WJ
W.J. Jansen
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1
Wave dissipation in a permeable structure
Experimental research on the physical processes causing energy dissipation inside an array of cylinders
Master thesis
(2019)
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Wiljan Jansen, Wim Uijttewaal, Alejandra Gijón Mancheño, Emma Sirks, Coen Kuiper, Robert Lanzafame, Han Winterwerp
Mangrove forests, which serve as a natural sea defence, have been degraded by human action during the last decades. To protect and recover these areas, different types of permeable structures consisting of bamboo have been proposed and applied. However, these structures are currently based on engineering judgement, as design rules are not available. The goal of this research is to make a step forward in the design optimization of permeable structures and gain a better understanding of the processes causing the wave dissipation inside the structure. In this study, scale experiments are conducted in the wave flume at Delft University of Technology. An array of aluminum cylinders is used as a schematization of the structure, with an element diameter of 4 cm and a minimum spacing of 2 cm. The tested wave height is 0.13 m,with a water depth of 0.6 m for the first set of experiments. These experiments evaluated wave transformation through a selected number of configurations. The water depth was 0.55 m for the second set, where the velocities and forces were also measured inside the structure. As the applied wave periods are short (T = 1 − 2 s), the tested wave conditions are in the range of small KC-numbers (4 < KC < 13). The first part of this research focuses on effect of different configurations and arrangements on the amount of energy dissipation, by measuring the incoming and reflected wave heights in front of and behind the structure. For the short waves, the horizontal arrangements dissipate more energy, as energy is dissipated by both the vertical and horizontal drag forces. However, the effect diminishes with increasing wave period. Considering the total dissipation and the amount of dissipation per element, the placement of the elements in rows perpendicular to the direction of wave propagation is found to be the most effective. To study the effect of the element diameter on the dissipation, the results are compared with previous research by Haage (2018) on a model with a diameter and spacing of 2 cm. As with the change in diameter also the structure porosity changed, a direct comparison was not possible and a comparison based on a simplified drag coefficient is done. No direct effect of the diameter is observed, as the obtained drag coefficients show the same trend and magnitude when plotted against an adapted Keulegan-Carpenter number (KC∗), which is based on the element spacing instead of diameter. Small KC∗-numbers result in large drag coefficients,which decrease when KC∗ increases. The second part of this research focuses on the processes inside the structure that cause the energy dissipation. A force and velocity sensor are applied at three locations inside the structure in separate experiments, to determine the relation of the force and velocity inside the structure with the undisturbed values. Two methods are applied for the analysis, based on two different principles. Method 1 is based on the assumption of a constant pair of force coefficients and an increase in velocity inside the structure, method 2 is based on the assumption of a constant velocity and an increase in force coefficients inside the structure.By comparing both methods, it is found that an increase in velocity is the most important factor for the increase in drag force, which is the driving factor for the energy dissipation. It is also found that the amplification factor for the velocity is dependent on both the structure porosity and frontal porosity.
...
Mangrove forests, which serve as a natural sea defence, have been degraded by human action during the last decades. To protect and recover these areas, different types of permeable structures consisting of bamboo have been proposed and applied. However, these structures are currently based on engineering judgement, as design rules are not available. The goal of this research is to make a step forward in the design optimization of permeable structures and gain a better understanding of the processes causing the wave dissipation inside the structure. In this study, scale experiments are conducted in the wave flume at Delft University of Technology. An array of aluminum cylinders is used as a schematization of the structure, with an element diameter of 4 cm and a minimum spacing of 2 cm. The tested wave height is 0.13 m,with a water depth of 0.6 m for the first set of experiments. These experiments evaluated wave transformation through a selected number of configurations. The water depth was 0.55 m for the second set, where the velocities and forces were also measured inside the structure. As the applied wave periods are short (T = 1 − 2 s), the tested wave conditions are in the range of small KC-numbers (4 < KC < 13). The first part of this research focuses on effect of different configurations and arrangements on the amount of energy dissipation, by measuring the incoming and reflected wave heights in front of and behind the structure. For the short waves, the horizontal arrangements dissipate more energy, as energy is dissipated by both the vertical and horizontal drag forces. However, the effect diminishes with increasing wave period. Considering the total dissipation and the amount of dissipation per element, the placement of the elements in rows perpendicular to the direction of wave propagation is found to be the most effective. To study the effect of the element diameter on the dissipation, the results are compared with previous research by Haage (2018) on a model with a diameter and spacing of 2 cm. As with the change in diameter also the structure porosity changed, a direct comparison was not possible and a comparison based on a simplified drag coefficient is done. No direct effect of the diameter is observed, as the obtained drag coefficients show the same trend and magnitude when plotted against an adapted Keulegan-Carpenter number (KC∗), which is based on the element spacing instead of diameter. Small KC∗-numbers result in large drag coefficients,which decrease when KC∗ increases. The second part of this research focuses on the processes inside the structure that cause the energy dissipation. A force and velocity sensor are applied at three locations inside the structure in separate experiments, to determine the relation of the force and velocity inside the structure with the undisturbed values. Two methods are applied for the analysis, based on two different principles. Method 1 is based on the assumption of a constant pair of force coefficients and an increase in velocity inside the structure, method 2 is based on the assumption of a constant velocity and an increase in force coefficients inside the structure.By comparing both methods, it is found that an increase in velocity is the most important factor for the increase in drag force, which is the driving factor for the energy dissipation. It is also found that the amplification factor for the velocity is dependent on both the structure porosity and frontal porosity.
Sedimentation in the mouth of the Magdalena river
Improving navigability in the port of Barranquilla
Student report
(2018)
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Lindert Ambagts, Wiljan Jansen, Nena de Jong, Anna Kosters, Cees Oerlemans, Anouk Slockers,
The city of Barranquilla can play an important role in the establishment in a multi-modal network. Currently however, issues with navigable depth in the river mouth (Bocas de Ceniza) make it difficult for the larger vessels to enter the port area. Resolving these issues will increase the navigability of the Magdalena river which in turn will help Barranquilla bloom as a port city. The aim of this research is to analyse the various sedimentation processes and to explore the possibilities for mitigation measures.
In order to gain more insight into the processes which affect the sediment transport in the first 38 km of the Magdalena river a morphological model is developed using Delft3D software. However, to understand and capture the essence of the issue also the socio-political context is researched. This is done by analysing both the stakeholders and the previous unsuccessful Magdalena recovery project. The combination of both the physical and more organisational aspects formed the input to come up with multiple intervention ideas to solve navigability issues. To evaluate these interventions a multi-criteria analysis (MCA) is conducted witFh both technical and managerial criteria. Finally, also an engagement plan is constructed as a guideline to have successful stakeholder management in a future river project.
From the exploration of the physical part of the issue it can be concluded that there are both natural and anthropogenic aspects which influence the river system, hence the sedimentation issue. With regard to the natural aspects, high discharge variability, stratification and wave action are selected to be further investigated with the Delft3D model. Model results show that sedimentation and erosion rates increase in the access channel above a discharge of around 5000 m3/s. For discharges below this value, the salt intrusion length increases faster than for higher discharges. This salt intrusion leads to a low flow velocity at the bottom that is directed upstream, making the reach over which salt intrudes susceptible to sedimentation. Waves mainly influence bar formation in the river mouth and are not important further upstream.
The analysis of the non-physical aspects leads to a few important conclusions as well. First of all it is concluded that the stakeholder network, in which this problem exists, is a fairly complex one. Therefore stakeholder and process management are key elements in making a future river project successful. Furthermore hypotheses on the success factors of setting up a public private partnership (PPP) in Colombia are formed based on research by Koppenjan (2003), cultural differences between the Netherlands and Colombia and interviews with important stakeholders. These hypotheses are that corruption, political stability and openness for change are also important factors in setting up a successful PPP in Colombia.
Possible interventions to mitigate the sedimentation issues in the first 38 km of the Magdalena river have been assessed separately for three different sections. For the first section (reaching from 0 km to 8 km), the most promising intervention resulting from the MCA is water injection dredging. This would efficiently improve the navigation conditions, capital investments are relatively low and it is also an adaptable solution. For the next section (from 8 km to 22 km), a fixed bottom layer is considered a promising intervention since it will lead to erosion of the relatively shallow inner bend and therefore increase the navigable width. As for the last section (from 22 km to 30 km) it can be concluded from the MCA that none of the assessed training works score better than the current situation.
...
In order to gain more insight into the processes which affect the sediment transport in the first 38 km of the Magdalena river a morphological model is developed using Delft3D software. However, to understand and capture the essence of the issue also the socio-political context is researched. This is done by analysing both the stakeholders and the previous unsuccessful Magdalena recovery project. The combination of both the physical and more organisational aspects formed the input to come up with multiple intervention ideas to solve navigability issues. To evaluate these interventions a multi-criteria analysis (MCA) is conducted witFh both technical and managerial criteria. Finally, also an engagement plan is constructed as a guideline to have successful stakeholder management in a future river project.
From the exploration of the physical part of the issue it can be concluded that there are both natural and anthropogenic aspects which influence the river system, hence the sedimentation issue. With regard to the natural aspects, high discharge variability, stratification and wave action are selected to be further investigated with the Delft3D model. Model results show that sedimentation and erosion rates increase in the access channel above a discharge of around 5000 m3/s. For discharges below this value, the salt intrusion length increases faster than for higher discharges. This salt intrusion leads to a low flow velocity at the bottom that is directed upstream, making the reach over which salt intrudes susceptible to sedimentation. Waves mainly influence bar formation in the river mouth and are not important further upstream.
The analysis of the non-physical aspects leads to a few important conclusions as well. First of all it is concluded that the stakeholder network, in which this problem exists, is a fairly complex one. Therefore stakeholder and process management are key elements in making a future river project successful. Furthermore hypotheses on the success factors of setting up a public private partnership (PPP) in Colombia are formed based on research by Koppenjan (2003), cultural differences between the Netherlands and Colombia and interviews with important stakeholders. These hypotheses are that corruption, political stability and openness for change are also important factors in setting up a successful PPP in Colombia.
Possible interventions to mitigate the sedimentation issues in the first 38 km of the Magdalena river have been assessed separately for three different sections. For the first section (reaching from 0 km to 8 km), the most promising intervention resulting from the MCA is water injection dredging. This would efficiently improve the navigation conditions, capital investments are relatively low and it is also an adaptable solution. For the next section (from 8 km to 22 km), a fixed bottom layer is considered a promising intervention since it will lead to erosion of the relatively shallow inner bend and therefore increase the navigable width. As for the last section (from 22 km to 30 km) it can be concluded from the MCA that none of the assessed training works score better than the current situation.
...
The city of Barranquilla can play an important role in the establishment in a multi-modal network. Currently however, issues with navigable depth in the river mouth (Bocas de Ceniza) make it difficult for the larger vessels to enter the port area. Resolving these issues will increase the navigability of the Magdalena river which in turn will help Barranquilla bloom as a port city. The aim of this research is to analyse the various sedimentation processes and to explore the possibilities for mitigation measures.
In order to gain more insight into the processes which affect the sediment transport in the first 38 km of the Magdalena river a morphological model is developed using Delft3D software. However, to understand and capture the essence of the issue also the socio-political context is researched. This is done by analysing both the stakeholders and the previous unsuccessful Magdalena recovery project. The combination of both the physical and more organisational aspects formed the input to come up with multiple intervention ideas to solve navigability issues. To evaluate these interventions a multi-criteria analysis (MCA) is conducted witFh both technical and managerial criteria. Finally, also an engagement plan is constructed as a guideline to have successful stakeholder management in a future river project.
From the exploration of the physical part of the issue it can be concluded that there are both natural and anthropogenic aspects which influence the river system, hence the sedimentation issue. With regard to the natural aspects, high discharge variability, stratification and wave action are selected to be further investigated with the Delft3D model. Model results show that sedimentation and erosion rates increase in the access channel above a discharge of around 5000 m3/s. For discharges below this value, the salt intrusion length increases faster than for higher discharges. This salt intrusion leads to a low flow velocity at the bottom that is directed upstream, making the reach over which salt intrudes susceptible to sedimentation. Waves mainly influence bar formation in the river mouth and are not important further upstream.
The analysis of the non-physical aspects leads to a few important conclusions as well. First of all it is concluded that the stakeholder network, in which this problem exists, is a fairly complex one. Therefore stakeholder and process management are key elements in making a future river project successful. Furthermore hypotheses on the success factors of setting up a public private partnership (PPP) in Colombia are formed based on research by Koppenjan (2003), cultural differences between the Netherlands and Colombia and interviews with important stakeholders. These hypotheses are that corruption, political stability and openness for change are also important factors in setting up a successful PPP in Colombia.
Possible interventions to mitigate the sedimentation issues in the first 38 km of the Magdalena river have been assessed separately for three different sections. For the first section (reaching from 0 km to 8 km), the most promising intervention resulting from the MCA is water injection dredging. This would efficiently improve the navigation conditions, capital investments are relatively low and it is also an adaptable solution. For the next section (from 8 km to 22 km), a fixed bottom layer is considered a promising intervention since it will lead to erosion of the relatively shallow inner bend and therefore increase the navigable width. As for the last section (from 22 km to 30 km) it can be concluded from the MCA that none of the assessed training works score better than the current situation.
In order to gain more insight into the processes which affect the sediment transport in the first 38 km of the Magdalena river a morphological model is developed using Delft3D software. However, to understand and capture the essence of the issue also the socio-political context is researched. This is done by analysing both the stakeholders and the previous unsuccessful Magdalena recovery project. The combination of both the physical and more organisational aspects formed the input to come up with multiple intervention ideas to solve navigability issues. To evaluate these interventions a multi-criteria analysis (MCA) is conducted witFh both technical and managerial criteria. Finally, also an engagement plan is constructed as a guideline to have successful stakeholder management in a future river project.
From the exploration of the physical part of the issue it can be concluded that there are both natural and anthropogenic aspects which influence the river system, hence the sedimentation issue. With regard to the natural aspects, high discharge variability, stratification and wave action are selected to be further investigated with the Delft3D model. Model results show that sedimentation and erosion rates increase in the access channel above a discharge of around 5000 m3/s. For discharges below this value, the salt intrusion length increases faster than for higher discharges. This salt intrusion leads to a low flow velocity at the bottom that is directed upstream, making the reach over which salt intrudes susceptible to sedimentation. Waves mainly influence bar formation in the river mouth and are not important further upstream.
The analysis of the non-physical aspects leads to a few important conclusions as well. First of all it is concluded that the stakeholder network, in which this problem exists, is a fairly complex one. Therefore stakeholder and process management are key elements in making a future river project successful. Furthermore hypotheses on the success factors of setting up a public private partnership (PPP) in Colombia are formed based on research by Koppenjan (2003), cultural differences between the Netherlands and Colombia and interviews with important stakeholders. These hypotheses are that corruption, political stability and openness for change are also important factors in setting up a successful PPP in Colombia.
Possible interventions to mitigate the sedimentation issues in the first 38 km of the Magdalena river have been assessed separately for three different sections. For the first section (reaching from 0 km to 8 km), the most promising intervention resulting from the MCA is water injection dredging. This would efficiently improve the navigation conditions, capital investments are relatively low and it is also an adaptable solution. For the next section (from 8 km to 22 km), a fixed bottom layer is considered a promising intervention since it will lead to erosion of the relatively shallow inner bend and therefore increase the navigable width. As for the last section (from 22 km to 30 km) it can be concluded from the MCA that none of the assessed training works score better than the current situation.