Properties and dynamics of suspended load and near-bed fine cohesive sediments in highly impacted estuaries.Case studies from the Weser, Ems and Elbe estuaries (Germany)
Properties and dynamics of suspended load and near-bed fine cohesive sediments in highly impacted estuaries.Case studies from the Weser, Ems and Elbe estuaries (Germany)
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受严重影响的河口悬浮载荷和近床细粘性沉积物的特性和动态。威悉河口、埃姆斯河口和易北河河口的案例研究（德国）
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发表时间：
2012
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通讯作者：
S. Papenmeier
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作者：
S. Papenmeier
河口通常被用作通往内陆城市和港口的运输通道，并已成为海岸研究的一个重要焦点。人类渴望了解和控制复杂的水沙动力学，以优化系统，由于社会经济的需求。在这一过程中，河流法规（如疏浚活动）可持续地改变了环境的自然动态。这项研究提供了新的知识，近床粘性泥沙动力学以及细粘性泥沙动力学不仅在“自然”潮流，但也在注水疏浚（WID）的影响下。
在德国易北河和威悉河河口测量的原位粒径分布（ISPSDs）和初级粒径分布（PPSDs）表明，有机和无机悬浮颗粒物（SPM）处于絮凝状态。絮凝过程所需的有机物基质主要从海滨输送到河口。在冬季，当淡水流量高，最大湍流带（TMZ）的延伸很大时，PPSD的区域差异已被观察到。在夏季TMZ延伸较小时，未观察到向海和向陆部分以及TMZ中的个体分选。PPSD的区域差异对ISPSD没有影响。后者主要由潮汐力和增加的悬浮泥沙浓度（SSC）驱动的颗粒碰撞频率控制。虽然，絮体破裂，由于剪切应力与进步潮流，他们不改变他们的PPSD。了解絮凝体的大小和组成对估计SPM的沉降速度是很重要的。颗粒沉降增加可导致近床细粘性泥沙浓度增加。在威悉河和埃姆斯河口近床SPM的高分辨率垂直采样表明，被广泛接受的3层模型，通常用于描述垂直，粘性沉积物分布是明显不完整的。通过聚类分析统计证实的沉积学和流变学参数表明，中间流体泥浆层必须细分为低粘度流体泥浆层（I）和高粘度流体泥浆层（II）。在多参数分析的基础上，可以确定两种流体泥浆类型的准确SSC极限。流体泥浆（I）的上边界的特征在于强SSC梯度（跃层），其用参数沉积物回声测深仪的低频通道检测。声界面的振幅与在声界面处采样的SSC梯度相关，这在文献中之前没有被量化。随着潮流的进行，SSC梯度逐渐减小，表明界面混合，但在静水后一小时，平滑床形态的区域不会发生显著变化。建议流体泥浆（II）层代表经常性的粘性沉积物堆积，这些沉积物必须经常在港口和航道中疏浚。在过去的几十年里，水力WID技术在潮汐控制环境中清除泥滩和水下沙丘方面引起了越来越多的关注。广泛的水声，光学和地面实况数据收集在WID在咸淡水和威悉河河口的河段表明，水下沙丘的波峰被完全删除在所需的高度。潜在的疏浚影响仅限于近似的疏浚现场。内部沉积物结构的破坏仅限于上部分米，流动的桑迪沉积物堆积在沙丘斜坡或邻近的槽中。在咸水区和淡水区均未观察到SSC或絮凝体大小的显著变化，但声学干扰表明疏浚设备当前背风面处数百米距离内的粘性增加。
Estuaries are often used as transport ways to cities and harbours in the hinterland and have emerged as an important focus in coastal research. Mankind aspires to understand and control the complex hydro- and sediment dynamics in order to optimize the system due to social-economic demands. In this process, river regulations (e.g. dredging activities) change the natural dynamics of the environment sustainably. This study provides new knowledge about near bed cohesive sediment dynamics as well as of fine cohesive sediment dynamics not only under ‘natural’ tidal flow but also under the influence of Water Injection Dredging (WID).
In-Situ Particle Size Distributions (ISPSDs) and Primary Particle Size Distributions (PPSDs) measured in the German Elbe and Weser estuaries indicate that the organic and inorganic Suspended Particulate Matter (SPM) is in a flocculated state. The substrate for the organic matter, which is needed for flocculation processes, is mainly transported from the seaside into the estuaries. Regional differences in PPSD have been observed in winter when the freshwater discharge is high and the extension of the Turbidity Maximum Zone (TMZ) is large. Individual sorting between the seaward and landward section as well as in the TMZ has not been observed in summer when the TMZ extension is small. Regional differences in the PPSD have no influence on the ISPSD. The latter is controlled primarily by the particle collision frequency powered by tidal forces and increased Suspended Sediment Concentrations (SSCs). Although, flocs break-up due to shear stress with progressing tidal current they do not change their PPSD. Knowledge about floc size and composition is important to estimate settling velocities of the SPM. Increased particle settling can lead to enhanced near bed fine cohesive sediment concentrations. High resolution vertical sampling of near bed SPM in the Weser and Ems estuaries indicates that the widely accepted 3-layer models, often used to describe vertical, cohesive sediment distribution is evidently incomplete. Sedimentological and rheological parameters, statistically proven by a cluster analysis, have shown that the intermediate fluid mud layer has to be subdivided in a low-viscosity fluid mud layer (I) and high-viscosity fluid mud layer (II). On the basis of a multi-parameter analysis it was possible to define the exact SSC-limits of both fluid mud types. The upper boundary of the fluid mud (I) is characterised by a strong SSC-gradient (lutocline) which is detected with the low frequency channel of a parametric sediment echo sounder. The amplitude of the acoustic interface correlates with the SSC-gradient sampled at the acoustic interface which had not been quantified before in literature. Decreasing SSC-gradients with progressing tidal currents indicate an interfacial mixing but significant changes do not occur in areas of smooth bed morphology until one hour after slack water. Fluid mud (II) layers are suggested to represent recurrent, cohesive sediment accumulations which frequently have to be dredged in harbours and navigation channels. Over the last few decades the hydraulic WID technique has gained increased interest for the removal of mud shoals and subaqueous sand dunes in tidal controlled environments. Extensive hydroacoustical, optical and ground-truthing data collected during WID in the brackish- and freshwater reach of the Weser estuary shows that the crests of subaqueous sand dunes were exactly removed at the demanded height. Potential dredging effects are restricted to the approximate dredging site. Destruction of the internal sediment structure is limited to the upper decimetres and mobilized sandy sediments are accumulated on the dune slopes or in the adjacent troughs. Significant variations in the SSC or floc size are neither observed in the brackwater nor in the freshwater reach although acoustic interferences suggest increased turbulences over a distance of some hundreds of metres at the current lee-side of the dredging device.