University of Twente Student Theses


Conceptual model of the seasonal inlet closure in the Da Dien Estuary

Posthumus, R. (2015) Conceptual model of the seasonal inlet closure in the Da Dien Estuary.

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Abstract:Estuaries are the places where river and sea interfere. Some estuaries are separated from the sea by a sand spit, so the water can only flow through a narrow inlet from the estuary to the sea and back. Along the Vietnamese coast, there are many estuaries with a sand spit. The river mouth of the Ba River is one of them; the Da Dien estuary. Among the several problems identified for this estuary, the seasonal inlet closure is studied in this thesis. Due to this closure, fishing boats cannot pass the inlet for harvesting trips. The goal of this study is to develop a conceptual model that describes the seasonal inlet closure in the Da Dien estuary. The conceptual model will form the basis of a simulation model, in which solutions for the closure can be implemented and evaluated. To reach this goal, the seasonal inlet closure is studied in detail and the factors that affect the closure are reviewed. Based on that knowledge, a conceptual model is developed. Next, the model is validated with collected data. A literature study of the factors that affect the seasonal inlet closure shows that the closure mainly depends on the river discharge. When the river discharge drops and only a small amount of water is flowing through the inlet channel, deposition occurs and the inlet gets filled up with sediment. Due to the large variation in river discharge in the Ba River, the closure can occur during the dry season. The sediment that is transported by large river discharges during the flood season (September-December), flows through the inlet channel and is deposited in front of the inlet. The sediment is transported back to the inlet during the dry season (January-August). Wind and waves play an important role in this process. They are influenced by the northeast monsoon from September till March and the southwest monsoon from April till August. Because the coastline of the area around the Da Dien estuary is oriented from the northwest to the southeast, the monsoon seasons cause both onshore and longshore currents. After the flood season, onshore currents are dominant from January till March, driven by wind and waves from the northeast. The sediment that is dropped in front of the inlet, is transported back to the inlet, so the inlet fills up. The longshore currents transport sediment along the shoreline. When they are interrupted by tidal currents in front of the inlet, the longshore current velocity drops and the sediment deposes. This process mainly occurs during the southwest monsoon. Other factors that affect the seasonal inlet closure are human activities and climate change. Since the conceptual model is the basis for further research, the model should be valid, credible, feasible and useful. In the conceptual model, boxes, lines and arrows show direct, indirect and interactive relations between the variables. The first sketch of the conceptual model includes all studied factors. However, the model is too complicated to use in a simulation and needs simplification. Based on the literature study and the first sketch of the model, the monsoon, wind, human activities and climate change are excluded from the model. This leaves a conceptual model in which the seasonal inlet closure is driven by river discharge, waves and tide. Satellite images and data of river discharge, wind, wave and bed topography have been collected to validate the model with a correlation analysis. Unfortunately, there is a great lack of data (and the data available are not always reliable). The validation shows that there is a direct relation between the river discharge and the seasonal inlet closure, whereby the correlation is stronger when the maximum discharge in each month is used instead of the average discharge. However, the validation also shows that the waves have less than expected influence on the inlet closure. Therefore, the model is adjusted. In the improved model, the seasonal inlet closure still depends on the river discharge, but the other variables in the model also depend on these discharges. This does not indicate that they do not affect the seasonal inlet closure, but their influence depends on the amount of river discharge. To conclude, a conceptual model has been developed that describes the seasonal inlet closure. Further improvements require more and reliable data, after which it can be used to make a simulation model.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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