| Estuarine delta is in the complex area of land,river and ocean interaction,which is influenced by the double action of material exchange between land and sea.Qingshuigou lobe-Qing8 channel period of the Yellow River Delta is the current branch to which the Yellow River reaches the sea,which brings a lot of sediment and runoff.After the diversion of the Yellow River,the influence of the ocean dynamic action is enhanced.Combined with the background of global climate warming and sea level rise,as well as the new situation of"low water and less sand"of the Yellow River in recent years,as well as the subsequent changes of waves,tides,landforms and other Marine dynamics,the geomorphic evolution of the Yellow River Delta has become more complex and unpredictable.In addition,the Qingshuigou lobe spout has a broad tidal flat and a large tidal trench system.Under the joint action of natural and human factors such as wetland reclamation,the amount of sand into the sea,ocean dynamics,storm surge disasters,the tidal trench system is sensitive and changeable,which will cause certain impact and harm to the evolution of Qingshuigou lobe and wetland protection area.Therefore,the change trend of shoreline and land morphology and the development process of tidal gully network were revealed,and the main factors affecting its evolution and development were revealed,which provided data support and decision-making basis for ecological protection and sustainable development of the gully spout area.Based on remote sensing image data,chart data and incoming sediment data of the Yellow River Delta from 1996 to 2022,this paper extracted the coastline of the Qingshuigou lobe spit area year by year,and quantitatively analyzed the change process of the Qingshuigou lobe spit shoreline and land morphology with the digital shoreline analysis system since 1996.Based on the data of water and sediment,precipitation and surface air temperature in the Yellow River Basin,the effects of water and sediment change,ocean action,climatic factors and human activities on the evolution of Qingshuigou lobe were analyzed.The tidal gully network in characteristic years was extracted,and the temporal and spatial variations of its quantity and length were analyzed.The density,branching rate and curvature of tidal gully were selected to quantitatively analyze the morphological changes of the tidal gully network and its response to the geomorphologic evolution of the sandy gully since 1996.The main conclusions of the study are as follows:(1)The landform evolution of the Qingshuigou lobe in the Yellow River has significant spatial difference.The flow path of the Qingshuigou lobe in the abandoned estuary is mainly affected by the enhancement of ocean dynamics and erosion receded.The average erosion rate is-112.53 m/a,and the land area decreases by-3.36 km~2/a annually.The current Qing8 channel period of estuarine was mainly affected by the sediment transport of the Yellow River and the water-sand transfer project,and silted to the sea.The average siltation rate was 150.50 m/a,and the land area increased by 3.81km~2/a annually.The erosion rate reached the peak within five years after the diversion,with an average annual erosion rate of-5.68 km~2/a,and the erosion rate stabilized after2001.Geomorphologic changes in Qing8 channel period are closely related to the water and sediment flux in the Yellow River.During 2001-2005,water and sediment transfer work reached the highest siltation rate in Qing8 channel period,with an annual siltation rate of 6.80 km~2/a.(2)A“U”shaped bay was formed in the west Qing8 channel period in August 2007after the Yellow River bough was naturally produced to the north.The hydrodynamic conditions in the bay were weak,and the west and south branches of the bay were in the balance of scour and silting,while the east branch was in the silting to the sea.Due to the phased changes of water and sediment flux in the Yellow River and water and sediment transfer activities,the bay west of Qing8 channel period was divided into three stages:a rapid growth stage from 2007 to 2013,a stable development stage from 2013 to 2018,and an accelerated maturation stage from 2019 to 2022.(3)During 1996-2008,the change of the Yellow River sediment into the seawater is the main source and driving factor of sediment deposition in the delta landform.Among the climatic factors,precipitation and surface temperature in the Yellow River basin have a positive response to the scour and silting evolution of the Qingshuigou lobe.However,the influence of climate factors on the Qingshuigou lobe evolution is less than that of the Yellow River water and sediment flux.The water transfer and sand transfer project can effectively increase the sand spit area of Qing8 channel period branch and respond positively to the scour and siltation evolution of the Qingshuigou lobe.The artificial diversion of water through the Yellow River caused the erosion area of sand spout to increase.During 2009-2021,the response of the Yellow River water and sediment flux to the evolution of Qingshuigou lobe was weakened.The influence of precipitation and surface temperature on scour and silting evolution of Qingshuigou channel period of Qingshuigou lobe in the Yellow River Basin has almost disappeared because the water and sediment flux in the Yellow River Basin has been completely dominated by human factors.The water and sediment transfer project will cause the Yellow River to have more water and less sediment and the ratio of water and sediment will continue to decline,resulting in the decrease of the correlation between the amount of sediment into the sea and the change of the spout area.There was no significant correlation between the artificial diversion water and the annual runoff,sediment discharge and spout area of the Yellow River,and the response to the landform evolution of the Qingshuigou lobe and spout was almost zero.(4)The tidal channels in the Qingshuigou channel period of Qingshuigou lobe in the Yellow River develops through random energy dissipation.From the perspective of space,due to the uneven distribution of tidal channels in salt marsh and mudflat and the irregularity of human reclaimed wetland,the total number and length of tidal channels in the Qingshuigou channel period of Qingshuigou lobe gradually increase from north to south and from east to west.From the perspective of time,the change of the number and length of tidal channels is mainly related to the flow path of the Yellow River,and then related to the flux of water and sediment and the activities of water and sediment transfer.The distribution of tidal channels have significant spatiotemporal heterogeneity.The main reason is that the Yellow River is diverted from the Qingshuigou lobe to the Qing8channel period,and the second reason is that the tidal channels are controlled by the elevation gradient of the beach surface,which causes the erosion on the beach surface.The average number and length of tidal gullies were ranked:South side of the Yellow RiverⅢ>East side of the Yellow RiverⅡ>North side of the Yellow RiverⅠ.(5)Qingshuigou channel period of Qingshuigou lobe in the Yellow River,the density of tidal ditches gradually increases from south to north.On the one hand,the development time of tidal channels in the north is relatively short while the network of tidal channels in the south is relatively mature.On the other hand,the tidal flat area in the south is rapidly reduced due to the effect of ocean dynamics and the increase of human reclamation activities has occupied part of the tidal flat.In the Qingshuigou lobe of the Yellow River,the curvature of the tidal channel changes little,the overall curvature is low,and the overall development of the tidal channels is straight and stable.There is a significant correlation between the bifurcation rate of tidal channels and the density of tidal channels,the length of tidal channels and the change value of spout area are linearly proportional to each other,and the density and bifurcation rate of tidal channels and the change value of spout area are inversely proportional to each other.There is a correlation between the development of tidal channels and the scour and silting evolution of sandy channel. |
