The Influence On Current Ridge, Sand Wave And Topography Of Dongfang Offshore By Typhoon

The continental currents can make the sea bottom sand to all kinds of seabed forms. The seabed form, which grows parallel to the main current, is called the submarine current ridges, and sand dunes or sand waves, which are developed vertical to the main current. The sand dunes are the typical forms on the sand seabed. The migration and movement of sand dunes can empty the sand overlaying the pipes or bury the pipes leading to the base destabilization of buildings, and are the marks of the sea environmental changes. The current ridges and sand waves have close relationships with the marine engineering, navigation and military affairs, so they have been a major research focus in geosciences.The study area is located in the southeast of the Beibu gulf in South China Sea, which is off-seashore to Dongfang in Hainan province(18?0'-19?0'N, 108?0'~ 108?0'E). There are two major kinds of sea bottom topographies: the current ridge and sand wave. The height of sand waves is 1-5.5m, the wavelength is 10~30m, and the height of the current ridge is 6~20m, the wavelength is 2~4km. Through comparing the lOOOkm's sonar data, VOOkm's multi-beam data, and 1200km's echo-sounding data, which is collected before and after typhoons, it is calculated that: (1) According to the tides in normal situation, the current ridges and sand waves are divided into three parts including east, middle and west region. By using the modified equations, the calculation results reveal that, in the shallow area (the east region), the movement speed of the sand wave is 5-50m/a. When the angle between the current ridge and water flow direction is 10 degrees, the movement speed of the current ridge in the region is 0.1-lm/a. The west region belongs to deep area. The movement speed is l-10m/a for the sand wave and 0.01-0. Im/a for the current ridge. Because the flow speed of tides is similar in the middle region, the sand wave is relatively stable. (2) According to the sonar data and multi-beam data, it can understand the influences of typhoons on the sea bottom topographies: based on the characteristics of the seabottom in shallow area, the erosion and grooves appear after typhoons, but the scales of them is small. In the area covered by the sand wave and current ridge, the typhoon can make the big dune lager, level the small dune, Another possibility is the displacement distance of the small sand wave is too far. Typhoon can also make sand waves move and the displacement is between 10-60m. According to the sonar data, the displacement of the current ridge is short. (3) The theoretical calculation results show that when the wave height of the seawater during the typhoon is from 5m to 10m and the 3m-height sand wave is in 30m water depth areas; the displacement is between 1 and 100m. Through calculating the displacement of the sand wave on different conditions including water depth, height, it is revealed that, on the same conditions, if the sand wave is higher, the water depth is deeper and the typhoon influence time is shorter, the displacement will be smaller. (4) Through comparing the theoretical calculation and the field investigation data, it is shown that the region covered by the current ridge and sand wave in Dongfang offshore are caused together by tidy currents and strong bottom currents mainly caused by typhoons. The function of tidy currents system is to maintain and restore the sand wave. The main reason for long displacement of sand wave is the strong bottom currents mainly caused by the typhoons.