Like many rivers around the world, the Loire river in France has a history of human interventions in order to facilitate navigation and port development. Next to affecting bed levels directly, the heavy modification of the river-estuary has induced significant changes in the hydrodynamic and morphodynamic behaviour of the Loire. As a result, the Loire river is experiencing an ongoing bed degradation. In the estuary, the interventions have led to an amplification of the tidal amplitude, a shift of the tidal and salt intrusion limit in upstream direction and a larger tidal asymmetry. According to Winterwerp and Wang (2013), this tidal deformation leads to an increased import of fine sediment in the estuary and a reduced hydraulic drag. This in turn enhances tidal deformation further. Due to this positive feedback mechanism the Loire estuary has evolved into a hyper-turbid state, associated with large suspended sediment concentrations and the formation of fluid mud.

The behaviour of the river-estuary on large temporal and spatial scales and its response to historical and possible future interventions is not yet sufficiently understood. Therefore, an idealized, computationally efficient, three-dimensional morphodynamic model of the river-estuary is developed with the FLOW module of the Delft3D software suite, in which the main processes influencing the morphology of the Loire river-estuary are included.

The current estuarine geometry, lacking the presence of large intertidal areas, induces a flood-dominant tidal signal. In the mouth of the estuary, the baroclinic pressure gradient introduces a mean landward-directed velocity near the bed and amplifies the mean seaward-directed velocity near the surface. This gravitational circulation causes the formation of an Estuarine Turbidity Maximum at the tip of the salt wedge, which is strengthened further by the flood-dominance of the tide. Model results confirm the feedback mechanism between tidal deformation, the import of fine sediment and the effective hydraulic drag as described above. However, next to tidal deformation, strengthening of the gravitational circulation also plays a large role in this mechanism for the Loire.

To mitigate bed degradation, several measures are simulated that decrease the transport capacity of the flow, increase the sediment supply or do both. Over the first 10 years, sediment nourishments are the most effective according to the model. However, after 100 years of simulation the removal of all groynes present within the domain leads to the most sedimentation relative to the reference scenario. To decrease the import of mud into the estuary, two measures have been investigated. Increasing bed levels in the main channel of the estuary mainly leads to a decrease of the tidal range, whereas restoring tidal flats turns the tide from flood- into ebb-dominant. In both cases, a decrease of the salt intrusion length and a significant weakening of the gravitational circulation occurs, leading to a tidally averaged export of fine sediment and very low mud concentrations.
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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.
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