Rheological Properties and Incipient Motion of Cohesive Sediment in the Haihe Estuary of China

The Haihe cohesive sediment, which is typical in China, is studied systematically for its basic physical and incipientmotion properties. Following the requirements of dredging works in the Haihe Estuary, cohesive sediment samples weretaken from three locations. Laboratory experiments were conducted to determine the rheological properties of these sam-ples and to examine the incipient motion of the cohesive sediment. It is found that the cohesive sediment has an obviousyield stress τb, which increases with the mud density in a manner of an exponential function, and so does the viscosityparameter η. The cohesive sediment behaves like a Bingham fluid when its density is below 1.38 ～ 1.40 g/cm3, andwhen denser than these values, it may become a power-law fluid. The incipient motion experiment also revealed that theincipient velocity of the cohesive sediment increases with the density in an exponential manner. Therefore, the incipientmotion is primarily related to the density, which is different from the case for non-cohesive sediment in which the incipi-ent motion is correlated with the diameter of sand particles instead. The incipient motion occurs in two different ways de-pending on the concentration of mud in the bottom. For sufficiently fine particles and a concentration lower than1.20 g/cm3, the cohesive sediment appears as fluidized mud, and the incipient motion is in the form of instability of aninternal wave. For a higher concentration, the cohesive sediment appears as general quasi-solid-mud, and the incipientmotion can be described by a series of extended Shields curves each with a different porosity for newly deposited alluvial mud.     

The role of sediment specific gravity and availability on cluster evolution

The objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics (spacing and size). To isolate the effects of sediment availability and specific gravity on cluster evolution, mono‐sized spheres were used to simulate the cluster evolutionary cycle. Overall, twelve experimental runs were carried out in the laboratory flume. Six of these tests were performed by using glass spheres (specific gravity, SG = 2·58) and the other six by employing an equal combination of glass and Teflon spheres (SG = 2·12) of the same diameter to evaluate the role of specific gravity on cluster evolution. The three sediment availability conditions that were investigated here simulated isolated gravel elements, pool–riffle sequences and densely packed gravel‐bed. An advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area. The results of this study showed that: (1) sediment availability affects the architecture and size of cluster microforms; and (2) clusters consisting of mono‐sized sediments start disintegrating at twice the incipient conditions. By performing complementary tests for the isolated gravel elements case, it was found that the evolutionary cycle of individual clusters could be described as follows, in order of increasing stress: no cluster→two particle cluster→comet→triangle→rhomboid→break up. Copyright © 2003 John Wiley & Sons, Ltd.     

东海舟山群岛海域表层沉积物运移特性

通过潮流和表层沉积物粒度参数等实测资料,对舟山群岛海域表层沉积物的运移趋势、净输运通量以及起动流速进行了初步分析.结果表明,舟山群岛对研究区表层沉积物分布影响明显;舟山群岛海域表层沉积物整体由湾口向外海输运,平均日单宽输沙通量由杭州湾内向外海迅速减小;从杭州湾向外海,泥沙起动越来越难,单纯潮流对表层沉积物再悬浮作用相对较弱.     

驯化水温及温升速率对三门湾三种虾蟹类热耐受性的影响

作者采用动态实验法与静态实验法相结合的方法,研究了三门湾脊尾白虾(Exopalaemon carinicauda)、口虾蛄(Oratosquilla oratoria)和日本(Charybdis japonica)在不同季节的基础水温即驯化水温(8~29℃)和温升速率(0.5~15.0℃/h)下的热耐受能力。结果表明,驯化水温和温升速率对各实验动物的热耐受性均有显著影响。实验动物的热耐受性与驯化水温总体上呈显著正相关,而温升速率对热耐受性的影响具有物种特异性,并受驯化水温制约;在不同驯化水温下,各实验动物的热耐受性随温升速率增大呈不同变化趋势。各实验动物的24 h高起始致死温度受驯化水温的影响显著,随着驯化水温从8℃升高到29℃,脊尾白虾、日本和口虾蛄的24hUILT50分别从24.2、34.6、24.9℃显著增大到35.3、37.4和34.4℃。结合3种实验动物的最大临界温度分析,它们的热耐受能力依次为:日本脊尾白虾口虾蛄。研究结果可为探究三门湾水域潜在的热污染状况及其生态环境效应提供科学依据。     

Numerical study of sand scour with a modified Eulerian model based on incipient motion theory

Application of the standard Eulerian model to simulations of sand scour results in unrealistic phenomena. Therefore, the present work develops a modified Eulerian model based on sand incipient motion theory. The modified model is applied for simulating a two-dimensional single vertical jet and a moving planar jet. The simulation results generally demonstrate fairly good agreement with published results of scour profiles and the velocity contours of the water and sand phases. In addition, equations to describe self-similar scour profiles for the moving planar jet cases are given. The results demonstrate that the modified model efficiently and accurately simulates the two-dimensional sand scour produced by jets, particularly for the moving jet cases.     

水压致裂煤层裂缝发育特点的研究

本文对煤岩基本力学性质、煤层水压致裂缝形成条件、裂缝形态以及裂缝开裂角方位等基本理论进行了研究与探讨，通过大量煤岩力学性质测试，证实了试验区目的层煤岩弹性模量低，泊松比较高，脆性大，易破碎，易压缩。文章还得出了目的层煤岩Ｍｏｈｒ断裂准则二次抛物线型包络线，煤层水压致裂裂缝形式判断，裂缝开裂角方位的计算公式以及有关结论。     

泥沙起动判别标准探讨

分析了现有起动标准的不足之处。在制定起动标准时,不但要考虑水流条件,还必须考虑泥沙颗粒在床面的相对位置。对同一粒径的泥沙颗粒,其起动条件不是一个常量,而是位于一个区间,它随颗粒在床面的相对暴露度而变化。根据力学原理,推导了泥沙颗粒起动时的临界无量纲切应力公式,并对其系数取值进行了理论计算。计算结果表明,对同一粒径、在同一起动状态下,泥沙颗粒的起动临界条件并非一个常量。并从理论上分析得到了临界值的范围,对个别及少量起动,其无量纲临界切应力为0.021～0.042及0.041～0.062。     

Changes in the critical erosion velocity for sediment colonized by biofilm

In flowing water the incipient motion of sediment can be affected by the presence of microbial biofilm growth. This article documents a series of flume experiments using non‐uniform sediments, in which sediment entrainment was investigated for cases where the sediment was immersed in deionized water, so that no biofilm developed, and for cases where a bio‐sediment was cultivated by placing the sediment in a mixture of natural water and nutrient solution. Differences in entrainment and the velocity at incipient motion were measured over an eight week period, as biofilm grew. It was found that the incipient motion phenomena were quite distinct between the two kinds of sediment. Sediment with biofilm was more stable and, over time, incipient velocity increased to a threshold level, before declining. Biofilm development is clearly an important control on the stability of sediments, especially in eutrophic water bodies. Two incipient velocity formulas were derived for sliding and rolling conditions. Film water theory was utilized to describe the cohesive force between sediment particles and the adhesive force generated by biofilm was introduced into the formula derivation; the time variation characteristics of biofilm strength and the features of the substrate were also taken into consideration. Such analyses can help to predict sediment transport changes due to biofilm presence in nutrient‐rich water bodies.     

波浪及波流边界层泥沙起动规律

基于波浪边界层理论及单向流泥沙起动Shields曲线,推证出波浪泥沙起动Shields曲线;基于波流边界层理论,提出表述波流边界层动力特征的波流比因子X及非线性作用因子Y,并建立了Y与X的相关关系;在此基础之上,结合单向流及波浪泥沙起动Shields曲线,推证出波流共同作用下泥沙起动Shields曲线。结果表明:波浪泥沙起动Shields曲线在层流区与单向流光滑紊流区曲线保持一致,粗糙紊流区与单向流粗糙紊流区曲线保持一致,过渡区线型为折线,由层流区及粗糙紊流区曲线延长交汇获得;X及Y能够合理地表征波流边界层动力对比特征及非线性作用特征;波流泥沙起动Shields曲线介于波浪及单向流泥沙起动Shields曲线之间,随着波流比因子X的不同,依据非线性作用因子Y,自动在波浪及单向流泥沙起动Shields曲线之间非线性过渡。建立的波流泥沙起动Shields曲线与试验结果吻合较好,且能够概括单向流、波浪及波流等不同动力及细沙、粗沙等不同粒径的泥沙起动条件。     

Transport of moderately sorted gravel at low bed shear stresses: The role of fine sediment infiltration

A reliable estimation of sediment transport in gravel‐bed streams is important for various practical engineering and biological studies (e.g., channel stability design, bed degradation/aggradation, restoration of spawning habitat). In the present work, we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses. The laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses, with detailed monitoring of the bed topography using a laser scanner. Effects of bed surface arrangements were documented by testing loose and packed bed configurations. Effects of fine sediments were examined by testing beds with sand, artificial fine sand or cohesive silt infiltrated in the gravel matrix. Analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement, the bed material properties (e.g., size and shape, consolidation index, permeability) and the concentration of fine sediments within the surface layer of moving grains. This concentration is directly related to the distribution of fine particles within the gravel matrix (i.e., bottom‐up infiltration or bridging) and their transport mode (i.e., bedload or suspended load). Compared to loose beds, the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments; in both cases, the bed shear stress for gravel entrainment increases by about 12%. On the other hand, the mobility of gravel increases significantly (bed shear stress for particle motion decreasing up to 40%) for beds clogged at the surface by non‐cohesive sand particles. Copyright © 2017 John Wiley & Sons, Ltd.