Spatiotemporal Dynamics And Driving Factors Of Suspended Sediment Concentration In The Yellow River Eatuary And Its Adjacent Waters Based On GOCI Images

As a key area of river-ocean interaction,the large river delta is one of the most frequent and active areas for material and energy exchange,and is also an area with an extremely fragile and sensitive ecological environment.Large rivers continuously transport abundant suspended matter to the ocean,which affects the distribution and diffusion of pollutants,promotes the circulation of carbon,oxygen and nutrients in the coastal waters,shapes the underwater topography,and affects the erosion and siltation of the coastline.In addition,it can significantly increase seawater turbidity and cause sunlight attenuation,which makes it an important regulator of primary productivity and oxygen cycle in coastal waters.Correspondingly,the complex dynamic environment of the sea near the delta also greatly affects the temporal and spatial dynamic changes of suspended sediment concentration(SSC),shoreline changes,terrain evolution,water discharge and sediment load,tidal currents,wind waves,storm surges and human activities.All these factors have driving effects on SSC.The Yellow River is famous for its less water and more sediment.In recent decades,a new situation of low water and less sediment has gradually formed.At the same time,the dynamic conditions of the coastal sea are complex,the temporal and spatial dynamics of SSC have new characteristics.Therefore,the study of the temporal and spatial dynamics of SSC and driving factors in the Yellow River Delta's coastal water has great significance for exploring the dynamic mechanism of the Yellow River Delta's coastal waters,the evolution of underwater topography,the construction of estuary and coastal waters,even the natural resources and protection of the ecological environment.This study takes the Yellow River Delta and its adjacent sea areas as the research object.Select about 1,000 GOCI images from 2011 to 2020,combined with water and sediment data,years of measured terrain elevation,wind speed and direction,waves,flow speed and direction,particle size data,utilize spatial analysis and mathematical statistics methods to discussed the spatial and temporal dynamic characteristics and driving factors of SSC in the adjacent waters of the Yellow River Delta.The main research results are as follows:(1)An empirical model is established using the measured SSC data and the reflection of the image bands in the study area.It is found that the nonlinear structure retrieval consequent is better than that of the linear structure.The combination of the near-infrared band is better than that of the visible light or near-infrared band,and the multivariate function model is better than the one-variable function model when using GOCI images to build a high-concentration water suspended sediment model.(2)In terms of the change of SSC,the sea area adjacent to the current estuary has always been a high value area.In terms of inter-annual variation,the diffusion range of high SSC(?100 mg/L)was mainly within 10 km from the estuary,and the inter-annual variation of SSC in Laizhou Bay is small.During the year,the current high SSC diffusion in the adjacent waters of the estuary can be as far as 15 km away from the estuary,and the SSC is the highest from July to August.The sand diffusion range is about 3-35 km from the shore.The tidal cycle changes are mainly concentrated in the coastal waters of the estuary.It is found that the difference between the high-concentration suspended sediment area is small by extracting the hourly high SSC boundary,but the boundary swing angle has a large difference.The trend is basically the same.The dispersion area,STD and CV value of suspended sediment with different concentrations on spring tide are higher than those at neap tide,which mainly occurs in the sea areas on both sides of the estuary.The measured data show that during the ebb and flow period,there is no good correspondence between the high sediment content of the surface water in the near-shore waters of the estuary and the center of the high flow velocity.The area of high SSC diffusion on spring tide day was significantly larger than that of neap tide,and the effect of sediment resuspension on the SSC value was more obvious due to the difference in tidal velocities between large and small tides.(3)Natural water discharge and sediment load are the main driving factors for the interannual and intra-annual variation of SSC in the coastal waters of the estuary.On the interannual scale,the main influence range of water discharge and sediment load is within 5 km from the estuary,within this range,the SSC is basically greater than 200 mg/L,and the farther from the mouth,the smaller the influence,the 15 m isobath is the limit of influence.In annual changes,the influence range of monthly water discharge and sediment load in the wet season can reach as far as 10 km(15 m isobath)from the estuary,and in the dry season,the influence range is about 5 km away from the estuary.Laizhou Bay is less affected by water discharge and sediment load.High SSC can diffuse to 12-20 km away from the estuary during water and sediment regulation,and the diffusion range of suspended sediment with different concentrations is significantly larger than that of natural situation.(4)The effect of wind on SSC is reflected in two aspects: monsoon and short-term strong wind.The monsoon has a greater impact on the waters in the northwest of Laizhou Bay,and the northerly strong wind is the main controlling factor for the monthly variation of SSC.The SSC in the northwest of Laizhou Bay will increase significantly under the influence of the strong winter monsoon.On the short-term strong winds,the whole process of SSC changes during the typhoon "Capri" passed through Shandong is analyzed,it is found that SSC is positively correlated with wind speed and has a great impact on Laizhou Bay.The impact can continue for 3-4 days;after the typhoon landed,the maximum wave height and effective wave height both increased significantly,the influence time is longer than wind speed.The wind and the waves generated by it caused strong resuspension of the bottom sand,resulting in a short-term and rapid increase in the SSC.(5)Comparing the underwater topography of 15 fixed sections and SSC from2012 to 2018,it is found that the high value area of SSC is generally located in the obvious erosion or deposition area.There are two high-value areas of SSC in the north of the estuary,corresponding to the deposition area and micro-erosion area respectively;there is a high-value area in the old estuary,corresponding to the terrain micro-deposition area.Topographic erosion and deposition are related to the complex marine dynamics of the estuary.The large amount of sediment provided in the deposition area increases the concentration of suspended sediment,while the high SSC in the erosion area is mainly caused by the strong scouring of the nearshore and the resuspension caused by the marine dynamic.Based on Landsat 8 OLI images and DSAS shoreline analysis system,it is found that shoreline erosion causes high SSC to retreat to shore,and shoreline siltation causes high-concentration suspended sediment to expand seaward.