Morphodynamic Processes And Its Mechanism In The Yellow River Delta

The confluence of rivers and seas gives birth to resource-rich deltas.Global major river deltas have become a focal point of attention for scholars both domestically and internationally,owing to their significant ecological and socio-economic importance.However,in recent decades,the combined impact of natural processes and human activities in the basin has resulted in a sharp reduction in sediment input,placing many of the world’s major river deltas at risk of erosion.Consequently,a thorough investigation into the dynamic geomorphology of these river deltas and their evolution mechanisms has become exceedingly urgent and essential to address the continuously changing landscape.The Yellow River Delta,renowned for its high sediment load,also faces similar challenges.In recent years,the hydro-sedimentary regime of the Yellow River has been affected by natural variations in the basin and intense human interventions,leading to the erosion crisis in the Yellow River Delta.To maintain the stability of the Yellow River Delta’s landscape and achieve ecological sustainability,it is imperative to conduct comprehensive research on the geomorphic responses and evolution dynamics of the Yellow River Delta in response to basin water and sediment changes.This article,based on an analysis of the multiscale variations in water and sediment discharge of the Yellow River into the sea and the overall geomorphic evolution patterns of the Yellow River Delta,establishes a numerical model for water and sediment transport in the Yellow River Delta.The model is employed to investigate the hydrodynamic changes induced by high nearshore runoff from the Yellow River and the dynamic geomorphic responses of the delta.The primary focus of the research lies in the following areas,along with a summary of the achieved results:(1)Multi-temporal variations in sediment discharge into the Yellow Sea by the Yellow River.At the monthly scale,significant intra-annual distribution disparities are observed between the Yellow River’s inflow discharge and sediment transport to the sea.Both monthly inflow discharge and sediment transport exhibit concentrated patterns during the flood season(July to October).There is a notable decreasing trend in the proportion of annual inflow discharge and sediment transport during the flood season over time,with synchronous interannual fluctuations between the two.Furthermore,the intra-annual distribution unevenness of sediment transport to the sea is more pronounced than that of inflow discharge.A power-law relationship is identified between monthly sediment transport to the sea and inflow discharge,indicating a power-law growth in sediment transport with varying inflow discharge.On an interannual scale,both inflow discharge and sediment transport show significant declining trends over time,with synchronous interannual fluctuations.Additionally,the interannual variability and decline magnitude of sediment transport to the sea are more pronounced compared to inflow discharge.Distinct periodic variations are identified in the annual inflow discharge and sediment transport processes,with significant cycles at 15,22,37,and 64 years for inflow discharge,and 7 and 16 years for sediment transport.The intensity of periodic variations in annual inflow discharge varies across different time periods due to the influences of natural changes in the basin and human activities.After 1980,the intensity of periodic variations in annual sediment transport gradually diminishes.The variability in annual sediment transport from the Yellow River is determined by the annual water discharge,displaying a power-law growth pattern with annual water discharge.(2)Quantitative assessment of the geomorphic deposition and erosion evolution processes in the Yellow River Delta.Over the past approximately 160 years,the Yellow River Delta has undergone a significant transition in its geomorphic state,shifting from an accretionary phase to a erosional phase.Specifically,the terrestrial portion of the Yellow River Delta experienced rapid accretionary growth before the year2000,but post-2000,it shifted to a slow erosional phase,closely linked to reduced upstream sediment supply.Additionally,the subaqueous portion of the Yellow River Delta also underwent a geomorphic transition from sediment deposition to erosion around the year 2000,with a net erosion rate of-1.5×10~8 m~3/yr.As a critical source of material for delta development,an annual sediment supply of at least 2.6×10~8 t is needed to maintain the morphological balance of both the terrestrial and subaqueous portions of the Yellow River Delta.(3)Analysis of key factors influencing geomorphic deposition and erosion evolution.Firstly,an ample supply of sediment exerts a direct and critical influence on river-dominated deltaic systems.At the decadal scale,the variation in sediment flux in the upstream Yellow River is strongly correlated with changes in sediment deposition and erosion in the subaqueous portion of the Yellow River Delta.Under conditions of abundant sediment supply,the subaqueous delta remained in a predominantly depositional state,while post-2000,due to inadequate sediment supply,it transitioned into an erosional phase.Secondly,the geomorphic evolution of a delta is closely related to the morphology of its river channels,especially the dynamic shift in the location of erosion/accretion centers within the channel network.Spatial changes in the erosion/accretion center of the Yellow River Delta demonstrate that,post-2000,the river channel shifted gradually from east to north,giving rise to a new sedimentation center associated with the emergence of a new river mouth,while the original eastern river mouth gradually contracted to form a new erosion center.Changes in river channel morphology directly impact sediment transport processes and long-term erosion/accretion patterns in the subaqueous delta.Finally,the Yellow River Delta exhibits distinct erosion and sedimentation changes in response to extreme storm events.By comparing the spatial distribution of suspended sediment under calm weather conditions and during storms,it was observed that high-suspended sediment regions expand significantly during storm conditions,with suspended sediment concentrations exceeding 200 mg/L in the northern part of the delta and the current river mouth.Additionally,suspended sediment in the isolated eastern coastal area is approximately double that of normal conditions,reaching up to 80 mg/L,and the weakening of tidal shear fronts during storms facilitates the effective dispersion of river sediment in the estuary.The resuspension of sediment in the nearshore region of the delta appears to provide a source of sediment for transport offshore and to the south.(4)Exploration of the hydrodynamic changes and geomorphic effects of high river discharge on the Yellow River Delta.High river discharge near the old Yellow River Mouth results in elevated water levels,increased flow velocities,and heightened riverbed shear stress,while its influence is relatively weaker in the isolated eastern coastal area.During periods of high river discharge,changes in tidal constituents lead to stronger tidal asymmetry,and ebb-dominant residual currents enhance the resuspension and transport of sediment,inducing the southward dispersion of suspended sediment.The hydrodynamic and sediment transport alterations induced by high river discharge during the flood season have significant implications for the geomorphic siltation and erosion patterns in the Yellow River Delta.Taking the high river discharge events in the flood years of 2018 and 2020 as examples,it was observed that the Yellow River Delta experienced significant erosion under the influence of high river discharge,resulting in a net erosion rate of-0.07×10~8m~3/yr.Notably,the most intense erosion occurred near the Old Qingmugou River Mouth,with a maximum erosion depth of up to 7 meters.Considering the dual pressures of future climate change and inadequate sediment supply,high river discharge events are expected to further exacerbate erosion risks in the river mouth delta.This research provides valuable insights into the hydrodynamic-geomorphic dynamics of deltas under high river discharge conditions and their implications for sustainable development.