| Bohai Bay(west of 118°20′),one of the three major bays in Bohai Sea,is～7,700 km~2 of the sea area.Bohai Bay locates in the west part of Bohai Sea.The Binhai New Area, west coast of the Bohai Bay,has been upgraded into the national strategic plan,and has becoming the Third Economic Pole after the Pear-River-Triangle and Changjiang-River-Triangle Regions.We had a chance to investigate the lower intertidal zone and shallow sea as well till the water depth of 10-13m,by using both EdgeTech 3100P and bathymetry HD-27.After three-year-investigation,a group of ridges,located in the north part of the Bohai Bay,with water depth of～4-7m had been revealed.They are parallel with eachother.The strike direction of the ridges is mainly NW-SE,roughly parallel to the circulation of the main regional trend.The strike direction of flood current is northwesterly,and the ebb current is southwesterly.The topography of sand ridge is ridge groove phase,that is,a washout between two parallel sand ridges.These two sand ridges were named No.1 and 2 form west to east.No.1 sand ridge is 6kms long,3kms wide,and about 2m thick;No.2 sand ridge is 5kms in length and 3kms in width,and 1～2m in thickness.The washout between these two sand ridges is NW-SE in strike direction,whose water depth is northwestwards decreasing gradually.Based on the sub-bottom profiles,the SW-oriented slope of No.1 Sand Ridge is relatively steeper in contrast with the NE-oriented slope,and the morphology of sand ridges is hummocky.From the sub-bottom profiles,ridges have downlapping patterns.The tendency of bedding is point to sand ridge migration direction.The strike direction of the ridges is parallel to the main trend of tidal,ridges show linear distribution.We have taken 118 samples in the shallow sea,by using MasterSizer2000,and the method of moment measurement to get data.The type of surficial samples in the surface of ridges is fine sand,positive skewness,good sorting,the mean size of sediment is 3～4φ. Composition of probability curve is mainly saltation component which is less than 5φ.Its content is about 60 to 70 percent,levitation components in about 30 to 40 percent. Saltation component is divided into two sections by 3φnode,the first saltation component of the election is better than the second saltation component,which may reflect that the type of sediment is controlled by two different directions and speed of the fluctuation current,and show it is related with current.Claysand locates in the south of the sea.the mean size of sediment is 6.5～7.9φ, sorting is 1.5～2.3,skewness is -0.2～0.2,kurtosis is 1～1.1,content of clay is 20%～35%, fine sand is 50%～70%,sand is 0～20%.Sandslit locates in 0～5m sea,the mean size of sediment is 4.5～6.5φ,sorting is 2～2.5, skewness is 0～0.4,kurtosis is 0.8～1.1,content of clay is 15%～20%,fine sand is 45%～60%,sand is 207/～40%.Composition of probability curve is bimodal,main peak is 3～4φ,another is 7φ.Slitsand locates in 0～5m sea,the mean size of sediment is 3.6～4.4φ,sorting is 1～2.4, skewness is 0.1～0.6,kurtosis is 1～1.4,content of clay is 10%～15%,fine sand is 25%～45%,sand is 45%～70%,Composition of probability curve is 3～4φ.Sand locates in 2～5m sea,the mean size of sediment is 3.1～3.3φ,sorting is 0.6～1.3, skewness is 0～0.2,kurtosis is 1.2～1.5,content of clay less than 5%,fine sand is 2%～20%, sand is 80%～95%,Composition of probability curve is 3～4φ.In the North of Bohai Bay,from east to west,marine sediment became smaller,sand and silty sand gradually turn into sandy and clay silt.In the last 26 years,No.1 Sand Ridge have moved 6km from southeast to northwest, i.e.240m per year.
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