The Tidal Geomorphology In The Eastern Bohai Sea And Its Controlling Factors

The submarine geomorphology in the eastern Bohai sea is a complete tidaldepositional system with relative factors. The geomorphic types and distributionsare dominated by tidal dynamic patterns. In this paper, research was based on thedata about the surface sediments and minerals analysis from “China’s coastal marineenvironment, a comprehensive evaluation” in Bohai section. And it used thesimultaneous observational data of sand ridges and current in the trenches, data of thedepth and terrain, sidescan-sonar investigation and the surface sediments, those werein the geomorphic survey of Liaodong shoal and its surrounding areas from NationalNatural Science Foundation of China “The evolutionary process of the tidal sandridges and its response mechanism in Liaodong shoal”. Research included thegeomorphic types and distributions of sand ridges-trenches-sand sheets,micro-geomorphic characteristics and the grain size distributions of differentgeomorphic units. Research adopted calculations of sand waves’ migration rates andnumerical modeling method to discuss the conditions of the sand waves’developments. According to the transverse and vertical observational data ofcurrents, the sand ridges-trenches distributed regularities of the currents wereresearched, and the geomorphic evolutional tendency about the tidal depositionalregion in the research areas was discussed. Four conclusions were made in thisresearch, as follow:(1) The length in E-W direction of the east side of Laotieshan water lane is about49km, the length in NE-SW direction of the west side is about43km. The deep trenchin the center area is88km in length, bout20km in width, with a maximum depth of86m. The main surface sediments are thick silty-sandy gravels, the sorting isextremely poor. The distances between each sand ridges of Liaodong shoal are7.0~16.8km, with differences of6.8~21.9km between ridges and trenches. The main sediments are sand with better sorting. Large-scale and middle-scale sand wavesare developed there. The sediments of the trenches are sandy silt, silty sand and siltetc. with poor sorting. The area of the center part of sand sheets in Bozhong shoal is670km2. The main sediments are silty sand with poorer sorting. The surfacesediments of Laotieshan water lane have a content of stable heavy minerals less than10%, such as garnet and ilmenite. The content of stable heavy minerals in Liaodongshoal is more than15%, relatively increased. The content of stable heavy mineralsin sand sheets of Bozhong is10~20%.(2) The sand waves of the sand ridges in Liaodong shoal are mainly developedon the spines of ridges, which has1.6~2.9φ of average grain sizes in bottom materials,more than50cm/s in maximum bottom flowing velocity, and approximately16~26min depth. The sand waves are mainly asymmetric, with2~23m in wave length and0.2~1.2m in wave height. The gentle slopes are at the southwestern side, and theabrupt slopes are just at the opposite side, with a strike of WNW-ESE direction. Thegrain sizes of bottom materials are3.2~5.3φ in the trenches, the maximum bottomflowing velocity is33.3~35.0cm/s, so the sand waves are not developed. Accordingto the calculation of Shinohara’s migration rate formula of sand waves, the migrationrates are about3.26×10-6~2.65×10-4cm/s, and the maximum migration distances areabout1.6~7.6cm/d. The geomorphic characteristics of sand waves and net migrationdirection reflect that the net migration direction of the sediments has generally thesame NNE direction as the main flowing direction of tide.(3) The main flowing direction of high and low tides to the sand ridges isNNE-SSW. The included angle between the flowing directions and the long axis ofsand ridges is9~30°. The maximum flowing velocity at the surface of ridges is103.9~118.8cm/s, and minimum flowing velocity of it is52.2~67.0cm/s. Themaximum flowing velocity at the surface of trenches is108.7~124.3cm/s, andminimum flowing velocity of it is33.3~35.0cm/s. The dividing depth of the bottomflowing velocity between ridges and trenches is about0.7H layer (H is the price ofdepth). The ridges are gradually under erosion, and the sediments of the ridges havea net migration direction along NNE, because the flowing velocity of ridges is fast. For the lower flowing velocity of the trenches, the velocity of the tide is slower thanthe sediment-moving incipient velocity, except the high and low tide during springtide times. So the trenches are under deposition in majority of times, and sometimesunder slight erosion.(4) Laotieshan water lane remains the origin of the sediments to the researcharea. In high velocity dynamic condition, the bottom of water lane will be erodedcontinuously, depth will be deeper, the sediments that transported through Laotieshanwater lane to Liaodong shoal and Bozhong shoal tend to decrease. The tidal velocitydifferences between the ridges and trenches result in the gradual erosion to the spinesof the ridges. The trenches are gradually deposited, the height differences aredecreasing. The ridge-trench geomorphologies tend to be flat. Sand ridges migratealong the main direction of tide, the sand ridge-trench geomorphologies come intodegraded period. According to the present analysis of the tidal patterns, depths andlandforms, the grain sizes of the sediments and sand waves’ migration rates, theridge-trench geomorphic patterns of Liaodong shoal will remain stable inhundred-year scale.