| The estuary delta is deeply compared with the power between land and sea,which has important practical value in resource utilization,green tourism and ecological protection.The Diaokou sub-delta was formed by the deposition of the Yellow River in the Diaokou River from 1964 to 1976.After being abandoned in 1976,the Diaokou sub-delta became a typical eroded coast under the influence of sediment cut-off,ocean erosion and extreme weather.In this paper,the Diaokou sub-delta in the north of the Yellow River Delta is taken as the research area.Based on historical datas,underwater topographic datas,remote sensing image datas and field measured datas,the formation and development process of estuary lobe and the geomorphological evolution and dynamic mechanism of abandoned estuary on land and underwater deltas are systematically studied by using RS and GIS technologies.The influence of ecological water replenishment strategy on the geomorphological evolution of abandoned estuary is analyzed.Exploring the development and evolution process and influencing factors of the Diaokou estuary under the new situation is of great significance for grasping the characteristics and laws of coastal erosion,scientifically formulating protective measures and promoting the high-quality development of the Yellow River Basin.(1)From 1964 to 1976,the runoff,average runoff,sediment transport and sediment concentration of Diaokou River showed a fluctuating downward trend.The annual variation coefficient of runoff and sediment discharge varies greatly,and the annual distribution is very uneven.The development of the lobe in the Diaokou estuary during the running water period can be divided into three stages: the initial stage of diversion,the straight and stable stage of the river channel and the swing of the branch again.According to the development form of the estuary,it can be divided into two stages: the vertical deposition to the sea and the deposition to both sides of the estuary.Estuary deposition is divided into delta deposition,offshore deposition and off shore deposition.After the formation of a single straight channel from 1968 to 1972,the sediment deposition on the land decreased,and the proportion of sediment deposition in the sea area increased rapidly.(2)The morphological changes of the abandoned estuary show that the lobe protruding from the original sea are gradually eroded and flattened,and gradually move east-west over time.The topography of the front edge of the estuary changed from gentle to steep,and developed into an eroded bay on the east side of the abandoned estuary.After 1976,the coastline erosion rate gradually slowed down,and the initial erosion was the most serious.After 1996,the coastline erosion curvature became larger,and the coastline became more tortuous and broken.However,the coastline under the construction of artificial coastal facilities advanced to the sea.In addition,a large area of central bar appears near the estuary during the flow period,and its formation was related to the interaction of runoff and tidal current.After the river was abandoned and the runoff conditions were lost,the river island gradually disappeared.The coastal erosion of the abandoned estuary is mainly affected by the combined effects of waves,tidal currents and residual currents,storm surges,sea level rise and human activities,among which waves and tides are the main effects.(3)The evolution of tidal creeks on abandoned tidal flats in the north has spatial and temporal differences,among which the development of tidal creeks in Lanni Bay is the most typical.The complex tidal current convergence erosion tidal creek system gradually formed under the action of fluctuating tide in this area.The development of tidal creeks leads to the gradual disintegration of tidal flats,making the bay larger.In addition,because the east coast of Lanni Bay is located in the main action area of the ebb and flow,the development of tidal creek is more obvious.From 1976 to 2021,the head of the tidal creek of the high tidal flat on the abandoned tidal flat was gradually abandoned and disappeared;the tidal creeks in the middle tidal flat show shorter length,more branches and larger density.The low tidal flat is gradually eroded to form an underwater deep trough.Human activities and ocean dynamics are important factors leading to the development and evolution of tidal creeks.During the ecological water replenishment period from 2010 to 2015,the tidal creeks in the northern region have generally undergone three replacements.The ecological water replenishment strategy has effectively alleviated coastal erosion.The erosion in the northern part of the Yellow River Delta is serious,and the implementation of the ecological coastal protection project has certain feasibility in alleviating coastal erosion.(4)From 1959 to 1976,the underwater sea area was generally fan-shaped and extended to the sea,but the nearshore deposition center shifted from the Shenxiangou estuary to the Diaokou estuary.After the abandonment in 1976,the underwater erosion and deposition changed to the state of nearshore erosion and offshore deposition,and the erosion intensity was strong in the east and weak in the west.At the same time,the maximum erosion area in the abandoned estuary area corresponds to the maximum deposition area in the water period.Since 1976,the erosion and deposition of underwater topography has experienced three stages: rapid erosion,fluctuation adjustment and secondary erosion.The average erosion intensity of underwater sea area takes 2002 as the node,showing a trend of increasing first and then decreasing.The balance of erosion and deposition was basically balanced during the ecological water replenishment period from 2010 to 2015.Different sections have similar regularity in morphological changes.The profiles are characterized by ’upward erosion and downward deposition’,and the erosion-deposition conversion generally occurs near 15 m,and the erosion-deposition conversion depth of different profiles gradually increases from west to east.The whole between 0～5 m isobath is affected by marine erosion.The middle slope section in the 5～15 m isobath transforms from erosion to deposition.The sea area with water depth greater than 15 m is dominated by sedimentation.The morphological changes of the profile indicate that the near-shore water depth changes sharply,and the underwater terrain changes from steep to slow.The profile of the Lanni Bay showed deposition trend during the ecological water replenishment period.The seabed profile at the northeast seawall is seriously eroded near the shore after the construction of the seawall,and the seabed in front of the seawall has a reverse profile of ’near-field water depth and far-field water depth’,and the near-shore seabed becomes steep.The storm surges of 1992 and 1997 intensified the processes of nearshore erosion and offshore deposition. |
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