Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel

Topographic interactions generate multidirectional and unsteady air?ow that limits the application of velocity pro?le approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin‐type surface‐mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp‐crested model dunes. Findings complement existing theories on secondary air?ow effects on stoss transport dynamics and provide new information on the in?uence of lee‐side air?ow patterns on dune morphodynamics. For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a signi?cant drop in time‐averaged measurements of streamwise shear stress at this location. This effect is believed suf?cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and ?ow acceleration cause an increase in ?ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60–70 per cent greater than at ?ow reattachment on the lower stoss of closely spaced dunes. Streamwise ?ow accelerations, rather than turbulence, have greater in?uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed ?ow within the separation cell generates signi?cant surface shear (30–40 per cent of maximum values) for both spacings. This supports ?eld studies that suggest reversed ?ow is competent enough to return sediment to the dune directly or in a de?ected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time‐averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and ?ow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to ?uvial estimates. The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost‐effective design shows promise for validating these ?ndings in natural dune settings. Copyright © 2003 John Wiley & Sons, Ltd.     

Optimal distribution of linear control intensity over the frequency range

In this paper, a control strategy for structural systems is proposed and developed in the frequency domain. The algorithm is substantially based on a linear derivative feedback and a convolution of the control parameter, whose distribution in the frequency field is chosen in such a manner as to comply with the requirements of an ad hoc formulated constrained optimum problem, with the response data monitored until the instant of control action application. Some numerical testing is carried out by referring to given recorded accelerograms, showing a good performance of the proposed approach. Copyright © 2002 John Wiley & Sons, Ltd.     

Sliding fragility of block‐type non‐structural components. Part 1: Unrestrained components

Motivated by the development of performance‐based design guidelines with emphasis on both structural and non‐structural systems, this paper focuses on seismic vulnerability assessment of block‐type unrestrained non‐structural components under sliding response on the basis of seismic inputs specified by current seismic codes. Two sliding‐related failure modes are considered: excessive relative displacement and excessive absolute acceleration. It is shown that an upper bound for the absolute acceleration response can be assessed deterministically, for which a simple yet completely general equation is proposed. In contrast, fragility curves are proposed as an appropriate tool to evaluate the excessive relative displacement failure mode. Sample fragility curves developed through Monte‐Carlo simulations show that fragility estimates obtained without taking into account vertical base accelerations can be significantly unconservative, especially for relatively large values of the coefficient of friction. It is also found that reasonable estimates of relative displacement response at stories other than the ground in multistorey buildings cannot in general be obtained by simply scaling the ground acceleration to the peak acceleration at the corresponding storey. Failure modes considered in this study are found to be essentially independent of each other, a property that greatly simplifies assessment of conditional limit states. Copyright © 2002 John Wiley & Sons, Ltd.     

A procedure for evaluating seismic energy demand of framed structures

Energy serves as an alternative index to response quantities like force or displacement to include the duration‐related seismic damage effect. A procedure to evaluate the absorbed energy in a multistorey frame from energy spectra was developed. For low‐ to medium‐rise frames, it required a static pushover analysis of the structure to determine the modal yield force and ductility factor of an equivalent single‐degree‐of‐freedom system for the first two modes. The energy spectra were then used to determine the energy contribution of each mode. A procedure was also developed to distribute the energy along the frame height based on energy shapes. This study showed that the second‐mode response in some cases needs to be considered to reflect the energy (or damage) concentration in the upper floors. Copyright © 2002 John Wiley & Sons, Ltd.     

Observing artificially induced strain: implications for subglacial deformation

By analysing a series of four successive thin‐sections from a ceramic clay that was subjected to uniaxial compression, we were able to monitor the development of microstructures in a fine‐grained sediment. The artificially induced microstructures, such as unidirectional clay reorientations and linear and circular grain arrangements, are identical to features that have been observed in thin‐sections of subglacially deformed tills, and therefore may be used as representative analogues. We argue that the structures, reflecting slip, planar shear displacements as well as rotational movements, can be explained by assuming a Coulomb‐plastic response to imposed shear. We conclude that sediments subjected to subglacial deformation behave as Coulomb materials, at least during the final stages of the deformation. The present study bridges the gap between field studies, experimental studies and theoretical modelling. The microscopic observations assist in visualising inferred subglacial processes and facilitate up‐ and downscaling between diverse methodological approaches. Copyright © 2003 John Wiley & Sons, Ltd.     

黄骅坳陷横向变换带的构造特征及成因

讨论了黄骅坳陷横向变换带的构造特征和成因。根据利用地震反射剖面对盆地构造几何学的研究，圈定了不同尺度的横向变换带，确定了伸展构造背景下的横向变换带是为了保持区域伸展应变调节构造变形的一种调节体系。这种伸展应变守恒是靠三维空间上断层位移沿走向的变化实现的。横向变换带的构造样式为横向地垒状凸起和鼻状凸起或鼻状背斜，并具伸展性质的正断层。横向凸起与非叠复的和叠复的分段断层系伴生及发育在断层位移最小处等事实表明，横向凸起或横向鼻状背斜是由于分段断层系位移沿走向变化形成的。     

Main models and distribution patterns of gas reservoirs in large-medium gas fields of China

Based on characteristics and trap types of gas reservoirs in large and medium gas fields in China, 4 gas reservoir models have been established:(i) structural trap gas reservoir model I, formed earlier than or simutaneously with generating of gases; (ii) structural trap gas reservoir model II, formed later than generating of gases; (iii) fossil weathered residuum gas reservoir model; and (iV) mud diapir abnormal temperature and pressure gas reservoir model. Distribution patterns of large-medium gas fields are described with the concept of “sealed compartment”. It is concluded that the inner-compartment area, marginal area of the compartment and the areas between two overlapped sealed compartments are the most favourable areas for discovering large-medium gas fields. Project supported by the “85–102” Chinese National Key Science and Technology Project.     

Toxic Cyanobacteria in Greek Freshwaters, 1987—2000: Occurrence,Toxicity, and Impacts in the Mediterranean Region

In a survey in Greece from 1987 to 2000 hepatotoxic cyanobacterial blooms were observed in 9 out of 33 freshwaters. Microcystins (MCYSTs) were detected by HPLC in 7 of these lakes, and the total MCYST concentration per scum dry weight ranged from 50.3 to 1638 ± 464 μg g^—1. Cyanobacterial genera (Microcystis, Anabaena, Anabaenopsis, Aphanizomenon, Cylindrospermopsis) with known toxin producing taxa were present in 31 freshwaters. From our data and a review of the literature, it would appear that Mediterranean countries are more likely 1) to have toxic cyanobacterial blooms consisting of Microcystis spp. and 2) to have higher intracellular MCYST concentrations. A case study in Lake Kastoria is used to highlight seasonal patterns of cyanobacterial and MCYST‐LR occurrence and to assess cyanotoxin risk. Cyanobacterial biovolume was high (> 11 μL L^—1) throughout the year and was in excess of Guidance Level 2 (10 μL L^—1) proposed by WHO for recreational waters and Alert Level 2 for drinking water. Further, surface water samples from April to November exceeded Guidance Level 3, with the potential for acute cyanobacterial poisoning. Intracellular MCYST‐LR concentrations (max 3186 μg L^—1) exceeded the WHO guideline for drinking water (1 μg L^—1) from September to November with a high risk of adverse health effects. Preliminary evidence indicates that in 3 lakes microcystins are accumulated in some aquatic organisms. Generally, a high risk level can be deduced from the data for the Mediterranean region.     

Bedload transport resistance in rough open‐channel flows

During bedload movement by saltation, streamwise momentum is transferred from the ?ow to the saltating grains. When the grains collide with other grains on the bed or in the ?ow, streamwise momentum is reduced, and there is a decrease in streamwise ?ow velocity and an increase in ?ow resistance, herein termed bedload transport resistance f_bt. Based on experiments in two ?umes with ?xed and mobile plane beds and previously published data, an equation is developed that may be used to predict f_bt for both capacity and non‐capacity ?ows. The variables in this equation are identi?ed by dimensional analysis and the coef?cients are determined by non‐linear regression. This equation applies to rough turbulent open‐channel ?ows, where the relative submergence is between 1 and 20 and the entire sediment load moves by saltation. An investigation of the relative magnitudes of f_bt and grain resistance f_c suggests that where dimensionless shear stress θ is less than 1 and saltation is the dominant mode of bedload transport, f_bt/f_c increases with θ but never exceeds 1. Copyright © 2004 John Wiley & Sons, Ltd.     

Hydrological connectivity of soil pipes determined by ground‐penetrating radar tracer detection

Soil pipes are common and important features of many catchments, particularly in semi‐arid and humid areas, and can contribute a large proportion of runoff to river systems. They may also signi?cantly in?uence catchment sediment and solute yield. However, there are often problems in ?nding and de?ning soil pipe networks which are located deep below the surface. Ground‐penetrating radar (GPR) has been used for non‐destructive identi?cation and mapping of soil pipes in blanket peat catchments. While GPR can identify subsurface cavities, it cannot alone determine hydrological connectivity between one cavity and another. This paper presents results from an experiment to test the ability of GPR to establish hydrological connectivity between pipes through use of a tracer solution. Sodium chloride was injected into pipe cavities previously detected by the radar. The GPR was placed downslope of the injection points and positioned on the ground directly above detected soil pipes. The resultant radargrams showed signi?cant changes in re?ectance from some cavities and no change from others. Pipe waters were sampled in order to check the radar results. Changes in electrical conductivity of the pipe water could be detected by the GPR, without data post‐processing, when background levels were increased by more than approximately twofold. It was thus possible to rapidly determine hydrological connectivity of soil pipes within dense pipe networks across hillslopes without ground disturbance. It was also possible to remotely measure travel times through pipe systems; the passing of the salt wave below the GPR produced an easily detectable signal on the radargram which required no post‐processing. The technique should allow remote sensing of water sources and sinks for soil pipes below the surface. The improved understanding of ?owpath connectivity will be important for understanding water delivery, solutional and particulate denudation, and hydrological and geomorphological model development. Copyright © 2004 John Wiley & Sons, Ltd.