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Sean J. BENNETT Carlos V. ALONSO 《国际泥沙研究》2005,20(4):281-294
On hillslopes and agricultural fields, discrete areas of intense, localized soil erosion commonly take place in the form of migrating headcuts. These erosional features significantly increase soil loss and landscape degradation, yet the unsteady, transient, and migratory habits of headcuts complicate their phenomenological and erosional characterization. Here a unique experimental facility was constructed to examine actively migrating headcuts typical of upland concentrated flows. Essential components of the facility include a deep soil cavity with external drainage, rainfall simulator, capacity for overland flow, and a video recording technique for data collection. Results from these experiments show that： （1） after a short period of adjustment, headcut migration attained a steady-state condition, where the rate of migration, scour hole geometry, and sediment discharge remain constant with time; （2） boundary conditions of higher rates of overland flow, steeper bed slopes, and larger initial headcut heights produced systematically larger scour holes with higher rates of soil erosion; and （3） during migration, the turbulent flow structure within the scour hole remained unchanged, consisting of an overfall nappe at the brink transitioning into a reattached wall jet with two recirculation eddies within the plunge pool. The systematic behavior of headcut development and migration enabled the application of modified jet impingement theory to predict with good success the characteristics of the impinging jet, the depth of maximum scour, the rate of headcut migration, and the rate of sediment erosion. These laboratory data and the analytical formulation can be used in conjunction with soil erosion prediction technology to improve the management of agricultural areas impacted by headcut development and ephemeral gully erosion. 相似文献
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根据我国滨海电厂发电现状与发展趋势，本研究评述了我国火电和核电厂直流冷却需水量以及电厂规模化直流冷却用水对海洋生态系统的影响,包括取水卷塞效应导致生物伤害及其机理，卷载和温排水对海洋生物和海洋生态系统的负面影响。本研究估算了取排水导致的卷塞和卷载损失量。同时，以大亚湾生态系统变化为例，探讨了取排水的可能影响，提出了底栖贝类食物链形成的假设。由于电厂取排水对海洋生态系统构成实际和潜在危害，因此，有必要采取措施缓解电厂取水造成的环境影响。在对电厂的取排水海洋环境评估的基础上，电厂可以利用最佳实践技术（Best Technique Available, BTA），避免或减轻取排水对海洋生态系统的影响，维护冷却海水资源开发的可持续性。减损措施不仅有利于海洋生态保护，也有利于电厂的冷源安全。 相似文献
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Frankie Thielen Uwe Weibel Jürgen Hirt Marcel Münderle Michael Marten Horst Taraschewski Bernd Sures 《Limnologica》2008,38(1):76-85
The fish fauna in the northern part of the upper river Rhine close to the city of Karlsruhe (Germany) was analyzed by collecting fish from the cooling-water intake of a power plant (impingement method) between 1989 and 2001. With this method a total of 36 fish species were recorded. The results were compared between the years as well as with other studies on the fish fauna in this area. Particularly with regard to the physical structure of the river bed and banks, the pre-technical situation of the river Rhine has not been restored. However, the number of fish species has been augmented since the 1970s mainly due to improvements in the water quality and almost all autochthonous fish species are present again. Altogether only three introduced fish species (Sander lucioperca L., Lepomis gibbosus L., and Carassius auratus gibelio were found in this study. Among the long-distance migrating species three freshwater spawners (Lampetra fluviatilis, Petromyzon marinus and Salmo trutta trutta) were detected in addition to the catadromous eel. The eudominant fish species varied between years, i.e. the European eel, Anguilla anguilla, was the eudominant species 1989, whereas this species occurred in much lower numbers during the subsequent sampling campaigns. This clearly indicates that the high dominance of eels is mainly caused by stocking activities in the upper Rhine. In the following years roach, Rutilus rutilus was the eudominant fish species. Overall a positive development of the fish fauna in the upper Rhine over the last 15 years could be observed in terms of fish populations as well as in terms of species richness. 相似文献
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Paul L. Broughton 《Sedimentology》2015,62(3):845-866
Small vertically oriented traction carpets are reported from the collapsed sandy fills of 100 m deep Devonian limestone sinkholes underlying the Lower Cretaceous Athabasca oil sands deposit in north‐eastern Alberta, Western Canada. Dissolution of 100 m of underlying halite salt beds caused cataclysmic collapse of the sinkhole floors and water saturated sinkhole sand fills to descend very rapidly. Turbulent currents flushed upper sinkhole fills of friable sandstone blocks and disaggregated sand and quartz pebble for tens of metres. Laminar deposits with inverse grading accumulated as many as six to eight curvilinear entrained pebble streaks, 10 to 30 cm long, vertically impinged against the sides of descending collapse blocks. These deposits were initiated as vertically oriented early stage traction carpets that interlocked fine sand grains and inversely graded overlying pebbles entrained below the dilute overlying turbulent flows. Vortexes that flushed these sinkhole fills and induced these depositional processes may have lasted only seconds before the very rapid descents abruptly halted. Some of the fabrics were suspended vertically in‐place and preserved from unlocking and obliteration. These small fabrics provide insight into the instability and ephemeral character of the transition from strong gravity‐driven grain falls to very early stages of traction carpet formation. These short‐lived deposits of very thin sand layers resulted from sufficient incipient frictional freezing that grain interlocking overcame, however briefly, the strong gravity drives of the vertical falls that would have otherwise dispersed grains and obliterated any organized fabric patterns. Tenuous frictionally locked grains were also suspended at the centres of hyperbolic grain fall flows that briefly developed between turbulent flow eddies, some of which were fortuitously preserved. Some of these suspended grain locking zones passed downward onto the relatively more stable surfaces of the rapidly descending block surfaces. The morphogenesis of these early stage traction carpets differ from more fully developed deposits elsewhere because of their short‐lived transport, dynamic instability and vertical orientation. 相似文献
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Shenn-Yu Chao Ping-Tung Shaw Ming-Kuang Hsu Ying-Jang Yang 《Journal of Oceanography》2006,62(6):811-823
We have investigated the reflection and diffraction of first-mode and second-mode solitary waves by an island, using a three-dimensional
nonhydrostatic numerical model. The model domain consists of a circular island 15 km in diameter in an ocean 300 m deep. We
use prescribed density anomalies in an initially motionless ocean to produce highly energetic internal solitary waves; their
subsequent propagation is subject to island perturbations with and without the effect of earth’s rotation. In addition to
reflected waves, two wave branches pass around the island and reconnect behind it. Island perturbations to the first-mode
and second-mode waves are qualitatively similar, but the latter is more profound because of the longer contact time and, in
the presence of earth’s rotation, the scale compatibility between Rossby radius of the second baroclinic mode and the island
diameter. Without earth’s rotation, reflected and diffracted waves are symmetrical relative to the longitudinal axis passing
through the island center. With earth’s rotation, the current following the wave front veers to the right due to Coriolis
deflection. For a westward propagating incoming wave, the deflection favors northward wave propagation in the region between
the crossover point and the island, shifting the wave reconnection point behind the island northward. It also displaces the
most visible part of the reflected waves to the southeast. In the presence of earth’s rotation, a second-mode incoming wave
produces island-trapped internal Kelvin waves, which are visible after the passage of the wave front. 相似文献
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Moqin He Brain Veitch Neil Bose Bruce Colbourne Pengfei Liu 《Ocean Engineering》2007,34(8-9):1197-1210
Potential flow based vortex numerical methods have been widely used in aerodynamics and hydrodynamics. In these methods, vortices shed from lifting bodies are traced by using vortex filaments or dipole panels. When the wake elements encounter a downstream body, such as a rudder behind a propeller or a stator behind a rotor, a treatment is necessary to divert the wake elements to pass by the body. This treatment is vital to make wake simulations realistic and to satisfy the non-penetration condition during wake body interaction. It also helps to avoid pure numerical disturbances such as when a vortex filament or an edge of a dipole panel passes through the collection point of a body element; this is a singularity for induced velocity and it will introduce a large numerical disturbance. This necessary treatment for three-dimensional problems with geometrical complexity has not been found to date. In this study, a wake impingement model was developed to divert wake elements to slip over the body surface, model the vortex/body interaction, and predict forces on fluctuating components. The model was also tested on configurations of oscillating foils in tandem with an existing panel method code. Simulation results with the wake impingement model are shown to be in closer agreement with limited published experimental data than those without the model. With the established wake impingement model, force fluctuations on the after body due to the wake vortex impingement were investigated based on a series of simulations. The series varied several parameters including distance between two foils, oscillating frequency, span, rear foil pitch angle, swap angle and vertical position. 相似文献
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目的:研究肩峰下撞击综合征与MSCT下肩峰分型的关系。方法:对2013年1月至2015年1月在我院确诊为肩峰下撞击综合征的110例患者行MSCT扫描,将所有患者按照不同年龄阶段、性别、肩峰的不同位置进行分组,比较不同肩峰形态在不同年龄组、不同性别组以及不同位置组的分布情况。结果:Ⅰ型、Ⅱ型肩峰随年龄增长,比例逐渐降低,Ⅲ型肩峰随年龄增长,比例逐渐增大。Ⅱ型肩峰A组患者比例为51.6%,较B组和C组明显高,Ⅲ型肩峰A组患者比例为22.6%,较B组和C组明显低(P<0.05)。男性组与女性组Ⅰ型、Ⅱ、Ⅲ型肩峰形态所占比例比较,差异均不具有统计学意义(P>0.05)。Ⅲ型肩峰形态右肩较多,所占比例为52.5%,明显高于左肩组(P<0.05)。结论:MSCT可以清楚显示肩峰的形态与结构,肩峰下撞击综合征患者弯曲型肩峰随年龄增大比例逐渐减少,钩型肩峰多在右肩,且随年龄增大比例逐渐增大。 相似文献
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介绍美国滨海18个核电厂址(共31台核电机组)取水设施特点及其生态影响。可以看出,在取水设施设计上,美国大部分核电厂采取了减小取水设施对水生生物的撞击和夹带的影响措施,包括采用闭式循环冷却系统;设置改进型滤网及滤网清洗和鱼回流系统;降低穿过滤网流速;设置离岸蘑菇头式取水口;减小取水流量;在取水设施外围设置屏障网等。虽然采取了一些控制措施,由历年监测数据可以看出,美国有些厂址的冷却水取水设施对鱼类、贝类等水生生物的夹带、撞击影响可能为中等水平,甚至是大的。与我国滨海厂址取水设施比较可以看出,我国大部分核电厂未采取减小取水影响的措施。并且,我国核电厂址机组数量较多,冷却水取水量较大,取水可能带来的生态影响不容忽视。因此,应结合厂址特性优化冷却水取水设施位置、设计、建造和能力,以反映最佳适用技术(BTA),最小化环境的负面影响。 相似文献
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