Sand-mud morphodynamics under combined tidal and wave actions

河口和海岸的形态动力学受河床侵蚀和沉积的复杂过程控制。在大多数河口和低能海岸，河床沉积物通常含有一些细砂，以及较高比例的粉砂和粘土颗粒，直径从0.0005 mm到0.065 mm不等。由于这些沉积物与污染物和营养物通量、底栖生物和远洋生物群落结构和健康密切相关，因此对岸面细粒沉积物沉积的了解尤为重要。因此，预测沿海、河口或内陆水域内粘性沉积物运动的能力在发展新工程和维护现有设施方面具有重要的经济和生态意义。然而，以前的研究主要集中在砂层或泥浆/淤泥层上。我们对砂泥混合物的运动和运移特性的认识是非常不足的。因此，大多数工程软件包中的多组分输运模块必须依赖于从单尺寸泥沙输运研究中获得的参数化。在这些模型中，絮凝体沉降速度通常被认为是一个时间无关的变量，而在现实中，絮凝体沉降速度与时间密切相关。迄今为止，既没有一个可处理的砂泥混合物沉降速度的历史影响模型，也没有在受控环境下对砂泥混合物沉降过程的直接测量。拟议的研究将以系统的方式解决这一问题。研究计划将包括沉淀池实验，以了解沉降、絮凝和固结过程，以及1-和2DV模型，以研究与波浪效应、沉积物分离和长期形态演变有关的问题。这些数据（以及将开发的数值模型）将支持建立更有效的战略，以预测沿海变化的长期演变。

The morphodynamics of estuaries and coasts are controlled by a complex process of erosion and deposition of bed sediments. In most estuaries and low-energy coasts, bed sediments usually contain some fine sand as well as a higher proportion of silt and clay grains ranging in diameter from 0.0005 mm to 0.065 mm. Knowledge of the fine-grained sediment deposition on the shoreface is particularly important due to the close association between these sediments and contaminant and nutrient fluxes, benthic and pelagic community structure and health. Therefore, the ability to predict the movement of cohesive sediment within coastal, estuarine or inland waters has a significant economical and ecological importance in the development of new engineering works and the maintenance of existing installations. However, previous investigations have focused primarily on either sand beds or mud/silt beds. Our knowledge on the motions and transport behaviour of sand-mud mixtures is grossly inadequate. As the result, multi-fraction transport modules in most engineering software packages have to rely on the parameterisations obtained from single-sized sediment transport studies. In these models the settling velocity of flocs is usually taken as a time-independent variable whereas in reality it is strongly time-dependent. To date, neither a tractable model of history effects on the fraction settling velocities of sand-mud mixtures nor direct measurements of settling processes of sand-mud mixtures in a controlled environment are available. The proposed research will address this problem in a systematic way. The research programme will include settling tank experiments to understand the settling, flocculation and consolidation processes and 1- and 2DV modelling to investigate issues related to wave effects, sediment segregation and long-term morphological evolution. These data (and the numerical models that will be developed) will underpin the establishment of more effective strategies for the prediction of lthe ong-term evolution of coastal changes.