Nearshore hydrodynamics at Hai Hau Beach, Vietnam : field measurements and wave modeling

Many of the coastal areas in Vietnam have elevations below one meter making them vulnerable to salt water intrusion, flooding, and erosion. The Red River Delta (RRD) located in northern Vietnam has suffered severe erosion along certain stretches of coastline during at least the last hundred years. At Hai Hau Beach in the RRD, a land area of 16 km in length and 2.5 km in width has been lost during the last century due to erosion. Several different hypotheses have been put forwarded concerning the main factors causing erosion at this beach, including the closing off of a sediment-transporting river branch, the construction of a large dam in the river system (Hoa Binh Dam), and sheltering of the beach by the Ba Lat river mouth causing gradients in the longshore sediment transport. Recent studies indicate that the third hypothesis is the most likely one, but detailed knowledge about the effect of the regional topography on waves and currents is needed. The main objective of this study was to investigate nearshore hydrodynamics at Hai Hau Beach in order to gain better understanding of wave transformation, nearshore circulation, and sediment transport in the area. The characteristics of the regional coastline shape together with the complex bottom topography were analyzed with focus on the impact on waves and currents along Hai Hau Beach. Implications for the longshore sediment transport and beach evolution were also considered. Data from two major field experiments at Hai Hau Beach, one during the winter monsoon and one during the summer monsoon (January and August 2005, respectively), were analyzed and used for numerical wave modeling. Both field campaigns involved the same general layout and procedure and were carried out at the same locations with similar instruments. The instruments were deployed at seven stations and were utilized to record waves, currents, and suspended sediment concentration. Four of the stations were located in a depth of 10-20 m, whereas the other three stations were closer to the shoreline at depths of about 3 m. Analysis of simultaneous measurement of local wave properties and sediment concentration indicated a distinct correlation for situations with strong waves, but little correlation when the waves were low and most of the sediment originated from the river discharge. The numerical spectral wave model, STWAVE, was used to simulate regional wave transformation and the propagation of waves into the Hai Hau Beach area. Input data for the simulations to validate the model consisted of measured data from the most offshore measurement station. Model results were compared with recorded waves at the stations closer to the beach, and satisfactory agreement was obtained between calculated and measured significant wave height and wave period. Sensitivity analysis regarding the influence of bottom topography and regional coastline shape on the wave transformation was performed using a larger grid that covered the Ba Lat river mouth. These simulations displayed distinct gradients in wave height along Hai Hau Beach for waves approaching from a sector north to east that may be responsible for the severe erosion observed. (examensarbetet är utfört vid Avd för Teknisk Vattenresurslära, TVRL)