Effects of typhoon on sedimentary processes of embayment tidal flatA case study from the “Fenghuang” typhoon in 2008

Typhoon, as a high energy dynamic event, has the obvious influence on the sedimentary processes of the tidal flat. The Luoyuan Bay is located in the north of Fujian Province, China and the intertidal flat is distributed extensively in the bay. In order to understand the sedimentary processes of the tidal flat during this typhoon event, the in situ measurements of the water level, the tidal current speed, the suspended sediment content (SSC) and the wave height were carried out on the tidal flat during the period when the "Fenghuang" typhoon influenced the region, and the surfacial sediments, under the different conditions were collected. Analyzing the obtained data shows that the water inundation time increases, and the value of storm surge can reach 1.1m; the variations of water current flow are very complicated, and the current velocity in the inner of Spartina alterniflora marsh is higher than that at the marsh edge. The influence of the typhoon on the SSC was highly significant, with the average SSC over the whole tide cycle being 7 times 2 d before the tyhhoo occurred. The decimus wave height can reach 1.76 m with the maximum water depth of 4 m. The repeated surveys of surface elevation show that the erosion occures on the whole bare flat along the section; only certain regions within the inner of marsh are eroded, with the erosion depth of 4.5~5.5 cm, however, the value only reaches 0.7 cm at marsh edge. The main reason of erosion on the marsh is that the plant roots are snapped off on the flat surface under the interactions among wind, wave and current, which carry off the sediment around the plant roots. With the decreasing of influencing intensity of the typhoon event, the whole flat experienced deposition. The rainfall intensity plays an important role on sediment activity.