Grain Size Characteristics Of The Modern Tidal Sediments Along The Tianjin Sector Of The West Coast Of Bohai Bay

As a transitional zone, in which marine and terrigenous activities happen reciprocally, the tidal zone is affected by a number of natural factors, including tide, tidal current, wave, river discharge, and human impact.Since 1950s, the human influence has dramatically increased causing powerful changes on the tidal zone of west Bohai Bay.To better understanding the reduction of runoff and sediment discharge, sea level rise and increase of oceanic hydrodynamics, combined with human economic activities and their potential influences, will be greatly significant to the sustainable development of the tidal zone environment.By using granularity method, this paper analyses 42 surface samples, taken from 8 sections, on the tidal zone. Results indicate that seven sorts of surface sediments exist including clayey-silt, sand, silty-sand, sandy-silt, silty-clay, sand-silt-clay and silt. The distribution of tidal sediments are characterized as (1) The clayey-silt prevails in the south of the study area; (2) Grain size becomes sand northward gradually from the mouth of Lujuhe R. to the mouth of Haihe R. (3) Again, clayey-silt spreads out in the north of Haihe R.Comparing the nowadays surface sediments with the counterparts measured in 1980s, change of the grain sizes indicate that both the south and north portions are getting fining while the mid-portion becomes coarser in the past double of decades.39 samples, taken from three undisturbed columns drilled on the upper tidal zone of Mapengkou, Lujuhe and Dashentang, respectively, show a buried changing boundary of grain size in depth of 30~40cm below the surface. Upwards from the boundary, the grain size becomes finer in Mapengkou and Dashentang while Lujuhe becomes coarser. Based on correlation between the grain size curves of the columns and the 1984's grain size, this paper figures out the accretion rates of the upper tidal zone are 1.82cm/yr in Mapengkou, 1.36cm/yr in Lujuhe as 1.64cm/yr in Dashentang as well.Although the content of heavy minerals, analyzed from the surface sediments, are low (<1%), the heavy mineral characteristics and first order isotimic lines of stability coefficients indicate the sediments in the estuary area come from rivers while the sediments of the Mapengkou in the south and the Caijiapu and Dashentang in the north on the tidal zone come from longshore current, and surface sediments of Lujuhe tidal zone come from subtidal zone by tidal current.