Short term morphological changes of Suang-SI estuary

Suang-Si estuary and adjacent beaches are one of the most popular beach resorts in TAIWAN. However, the spit of the river mouth sometimes was totally swept away. In few cases, it did not return to its commonly recognized state. Investigations including field surveys, numerical modeling and physical modeling were conducted to look into the mechanisms causing these morphological changes. Results show that the flood scale and the magnitude and direction of waves characterize the short term morphodynamic processes of this estuary. As the peak discharge less than about 1,000 m³/s, flows would still follow the original main channel confined by the geometry of the spit. As the peak flood discharge larger than 1,000 m³/s, it would create a new path discharging into the sea. In cases for even larger flow rate, the spit would be totally swept away. Meanwhile, if the flood happens simultaneously with a typhoon induced waves coming mainly from NE or NNE, the sediment would be transported to deeper water, where it is hardly to be transported back to the coast by natural processes. In cases the typhoons take place in autumn or even later the year, the combined effect by monsoon waves coming from NE and by the storm waves would worsen the situation.