Sediment transport and coastal evolution at Thuan An Inlet, Vietnam

Abstract: The Tam Giang-Cau Hai lagoon is located outside of Hue in central Vietnam. Southeast of Thuan An inlet, one of the two inlets of the lagoon, a groin was constructed in 2008 as a measure to reduce the sediment transport to prevent the inlet from closing. This groin has caused erosion of the eastern sand spit of the inlet and accretion of sediment on the coastline south east of the groin. The objectives of this study was to make a model of the sediment transport on the southeast side of the groin to be able to analyse how different measures to reduce the erosion of the inlet would affect the sediment transport past the groin. The field measurements and parts of the data collection took place in Vietnam during nine weeks in the beginning of 2013. The position of the coastline at Thuan An inlet was measured and the rest of the data, such as e.g. wave data, bathymetry data, previously measured coastlines and sediment transport was obtained from Institute of Mechanics in Hanoi. Other data used in this thesis was also obtained from The University of Agriculture and Forestry in Hue and from The Oceanographic Institute in Nha Trang. The study area has a tropical monsoon climate with two monsoon season per year – the southeast monsoon and the northwest monsoon. The micro tidal climate in the area is fully semi-diurnal and gives that the main sediment transport is wave induced. The modelling software GENESIS was used to model the sediment transport past the groin. The model was calibrated and validated using measured data and then the sediment transport on the southeast side of the groin was modelled during the years 2013 to 2017. First, the case where no measures were taken was modelled, to see what will happen if no changes of the groin were made. After that, three different measures to decrease the accretion of sediment on the southeast side of the groin (i.e. increase the sediment transport past the groin in order to reduce the erosion of the Thuan An inlet) were modelled and analyzed. These three measures were making the groin shorter, increasing the permeability of the groin and dredging sand from southeast side of the groin. The result of the model and the different measures to reduce erosion were discussed. The conclusion was that interventions to reduce the erosion are needed to protect the lagoon and the people living around it and making their livelihood from it. Many people work with e.g. fishery, aquaculture and agriculture and these occupations could be severely affected of the results of continuous erosion, such as changed water environment in the lagoon and flooding.