| The accumulation of fine-grained sediments forms broad and mildly sloped tidal flats in intertidal zones.Tidal flats are widely distributed in coastal areas,especially in areas that are characterized by large tidal ranges,abundant resources of fine-grained sediments,seabeds with small slopes,and weak wave forces.Tidal flats are important sedimentary coastal environments,and tidal flat processes are closely related to the patterns of human activity and economic development.Progradational tidal flats provide space for both the expansion of salt marshes and human activity.For example,the land area of Chongming Island in the Yangtze Estuary has increased by 120%via the reclamation of salt marshes.In addition,tidal flats serve as flood defence systems by dissipating waves to protect large populations along coastline areas.However,tidal flats have been subjected to pressures by a drastic decrease in sediment supplies with sea level rise due to climate change and human impacts?e.g.,damming in the river basin?.Knowledge of sediment dynamics is fundamental to developing a better understanding of mechanisms of erosion and deposition.These are crucial for predicting future evolutionary patterns the protection and re-construction of tidal flats.In situ measurements serve as an important and useful approach to studying the sediment dynamics of tidal flats.Most studies on sediment dynamics in intertidal zones have focused either on intra-tidal and neap-spring variations in current velocity and suspended sediment concentrations?SSCs?based on instrument records collected during tidal submersion or on bed-level changes based on surveys undertaken when the tidal flats are emerged.However,the way in which intra-tidal bed-level changes are associated with hydrodynamic and sedimentary processes is key to understanding the mechanisms of tidal flat erosion and deposition.Furthermore,tidal flats are shallow environments subjected to both wave and current forces.It is thus critical to study sediment dynamics on intertidal flats based on integrated measurements of in situ waves,tidal currents,SSC and bed-level changes.Additional to in situ measurements,numerical models will be essential to integrate the knowledge and make predictions for the development of intertidal areas.The overall objective of this study is to understand the sedimentary processes and bed-level changes under combined wave-current action forces on intertidal flats.Therefore,we use an integrated approach with in situ measurements of waves,tidal currents,SSC and bed-level changes at high temporal and spatial resolutions using advanced instruments under different wind conditions in combination with numerical modelling.First,we identify and quantify erosion and deposition stages by comparing and linking the total bed shear stress under combined wave-current action(?cw)with the critical bed shear stress for erosion??e?,which is derived from measured soil mechanical characteristics of bed sediment.Second,we simulate intra-tidal bed-level changes by calculating the erosion and deposition rates,using the data as obtained from the field campaigns.A combination of measurements and simulations is used to further our understanding of erosion-deposition mechanisms and improve the given model.The main research question is to find a sound relation between the bed-level changes and sediment transport under combined wave-current action.The key techniques applied to address this question are:operating instruments measuring in situ waves,tidal currents,SSC levels,and bed-level changes;quantifying ?cw and ?e;and choosing appropriate erosion and deposition rate modelling strategies.In this study,we examine three intertidal flats of different exposure levels:the Nanhui Foreland mudflat,which faces an open sea area with a fetch of at least 100 km and which forms an absolute open flat;the Southeastern Chongming mudflat,which is a moderate open flat with offshore shoals of several kilometres between the flat area and sea;and the Kapellebank mudflat,which is a weakly open?sheltered?flat located at the outer bend of a channel in the Westerschelde Estuary.On the three intertidal flats,we carried out integrated observations of water depths,wave parameters,current profiles,turbulent velocities,SSC levels at specific heights,SSC profiles,and bed-level changes using wave-loggers,ADVs?Acoustic Doppler Velocity meters?,ADCPs?Acoustic Doppler Current Profilers?,OBSs?Optic Backscatter Sensors?,and ASM?Argus Surface Meter?.During the observation periods,we collected suspended sediment samples,surface bed sediment samples,and mini-core samples;we carried out instrument data calibrations and measurements of grain size distributions,water content,and diatom biomass.By applying widely used calculation models,we calculated the total bed shear stress levels under combined wave-current action ?cw?via Grant-Madsen,van Rijn,and Soulsby wave-current interaction models?,critical shear stress for erosion ?e,erosion rates E,settling velocities cos,deposition rates D,and sediment transport rates Qs.The measured and calculated parameters were used to demonstrate variations in hydrodynamic forces at intra-tidal,spring-neap,and storm scales;temporal and spatial variations in SSC;and bed stability distributions and hydrodynamics and their mechanical interpretations.The findings were further used to formulate the bed-level change?BLC?model.1.Dynamic variations on intertidal flatsThe measurements on the mudflats resulted in large data sets?1680 hours?for various parameters at the three locations.The results can be summarized as follows.Nanhui Foreland mudflat:wind speed was 1.7-19.7 m/s and averaged 5.9 m/s during the observation period.Tidal range varied from 1.4-5.0 m and averaged 3.7 m.Near-bed current velocity was from 0.001-0.51 m/s and averaged 0.17 m/s.Significant wave height?Hs?ranged from 0.01-3.92 m and averaged 0.32 m.?c,ranged from 0.0005-3.58 Pa and averaged 0.56 Pa;?w ranged from 0-1.62 Pa and averaged 0.15 Pa;and ?cw ranged from 0.02-4.79 Pa and averaged 0.72 Pa.?cw was dominated by tidal currents during much of the tidal submersion period and was dominated by waves when the water depths were less than 1 m.?cw followed a V-shaped time series,whereby the?cw values were high during flood and ebb peaks and low during high water periods.The average ?cw value during spring tides was 0.65 Pa,which was greater than that observed in neap tides?0.35 Pa?.During storm periods,?cw significantly increased to 2.13 Pa.Southeastern Chongming mudflat:wind speed was 0.4-10.8 m/s and averaged 6.0 m/s during the observation period.Tidal range varied from 0.9-4.7 m and averaged 2.7 m.Near-bed current velocity ranged from 0.004-0.51 m/s and averaged 0.18 m/s.Hs ranged from 0.07-0.46 m and averaged 0.21 m.?c ranged from 0.01-2.43 Pa and averaged 0.52 Pa;?w ranged from 0.01-1.68 Pa and averaged 0.22 Pa;and ?cw ranged from 0.04-2.47 Pa and averaged 0.66 Pa.The average ?cw during strong wind and normal weather periods?average wind speeds of 6.8 m/s and 2.6 m/s,respectively?were 1.07 Pa and 0.35 Pa,respectively.Kapellebank mudflat:wind speed was 0-18.0 m/s and averaged 5.6 m/s during the observation period.Tidal range varied from 3.2-5.3 m and averaged 4.5 m.Near-bed current velocity ranged from 0.001-0.47 m/s and averaged 0.20 m/s.Hs ranged from 0.005-0.43 m and averaged 0.06 m.?c ranged from 0.0004-2.27 Pa and averaged 0.14 Pa;?w ranged from 0-1.26 Pa and averaged 0.08 Pa;and ?cw ranged from 0.002-2.55 Pa and averaged The 0.18 Pa.?cw values before,during and after storm periods were 1.32 Pa,2.10 Pa,and 0.57 Pa,respectively.2.Critical shear stress for erosion ?e on intertidal flats and its comparison with?cwNanhui Foreland mudflat:the median grain size?dso?of surface sediment on the middle flat was 34 ?m;the water content of the surface 2 mm and 10 mm layers was 72%and 34%,respectively,and those of ?e were 0.085 Pa and 0.119 Pa,respectively.On low flats,the d50 of surface sediment was 21 ?m;water content was 73%;and ?e was 0.084 Pa.The flat showed an erosion tendency,as ?cw was larger than ?e by over 90%during the observation period.Under normal weather conditions,?cw<?e was found during high slack water periods,and this lasted for 15%of the tidal submersion period;however,this was the case for 0%of the storm period and for 19.3%and 26.5%of the spring and neap tide periods,respectively.Southeastern Chongming mudflat:the d50 of surface sediment was 34 ?m;water content was 32%;?e was 0.29 Pa.Freshly deposited sediment was looser and finer,d50 was 26 ?m;water content was 98%;and ?e was 0.14 Pa.?cw was greater than re for 88.3%of the observation period.?cw<?e was found for 6.1%of the tidal submersion period during neap and moderate tide periods,and this coincided with strong wind events.The same condition was found in 23.1%of the normal weather periods.Kapellebank mudflat:the d50 of surface sediment was 20.2-30.6 ?m;water content was 106-148%;and re was 0.11-0.13 Pa.In the vertical dimension,?e increased from 0.11 Pa to 0.75 Pa within the uppermost 11 cm layer.Without biological effects,?cw was greater than ?e for 21.6%of the observation period.?cw<?e conditions were found in 53.9%and 85.9%of tidal submersion periods during spring and neap tides,and these conditions accounted for 86.9%of the storm period.Diatom presence enhanced ?e to 0.44 Pa,representing 4 times the value of abiotic ?e.3.SSC variations on intertidal flatsNanhui Foreland mudflat:near-bed SSC in the middle flat area varied from 0.4-4.8 kg/m3;average SSC values at 6 cm,15 cm,35 cm and 75 cm were 3.1 kg/m3,1.8 kg/m3,1.4 kg/m3,and 1.0 kg/m3,respectively.Near-bed SSC in lower flat areas varied from 0.02-19.9 kg/m3,and the average SSC values were measured at 10 cm,35 cm,50 cm and 100 cm as 4.4 kg/m3,2.4 kg/m3,2.2 kg/m3,and 1.7 kg/m3,respectively.Vertical SSC profiles identified during typical flood and ebb stages following a logarithmic trend,i.e.,SSC increased gradually with height,whereas vertical SSC profiles were L-shaped during slack water periods.The background SSCs in middle and low flats were 1.4 kg/m3 and 2.0 kg/m3,respectively.Substantial volumes of suspended sediment that settled during slack water periods formed a near-bed fluid mud layer.The thicknesses of fluid mud layers lasting for 0.5-1 h under normal weather conditions in middle and low flats were<6 cm and 20-40 cm,respectively.During storm periods,the thickness of fluid mud layers increased to 48 cm when the background SSC value increased to 3.3 kg/m3.In addition,30 cm fluid mud layers likely formed during peak flood/ebb periods.Southeastern Chongming mudflat:near-bed SSC varied from 0.005-3.64 kg/m3 and averaged 0.91 kg/m3.There were three stages in the measurement duration:strong wind event?average wind speed of 6.9 m/s?,bed recovery stage when winds were weak?average wind speed of 3.5 m/s?,and second wind event?average wind speed of 5.4 m/s?.Average SSC values during these three stages were 0.44 kg/m3,1.53 kg/m3,and 2.4 kg/m3,respectively.Kapellebank mudflat:near-bed SSC values varied from 0.01-8.5 kg/m3 and averaged at 0.53 kg/m3.The average SSC values were 0.48 kg/m3 and 0.58 kg/m3 during normal weather and storm periods,respectively.4.Bed-level changes on intertidal flatsIntra-tidal bed-level changes in the Nanhui Foreland mudflat show that erosion occurred during peak flood and ebb periods while accretion occurred under high slack water conditions during periods of normal weather.The maximum intra-tidal erosion depth was 2-3 cm,exceeding the net intertidal bed-level change of 0.7 cm.On low flats,the bed levels were in equilibrium during periods of calm weather.The maximum bed variations was only 2.2 cm.During storm periods,bed-level changes measured via ADV and buried-plate methods showed maximum variations of 8-9 cm,but they were found to be in poor agreement due to the occurrence of sheet erosion.ADV measurements show that rapid accretion?9.5 cm in two tidal cycles?and degradation?5.6 cm in one tidal cycle?occurred during later stages of storm periods and after storms.These processes were closely related to the formation and migration of edges resulting from sheet erosion.On the Southeastern Chongming mudflat,bed degradation of 10.6 cm occurred over 11 tidal cycles during strong wind events,whereas bed accretion levels of 8.3 cm occurred in the following 5 recovery tidal cycles.Overall,bed-level changes in the mudflat were more significant than those found along the salt marsh,with maximum values observed in the middle flat area.Buried-plate measurements show that during erosion,3 cm of bed degradation occurred in the middle flat area,whereas lower and upper flat areas showed bed-level variations of ±0.5 cm;during recovery stages,accretion was greatest in the middle flat area?2.3 cm?,followed by those in the lower flat area?1.5 cm?and in the transitional zone between flat and marsh areas?0.8 cm?.The lowest value was found in the salt marsh area?0.4 cm?.The maximum variations in low,low-to-middle and middle Kapellebank mudflat area were 11.8 cm,4.3 cm and 3.6 cm,respectively.Significant bed degradation processes occurred only in low flat areas during storm events.Intra-tidal bed-level changes show that bed degradation only occurred when water depths were less than 1 m,and the bed levels were stable or accretional during the remaining tidal submergence period.5.Relationships between sediment dynamic processes on intertidal flatsSediment dynamic processes respond to tides,wind conditions and sediment properties at intra-tide,neap-spring tides,and storm time scales.Erosion and deposition stages alternate due to the balance between ?cw and ?e and as a result of background SSC values.During tidal submergence periods,erosion occurred during peak flood and ebb periods,whereas deposition generally occurred during slack water periods.Deposition stages were shortened and even disappeared under storm conditions.Sediment dynamic processes on intertidal flats exhibit neap-spring cyclicity in calm weather conditions.Wind events,which are random,interrupt this neap-spring cyclicity pattern.Our study shows that the effects of waves on intertidal flat sediment dynamics cannot be ignored,even for meso-macrotidal sheltered mudflats subjected to strong wind conditions,which are weaker than those of storm conditions.The effects of waves become more pronounced when flats are open.Bed degradation during storm periods results from an increase in ?cw.Wind enhances ?cw in the following two ways:?1?wave orbital velocities are increased due to enhanced wave heights driven by winds,which lead to an increase in ?w;and?2?wind driven flows increase ?c,in the ?cw.model.One form of wind-driven flow involves extra turbulence superimposed on original turbulence resulting from tidal currents,with mean flow speeds maintaining the neap-spring cyclicity.The other form involves wind-driven flows resulting from the abnormal flow structures found on at interface of a flat and channel,which breaks the intra-tidal cyclicity of current speeds.Bed accretion is favourable after wind events under the following conditions:?1?when abundant sediment is delivered through high-energy tidal flows,and often during spring tidal periods;?2?when tidal asymmetry favours flood dominance,leading to higher levels of onshore sediment transport;?3?when ?d exceeds ?cw.long enough to promote sediment deposition;and?4?when high SSC levels enhance flocculation,in turn promoting sediment settlement.6.BLC?bed-level change?model and its applicationsBased on our understanding both of the relationships between ?cw,?e,?d,SSC and bed-level changes and of the classical Partheniades-Krone erosion-deposition model,we constructed a BLC model using the measured parameters as input.Measurements and simulations were used to investigate effects of strong wind events on sediment dynamics in intertidal mudflats,to quantify the erosion parameter re and M?erosion coefficient?and to study the influence of wind events on these variables.Our results show that the sediment that was freshly deposited after wind events is much looser:the water content was found to be 3 times that of more consolidated bed sediment before the examined wind event,and M decreased by 60%.Vertically,?e decreased following a power function,and the magnitude of M was 10-3?10-4 s/m without presenting a significant tendency with depth.This result indicates that ?e cannot be simplified as a constant in depth in erosion-deposition and morphological models and that M can be used as a constant in depth.The BLC model was used to determine whether ?d should be introduced into the deposition model.We define falling sediment motion as 'settling' and the product of sediment settling as 'deposition':sediment always settles under the influence of gravity,whereas sediment deposition resulting in bed accretion occurs when ?cw falls below a certain threshold,which is ?d.Therefore,we suggest employing ?d,which is valued at 0.5-1 ?e.This study furthers our understanding of characteristics and mechanisms of physical processes in intertidal mudflats by investigating large-to small-scale processes using qualitative and quantitative measures based on single indicators and well as a combination of multidisciplinary parameters.We also construct and improve a model that simulates morphological changes in intertidal mudflats and provide suggestions for the model's parameterization.In addition,part of the work described in this study provides insight into studies conducted in similar disciplines,e.g.,coastal engineering,which is related to sediment dynamics,coastal ecology,which concerns bed stability,and coastal environmental studies,which concern fine-grained sediment transport processes.The novelties of this thesis are as follows:?1?we proposed and deployed a combined approach to in situ measurement and numerical modelling to improve methodologies for studying sediment dynamics in intertidal mudflats.First,technical difficulties experienced when measuring simultaneous waves,current velocities,SSC values and bed-level changes in intertidal flats at a high resolution were addressed.Second,the BLC model was designed to apply ?cw,?e,?d,E and D calculations based on in situ measured data,and simulated BLC values were found to be in good agreement with measured BLC values.?2?We focused on wind effects not only at the storm level but also at weaker levels?strong wind;speed<10 m/s?.We found that even for semi-open tidal flats,strong winds substantially affect sediment dynamics on intertidal flats.We recommend that future works highlight the importance of integrated in situ measurements of bottom boundary layers under combined wave-current action conditions to the study of sediment dynamics.This study shows that:i)as intertidal flats are shallow water environments,the Hs/h ratio is likely to be larger than 0.25 in these areas,causing near-bed velocities in three dimensions to be influenced by surface waves;ii)in highly turbid areas,fluid mud layers often form within 50 cm above the bed.Because fluid mud layers are occasionally only several centimetres thick,this is where the OBS probe must be placed;and iii)70%of sediment transport occurs within 50 cm above the bed,indicating that estimations of sediment flux in the bottom boundary layer are vital to understanding sediment transport processes occurring in intertidal flats.This study also presents ways to better describe bottom boundary conditions in BLC or other morphological models:i)wet-dry treatments?or critical depths?in numerical models should be improved,as our results show that significant bed-level changes occur under very shallow conditions?0.3-1 m?both in open and sheltered flats;ii)values of ?e and M should be specified,and their vertical distributions should be considered in the layered bed model;and iii)seasonal variations in diatom biomass,which is a stabilizer,should be considered when formulating bed descriptions.Further work must take destabilizers?e.g.,macro benthic animals?into account and should quantify biotic effects on ?e. |
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