Sediment Dynamic Process On Tidal Flat Under Windy Conditions

The sediment dynamic process plays an important role in material transport,geomorphologic evolution and ecological environment of tidal flat and is one of the hottest research topics on land and ocean interactions.Since the 21 st century,extreme weather events,such as typhoon,cold wave,occurs frequently.A thorough understanding of tidal flat sediment dynamic process in windy condition is very necessary.In this thesis,we systematically compared the sediment dynamic characteristics of Rudong tidal flat in both windy and normal weather conditions,including tidal currents,waves,boundary parameters,etc.Meanwhile,different methods of estimating bottom shear stress,such as log-profile(LP),turbulent kinetic energy(TKE),modified TKE(TKEw),Reynolds stress(RS),and inertial dissipation(ID),were compared to discuss the impact of waves on currents,and the validation of these methods were also evaluated.This thesis provided a typical case to study the development and evolution of tidal flat under extreme weather conditions,as well as references for the coastal engineering construction,management,and coastal zone sustainable development.From 21 September to 22 October 2014,oceanographic instruments attached on three tripods were deployed in a seaward cross-shore transection over the tidal flat.Observation results shown that the tidal currents were rectilinear.The flood duration was shorter than ebb duration(with a ratio of 0.70 to 0.85),and their difference increased gradually from land to sea.In calm weather,the flood velocity was stronger than the ebb during spring tide,and they have the same magnitude during neap tide.While in the windy condition,the ebb velocity increased obviously.Waves in the study area were mainly caused by local wind,and the significant wave height could reach over I m in windy weather,with a maximum value of 1.4 m.The direction of wave propagation was 200°?260°.Suspended sediment concentrations were below 1400 mg/L in calm condition,but they were over 2000 mg/L and remained above 1400 mg/L during ebb in the windy condition.During the observation period,the magnitude of wave orbital velocity ranged between 10-2 to 100 m/s and wave shear stress magnitude was 10-2 to 100 N/m2;both of them were correlated well with the significant wave height.The total bottom shear stress of combined wave and current interactions ranged between 0 N/m2and 1.8 N/m2,and the friction velocity was from 0.01 to 0.04 m/s.According to the Soulsby(1997)eqation,the settling velocity of bottom sediments was from 2.9×10-3 to 8.8×10-3 m/s,and for sediments in water column,the settling velocity was 1.4×10-4m/s,which was one order of magnitude less than bottom sediments.Suspended sediment transport in our study area was mainly controlled by resuspension.The magnitude of instantaneous erosion and deposition flux were 0?0.02 kg·m-2·s-1 and 0?0.01 kg·m-2·s-1,respectively.Sediments were transported landward during flood and seaward during ebb.Strong winds could lead to significant increase of water depth in tidal flat,with an increasing amplitude of more than 1 m.Tidal cycle also increased 0.5 hour and the current velocity increased 25%to 50%in windy conditions.In normal weather,the significant wave height in the study area is below 0.3 m,while in windy period,it could reach 1.4 m,nearly 30%of the max water depth.Suspended sediment concentrations also increased 40%to 60%in the windy weather.When the wave action was much stronger than the current action,even small wave orbital velocity could cause large variation of tidal current.Therefore,it was necessary to remove wave's influence to estimate the Reynods shear stress when the wave action was significant.We have adopted different method to decompose wave from tidal current,results shown that the energy spectrum analysis(ESA)method was much closer to in situ measurement than the moving average(MA)method,but the MA method was easier to execute.The total bed shear stress was determined based on the modified Grant-Madsen wave-current interaction model(WCI).When using low-frequency item to calculate the bottom shear stress,the result of log-profile(LP)method had a better consistency with the other methods.Therefore,in the case where the velocity was only measured at one point,LP method could effectively estimate the bottom shear stress.By using high-frequency item,the result of modified TKE(turbulent kinetic energy)method(TKEw)had the best consistency with other methods.The consistency between Reynolds stress(RS)method and TKE method was great when the turbulence was strong.In addition,on the basis of the total bottom shear stress,we found that suspended sediment concentration cold influence the eddy kinematic viscosity of water.When the suspended sediment concentration was higher than 1000 mg/L,suspended particles significantly reduced the internal resistance of the water.