Seabed Erosion And Its Mechanism In The Littoral Area Of Yellow River Delta

River deltas accumulate abundant water and soil resources.They have vital ecological and social-economic value in increasing potential land resources,port and waterway construction,wetland tourism,resources development,aquaculture and hyper species diversity.However,the evolution of delta seabed has become more complicated with the influence of decrease in basin inflow and sediment,high-intensity development and construction in coastal zone and frequent natural disasters in recent years.The original balance of erosion and accretion has been destroyed,and the erosion effect has gradually become prominent.Exploring the coastal seabed erosion and its dynamic mechanism in the new environment has become one collective focus of researchers,the government and the society.In this paper,the seabed in the littoral area of the Yellow River Delta(YRD),which is affected by estuarine flow,sediment discharge,ocean dynamics and human intervention,is taken as the research object.Based on the analysis of the process of seabed erosion and shoreline retreat of the YRD,an improved high-reliable morphodynamic model is established.The mechanism of seabed erosion responses to main driving factors,such as sedimentary dynamics and storm surge,are investigated with an improved high-reliable morphodynamic model.Avulsion is the cause of delta lobe retreat and seabed erosion.Abandoned delta lobe(sand spit)and protective zone of Dongying Port-Gudong coasts suffer intense coastal erosion.Engineering-protected coastal seabed erosions mainly distributed within 10 km of the coast,and developed toward the shore.The seabed instabilities in front of the dyke grew gradually.In 2015,84.20%of the seabed in front of the dyke was in a strong or extremely unstable state.In Diaokou River Delta lobe,the rate of shoreline retreat was the fastest at the initial stage of mouth abandonment,and it entered a slow stage after 1996.The erosion intensity of the coastal seabed decreased gradually with the increase of water depth,and the critical depth of erosion increases gradually from west to east,within 6.5-14.5 m.The shoreline of old Qingshuigou spit and old Qing 8 spit began to retreat in 1996 and 2007,respectively.Since 2013,the rate of shoreline retreat in this two regions has been still over 300 m/yr.The coastal seabed erosion of the old Qingshuigou spit and old Qing 8 occurred mainly in the shallow sea area less than 10 m,and the former gradually developed toward the shore,while the latter gradually shifted to the south.From the relationship between the erosion degree of abandoned river mouth and the extent of avulsio,the newly developed mouth have sediment recharge to abandoned mouth,but the supply is limited.In order to explore the supply constraints,a morphodynamic model is established for YRD,which considers realistic bed material distribution and multi-sediment fractions.The model is validated with measurements of long-term water level,tidal current and sediment data,showing a high reliability in morphodynamic modeling.Based on this model,the sediment dynamics affecting source transport are studied.It is found that the spatial distribution of tidal shear front and its shear strength jointly restricts the supply of sediment sources in the erosion zone of the seabed.The distribution pattern of shear fronts is a"meniscus"and mainly appear near the active river mouth and the old Qingshuigou mouth.In the northern end of the shear front,the shear strength in the shallow water is greater than that in the deep water.During ebb tide,the north-diffused sediment is mostly blocked in the shallow water near the coast,and the seabed outside the Gudong dyke in the deep water(far from the coast)can not be supplied by sediment.In the southern end of the shear front,the shear strength in the deep water is greater than that in the shallow water.During ebb tide,the south-diffused sediment can diffuse to the deep water.The seabed of old Qingshuigou mouth can not be supplied by sediment.The distribution of shear front is highly consistent with the erosion and deposition changes.The sea area with shear front distribution is dominated by deposition,while the sea bed without shear front is dominated by erosion,and the outer boundary of the shear front is roughly the seabed erosion-siltation transition zone.The response of the long-time-scale geomorphological evolution to the short-time-scale estuarine sedimentary dynamic process is an important factor affecting the spatial pattern of seabed erosion.Based on the dynamic geomorphological model,the hydrodynamics and sediment transport along the coast of the YRD under the conditions of light wind and storm surge are studied comparatively.During the storm,the tidal shear fronts near the river mouth and Gudong coast were weakened,and their trends perpendicular to the coast with insignificant low velocity zones.The transport rate of water was generally greater than0.2 m~3/s,which was twice as greater as that in light weather condition.The effective wave heights in the littoral area of YRD were generally over 1 m,and that off the Dongying Port-Gudong was as high as 1.5~2 m.Under the combined action of wave and tide,the enhanced bottom shear stress caused significant sediment resuspension.The suspended sediment concentration(SSC)in Dongying Port-Gudong offshore area during storm was three times as high as that in light weather,with a maximum value of2.5 kg/m~3.During the storm,the sediment transported offshore and southward.The dynamic and sediment transport changes under storm conditions caused significant changes in seabed erosion and siltation.The area of seabed erosion area was nearly three times as large as that under light wind conditions,and the erosion volume was nearly 20 times as large as that under light wind conditions.The frequent northerly storms in winter half year are the power source of seabed erosion.In conclusion,based on multi-source data and using numerical simulation method,this thesis systematically studied the coastal erosion process and its dynamic mechanism in the littoral area of YRD,which deepened the understanding of the geomorphological evolution patterns of the YRD.The relevant achievements also provide valuable information for sustainable management of coast systems and improving the capability of defending disasters in delta areas.