The Role Of Seabed Dynamic Response In Sediment Resuspension Under Waves In Modern Yellow River Delta

The resuspension of sediment by marine hydrodynamics in the modern YellowRiver delta plays an important role in the long-distance transportation and destinationunderstanding of the Yellow River-derived sediments into sea. Existing research onsediment resuspension process and its driving mechanism only focuses on interactionof the wave-current bottom boundary layer, but ignores the role of wave-inducedseabed sediment dynamic response, which is necessary for further research on themodern sedimentary dynamic process. Jointly supported by the National NaturalScience Foundation of China "Storm control on seabed erosion and liquefaction in thesubaqueous Yellow River delta"（Contract No.41072215; Awarded:2011²013） and"Investigation of sources and governing factors of seabed erosion and resuspension inYellow River estuary"（Contract No.41272316; Awarded:2013²016）, thisdissertation reveals the controlling role of seabed dynamic response in sedimentresuspension under waves.Dynamic triaxial experiments and wave flume experiments in laboratory, andwave loading experiments in field are conducted to study sediment dynamic responseto waves in Yellow River Delta. In-situ observations in field and wave flumeexperiments in laboratory are carried out to study sediment resuspension in marinehydrodynamic conditions. Field survey and pit tests are proceeded to study spatial andtemporal distribution characteristic of sediment erodibility of the Yellow River Delta.Dynamic triaxial-scouring flume simulating experiments in laboratory, wave loading-recirculating flume experiments in field, and mathematics statistic analysis andtheoretical calculation are explored to study the control mechanism of waves insediment resuspension. Main findings are demonstrated as follows:（1） Under extreme sea conditions of50-year return period, for various sedimentary areas of the modern Yellow River delta, different levels of pore pressureaccumulation occur in the sediment within4m depth below the seabed surface; forareas in old sedimentation age, the accumulated pore pressure can almost be up to theoverlying sediment effective stress and is characterized by two phases, i.e. slowgrowth and rapid growth; whereas for the other areas, the accumulated pore pressureis less than50%of the overlying effective stress, and the accumulation develops bythree phases, i.e. rapid growth, slow growth and almost flat phase. The pore waterpressure generally presents development patterns of cumulative rise-drasticfluctuation-slow dissipation within30cm depth inside the seabed under heavy waveaction; fine particles inside the seabed are transported upward due to the pore waterpressure accumulation and hence the seabed is coarsened; the older sedimentary ageand the worse size gradation of the undisturbed tidal flat sediment, the strongercoarsening effect.（2） Small-scale sediment resuspension occurs when the nearshore waves arerelatively bigger but the pore water pressure accumulation degree is little, andwave-induced shear stress is the significant driving force; large-scale sedimentresuspension only occurs with high-level pore water pressure accumulation underrough sea condition, when the sediment liquefaction due to waves dominates itsgeneration process. Fine particles inside the seabed that under wave actions aretransported upward and resuspended into water can contribute to a maximum of50%of the total suspended matters.（3） Multi-spatial-temporal scale variations exist in the resuspensioncharacteristics of tidal flat sediments in the modern Yellow River delta. Comparedwith high and middle tidal flat, the sediment in low tidal flat is more prone toresuspension and has higher spatial distribution ununiformity. The resuspension oftidal flat sediments can more easily occur in the north and south part but less in theeast and northeast part of the modern Yellow River delta. The freshly depositedsediments show the similar resuspension characteristics to the undisturbed tidal flatsediments in3to4days due to marine hydrodynamic effects. Sediment resuspensioncharacteristics of the modern Yellow River delta are speculated to rise and fall undergo an evolutionary trend of first increasing and then decreasing over centuries ofsedimentary history.（4） Large-amplitude wave loads can cause linear attenuation of sediment criticalshear stress and threshold velocity, which is closely related to the sediment structuralstrength loss under waves. Sediment critical shear stress and threshold velocity isreduced during the early period of relatively small waves, and stable in smallfluctuations thereafter; the softening of seabed sediments due to pore pressureaccumulation is an important factor in the significant reduction of sediment erodibility,and the later change is accounted for the variation of sediment particles under waves.An estimated formula for seabed internal sediment resuspension flux driven by theaccumulated pore water pressure is established based on the field observations andlaboratory experiments data. The internal sediment resuspension flux is about17⁹1g·m^-2·s^-1under waves in different return periods （5to50years） in Chengdao sea area;the maximum values of internal sediment resuspension flux under waves of differentreturn periods appear in the water at depths of7⁸m.Innovation of this research is composed of the following three aspects:（1） Revealing the diversity of sediment dynamic response to waves for YellowRiver Delta sediments deposited in different ages, and the rework of waves and flowsto sediment composition, structure, physical and mechanical properties, and sedimenterodibility.（2） Constructing the model of pore water pressure accumulation, and therelationship between the degree of excess pore water pressure accumulation and theparameters of sediment resuspension.（3） Creatively putting forward the controlling role of wave-induced sedimentliquefaction in seabed sediment resuspension in the Yellow River Delta, andquantitively describing the contribution of seabed internal sediment upward transportto sediment resuspension.