Study Of Near-bed Suspended Sediment Concentration Change And Monitoring Method In The Yellow River Delta, China

The Yellow River carrying large amounts of sediment into the sea, and about70-80%of the sediment into the sea deposited at the coastal region of the delta, thebed sediment will resuspend and transport continuously under the hydrodynamic force.Variability of near-bed suspended sediment concentration in the Yellow River Delta,under the effect of hydrodynamic action, is still unclear, and that the existingmonitorig methods unrealized the real-time and automatical monitoring of thenear-bed suspended sediment concentration profiling under field situation. Thefollowing research was based on the existing studies of the National High TechnologyResearch and Development Program of China (863Program)“Marine SedimentRe-suspension Flux in-situ Monitoring Techniques during the Storm Process”(No.2008AA09Z109), and funded by the project of National Nature ScienceFoundation Projects “Study on Consolidated Sediment Re-suspension andTransportation in the Yellow River Estuary”(Project Number,40876042) and “Thestorm against erosion control and seabed liquefaction to the Yellow River SubaqueousDelta”(Project Number,41072215). It aimed to not only study the dynamic change ofbed sediment under the effect of hydrodynamic action, but also apply the in-situ andreal-time antomatic monitoring system to estimate suspended sediment concentration(SSC), and provide a new way for the in-situ research of sea-bed sedimentresuspension and transport processes.The resuspension and settling process of the sediment under different wavesimposed on the seabed soils, collected from the modern Yellow River delta，areobserved in a laboratory flume experiment. Through the flume experiment and thefield test in the Yellow River Delta, it not only established the correlative equationbetween SSC and probe resistivity, but also analysis the influence of salinity. Based on data on the monitored electrical resistivity, waves, tides and currents collectedfrom long-term observation experiment in the adjacent Bohai Sea, effect ofhydrodynamic factors on the resuspension, erosion and deposition of seabed sedimentis investigated. It is showed that:(1) With the wave flume model experiment, we have come to the followingconclusions: resuspension flux of seabed sediments increases with wave height undercertain water depth with a good linear fitting relationship；the resuspension of seabedsediments experiences about5000-6000wave cycles under the continuous action ofcertain height waves. At the primary stage of wave action, the difference between lowlayer concentration and the middel-above layer concentration is significant, thevertical concentration structure presents a slanted type. After the distribution of SSCin each water layer is stable，the ratio of SSC at the bottom to that at the surface isabout0.96-1.98and the SSC uniformly distributes in the whole water．During thehydrostatic sedimentation process，the SSC presents a law of exponential dampingover time and there is a good negative linear correlation between SSC and settlementflux when the SSC is high (greater than1g/L).(2) The feasibility test that using electrical resistivity probe to monitor SSC areobtained the follwing conclusion: the electrical resistivity of the probe is sensitive towater salinity. The electrical resistivity of the probe decreased exponentially withsalinity under water. Therefore, monitoring suspended sediment concentration heve tobe made under salinity constant conditions. The results showed that the electricalresistivity increases linearly with SSC at the same salinity. The influence of SSC uponthe electrical resistivity will increase along with the ruduce of water salinity. This newsystem can monitor variations of SSC real-time, antomatically and in situ for longtime. It provide a new way for monitoring SSC in situ and automatically. The systemcan monitor sediment deposition process in real-time, and distinguish seabed-highconcentration suspended sediment–water interface accurately. The ratio of soilresistivity to water resistivity is about3.6. The resistivity for silty soil would increseas the pore water discharge form sediment. The soil resistivity is closely related withit’s porosity. (3) The field test of using electrical resistivity probe to monitor near-bedsuspended sediment concentration in the Yellow River delta show that: the system canaccurately monitor the sea tide, the variation of the suspended sediment concentrationin ebb cycle showed a good correlation with the tidal water level and the currentvelocity in this survey station. In general, SSCs will increse during spring tides, butreduce in the ebb process in certain water depths. The seabed erosion and depositionis closely related to the near seabed flow velocity. The surface sediment depositedwhen the flow velocity is lower than20cm/s. When the flow velocity is greater than20cm/s, bottom sediment will wash out and suspend, result in the quick errosion ofthe new siltation sediment. Because the wind wave is big in this area, the verticaldistribution of the suspended sediment concentration in the Yellow River delta is veryasymmetric.The innovations of this thesis were as follows:(1) Based on the wave flume model experiment, the paper presents theresuspension law of bed sediment under vave action, and the impacts of wave heighton the vertical distribution of suspended sediment concentration.(2) Using electrical resistivity probe to monitor suspended sedimentconcentration, we obtained the relation equation between electrical resistivity of theprobe and SSC under field situation. It provide a new way for monitoring SSC in situand automatically.(3) The seabed erosion and deposition process, and near-bed sedimentconcentration change process were monitored by electrical resistivity probe throughlong term observation test which was conducted on the sea. It studied the effect ofhydrodynamic factors, such as waves and currents, on the variation of near-bedsediment concentration.