Morphological Variability Of The Yellow River Delta And Its Dynamic Mechanism Under The New Regime Of River Delivery

The estuaries and river deltas are critical interaction zones between land and coastal oceans,with densely distributed population,a rich diversity of species,high ecological and social-economic values.At the same time,this dynamic unit is at the potential transformation in morphodynamic processes under environmental changes.Basin-scale human interventions have induced a drastic reduction in river delivery in recent decades,which has shifted the hydrodynamics and sedimentary processes in estuaries and triggered erosion threats in river deltas.The Yellow River Delta is famous for its high sedimentation rate in history and has experienced severe erosion in recent years under the transition of the water and sediment regime.The deltaic erosion and shoreline retreat has close relations with land use,stability of coastal structures,and wetland ecosystem variabilities.Thus it is urgently needed to capture the morphodynamic adjustment of the Yellow River Delta under the new regime of river delivery.Further evaluation of the dynamic mechanism of the typical geomorphic unit under the integrated influence of river and ocean dynamics is significant for the sustainability of geomorphology and ecology of the deltaic system.This dissertation takes the lowermost channel,the active river mouth,and adjacent coastal waters of the Yellow River Delta as research areas.The morphological evolution processes of the Yellow River Delta under the new regime of water and sediment delivery are calculated by the GIS technique.The well-validated and fully-coupled TELEMAC suite of the Yellow River Delta is built to investigate the characteristics of hydrodynamics,fine sediment transport,and dispersal patterns of the Yellow River estuary.Quantification of sediment source-to-sink variability at the lowermost channel,estuaries,and to the outsea is also achieved.And the impact of river discharge on hydrodynamics and sedimentary processes of the Yellow River Delta is also discussed.The main findings and conclusions are as follows:(1)The multi-scale characteristics of the Yellow River water and sediment delivered to the sea and the new discharge regimeThe wavelet transforms analysis indicates a strong oscillation of river delivery at the interannual scale and decadal scale.The water discharge of the Yellow River to the sea occurred periodic oscillations at 10yr,16yr,21yr.And the sediment delivery shows a significant impact at 4yr and 6yr and 20yr.With human activities in the river basin intensified,the oscillations of each period dramatically decreased after 1975.Based on the abrupt change of water and sediment delivery and the beginning time for the implementation of the Water-Sediment Regulation Scheme,we divided the decadal-scale variability of river input into three stages,namely:the slight human-disturbed stage(1976-1985),the highly human-disturbed stage(1986-2001)and the new regime of river delivery stage(2002-2016).The new discharge regime since 2002 is strongly interfered by the Water-Sediment Regulation Scheme,characterized by a more drastic decline of sediment load than that of water discharge;uneven distributions of monthly sediment delivery within the year;the more harmonious relationship between water and sediment discharge in the lower reach of the river to the sea;coarser sediment delivery and low suspended sediment concentrations.The natural regime of river delivery between July and October is replaced by a limited 20-day man-made flood peak,triggering uneven distributions of monthly water and sediment input within the year,whilst the flood and sediment peak value has a drastic decrease compared with the natural regime of Yellow River.(2)The morphological variability of the Yellow River lowermost channel and the active Yellow River mouthBefore the operation of the Water-Sediment Regulation Scheme in 2002,the lowermost channel of the Yellow River was in net accretion in flood seasons and retrogradation in dry seasons.After the implementation of the Water-Sediment Regulation Scheme,the erosion-deposition pattern is reversed with erosion in flood seasons and net deposition in dry seasons.At the decadal timescale,the bankfull area of the lowermost channel has increased with a more harmonic relationship in water and sediment,relieving the continuous heavy deposition of the riverbed.The lowermost river has turned from sediment sink into sediment source.On the decrease of sediment delivery,the propagation rate of the lowermost channel to the sea has decreased after the migration of the mouth channel at Qing 8 section.The morphological variability of the active Yellow river mouth and its adjacent coastal region is closely related to the new regime of river input,with significant spatial variability.The active river mouth experiences a net accretion in recent years with a net vertical deposition rate of 0.15 m/yr.Besides,recent morphological evolution of the active river mouth is found to have undergone four stages,namely:moderate accretion(1996-2002),rapid accretion(2002-2007),reduced accretion(2007-2015),and rapid erosion(2015-2016).The building of the active river mouth is also believed to be shaped by the event-scale WSRS-induced floodwater,the implementation of the Water-Sediment Regulation Scheme,and frequent mouth channel migrations.At least 41.4–62.3 Mt/yr of sediment input is required to achieve the erosion-accretion balance of the active river mouth.However,the Gudong littoral area has experienced erosion due to the insufficient sediment supply,with a net vertical erosion rate of 0.1 m/s.(3)The characteristics of hydrodynamics and sediment transport at the Yellow River Delta and quantification of sediment source to sink transferWe establish a full-scale numerical model based on TELEMAC,covering the Yellow River lowermost channel,the subaqueous delta,the Bohai Sea,and part of the Yellow Sea.The model is well-validated in water level,flow velocities,and suspended sediment concentrations,and the distribution of modeled high turbidity zones is similar to the suspended sediment distributions inversed from satellite images.The model shows the Yellow River Delta is dominated by micro-tides and mixed types.The near-coast of the Yellow River Delta is dominated by shore-parallel reciprocating flow,with southeastern current during flood tide and northwestern current during ebb tide.The residual current and net sediment flux show a southeastern transport of the water and sediment from the river mouth to the old Qingshuigou mouth and northern Laizhou Bay,and then tend to transport offshore towards the middle of the Bohai Sea.The Yellow River Delta develops a reverse tidal shear front,which originates from the northern coast of the Yellow River Delta,and gradually propagates to the south.Meanwhile,it originates from the nearshore near the active Yellow River mouth and propagates offshore,which lasts about 3-4 hours over a tidal cycle.The duration of the IEOF(inner-ebb-outer-flood)shear front is longer than the IFOE(inner-flood-outer-ebb)front at the active river mouth.After the operation of the Water-Sediment Regulation Scheme in 2002,the sediment scoured from the lowermost channel of the Yellow River Delta reaches about8.6%of the total sediment load of the Lijin station,becoming a new sediment source to the building processes of the active river mouth.It is estimated that about 60.5%of the sediment delivery participates in the building of the new land and subaqueous vicinity of the active river mouth.From the model results,we find that about 28.40%of the total sediment load transport southward to the Laizhou Bay,5.88%eastward,and 5.22%northward to the outsea,respectively.(4)Quantification of the impact of water discharge on tide dynamics and depocenterBased on the significant interannual variability of river input from the Yellow River to the sea,we investigate the variations of river input to the nearshore tide dynamics,sediment transport processes,and depo-center.The variation of runoff has played a critical role in shaping tidal amplitude at the Yellow River Delta lowermost channel and tidal flats.With the increase of the runoff,the amplitude of M₂ and K₁follows a decreasing trend,but little influence in the open sea.O₁,M_2,and K₁are the three main tidal constituents near the active Yellow River mouth and their decrease accounts for the most compared with other constituents.The model results also indicate that the river discharge affects the location and intensity of the shear front that occurs in the nearshore areas of the Yellow River Delta.Increasing the river discharge can induce a seaward movement of the shear front,reduce its width and concentrate its shear intensity.It is found that the reverse of the flow direction at each side of the shear front and strong longshore tidal current can act as a barrier for the sediment dispersal process by keeping suspended sediment in the inner zone,thus forming a particular sediment deposition zone and the depo-center.The water delivery has a significant impact on sediment transport and depo-center.Increasing the water discharge can induce a larger range of sediment plumes and a higher sediment transport rate.The depo-center is restricted near the east river mouth with relatively low water discharge,and it spans to the north river mouth and moves seaward when the water delivery increases.