This thesis research has been conducted to analyse the sediment dynamics in the Yangon Estuary, in order to define the cause for the sedimentation in Monkey Point Channel. For years, Yangon Port has been suffering from limited accessibility due to the sedimentation in the channel and currently daily dredging is required in order to let ships enter the port.
The main characteristics of Yangon Estuary are; tidal range of 6 m, high suspended sediment concentrations (up to 6 g/l), a monsoon climate and the presence of a confluence within the tidal range. Research to the cause of sedimentation is done by making a conceptual model of the estuary based on a study area analysis and a literature study to estuarine, riverine and sediment processes. In addition, numerical models are used to improve the conceptual model in a quantitative manner. The conceptual model resulted in the following findings.
The fluctuations in SSC are dominantly determined by the tide. During the monsoon season suspended sediment is brought in the system mainly by the Yangon River discharge, causing for averaged SSC values of 1 g/l during the season. In dry season, the SSC shows a fortnightly rhythm caused by the spring/neap cycle. Strong spring tidal currents cause for high bed shear stresses on the shoals, exceeding 1 N/m2, which erodes the stationary fluid mud layers. Due to the dispersion of the mud, the SSC rises rapidly up to 5 g/l. During neap tides the bed shear stress is much less, enabling the fine sediments to settle on the shoals and restore the fluid mud layers. Hence, the SSC values reduce to less than 0.5 g/l. During the monsoon, the spring flood current is reduced due to the seaward directed river discharge. Therefore, the spring flood currents are insufficient to erode the stationary fluid mud layers, hence the SSC are less than during spring tide in the dry season.
In contrast to the fluctuations in erosion and deposition of mud on the shoals, almost continuously deposition and consolidation of mud is possible in Monkey Point Channel. This is possible because the channel is located in the stagnation-zone where both during flood (bifurcation) and ebb (confluence) the flow velocities and bed shear stresses sufficiently low to let mud settle. After deposition the mud is not resuspended, and consolidation causes the mud layer to become thicker and become problematic for the ship traffic.
To conclude; the combination of high SSC with the possibility of consolidation of deposited mud in the stagnation-zone results in too hard layers to sail through. This potential cause corresponds with the dredged material from the channel bed and the sedimentation rate in the channel.
It is recommended to relocate the navigation channel outside the stagnation-zone. The combination of land reclamation at the western estuary bank downstream of the confluence and a dredged channel shows the most promising results. However, more research is required to determine the long-term sedimentation rates in the new channel.
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Valdivia is a Chilean city located near an estuary system, 800 km south of Santiago. The navigation capacity of the Valdivia river mainly determines the present state and future possibilities for the welfare of the city. Since sedimentation problems arose in the river, the capacity for navigation became limited. The ministry of Valdivia therefore desires a solution for this problem in order to create possibilities for future growth. However, it is not known to what extent solutions for the sedimentation problem will actually contribute to an increase of welfare. By identifying the problem for the Ministry of Public Works of Chile, the boundaries and goal of the project could be set. In this research an attempt is made to answer the following research question: How can the surplus of sediment in the Valdivia river be remedied and to what extent will this contribute to the economic and social values of the city of Valdivia? In order to know how to remedy the surplus of sediment, a qualitative analysis of probable causes of the sedimentation was necessary. The main subjects which were researched by means of data and literature are tidal influence, river discharges, sediment composition and salt intrusion. With this knowledge it was concluded that the directions and magnitudes of flow lines in the river system can give good indications on what locations sedimentation can occur. The interaction between tidal currents and river discharges are the main drivers behind these flow characteristics. To verify the theoretical analysis, a basic Delft3D model was set up containing only tidal movement and river discharges. The pattern of the flow lines which was obtained from the Delft3D model supports the possibility that sedimentation occurs on the current identified sedimentation locations. Although other hydrological and morphological processes were found to possibly influence sedimentation rates in the river, qualitative data for studying these processes were missing. The current Delft3D model is therefore a good result regarding the available data and can be seen as a part of preliminary research in order to support further studies. Because the exact causes of the sedimentation were not identified in this research, it was not possible to come up with suitable solutions and to research how these solutions could affect the system. However, in order to gain insight in the effects of a river system without sedimentation problems, a fictitious scenario was studied by means of a social cost benefit analysis. The goal of the social costs and benefits analysis was to give insight in what factors should be taken into account when considering a project plan which solves the sedimentation problem. A dredging design was made in order to make the Valdivia river navigable for larger cargo vessels to increase the trading capacity of Valdivia. This dredging design comprises the deepening of the Valdivia river to a minimum water depth of 10 meters. Considering investment and maintenance costs of the dredging activities of this magnitude, it is concluded that the costs are not profitable compared to the social and economic effects of a higher traffic intensity on the river. With the social costs and benefits analysis all effects of solving the sedimentation problem are inventoried. When the Delft3D model can identify what the remedy of the sedimentation could be, the impact on the economic and social values of Valdivia can be determined. This research was not sufficient to answer the research question. Both the Delft3D model and the SCBA model were lacking accurate and reliable data. To expand this research, it is necessary to collect the data of which an overview has been made. Therefore, this research can serve as a preliminary study for further research. It is recommended that after data collection, solutions will be identified which can solve the sedimentation problem. These solutions need to be compared with a scenario analysis in order to decide on which is the most suitable solution for the sedimentation problem. Besides this, it is advised to research whether it is actually necessary to expand the river for transport when looking at all actors involved. ... Read more