The Asymmetry Of Flood And Ebb And Geomorphic Dynamic Mechanism In Coastal Area Of The Yellow River Estuary

The estuaries and river deltas are located in the transitional zone between land and sea,which have both natural and social values.Sediment is the material basis of the delta,and the study of sediment transport and deposition in coastal area of the estuary is the key to reveal the development of the delta.The Yellow River Delta has become one of the most rapidly changing deltas due to the characteristics of the Yellow River's high sediment flux into the sea.However,the evolution of coastal area of delta landforms have appeared new trends in recent years,due to the decrease of inflow and sand into the sea and the concentration of suspended sediment,the development and construction of the coastal zone.It is urgent to re-understand the dynamic mechanism of sediment transport and erosion and accretion.The asymmetry of flood and ebb is a significant feature of the flow field in coastal area of the estuary,which is an essential dynamic mechanism for the transport of sediment and other materials,and it plays an important role in the evolution of the geomorphology.The research on the asymmetry of flood and ebb and its relationship with geomorphological evolution has important scientific significance,which is helpful to understand the formation and evolution of the erosion and deposition pattern in coastal area of the Yellow River Estuary(YRE).In this paper,we use surface sediment particle size trend analysis,suspended sediment flux mechanism decomposition and other methods to analyze characteristics of water and sediment movement based on a large amount of measured water and sediment data.A Delft 3D hydrodynamic numerical model is established to simulate the flow field in coastal area of the YRE,used to explore the temporal and spatial distribution,internal mechanisms and influencing factors of the asymmetry of flood and ebb in coastal area of the YRE.Combining the characteristics of water and sediment transport and the spatial distribution of asymmetry of flood and ebb,we explain in detail the role of the asymmetry of flood and ebb in the geomorphological evolution in coastal area of the YRE.We also analyze the changes of asymmetry of flood and ebb caused by topographic changes from 2007 to 2015.The main results are as follows:(1)From 2007 to 2015,there are multiple siltation and erosion centers distributed alternately in the coastal area from the active Yellow River Mouth to the Laizhou Bay.The eroded sediment on the front edge of the old Qingshuigou spit is an important source of sediment in the siltation area both on north and south sides.The direction of net sediment transport is opposite to the direction of residual flow outside the active mouth.Advection is the main form of sediment transport when the runoff is high,and tidal flow transport is the main form when the runoff is low.The magnitude of the tide pump sediment delivery item is large in nearshore,and the direction is consistent with the direction of flood,while the magnitude of the tide pump sediment delivery item is small in far shore,and the direction is consistent with ebb.(2)There is significant tidal asymmetry in coastal area of the Yellow River Estuary.The flow velocity asymmetry is flood-dominated outside the prominent spit,and stretches to the Laizhou Bay in the south.The main body of the predominant area of slack water asymmetry is located in the sea area northeast of the old Qing 8 River Mouth.There is mainly ebb-dominated nearshore.The flow velocity asymmetry is dominated by K₁/O₁/M₂ in deep water area,and by M₂/M₄ in nearshore.The contribution of each tidal components group to the tidal flow velocity asymmetry changes in the nearshore affected by the topography of the headland.The tidal components group in the Laizhou Bay is generally conducive to the flood-dominance in the sea area near the mouth of the Laizhou bay,and is conducive to the ebb-dominance in the side close to the top of the bay,and the position of the transition is same.The spatial variation trend of the contribution of tidal components group which are same type to the tidal flow velocity asymmetry is roughly same.The tidal asymmetry has obvious spring and neap cycle.The contribution of M₂/M₄ and K₁/O₁/M₂ to the flow velocity asymmetry has long period of half a year,while M₂/M₄ also has obvious lunar cycle.(3)Tidal flats,runoff and wind fields will all affect the flow velocity asymmetry and slack water asymmetry.Tidal flats help to form ebb-dominance.Runoff has a greater impact on the flow velocity asymmetry than on the slack water asymmetry,and surface runoff's residual current plays a major role.The effect of wind direction on asymmetry in different regions is different.The southwest wind and the south wind contribute to ebb-dominance in the shallow waters south of old Qingshuigou,and the south wind and the southeast wind contribute to ebb-dominance in the Gudong sea area.(4)The erosion and accretion in coastal area of the Yellow River Estuary are largely controlled by the spatial distribution of asymmetry of flood and ebb.The area where dominance is converted in the shallow sea near the Yellow River Estuary which depth is smaller than 10 m has become significant accretion area.Because the flow velocity asymmetry in the sea area southeast of Old Qingshuigou changed from flood-dominance to ebb-dominance,a large amount of sediment transported southward deposited in the central area of Laizhou Bay.The flood-dominance of slack water asymmetry and the local ebb-dominance of flow velocity asymmetry between headland of spit are important reasons for the accretion between Old Qing 8 mouth and Old Qingshuigou mouth.The enhancement and the expansion of the advantage of flood caused by strong northerly wind are the main driving mechanism for the accretion and coarsening of sediments in the northwestern Laizhou Bay.The strong northerly wind is also the reason for the formation of the long strip accretion area south of old Qingshuigou.The eroded sediment in the front edge of the old Qingshuigou spit is the main source of sediment in the accretion area.(5)While the asymmetry of flood and ebb affects erosion and accretion,the geomorphological evolution also affects the asymmetry of flood and ebb.From 2007to 2015,the predominant area of flood at the front edge of the Old Qingshuigou Spit expanded to the southwest because of the erosion and accretion.The decrease of the contribution of M₂/M₄ interaction and the residual current and the interaction between the residual current and the tidal constituents weakens the flood-dominance in the erosion sea area;the decrease of the contribution of M₂/M₄ interaction and the increase of the contribution of K₁/O₁/M₂ interaction contributed to the change from ebb-dominance to flood-dominance in the accretion sea area.