【作者】 刘建华；

【导师】 杨世伦；

【作者基本信息】 华东师范大学 ， 自然地理学， 2014， 博士

【摘要】 悬沙是河口(特别是高浊度河口)水体的重要物质组成之一。悬沙的空间分布、时间变化和输运对河口和相邻海域的生态环境和工程具有重要意义。尽管国内外对河口悬沙做过大量研究,但是鉴于径流、潮流、波浪、沿岸流和人类活动多因子和多尺度的复杂影响以及不同河口的自然背景差异,河口(尤其是口门水域)悬沙时空变化规律和输运机制的研究还有待深入。本文以长江口崇明岛向海侧近岸水域9个不同测点(从潮间带至20m水深)上18个测次(非风暴天气条件下)的实测资料为基础,探讨悬沙浓度和粒径的垂向剖面特征和变化规律；揭示悬沙浓度和粒径变化的周期性(潮周期、大小潮和季节性)和趋势性(悬沙浓度对流域来沙减少的响应)；通过计算和对比水动力剪切应力及底床沉积物临界侵蚀剪切应力,初步探讨水体和底床泥沙交换的的动力机制；通过单宽输水、输沙量分析,了解泥沙的水平净输运方向和通量。主要的结果和结论有：(1)悬沙浓度的垂向变化。402条单一(即瞬时测量)悬沙浓度剖面形态类型多种多样,可分为“L”型、阶梯型、斜上凹型、斜上凸类型、“S”类型、反“S”类型、斜线型、垂向准直线型、“弓”型等9种基本类型,它们只占总剖面数55%,另45%的剖面是过渡类型。但是,402条单一剖面的平均剖面呈现显著非线性向下(自表层向底层)增大的趋势(越向下增长率越大)(R2=0.99)。基于平均悬沙浓度剖面的近底层悬沙浓度和表层悬沙浓度的比率(R近底床/1H)为2.8,与欧洲学者提出的该比值的假定值3相近)。传统的Rouse公式不能很好地拟合本研究区的平均悬沙浓度剖面,但本文在Rouse公式基础上改进的一个新公式与实测剖面之间的相关系数R2达到0.99。(2)悬沙粒径的垂向变化。尽管41条单一悬沙粒径剖面变化复杂,但其平均剖面呈显著线性向下变粗趋势(R2=0.99),平均剖面上底层悬沙粒径(7.2μm)是表层悬沙粒径的1.6倍。这表明,悬沙浓度的向下增大趋势中包含粒径向下增大的贡献。(3)悬沙浓度变化的周期性和趋势性。潮周期内悬沙浓度变化明显,不同测点和测次的潮周期内变化特点存在一定差异,落急时段出现悬沙浓度极大值,而涨憩时段会出现悬沙浓度极小值,反映潮周期内不同阶段潮动力、径流-潮流相互作用和垂向混合差异的复杂影响。大潮悬沙浓度通常大于小潮悬沙浓度,不同测点和测次的大、小潮悬沙浓度值为0.05～3.10g/l,平均0.36g/l；主要原因是大潮流速明显大于小潮。枯季悬沙浓度一般大于洪季,测点H枯/洪季悬沙浓度比值为1.62,主要原因是洪季径流的稀释作用大于枯季(径流的悬沙浓度明显低于口门研究区悬沙浓度)以及枯季风浪引起的泥沙再悬浮作用大于洪季。目前研究区的悬沙浓度与三十年前相比下降了约25%,反映研究区对流域人类活动引起的长江入海悬沙通量下降有敏感响应。(4)泥沙沉降-再悬浮的动力机制。潮流剪切应力(τc)与流-浪联合剪切应力(τcw)之比值均大于0.82,说明在非风暴天气条件下研究区的水动力以流为主。τcw既有大于(底床沉积物的临界侵蚀剪切应力τcr的时间段(约48%),也有小于τcr的时间段(52%),说明研究区底床和水体之间存在频繁的泥沙交换过程。但是,τcw与悬沙浓度之间的统计相关性不显著,说明潮周期内悬沙浓度的时间变化可能主要受平流作用而不是受局地剪切应力引起的泥沙沉降-再悬浮影响。波浪在研究区内沉积动力的贡献率随着测点水深的减小而增大。(5)悬沙的输运。从表层至底层减小的流速剖面和向下增大的悬沙浓度剖面导致作为两者乘积的悬沙输运率剖面具有不确定的垂向变化趋势：最大悬沙输运率可出现在表层、底层或中层,以中层较为普遍。各测点和测次的潮周期余流速变化于0.07和0.28m/s之间。近口门测点的水沙净输运方向主要为东向(即向海),反映径流的控制作用。在口门以外40～50km的测点水沙净输运方向受风向的影响较大,有时表现为南向或北向,反映风生流的影响。更多还原

【Abstract】 Suspended sediment is one of the important material compositions in the water of the estuaries, especially the turbid estuaries. Spatial distribution, temporal variation and transport of suspended sediment play a vital role to the ecology system and engineering in the estuaries and adjacent waters. Numerous researches have been conducted on the suspended sediment. The complexity of multi-factor and multi-scale (e.g. variations of the runoff, current, waves, coastal currents, and human activities) of the estuaries and their natural different characteristics, however, lead to further study of temporal and spatial variations of suspended sediment and transport mechanisms in the estuaries (especially the water column near estuary mouth). Based on data collected from observations of eighteen observation voyages at nine measuring sites (from the intertidal zone to twenty meters depth) located in offshore area in front of Chongming Island, the present study aims to:discuss the variation patterns of vertical profile of suspended sediment concentration and grain size; reveal single tidal cycle, neap-spring cycle, and seasonal periodicity of suspended sediment concentration and grain size, as well as their trends caused by decrease of sediment discharge; discuss the dynamic mechanism of sediment exchange between water column and seabed by comparison between the calculated bottom shear stress and critical bed shear stress for erosion, understand the horizontal net sediment transport flux through the analysis of net water and sediment transport rate per unit width. The main results and conclusion are:(1) Vertical variations of suspended sediment concentration. The402individual SSC profiles can be classified as nine basic types:L-shaped, stepwise, slanting concave-up, slanting concave-down, S-shaped, reverse S-shaped, slanting line-shaped, vertical stable, and arc-shaped.55%of the SSC profiles are belonging to the above nine types, while the rest45%are transitional type. The averaged SSC profile, however, shows a nonlinear downward (from the surface to the bottom) increasing trend with greater gradient as the depth (R2=0.99). The ratio between near-bed SSC and water surface SSC was about2.8, which is close to the value of3suggested by Whitehouse et al.(2000). A new approach has been provided to simulate SSC, which is closer to the measured SSC (the correlation coefficient between the simulated and measured SSC exceeds0.99), rather than the classical Rouse equation.(2) The vertical variations of suspended sediment grain size. The averaged profile of41single profiles showed a significant linear downward coarsening (R2=0.996), although each profile showed some different patterns. The averaged bottom suspended sediment grain size,7.2μm, was1.6times as the surface suspended sediment grain size. This suggests that the downward increasing of suspended sediment concentration contained contribution in the particle size downward Increases.(3) The periodicity and tendency of the SSC changes. The SSC changes significantly within the tidal cycle, although some differences exist in the tidal cycle of different sites and observation time. The maximum SSC appears in the peak ebb phase, and the minimum value appears at slack flood tide, indicating that the effects of tidal current, runoff-current interactions, and vertical mixing in different tidal stages. The SSC in the present study varies from0.05to2.80g/1, with an average value of0.36g/1. SSC of spring tides is usually greater than that of neap tide, probably because the current velocity of spring tide is significantly greater than the neap tide. The SSC of dry season was bigger than that of flood season (i.e., the ratio of dry season SSC and flood season SSC was1.62at site H). That’s mainly due to the dilution of the runoff, whose sediment concentration is lower than the water column of the estuary mouth, and which is smaller in dry season than in flood season, as well as stronger resuspension in dry season caused by the wind wave. The SSC of the present study area has decreased by about25%compared with thirty years ago, which reflects the response of estuarine SSC to the decline in riverine sediment supply due to human activities.(4) Dynamic mechanism of sedimentation and resuspension of sediment. In the present study area, the hydrodynamic condition was controlled by currents during calm weather as the contribution of τc to τcw was more than0.82. The frequency of τc greater than τcr (critical bed shear stress for erosion) was48%, while frequency of τc less than τcr was52%, which means that sediment exchange between water column and the seabed was frequent. Correlation between near-bed SSC and τc was not significant, suggesting the SSC change based on the observations once an hour is reflected by advection rather than the deposition and erosion processes caused by local hydrodynamic conditions.(5) Suspended sediment transport. Current velocity decreasing downward and SSC increasing downward lead to vary probability for the vertical profile of product them. The maximum product value can occur at the surface, the bottom, or the middle layer, whereas commonly occur at the middle layer. The residual current of each site and voyage varied from0.07to0.28m/s. Net transport direction of the water and sediment at the site near the estuarine mouth is eastward, namely seaward, reflecting sediment transport in this area was controlled by runoff. Net transport of water and sediment are greatly influenced by the wind direction. Wind-induced longshore currents cause southward or northwards net water and sediment transport at the sites40-50km seaward from the estuary mouth.更多还原