Study On Formation Mechanism Of Consolidated Silt In Yellow River Delta

Consolidated sand is a kind of submarine dynamic landform,ofen developing as hard crust in tidal flat or shallow seabed.To reveal the physical mechanism of the formation and evolution of it is significant for further understanding the dynamic sedimentary process of silt shoal estuaries and coasts,ensuring shipping safety,and guiding the construction of marine engineering.In this paper,the Yellow River Delta,with wide consolidated sand,was taken as the research object.The natural tidal flat with typical micro-geomorphic units was selected to conduct field test,and the physical properties and micro-structure of sediment samples were tested in the laboratory.From the perspective of micro-geomorphology,the distribution and formation and evolution mechanism of consolidated sand in tidal flat are revealed;Based on the comprehensive analysis of the flume test results published.The formation mechanism of consolidated sand under the coupling action of horizontal scour and vertical seepage of seabed liquefaction under wave was proposed,and the quantitative evaluation method was constructed.The preliminary verification was carried out by taking Chengdao sea area of Yellow River Delta as an example.The main work and conclusions are as follows:Survey lines perpendicular to the tidal creek were arranged in Yellow River Delta natural tidal flats which with abundant micro-geomorphic units.Field tests were carried out,and samples were taken for particle size,physical properties and microstructure analysis.It was found that the tidal flat micro-topography is important in controlling the distribution and properties of consolidated sand.It was shown in the following: the tidal creek area covered by wide seawater,possessing weak bottom erosion and sludge,so it does not develop consolidated sand;The consolidated sand is widely developed in tidal slope ripple area for its strong hydrodynamics,large sediment density,and coarse particles;the water content of consolidated sand in the biological disturbance zone is lower than other areas,because the biological activity has enhanced the structure and drainage consolidation of the seabed.The larger the micro-geomorphic roughness is,the more obvious the consolidated sand is,but the strength and grain size of consolidated sand in this area are smaller than sand ripple area.Based on the rules found in this paper,the distribution and properties of consolidated sand can be preliminarily judged by the micro-geomorphic units of tidal flat.Based on the analysis of literatures,the formation mechanism of consolidated sand under the coupling action of erosion and liquefaction was proposed,that is,the horizontal sediment scored in seabed surface and the vertical fine particles transported caused by liquefaction play role simultaneously,causing fine sediment to be resuspended and coarse particles to remain dense,and forming consolidated sand.Based on the flume test results in literatures,it was shown that the contribution of seabed liquefaction to sediment re-suspension is more than 80%,and seabed scour and liquefaction have coupling effect under wave.Based on the fact that liquefaction and re-suspension are caused by pore water and carrying fine particles out of seabed,and the volume of pore water and fine particles loss in the bed is equal to the total settlement of the bed,a method based on the maximum excess pore water pressure as additional stress,through calculating the seabed subsidence for pore water quantity,and then calculate the liquefied re-suspension was proposed.Taking the Chengdao area of the Yellow River Delta as an example,it was shown that the calculation method of liquefaction re-suspension can well reproduce the process of fine particles re-suspension in Chengdao,and sediment changing from sludge to silt and forming consolidated sand in a certain range of seabed,which proves that the method has certain feasibility for the quantitative evaluation of the formation and evolution of consolidated sand.