The Observations And Models Of Bottom Boundary Layer For Fine Sediment

The observation systems for Bottom Boundary Layer(BBL)are designed and successful measurements for BBL are made in the South Passage and Chongming East Shoal of Changjing Estuary.Data of current,wave and sediment were obtained.With these observed data,the relevant analysis is carried out.Combing the Bottom Boundary Layer Model(BBLM)and one dimensional vertical(1DV)point model,the calculated method and mechanism about sediment transport are also discussed.The main contents could be reviewed as follows:Response of near-bed suspended sediment concentration(SSC)to dynamics in South Passage of Changjiang EstuaryThe analysis shows that the responses of near-bed SSC to the dynamics are complicated and affected by many factors,such as the hydrodynamics,the critical shear stress for erosion related with the sedimentation and consolidation of the bed sediment,the settling down of suspended sediment,as well as the flocculation of cohesive sediment,among others.The response of near-bed SSC to dynamics can be summarized as belonging to four types of response process,i.e., the first types of positive-response process in which the SSC increases with the increase in tidal current speed;the second types of negative-response process in which the SSC decreases with the increase in tidal current speed;the third types of negative-response process in which the SSC increases with the decrease in tidal current speed;and the fourth types of positive-response process in which the SSC decreases with the decrease in tidal current speed.In accordance with the velocity and SSC processes calculated by 1DV point model,those four response processes of near-bed SSC to dynamics are analyzed.The results also show the four stages of near-bed SSC to dynamics,which indicates that those four response processes of near-bed SSC to dynamics are typical sediment movement.As illustrated by the generalized model, the near-bed SSC are the synthesized operations of hydrodynamics,the available erosion flux and the settling flux of sediment et al.Response of SSC to dynamics in Chongming East ShoalOne of the most significant characteristics of SSC in Chongming East Shoal is the appearance of maximum SSC that comes forth at the beginning of flood tide and ending of ebb tide.The analysis of response of SSC in Chongming East Shoal to dynamics indicates that the response of SSC to tidal speed is not so visible as we supposed.During the observation period,only in several tidal period,SSC and tidal speed demonstrates the obvious positive relation.Although the relationship between the SSC and the tidal speed is feeble in one tidal cycle,the relationship between the tidal averaged SSC and tidal averaged current speed is obvious.The relationship coefficient R² in the mud flat and saltamrsh are 0.8 and 0.87,respectively. The SSC varies inversely with root-mean-square wave orbital velocity in each tidal cycle.The relationship between the SSC and the root-mean-square wave orbital velocity is not caused by their reciprocity,i.e.it is not the response of SSC to root-mean-square wave orbital velocity.One supposed explanation is:the relationship is due to the fact that the root-mean-square wave orbital velocity changes with the water depth.The other possible explanation is:the maximum SSC occuring at the beginning of flood tide is higher than the sediment-laden capacity of flow,i.e.this phase is over-saturation sediment transport.Therefore,the SSC decreases gradually although the root-mean-square wave orbital velocity increases with the increase of tidal level.Similarly with tidal speed,the tidal averaged SSC and the tidal averaged root-mean-square wave orbital velocity represents the direct ratio relationship.In the mud flat,the relationship is not so tied up strongly than that in the saltmarh whose the relationship index is 0.79.The positive relationship between the tidal averaged SSC and tidal averaged current speed/root-mean-square wave orbital velocity shows that the response period of SSC to dynamics in Chongming East Shoal is tidal one.The source of flow energy and the erosion-deposition mode in Chongming East ShoalNo matter what category of the tidal dynamics is,the tidal speed in the salt marh is much lower than that in the mud flat,whereas the root-mean-square wave orbital velocity in the salt marsh is still very high.The fact tells us that the main source of fluid energy comes from tidal movement and wave movement in the mud flat.However,wave is the main source of energy.The erosion or deposition in the mud flat is influenced by tidal averaged dynamics.With the increased tidal averaged dynamics,the mud flat is eroded more intensively,while the tidal averaged dynamics decreased,the mud flat is silted.During the observation period,the biggest erosion-deposition range reached 8.7cm.The erosion-deposition range in the saltmarh is far smaller than that in the mudflat. Erosion-deposition range in Site C-1 and Site C-2 is 0.2cm and 0.3cm,respectively,meanwhile the erosion-deposition range in the mud flat(Site A)ranged from 1.1cm to 6.5cm.Compared with Site C-1 and Site C-2,after the human weeding,the Site C-2 is eroded immediately with the erosion-deposition range of 0.8 cm.Upon the above analysis,we may conclude that the Spartina plays the important role to avoid erosion.Based on the analysis of observed data,the erosion-deposition mode in Chongming East Shoal is proposed:1)Combined wave and current.Due to the wave movement,the flow energy is higher both in the mud flat and the saltmarh,so the high SSC can reach to the saltmarh.It is the shoal protection and sediment trap of the vegetation in the saltmarh that make the shoal with vegetation occurs high siltation and extends offshore fast;2)No wave movement.The suspended sediment will deposit onto the ground on the flow way with the decrease of flow energy,and the high SSC might not appear in the saltmarsh deterring the fast extension of the saltmarh. The estimation of combined wave and current shear velocityThe shear velocity calculated by LP method shows that the variety range of u*_c is 0.5～6.7cm/s with an average value of 2.8cm/s.The combined wave and current shear velocity u*_cwcalculated by WKE method ranges from 4.1～10.7cm/s,with an average value of 6.5cm/s.The combined wave and current shear velocity is 1.15～5.22 times higher than the current only shear velocity,with an average of 3.03.It illustrates that due to the action of the wave,the shear stress increases a lot. Therefore,the action of waves cannot be ignored when taking the dynamics into account.The roughness length(z_0a)calculated by BBLM presents the negative power relationship with the relative quality of current and wave(C_r0),which illustrates that along with the current increasing, the bottom roughness(z_0a)is enforced and the bed friction force on the flow is strength accordingly..Making the analogy of the turbulent kinetics energy(TKE)method,a new method to calculate the combined wave and current shear velocity u*_cwi.e.(wave kinetic energy)WKE method is put forward.The result of Bottom Boundary Layer Model(BBLM)indicates that WKE method is applicable for the calculation of combined wave and current shear velocity.There is difference between the WKE method and You method to calculate the combined wave and current shear velocity u*_cw.The result indicates that with the change of the tidal speed,the "constant coefficient C" in the You methods could not be constant anymore.Therefore,we may improve the YOU method by amendment of the "constant coefficient C" with the introduction of a function related with the tidal speed.The results shows that the difference between the combined wave and current shear velocity calculated by the improved You method and the ones calculated by the BBLM is very small,which shows that the improved You method is applicable for the calculation of combined wave and current shear velocity.