Effects Of Wave-induced Seabed Liquefaction Degree On Sediment Re-suspension In The Yellow River Delta

The sediment in the subaqueous Yellow River Delta is eroded seriously undermarine hydrodynamic conditions. The existing studies have shown that the combinedaction of waves and currents leads to the surface sediment in the estuary erosion andresuspension, i.e. that waves lift sediments and currents transport sediments.However, recent studies have found that liquefaction and seepage force can leadsome internal clay grains in some depth to the seabed surface, and seabedliquefaction is the crucial reason resulting in the sediment erosion and resuspension.The research on the liquefaction degree of seabed sediment under the effect ofhydrodynamics in the Yellow River Delta is still unclear，and the quantitativedescription of the contribution of soil liquefaction to re-suspension quality is almostunknown. The following research is funded by the National Natural ScienceFoundation of China "Study on Consolidated Sediment Re-suspension andTransportation in the Yellow River Estuary"(Contract No.40876042) and "Thestorm against erosion control and seabed liquefaction to the Yellow RiverSubaqueous Delta"(Contract No.41072215). It is aimed to discuss the effects ofwave-induced liquefaction on sediment re-suspension and granulometriccomposition of the seabed in the Yellow River Delta, and reveal the relationshipbetween liquefaction degree parameters and the quantity of re-suspended sediment.The achievements obtained in this research can give a better understanding on thedynamic process of erosion, re-suspension and transportation of benthic sediments,and play a significant role in the prediction and control of geologic hazardous in thecoastal zones of the Yellow River Delta.The pore pressure accumulation and re-suspended sediment concentrations afterthe wave action onset was observed in the flume simulation experiments inlaboratory and in-situ investigation on different tidal flat with depositional age in the Yellow River Delta; The parameter "liquefaction degree"s was introduced todescribe the liquefaction process and evaluate degree of liquefaction, and reveal therelationship between liquefaction degree and the quantity of re-suspended sediment;The samples from these experiments were tested by grain size analysis to obtainvariations in the seabed granulometric composition under wave-induced seabedliquefaction. Some conclusions can be drawn as follows:(1) From the laboratory flume experiments, the results indicate that under differentwave action, the re-suspended sediments are resulted from the surface sedimenterosion and the internal seabed sediment upward transportation, and the initialincrements of sediment re-suspension mainly come from the re-suspension ofsurface sediment erosion, accounting for approximately50%to80%of the total.Whereas with the sustained action of waves, the later increments of sedimentre-suspension are from the internal seabed sediment upward transportation due to thewave-induced liquefaction and seepage, accounting for approximately20%to50%of the total; Liquefaction degree directly affects the amount of sediment frominternal seabed transport into the water accounting for the total suspended sediment,and the proportion of internal re-suspension increases as liquefaction degreeincreases, revealing that liquefaction contributes to the quantity of re-suspendedsediment. The granulometric composition changes significantly within the liquefieddepth inside the seabed before and after seabed liquefaction. The internal seabed finesediment migrates upward in the liquefaction process, so that the seabed gradationbecomes poorer and sorting becomes better, revealing that liquefaction effects thecomposition and structure properties of the soil bed significantly.(2) From the in-situ wave generation experiments on intertidal seabed in theYellow River Delta, it is found that excess pore pressure accumulates under cyclicloading on each tidal flat with different depositional age in the Yellow River Delta.The quantity of re-suspended sediment develops from a stable state to the maximumshortly after the excess pore pressure reaches maximum. There is a strong linearcorrelation between the average value of maximum liquefaction degree and the laterre-suspension increment, showing that liquefaction is contribution to the quantity of re-suspended sediment, and it is consistent with the flume experiments in laboratory.The internal fine-grained material will take more time to transport to the seabedsurface, so that the variety of the quantity of re-suspended sediment lags behind theexcess pore pressure. There is an interaction relationship between the seabedgranulometric composition and liquefaction. The seabed granulometric compositionis an important factor to liquefaction, and seabed liquefaction transformed thegranulometric composition inversely, that the coefficient of uniformity Cuis changedafter seabed liquefaction at each site.(3) The effect of liquefaction degree on the quantity of re-suspended sediment canbe explored using two methods to calculate the contribution degree of seabedliquefaction quantitatively to the quantity of re-suspended sediment. The twocomputational results show that all the quantity of re-suspended sediment increasesto a certain extent after seabed liquefaction, i.e., the quantity of re-suspendedsediment is larger under the seabed liquefying conditions than that undernon-liquefying conditions, showing the effect of liquefaction to the quantity ofre-suspended sediment.The innovations of this thesis are as follows:(1) Based on the wave flume experiments and in-situ wave generation experiments,the paper establishes the evaluation method of liquefaction degree and reveals therelationship between liquefaction degree and the quantity of re-suspended sediment.This has not yet been carried out by other researchers.(2) The results reveal that the quantity of re-suspended sediment not only from"Waves lift sand and currents transport sediment " of the surface sea-bed, but alsocome from the seepage upward transport within the soil fine-grained materials.(3) The impact mechanism of liquefaction degree on the quantity of re-suspendedsediment is explored, and quantitative calculation of the contribution degree hasbeen first realized using two methods.