riverbanks.

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.

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.

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.

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.

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 141 centimeters, 60 centimeters, 15 centimeters, and 8 centimeters, respectively, while the maximum erosion decreases to values of 28 centimeters, 25 centimeters, 22 centimeters, and 11 centimeters. 
For a flow rate of 200m³/s, the maximum deposition decreases to values of 150 centimeters, 72 centimeters, 42 centimeters, and 23 centimeters, respectively, and the maximum erosion decreases to values of 31 centimeters, 28 centimeters, 25 centimeters, and 22 centimeters,

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.

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