Sediment Dynamic Processes In Meso-macro Tidal Beaches

Located in the dynamic sensitive and vulnerable zone of land-sea interaction,beach provides vitally important resources of material,mineral and tourism for human beings.However,under the multiple pressures of global sea-level rise,fluvial suspended sediment decline and storm surge aggravation,global beaches exhibit shoreline erosion,surface sediment coarsening and beach face narrowing and steepening,etc.,which cause serious damages to people's lives and properties security,ecological environment and local socio-economic development in coastal areas.The study of beach sediment-dynamic processes,such as surficial sediment transport,cross-shore suspended sediment transport(SST)and beach elevation changes,can provide scientific theoretical basis for beach and coastal protection and management.Taking Yintan Beach,located on the northern Beibu Gulf,as an example and based on the field measurements of high-frequency turbulent kinetic energy(TKE),waves,suspended sediment concentrations(SSC),surficial sediments and beach face data,this study detected the linkage between SSC,waves and TKE,and explored the influence of waves,tides(tidal range),current and suspended sediment concentrations on the cross-shore sediment transport and the transverse distribution of surficial sediments,as well as the beach face changes,by means of suspended sediment flux calculation,bed shear stress calculation,extreme SSC/TKE event statistics,grey relation analysis and Mc Laren model,aiming to reveal the sediment-dynamic mechanism of meso-macro tidal beach.The main findings include:(1)Wave and turbulent kinetic energy jointly controlled the near-bed SSCs in surf zone.Under shoaling wave conditions,wave could well predict near-bed SSC variation at both incident wave scale(relative wave height)and wave group scale(wave group).However,the magnitude of near-bed TKE was determined by the relative wave height no matter whether the waves were broken or not.In addition,during the observation period,approximately 12.2%of the total TKE events caused 42.9%of near-bed SSC events that were over 1.86 kg/m~3.Moreover,TKE played the most important role in the variations of SSCs for all wave conditions,followed by wave group and flow acceleration,while the impacts of single wave(incident wave and infra-gravity wave)and mean current to SSCs were minimum in general.(2)Near-bed suspended sediments were characterized by net seaward transport under different tidal regimes for the area landward of breaker line.The net cross-shore suspended sediment flux over the entire tidal cycle was dominated by rising tide process which has a larger average SSC and a longer flood duration.Moreover,the mean cross-shore velocities,affected by the offshore-directed undertow from surf zone,were in offshore direction for almost all the flood and ebb processes,which led to overall offshore SST in the near bed of breaker zone throughout different tidal regimes.In the meantime,abruptly enhanced incident wave height and low wave period during the flood tide caused the beach in highly dissipative condition and the reversal of mean current,which resulted in net onshore SST in the near bed over the tidal cycle.However,within the surf zone,offshore-directed tidal cycle-averaged mean current gave rise to seaward net SST in the near bed throughout different tidal regimes and wave conditions.In specific,tidal cycle-averaged suspended sediment fluxes within the surf zone were-12.29 kg/m~3·m/s,-38.63 kg/m~3·m/s and-0.60 kg/m~3·m/s during spring tide,middle tide and neap tide,respectively.Meanwhile,the larger incident wave height was,the larger offshore-directed net tidal-cycle suspended sediment fluxes were.(3)Surf zone shifts,induced by tide level changes,combined with incident wave intensity caused periodic onshore/offshore shift of coarser sediments for the area landward of breaker line.When the tide level(tide regime)changed from middle tide to spring tide,the coarser sediments were distributed in the high tide zone.The coarser sediments on beach surface tended to migrate landward(corresponding to the high-energy sediment transport events),which became more obvious as tidal range increased.When the tide level changed from middle tide to neap tide,the seaward shift of surf zone led to offshore movement of coarser surficial sediments to the middle tide zone,which generated a distinct seaward-directed high-energy sediment transport event.During neap tides,the coarsest sediment within middle tide zone continued seaward moving,approaching the mean low water level of neap tide.During this period,if local(middle tide zone)sediments coarsening induced by large wave events(e.g.,enhanced southwest waves)were considered,the interaction between seaward moving of the coarsest sediment and seaward-directed sediment coarsening within middle tide zone would lead to the concurrence of landward/seaward-directed high-energy sediment transport events or insignificant transverse sediment transport.In other words,both large incident waves and the movement of surf zone were primary reasons for local sediment coarsening.(4)Beach face exhibited spring/neap tides and flood/ebb tides variation characteristics at tidal cycle scale.When the tide level(tidal regime)changed from middle tide to neap tides,coupled with moderate wave condition,the beach showed strong erosion.When the beach was subjected to spring tides during which low wave energy condition prevailed,the beach presented slight accretion.In summary,beach face displayed net erosion overall throughout the observation period.The variation amplitude of intra-tidal bed level was more prominent during neap to middle tides than during middle and spring tides,which was jointly caused by wave breaking probability regulated by water depth and the relative residence times of shoaling wave zone,breaker zone and surf zone that were determined by relative tidal range.The net erosion over the entire tidal cycle was mainly resulted from larger incident wave height and the consequent strong offshore-directed sediment transport by mean current(undertow).At the same time,the variation of beach face was macroscopically controlled by water depth,which showed an overall trend of accretion during rising tide and erosion during falling tide.During middle and spring tides in low energy wave conditions,the aforesaid intra-tidal trend of bed level was mainly determined by the balance between the sediment erosion flux caused by wave stirring and the sediment deposition flux induced by the combination of suspended sediment lag deposit and terrestrial sediments carried by offshore-directed mean current.However,during neap and middle tides in moderate wave conditions,the coupling between the strong offshore-directed mean current induced by enhanced southwest waves and terrestrial sediments could change the relative balance between the sediment erosion flux caused by wave stirring and the sediment deposition flux induced by suspended sediment lag deposit,which made the offshore-directed mean current become a main influence factor of accretion during rising tide and erosion during falling tide under moderate wave conditions.(5)Sediment-dynamic processes,including waves(relative wave height),tide(tidal range),turbulent kinetic energy and suspended sediment concentration jointly controlled the cross-shore sediment transport,the cross-shore distribution of beach surficial sediments and the beach face changes.Under low energy wave conditions(during middle and spring tides),waves and TKE superimposed on the tide level respectively stirred bed sediments and kept suspended sediments in resuspension.At the same time,coarser bed and saltation loads(mainly consisted of medium sand)close to the beach surface were forced to move landward by the wave asymmetry,whilst the finer components(mainly consisted of very fine sand)in the suspended sediments were transported seaward by the near-bed offshore-directed mean current and finally deposited on the beach surface under relatively weak-dynamic environment.As a result,the contents of very fine sand increased seaward and the contents of medium sand increased landward,which led to a cross-shore distribution trend of beach surficial sediments coarser in landward direction.In the processes of bed loads,saltation loads and suspended sediments movements,the surf zone shifted landward and seaward as the tidal range increased and decreased,which forced the coarsest(coarser)sediments to reciprocate between high and middle tide zones.Meanwhile,the rise and fall of tidal level would lead to the supplement rich terrestrial sediments,which caused net accretion of beach face under low energy wave condition.As the southwest waves enhanced and the neap tides encountered with moderate wave condition,stronger wave asymmetry transported coarser bed and saltation loads landward.Meanwhile,intensified incident waves and TKEs generated coarser suspended sediments,which were transported seaward by strong offshore-directed mean current and gradually deposited on the beach surface.Consequently,the cross-shore distributions of very fine sand and medium sand both became more uniform while the beach surficial sediments turned coarsening.During this period,strong mean current combined with a lack of coarser suspended sediments from terrestrial sediments resulted in larger net erosion during falling tide(compared to the net accretion during rising tide and a net beach face erosion under moderate wave condition.It was expected that sea level rise would lead to an overall landward shift of surf zone,which could lead to the continuously landward shift of sediment coarsening zone.Once storm wave events occurred with strong incident waves,the coarsening of beach surficial sediments,erosion of bed level and retreat of shoreline would be further intensified.