Effect of Channel Restoration on Flood Wave Attenuation

Stream channel restoration can increase flow storage and energy dissipation of passing flood waves. Elements of restoration design that can enhance attenuation include remeandering, which reduces channel slope and increases channel length relative to the floodplain; restoring channel-floodplain connectivity; and revegetating banks and the floodplain. Reestablishment of floodplain hydraulic function is increasingly a goal of restoration programs, yet the approximate magnitude of possible change to attenuation due to reach-scale restoration remains poorly quantified. We examined the efficacy of channel restoration on flood attenuation using restored reaches and synthetic reaches representing median dimensions of channel restoration projects in North Carolina (e.g., ～ 1?km in length). We applied an industry standard dynamic flood routing model (UNET in HEC-RAS) to route floods in impaired and restored reach models. Floods routed through field-based reach models either exhibited very small increases in attenuation, largely due to assumed increases in floodplain roughness, or a decrease in attenuation. Analysis demonstrated that attenuation of peak discharge is overall most sensitive to channel and valley slope, channel and floodplain roughness, and channel and valley length in decreasing order, but is dependent on flood magnitude. Restoration most impacted floods of intermediate magnitude (between 2- and 50-year return interval), particularly those confined to the channel under the impaired morphology but able to access the floodplain under the restored morphology. Restoration may rehabilitate a channel’s ability to attenuate small to intermediate floods by augmenting flood access to the floodplain, changing channel geometry, and enhancing channel and floodplain roughness over time. However, our study shows that the predominantly small scale of current channel restoration will provide minimally quantifiable enhancement to flood attenuation.     

Mass Angle of Repose of Open-Graded Rock Riprap

Mass angle of repose (that is, the angle at which a mass of sliding particles will come to rest) is a fundamental material property that is needed to design drainage channel linings composed of open-graded riprap stone, also called dumped rock riprap, which protect earthen slopes from erosion by flowing water. Multiple regression analysis of measurements at 74 stockpiles of dumped natural and crushed rock shows the angle to depend primarily on rock particle angularity and, to lesser extents, on the gradation and the median particle diameter of the stone mixture, the angle increasing with angularity, mixture nonuniformity, and particle size. Uncomplicated expressions are developed from the data to calculate the expected mass angle of repose of open-graded rock riprap along with prediction intervals.     

Effect of Flow Pulses on Degradation Downstream of Hapcheon Dam, South Korea

The changes in channel geometry downstream of Hapcheon Dam, South Korea, are closely examined. Daily pulses of water from peak hydropower generation and from sudden sluice gate operations affect the 45-km reach of the Hwang River between the Hapcheon Reregulation Dam and the Nakdong River. From 1983 to 2003, the median bed-material size, d50, increased from 1.0 to 5.7 mm, and the bed slope of the reach decreased from 94 to 85 cm/km. The vertical riverbed degradation averaged 2.6 m for a distance of 20 km below the reregulation dam. A simple analytical model is developed to predict the increase in sediment transport and the river bed adjustments from flow pulses in comparison with steady flow discharges. Numerical model simulations confirm the theoretical prediction that sediment transport rates from daily pulses are 21% higher than for steady flow discharges. Unsteady sediment transport simulations indicate that the channel bed degradation should extend mostly 20–25 km below the reregulation dam and should not change much after 2013.     

通道轮式换热器结霜工况下的换热研究

通道轮式新风换气机作为一种新型的通风换气设备近年来得到了广泛的应用。然而,其室外排风侧结霜是影响其在严寒地区冬季运行的主要问题,结霜严重时会影响该设备的运行性能。因此,有必要对通道轮式新风换气机在结霜工况下的运行规律进行深入的研究以指导其在严寒地区冬季的应用。文章在能量守恒、含湿量守恒的基础上建立了通道轮式换热器排风侧结霜工况下的数学模型,该模型耦合了结霜子模型和热交换子模型。利用该模型分析了不同工况下结霜量对该换热器换热的影响。计算出了不同工况下融霜的时间间隔,为采取有效的除霜控制方法提供了依据。将模拟结果与实验数据进行了比较,两者吻合很好,进一步验证了所建模型的可靠性。     

Behavior of channel shear connectors, Part I: Experimental study

In composite beams, shear connectors are commonly used to transfer longitudinal shear forces across the steel-concrete interface. This paper presents an experimental study on the behavior of channel shear connectors embedded in a solid concrete material slab under monotonic and low-cycle fatigue loading. The latter would be applicable to composite structures subjected to seismic events. Of specific interest are the behavior and effects of different concrete materials. A series of push-out specimens made of plain concrete, reinforced concrete (RC), fiber reinforced concrete (FRC) and engineered cementitious composite (ECC) were tested for this evaluation. The results show that the reversed cyclic shear strength of most specimens is 10%-23% lower than their monotonic strength. Also, using the polypropylene fibers (FRC specimens) has a slight effect on the shear strength and load-displacement behavior of the specimens; however, using the polyvinyl alcohol fibers (ECC specimens) causes considerable increase in ultimate strength and ductility of channel shear connectors. Finally, the experimental load capacities are compared with that suggested by North American design codes.     

Numerical investigation of channel columns with complex stiffeners—part II: parametric study and design

A parametric study of cold-formed steel channel columns with complex stiffeners is performed using finite element analysis. An accurate and reliable finite element model is used for the parametric study in which different sizes of complex stiffeners are investigated. Column strengths predicted by the finite element analysis are compared with the unfactored design column strengths calculated using the American Specification and the Australian/New Zealand Standard for cold-formed steel structures. It is shown that the design strengths obtained from the specification and standard are generally conservative for fixed-ended cold-formed steel channel columns with complex stiffeners for the more slender sections having a plate thickness of 1 mm with the flat flange width to thickness ratio of 57, but unconservative for sections having a plate thickness of 2 mm with the flat flange width to thickness ratio of 27.     

Hydraulically Efficient Power-Law Channels

A power-law channel is a generalized form of a channel and includes parabolic and triangular cross sections. For an exponent m<0.5 in the power law, the relative wetted perimeter has been estimated from a series expansion truncated to four terms. For values of the exponent m ≥ 0.5 the relative wetted perimeter has been estimated using an appropriate non-linear interpolation expression. A table to estimate relative wetted perimeter based on these expressions is presented for design purposes. With these expressions for relative wetted perimeter, and using the Lagrange method of undetermined multipliers, for any given maximum side slope, the area and/or wetted perimeter is minimized subject to the equality constraint of a uniform flow (Mannings) equation. Using this technique, for any given side slope, the exponent of the power-law channel can be determined and hydraulically efficient power-law channels can be designed. Optimized power-law channels are compared with trapezoidal and parabolic channels. The existing parabolic design of the Pehur High Level Canal, Pakistan is compared with an optimum power-law channel.     

Novel Metal Substrates for High Power Metal‐supported Solid Oxide Fuel Cells

Novel high permeable porous Ni‐Mo substrates with different area densities of straight gas flow channels are successfully developed to improve the hydrogen fuel gas and the water byproduct diffusion in the anode and supporting substrate. Metal‐supported cell A, cell B and cell C with 5 × 5 cm² supporting substrates are fabricated by atmospheric plasma spraying processes, these cells have the material structure of Ni‐Mo/LSCM (La_0.75Sr_0.25Cr_0.5‐Mn_0.5O_3–δ)/NiO‐LDC(Ce_0.55La_0.45O_2–δ)/SDC(Sm_0.15Ce_0.85O_3–δ)/LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ)/SSC(Sm_0.5Sr_0.5CoO_3–δ). Cell A is supported by a conventional porous Ni‐Mo substrate without straight gas flow channels, cell B and cell C are supported respectively by the novel high permeable porous Ni‐Mo substrates with 1.5 and 2.73 channels per square centimeter. The power densities at 0.8 V and 750 °C are 550, 998 and 1,161 mW cm⁻² for cell A, cell B and cell C respectively. The 100 h durability test at the constant current density of 400 mA cm⁻² and 650 °C shows cell B and cell C have smaller degradation rates than cell A. The results obtained from AC impedance and circuit model analyses indicate that the electrolyte ohm and the cathode polarization resistances are significantly reduced by introducing straight gas flow channels into the supporting substrate.     

工程计价模式与建筑技术发展关系探讨

 本文针对我国建筑技术发展缓慢的现状,对目前新技术、新工艺、新材料、新设备的推广方式、推广通道以及推广应用过程中的问题进行了分析。文章认为,现阶段计价模式存在着一定的缺陷,没有反映市场经济条件下“谁受益、谁付出”的费用承担原则,影响了施工单位对“四新”的推广应用的积极性。文章建议在计价模式中增设“技术应用支持费”计费口,使工程造价的高低与“四新”应用相联系,发挥施工单位在“四新”应用方面的主动性,加快我国“四新”研发、推广、应用的步伐。     

Water Evolution in Direct Methanol Fuel Cell Cathodes Studied by Synchrotron X‐Ray Radiography

Water evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X‐ray radiography. Radiographs with a spatial resolution of around 5 μm were taken every 5 s. Special cell designs allowing for through‐plane and in‐plane viewing were developed, featuring two mirror‐symmetrical flow field structures consisting of one channel with the through‐plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In‐plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross‐section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar.