Improved prediction of cohesive sediment erosion based on inter-particle forces

粘性沉积物（通常称为泥浆）形成许多水道和沿海环境的河床，因此是建造至关重要的工程结构的基础。这些结构改变了当地水流，可能引起沉积物的侵蚀（或冲刷），导致桥墩、桥台和护岸被破坏，损害离岸风力涡轮机的基础，并导致海底管道自埋。科学家们早就知道沉积物的成分会影响其抗侵蚀能力，但一直未能开发出通用模型来预测侵蚀阈值、速率或冲刷深度。这一专题的研究受到以下因素的阻碍：（一）侵蚀的定义和测量方法不一致，以及（二）控制粘性沉积物的凝聚力和粘附力的大量沉积物特性。迫切需要更好地预测侵蚀阈值和侵蚀速率，以改善冲刷风险评估，保护重要的交通和能源基础设施，特别是气候变化预计会增加风暴的频率和严重程度，这些风暴会驱动导致沉积物侵蚀的水动力。这项创新研究将推动物理-基于颗粒-颗粒和颗粒-流体相互作用的粘性泥沙侵蚀的预测模型，粘性泥沙内的凝聚力和粘附力的基础。它将结合联合收割机（一）关于粘性沉积物侵蚀阈值和侵蚀速率的新的实地和实验室研究和（二）一个新的计算动力学模型来模拟粘性沉积物力学和侵蚀动力学。该研究由三个工作包（WP）组成。在WP 1中，将在克兰菲尔德大学新的EPSRC资助的UKCRIC设施中测量自然和人工沉积物的侵蚀阈值和速率，这些沉积物覆盖了沉积物和水的各种性质。这将是第一次利用能够高精度监测水流、沉积床表面以及表面颗粒和聚集体的侵蚀的设施，在一项单一研究中系统地调查沉积物和水的性质对粘性沉积物侵蚀的影响。在WP 2中，将粘性沉积物表示为刚性、非粘性元素和较小的“软”粘性元素的混合物的离散元素模型（DEM）将与计算流体动力学模型（CFD）相结合。这一新的耦合模型结合了材料建模（具有不同物理、化学和机械特性的颗粒大小的混合物）方面的若干最新进展，并允许在与WP 1中的实验室分析相似的空间和时间尺度上模拟侵蚀。最后，WP 3将分析、评价和比较经验研究（WP 1）和耦合数值模型（WP 2）的结果，采用统计和概率方法来推断可蚀性和沉积物特性之间的重要关系，并评价模型的性能。这项研究将使人们对粘性沉积物力学的科学认识以及对侵蚀阈值、侵蚀速率和冲刷深度的预测发生重大变化。粘性沉积物侵蚀的预测大大改善，将有广泛的应用，这将有助于保护至关重要的水生环境和水资源，从污染的沉积物和工程基础设施，从洪水引起的故障。

Cohesive sediment (more commonly known as mud) forms the bed of many waterways and coastal environments and is thus the foundation on which critically important engineering structures are built. These structures alter local water flows that can induce the erosion (or scour) of sediment, resulting in the undermining of bridge piers, abutments and revetments; compromising the foundations of off-shore wind turbines; and causing the self-burial of submarine pipelines. Scientists have long known that the composition of the sediment will affect its resistance to erosion, but have not been able to develop universal models to predict erosion thresholds, rates or depths of scour. Research in this topic has been hindered by (i) inconsistencies in how erosion is defined and measured and (ii) the large number of sediment properties that control cohesion and adhesion in cohesive sediment. Better predictions of erosion thresholds and rates are urgently required to improve the assessment of scour risk to protect essential transportation and energy infrastructure, particularly as climate change is predicted to increase the frequency and severity of storms that drive the hydrodynamic forces responsible for sediment erosion.This innovative study will advance the development of a physically-based predictive model of cohesive sediment erosion by focusing on the particle-particle and particle-fluid interactions that underlie cohesive and adhesive forces within cohesive sediment. It will combine (i) new field and laboratory research on erosion thresholds and rates of cohesive sediment and (ii) a novel computational dynamics model to simulate cohesive sediment mechanics and erosion dynamics. The research is composed of three work packages (WPs). In WP 1, erosion threshold and rates of natural and artificial sediments covering a diversity of sediment and water properties will be measured in the new EPSRC-funded UKCRIC facilities at Cranfield University. It will be the first time that the effects of sediment and water properties on cohesive sediment erosion will be investigated systematically in a single study using facilities capable of high-precision monitoring of water flow, the sediment bed surface, and the erosion of surface particles and aggregates. In WP 2, a Discrete Element Model (DEM) that represents cohesive sediment as a mixture of rigid, non-cohesive elements and smaller 'soft' cohesive elements will be combined with a Computational Fluid Dynamics model (CFD). This new coupled model incorporates several recent advances in the modelling of materials (a mixture of particle sizes with varying physical, chemical and mechanical properties) and permits the simulation of erosion at similar spatial and temporal scales as the laboratory analyses in WP 1. Finally, WP 3 will analyse, evaluate and compare the results of the empirical study (WP 1) and coupled numerical model (WP 2), employing statistical and probabilistic approaches to infer significant relationships between erodibility and sediment properties and evaluate the performance of the modelling. The study will produce a step-change in the scientific understanding of cohesive sediment mechanics and the prediction of erosion thresholds, rates and depth of scour. The greatly improved predictions of cohesive sediment erosion will have wide ranging applications that will help to protect critically important aquatic environments and water resources from contaminated sediment and engineering infrastructure from scour-induced failure.