Erosion tests with the annular flume

To predict the changes of natural systems like estuaries and tidal lagoons as a consequence of human interferences often numerical models are applied. An important aspect is the erosion and transport of sediment. Therefore, these models require formulations that describe the erosion of the sediment bed consisting of sand, mud and organic matter. A physically founded and validated process-based description of these mixed sediments is still missing. Recently, a theoretically derived formulation for erosion of mixed sediments has been proposed. However, a proper validation of this new formula is lacking. Therefore, a systematic study has been undertaken. First, studies have been executed to investigate the individually physical properties in this formula. Next, erosion tests have been executed on a large number of varying sample compositions using a straight, re-circulating flume. The effect of sediment structures and cohesiveness of the sediment bed has been investigated, from which the new formulations could be qualified. However, this experimental set-up was relatively small, with possible large influences of e.g. boundary effects. Therefore, another set of erosion experiments had to be carried out to supplement the results of the mentioned tests. The objective of this study was to quantify the newly defined theorem for the erosion behavior of sand-mud mixtures. The experiments were carried out on three different artificially generated sand-silt-clay mixtures, using a large annular flume. The mixtures were homogeneously mixed and 100% saturated and, subsequently, placed in the flume. A unidirectional flow was generated by rotating the top-lid and the flume in opposite directions. The flow-induced shear stresses near the bottom were varied by increasing the rotational speeds step by step. An observation on the erosion behavior of the beds was carried out, as well as the measurement of the concentration of suspended fines over the vertical. Due to the occurring secondary currents, the eroded sand accumulated along the outer bend of the flume. By collecting this the amount of eroded sand was quantified. The shear stresses near the bottom were not measured directly, but were determined by means of a large eddy simulation model. The results show two parts regarding the erosion behavior of the mixtures. Before the complete failure of the bed (part 2), different modes of erosion were identified in which only small amounts of material were eroded (part 1). Mainly due to the placement of the bed, the bed strength was slightly varying in the upper part of the sediment bed concerning both the horizontal as the vertical direction. In this first part floc erosion occurred for the fines. Sand was transported as bed load as well as in the sheet flow regime. For this part, the concentration of eroded fines showed a typically non-linear increase as usually occurred for erosion tests with deposited beds. Several possible explanations for this unexpected behavior are discussed. For part 2, surface erosion was observed, during which the concentration was increasing linearly with time. The erosion rate as function of bed shear stresses revealed that the threshold of erosion for samples with a sand-silt skeleton was relatively high, while the threshold for the sample with the clay-water matrix was lower. A transition in erosion behavior was observed when regarding the erosion parameters for both fines and sand. This transition occurred for a plasticity index of around seven, which agrees well with the transition in mechanical behavior for sand-dominated to clay-dominated sediment as found in previous empirical geotechnical studies.

为预测人类活动干扰下河口与潮汐泻湖等自然系统的变化，常需采用数值模型（numerical models）。其中，泥沙的侵蚀与输运是核心环节，因此此类模型需要能够描述由砂、泥与有机质组成的床面沉积物侵蚀过程的数学表述。然而，目前仍缺乏针对这类混合沉积物的、具备物理基础且经过验证的基于过程的描述方法。近期，有学者提出了一种理论推导得到的混合沉积物侵蚀公式，但该新公式尚未得到充分的合理验证。为此，本研究开展了系统性工作：首先，针对该公式中的各项物理属性开展了单独的实验研究；其次，利用直形循环水槽（recirculating flume）针对大量不同组成的样品开展了侵蚀试验，探究了床面沉积物的结构特征与黏聚性的影响，进而对新公式进行了初步验证。但该实验装置规模相对较小，且可能存在较为显著的边界效应等影响，因此需要补充开展另一组侵蚀实验以完善前述试验结果。本研究的目标为量化砂-泥混合物侵蚀行为的新定义定理。实验采用大型环形水槽（annular flume），针对三种人工配制的砂-粉砂-黏土混合物开展。所有混合物均经均匀混合并达到100%饱和后置入水槽。通过反向旋转水槽顶盖与水槽本体生成单向流，通过逐步提升转速来改变床面附近的水流诱导剪切应力。研究中不仅观测了床面的侵蚀行为，还测量了垂向上悬浮细颗粒的浓度。由于次生流的存在，被侵蚀的砂会在水槽弯道外侧堆积，通过收集该部分沉积物可量化被侵蚀砂的总量。床面附近的剪切应力并未直接测量，而是通过大涡模拟（large eddy simulation）模型计算得到。研究结果表明，该混合物的侵蚀行为可分为两个阶段：在床面完全失稳的第二阶段之前，存在仅发生少量物质侵蚀的第一阶段。受床面铺设方式的影响，床面强度在床体的水平与垂向上均存在小幅波动。在第一阶段中，细颗粒发生絮凝侵蚀，砂则以推移质与片流两种形式输运。该阶段内，侵蚀细颗粒的浓度呈现出沉积床侵蚀试验中常见的典型非线性增长特征，本文对这一异常现象的多种可能成因进行了讨论。在第二阶段中，观测到表面侵蚀现象，此时浓度随时间呈线性增长。以床面剪切应力为自变量的侵蚀速率曲线显示，具备砂-粉砂骨架的样品的侵蚀临界剪切应力相对较高，而以黏土-水基质为主的样品的侵蚀临界剪切应力则更低。当针对细颗粒与砂的侵蚀参数进行分析时，可观测到侵蚀行为的转变点，该转变点对应的塑性指数约为7，这与既往经验岩土工程研究中得到的砂主导沉积物向黏土主导沉积物转变的力学行为转变规律吻合良好。