| Arc-shaped coast located at head-land bay is widely distributing in South China. The equilibrium and stability of this kind of coast is virtually a processes of dynamic changes of erosion and accretion due to adjustment, adaptation and feedback of coast geomorphology in response to changing driven actions. In this paper, based on the data including field measured beach profiles, sediment samples, tide and wave of Nanwan coast of Hebao Island, the characteristics of dynamical behavior, grain-size distributions and sediment transport patterns, the appearance of coastline planform, the short-time spatial and temporal changes of the beach profiles and the response of coast geomorphology to dynamical actions were analyzed. Thereafter, the equilibrium and stability of arc-shaped coast was further discussed.The states of refraction and attenuation of wave from three different incident directions were calculated by using wave numerical model. It showed that the attenuated intensity of the wave from SW direction was relatively large and its longshore distribution of wave energy was gradually weak from the shadow zone to the tangent zone. There was a reversely condition for SE wave. Thus, it was a key sculptured action of the Nanwan coastline for the SE wave perpendicular to the tangent zone, which was consisted with the previous result of geomorphological analysis. Thus, the static equilibrium coastline of Nanwan was simulated by adopting the static equilibrium equations and parabolic bay shape equation based on parameters of beach platform related to SE wave. It could be found that the coastline planform of Nanwan was in stable state.The results of grain size analysis were as follows: the sediment was composed of medium sand and coarse sand; the grain size was prejudiced coarse, and its size classification was fairly good; the distributed range of the coarse sand located in the shadow zone was narrow; it was centralized in the middle-tide zone for the sediments that have relatively large value of the medium size; the coarse sand component percentage over the swash zone from the shadow zone to the tangent zone was gradually increased. In addition, the sediment transport trends of transverse movement could be found in the region of from low-tide zone to the adjacent shallow zone. There was a weakly transport trends for the sediment movement from shadow zone to the tangent zone. The temporal changes of the medium size in sediment in the middle-tide zone of three beach sections were of distinct difference, which was a maximal change of the medium size in sediments of the middle zone, the tangent was next and the shadow zone was minimum.Due to the difference of grain size distribution patterns and different coast sections' response to dynamical behavior, the high-tide zone of all beach profiles showed different reflective and dissipative characteristics, while the most intensive reflective state was founded in the tangent zone. The characteristics of the low-tide zone and adjacent area of the shadow zone was described as dissipative state and had a trends toward accretion and more dissipative. By contraries, the tangent zone was transferring to reflective state. The intermediate zone was relatively unstable since its state swinged around reflective and dissipative.Due to the impacts of extremely high wave when a typhoon landing, the characteristics of beach geomorphology responded to dynamical action appeared as erosion in foreshore and accreted in backshore. Beach profile of the shadow zone was more stable than that of the tangent zone; the latter which was relatively stable under usual wave climate got destroyed worst during. Moreover, the adjustment process of sediment was also found over the beach due to the effect of typhoon action. All the above related were similar to the beach process occurred in winter, but much more violently. The beach recovered to the previous state soon after the typhoon died out, the subaqueous bars appeared again and moved to the land.
|
PDF Full Text Request