【作者】 王元叶；

【导师】 何青；

【作者基本信息】 华东师范大学 ， 自然地理学， 2007， 博士

【摘要】 本论文设计和实现了长江口滩和槽的近底水沙现场观测系统,利用该系统在河口滩、槽典型区域获取了包括流、浪和泥沙变化过程的系列宝贵数据;以观测数据为主开展分析研究;结合边界层和一维数学模型探讨了近底泥沙运动的计算方法和变化机制。获得的主要认识包括:长江口南槽近底悬浮泥沙浓度随动力的响应近底悬浮泥沙浓度随动力的响应过程受到多方面因素的影响,主要有水流的动力、床面泥沙的供应量（涉及到床面泥沙的沉积和固结过程）、泥沙的沉降以及由此涉及到的粘性细颗粒泥沙的絮凝现象等。近底悬浮泥沙浓度的变化和动力的响应关系总体可以归为四类过程:第一类过程——悬浮泥沙浓度随流速的增大而增大,原因是:该阶段床面泥沙的侵蚀量占据该过程的主导地位,它远远要大于向上的紊动扩散及泥沙的沉降的综合作用;第二类过程——悬浮泥沙浓度随流速的增大而减小,原因是:该阶段水体紊动作用增强,向上的紊动扩散作用加大,然而床面可供侵蚀的沉积物不多,使得该阶段的侵蚀量减小了,向上的紊动扩散作用成为该阶段的主导因素;第三类过程——悬浮泥沙浓度随流速的减小而增大,原因是:由于该阶段流速减小,水体的紊动扩散作用减小,中上层水体向近底层水体沉降的泥沙量增大,使得其成为该阶段的主导因素;和第四类过程——悬浮泥沙浓度随流速的减小而减小,原因是:中上层水体的悬浮泥沙大部分已经在上一过程中沉降到近底层,近底层悬浮泥沙的沉降量成为该阶段的主导因素造成的。其中第二类过程,即悬浮泥沙浓度随流速的增大而减小,是垂线六点法和垂线平均水沙观测所难以发现的。根据1DV点模型计算得到的泥沙浓度过程和水流过程数据,对近底悬浮泥沙浓度随动力的响应过程进行了分析。分析结果同样得到了近底悬浮泥沙浓度随动力响应的四个过程。这说明近底悬浮泥沙浓度随动力的响应过程代表了一类较为典型的过程,正如概化模型所分析,它是水流动力、床面泥沙供应量、泥沙沉降等多因素的综合作用结果。滩地悬浮泥沙浓度动力响应过程滩地悬浮泥沙浓度的一个显著特征是泥沙浓度峰值出现在涨潮初期和落潮末期。滩地悬浮泥沙浓度随动力的响应分析表明:滩地悬浮泥沙浓度随流速的响应关系不明显。观测时段内,只有少数几个潮周期悬浮泥沙浓度和流速的关系呈现比较明显的正比关系。尽管当时当地的悬浮泥沙浓度和流速呈现很微弱的相关关系,但是潮周期平均悬浮泥沙浓度和潮周期平均流速却呈现出很好的正相关关系。在光滩上和盐沼中,它们的相关系数R²分别为0.80和0.87。在每一个潮周期内,滩地悬浮泥沙浓度和均方根波浪轨迹流速呈反比关系。悬浮泥沙浓度和均方根波浪轨迹流速的这种反比关系并不是由它们之间的相互作用导致的,即不是一种悬浮泥沙随均方根波浪轨迹流速的响应关系。它们之间的这种关系的形成的一种解释是:他们的这种关系实际上完全是由均方根波浪轨迹流速自身随水深发生同步相应的变化而形成的。另一种可能的解释是:涨潮初期的高悬浮泥沙浓度峰值要大于此时的水流挟沙能力,即此时为过饱和输沙,所以在接下来均方根波浪轨迹流速逐渐增大过程,看到了悬浮泥沙浓度反而降低这种现象。和水流类似,潮周期平均悬浮泥沙浓度和潮周期平均均方根波浪轨迹流速呈现正比关系。在光滩上,它们之间的关系差一些。但是,在盐沼中,它们之间的相关性较好,相关系数为0.79。潮周期平均悬浮泥沙浓度随潮周期平均的潮流速和均方根波浪轨迹流速的正相关性,说明了滩地悬浮泥沙的一类周期为“潮周期”的动力响应关系。滩地水流能量来源和滩地冲淤模式不论何种潮型,在盐沼中的潮流速要远小于光滩上的潮流速,但是在盐沼中的均方根波浪轨迹流速仍然较大。说明在光滩上,水流的主要能量来源于潮流和波浪。然而,在有植被覆盖的盐沼中,水流的能量来源主要是波浪。崇明东滩东面是开阔的海域,一年四季都处于风浪的作用下,波浪成为水流能量的主要来源。光滩的冲淤变化受潮周期动力情况影响。潮周期动力增大,滩地发生冲刷;潮周期动力减小,滩地发生淤积。整个观测期间冲淤幅度最大达8.7cm。盐沼冲淤幅度要远小于光滩,其冲淤变化周期也要更长。站点C-1和站点C-2的冲淤幅度分别为0.2cm和0.3cm,而相应时段内光滩（站点A）的冲淤幅度变化为1.1cm和6.5cm。当站点C-1和站点C-2经过人为割草之后,站点C立即发生冲刷,冲刷幅度达到0.8cm。从这里可以看出,互花米草（Spartina）在护滩方面起到了相当重要的作用。基于观测资料的分析,提出了崇明东滩冲淤模式:1)有风浪作用。由于波浪的作用,光滩和盐沼中的水流能量都较高,高含沙浓度能够出现在盐沼带中。盐沼植被发挥对悬浮泥沙的捕集及其护滩作用,有利促使盐沼植被带滩地发生迅速淤涨;2)无风浪作用。悬浮泥沙随着上滩水流和水体能量的降低,发生沿程淤积,高含沙浓度到达盐沼带较困难,从而减缓了盐沼植被带滩地的快速淤涨。波流相互作用近底剪切应力估算LP方法计算得到的水流摩阻流速统计表明:u_*c变化范围为0.5-6.7cm/s,平均为2.8cm/s。WKE方法计算得到的波流相互作用的摩阻流速U_*cw变化范围为4.1-10.7cm/s,平均为6.5cm/s。波流相互作用的潮周期摩阻流速u_*cw要比潮周期水流摩阻流速u_*c的大1.15～5.22倍,平均为3.03倍。这说明由于波浪的作用,近底剪切应力大大增加,所以在对动力因素考虑的时候,波浪作用不能忽略。BBLM计算得到的糙率长度(z_0a)同水流和波浪相对大小比例(C_r0)呈乘幂的关系,并且为负相关关系。这表明随着波浪作用的增强,糙率长度(z_0a)增大,也反映了由于波浪的作用,增大了床面对水流的阻力。类比水流作用下的紊动能量（TKE）法,作者提出了一种新的用于计算波流相互作用下的摩阻流速U_*cw方法,即波浪运动能量法（WKE）。底部边界层模型（BBLM）的计算结果表明WKE方法可以用于计算波流相互作用的摩阻流速。用于计算波流相互作用下的摩阻流速U_*cw的WKE方法和You方法计算结果有差异。认为在水流发生变化的情况下,You方法中“常系数”C,不能再取常数。通过引入一个和流速有关的函数来修正You方法中的常数C,进而对You方法进行了改进。结果表明,改进后的You方法计算得到波流相互作用摩阻流速和BBLM计算得到的摩阻流速比较吻合,可以用于波流相互作用下的剪切应力计算。更多还原

【Abstract】 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.更多还原