Preliminary Study On Sedimentary Dynamics Of The Subaqueous Delta In Nanliujiang Estuary

Understanding the sediment transport process and it's control mechanism in the Submerged Delta of the estuary is very important for the coastal engineering construction and estuarine ecological environment protection.In this study,the Nanliujiang River subaqueous Delta in Guangxi was taken as the research object.The spatial distribution and transport mechanism of suspended sediment in estuaries are analyzed,inversion of Sedimentary Dynamic Environment in Estuary Area by End Element Model Analysis,and the erosion-deposition process of the bottom boundary layer is analyzed by means of the observation of the bottom quadrupod instrument.The sectional survey of temperature,salinity and suspended sediment concentration from mouth to Submerged Delta front shows that the suspended sediment concentration was mainly affected by the material input by river near the mouth,while the temperature and salinity stratification in water becomes more and more important in the process of advancing toward the sea.The mooring observations of temperature,salinity,suspended sediment concentration during spring and neap tide at the Submerged Delta front reuslts show the tide types were distinguish differents between spring tdie and neap tide.The spring tide type was diural tide,while the naep tide was semi-diural tide.Thersfore,the times series of current speed,temperature and salinity also have distinguish difference between spring and neap tide.These factors further affect the distribution of suspended sediment concentration.The analysis of suspended sediment transport mechanism during spring tide shows that Euler's residual sediment transport was the dominant factor for net suspended sediment transport,followed by Stokes' s residual sediment transport.The results of sediment dynamics analysis by in-situ observation data show that: waves play an important role in sediment erosion in the study area.However,when the seabed was free eroded,the lateral transport caused by tidal currents maintains the suspended sediment concentration in the near seabed.There were obvious differences in the floc size frequency distribution betweem four pharse,e.g.ebb tide,low tide,flood tide and high tide,during observation.The results of correlation analysis between mean size of floc and shear stress show that,there was significant positive correlation between mean of floc size and stress,which mean that that the increase of shear stress can facilitate the flocculation of floct.And by further performing correlation analysis between shear stress and relative volume concentrations of 32 classes measured by LISST-100 X also show that the microfloc can be convertd to macrofloc when the shear stress increase.Correlation analysis result show that,there was a significant negative correlation between the mean size of floc and salinity,indicating that "salt flocculation" did not play a dominant role in the salinity range of the estuary,and the increase of salinity would facilitate the flocs broken-up.Correlation analysis resuts also show that,the SSC and temperature did not influence the flocculation or broken-up of floc with significiance.In addition,the process of seabed erosion-deposition during the observation period was reproduced by Partheniades-Krone model with in-situ instrumental observation data.It is also proved that it is feasible to determine the erosion coefficient of cohesive sediment by using the index of agreement I between the field observation data and the model simulation result.The sediment grain size distribution can be used to reveal the sedimentary dynamic environments.Based on the garin size ananysis of the Nanliujiang River subqueous Delta,Guangxi Province,we identified the sediment sources and discussed associated sedimentary dynamic environments of the study area using methods of the end-member analysis conbined function fit and Flok classification.The analysis results indicated that the surficial sediment can be divided into 9 types,including 5 types of sediment wih gravel content and 4 types of sediment without gravel content.The spatial distribution of surficial sediment type is coincident with environmental dynamic characteristics near the estuarine environment.According to the end member analysis,there are 5 end members can be distinguished,which represents 5 types of sedimentary dynamic environment.The EM1(end member 1)reflects dynamic effects of wave induced by wind and tidal currents on surficial sediments,the EM2 indicates the dynamic process of surges on surficial sediments,the EM3 reveals the effects of Nanliujiang River diluted water on surficial sediment transpot processes,the EM4 represents the relicts sediment in near continental shelf,the EM5 is an noise end member,which may be induced by anthropogenic activities,such as coastal engineering construction,ect.Based on the experiences of paper,if we try to use the end member analysis to study the sedimentary dynamic environment in an area with complex environments(e.g.small mountainous river estuary-shelf system),we need to note two aspects: on the on hand,we should take into account that sediment sources and environmental dynamics characters in the study area,on the other hand,we can also combine other methods to corroborate mutually,such function fitting or grain size trend analysis.