The movement of cohesive sediment in a large combined sewer
The movement of cohesive sediment in a large combined sewer
复制标题
大型合流下水道中粘性沉积物的运动
DOI：
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
D. Wotherspoon
中科院分区：
文献类型：
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作者：
D. Wotherspoon
污水系统内沉积物的存在可能导致运行（过早超载和表面洪水）和潜在的环境问题（沉积物作为污染物的储存，在侵蚀事件中可能被释放）。国际社会已经认识到让这些问题持续存在的后果。在英国，水行业促进了基础研究和应用研究，以开发必要的业务和分析工具来管理这些问题。根据城市污染管理研究方案，对下水道沉积物的主要方面进行了研究，并将其影响纳入新的方法和工具。英国的大多数研究和其他地方都集中在非粘性沉积物的运动，而人们已经认识到，合并下水道沉积物具有粘性特征（尽管这种粘性主要来自合并下水道中的凝集和生物过程，而不是传统的粘性概念）。新的基于计算机的模型，如Mosqito（Moys 1987）和MOUSETRAP（WRc 1993），是基于泥沙输运能力理论，通过储存层识别系统内泥沙的有限可用性，只有当超过一定的剪应力阈值水平时，储存层才可用。在英国学习为了估计存储在下水道沉积物床中的污染物的释放，还需要了解沉积物床将被侵蚀并被夹带到流中的水力剪切应力条件。报告的研究探讨了明显的凝聚力性质的沉积物床在一个大直径的下水道同时与流动水力学，沉积物床存款深度和悬浮固体通量为一些干燥和潮湿的天气时期。仪器的开发和评估的研究下水道系统内的水力测量和（i）特别是，一种新的系统设计，以提高流量测量精度在大直径下水道。还进行了超声波装置的研制工作，以监测某一点沉积物存款深度的时间变化。沉积物床的组成材料进行了检查和流变技术进行了评估的研究下水道中存在的沉积物床的结构强度。结果证实了沉积物床的明显粘性性质，床的结构强度远远超过污水系统中遇到的正常水力剪切应力范围。通过流变试验，得到了沉积床的表观屈服强度与含液量之间的关系。然后将床层结构强度与流致剪切力的时间变化进行比较。建立了一个经验模型来预测所研究的合流下水道中粘性沉积物的侵蚀可用性。该模型进行了测试，对进一步的时间变化的数据集从下水道，并发现预测侵蚀的沉积物床下施加的剪切应力的变化，以及泥沙输运通量的变化。得出的结论是，当干燥的天气流引起超过1-2 N/m的床剪切应力时，可能会引起沉积物床结构的侵蚀，而引起超过4-6 N/m的剪切应力的风暴流则会将床侵蚀到更深的深度。据观察，沉积床在侵蚀事件发生后迅速恢复。调查和模型开发的显着贡献的知识，在下水道中的沉积物的行为，并首次提供了一个模型，以模拟侵蚀的沉积物床明显的粘性和随之而来的增加沉积物和污染物的传输速率。
The presence of sediment deposits within sewerage systems may lead to operational (premature surcharging and surface flooding) and potential environmental problems (sediments act as a store of pollutants which can be released during erosion events). The consequences of allowing these problems to persist have been recognised internationally. In the U.K., the water industry has promoted fundamental and applied research to develop the necessary operational and analytical tools to manage these problems. Under the Urban Pollution Management Research Programme the major aspects of sediments in sewers have been studied and their effects included in new methodologies and tools. Most studies in the U.K. and elsewhere have concentrated on the movement of non cohesive sediments, whilst it has been recognised that combined sewer sediment deposits possess cohesive characteristics (although this cohesion primarily arises from agglutination and biological processes in the combined sewer rather than classical concepts of cohesion). New computer based models, e.g Mosqito (Moys 1987) and MOUSETRAP (WRc 1993) , are based on sediment transport capacity theories with the limited availability of sediment within the system recognised through storage layers which become available only when certain threshold levels of shear stress are exceeded. Studies in the U.K. to estimate the release of pollutants stored within sewer sediment beds also require a knowledge of the hydraulic shear stress conditions at which the sediment beds will erode and become entrained into the flow. The reported study examines the apparent cohesive nature of a sediment bed in a large diameter sewer concurrently with flow hydraulics, sediment bed deposit depth and suspended solids flux for a number of dry and wet weather periods. Instrumentation was developed and assessed for hydraulic measurements within the study sewer system and in (i) particular, a novel system was devised to improve flow measurement accuracy in large diameter sewers. Development work was also undertaken on an ultrasonic device to monitor the temporal variation in sediment deposit depth at a point. The constituent materials of the sediment bed were examined and rheological techniques were employed to assess the structural strength of the sediment bed present in the study sewer. The results confirmed the apparent cohesive nature of the sediment bed, with the structural strength of the bed far exceeding the normal hydraulic shear stress ranges encountered in the sewerage system. A relationship between apparent yield strength and liquid content of the sediment bed was obtained from the rheological tests. The bed structural strength was then compared with temporal changes in the flow induced shear forces. An empirical model was developed to predict the availability for erosion of the cohesive deposits in the combined sewer studied. This model was tested against further temporally varying data sets from the sewer and was found to predict the erosion of the sediment bed under varying levels of applied shear stress together with changes in the sediment transport flux. It was concluded that when Dry Weather Flows induce bed shear stresses in excess of 1-2 N/m erosion of the sediment bed structure can be caused, with storm flows which induce shear stresses in excess of 4-6 N/m eroding the bed to a greater depth. The sediment bed was observed to be rapidly re-established following an erosion event. The investigation and model developed contribute significantly to knowledge about the behaviour of sediments in sewers and provide for the first time a model to simulate erosion of a sediment bed with apparently cohesive properties and consequent increase in sediment and pollutant transport rates.