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为探析三峡水库下游河床冲淤与水沙输移之间的关系以及河床冲淤对洪枯水位的影响,采用三峡水库运用前后长江中下游水文泥沙和河道地形资料,分析归纳了主要水文站水沙变化、沿程河道冲淤分布和洪枯水位变化特征。研究表明:冲刷集中于枯水河槽既与洪水频次减少、中等流量持续时间加长有关,也与中等流量下河道冲刷率大、洪水流量下河道冲刷率小的特点有关;主冲刷带自上而下推移,目前仍位于荆江河段;粗沙(d > 0.125 mm)的年均输移量至监利基本可恢复至蓄水前的多年均值,城陵矶以下长河段冲刷主要是细颗粒冲刷导致;河道冲刷导致中枯水位明显下降,并在城陵矶以上沙质河段呈现加速下降趋势,但当流量大于平滩流量附近的临界值时,高水位变化不大,初步预估这种变化特点将在较长时期内保持。 相似文献
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水沙条件变化对三峡水库泥沙淤积的影响 总被引:2,自引:0,他引:2
三峡水库上游来水来沙变化必然影响水库淤积发展过程,进而影响航运状况。本文运用一维及二维泥沙数学模型计算分析了上游水沙条件变化对三峡水库泥沙淤积及航运的影响,一维计算结果表明,90年代相对于60年代来沙变少变细后,三峡水库的淤积发展过程将明显减慢,淤积部位偏下,并且平衡淤积量也相对于60年代水沙情况下减小;上游建库的综合作用将延缓三峡水库的淤积发展过程,上游建库情况下,变动回水区河段在三峡水库运用40年内不会出现泥沙淤积问题;重庆河段整体呈现冲的趋势,100年内基本不发生淤积。二维计算结果表明,相对于上游无库情况下,上游干流建库后,重庆河段长江干流泥沙淤积量及淤积厚度明显减少,甚至出现冲刷,航运条件明显改善;金沙碛河段受上游干流建库影响较小,仍会出现碍航问题,需要进一步整治。 相似文献
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《水力发电学报》2017,(1)
针对三峡蓄水过程中的荆江典型碍航河段,研究了航道整治工程对蓄水过程的适应性及工程效果。通过实测水沙、地形资料,分析了三峡蓄水后荆江河段河床冲淤规律及航道条件变化,以窑监河段为研究对象,采用二维水沙数学模型模拟了守护型工程和调整型工程对于汛后蓄水过程的适应性。结果表明:在三峡水库蓄水后"清水冲刷"的前提条件下,守护型工程对改善航道效果有着积极作用,但需满足一定条件,主要是浅水道处于较好的演变阶段,并且整治目标与现状条件差距不大;而对三峡水库汛后蓄水过程较为敏感的河段,彻底改善或提高航道条件,需采用一定的调整型整治工程。研究成果可供三峡工程坝下河段航道整治研究等参考。 相似文献
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关于三峡工程对城陵矶附近防洪能力影响有关研究的讨论 总被引:4,自引:2,他引:2
本文根据长江城陵矶上下河道的历史冲淤情况,分析和讨论了有关单位对三峡工程建成后本段冲淤研究成果的合理性。特别讨论了城陵矶至汉口段的冲淤情况和三峡工程对长江中游防洪能力的影响。三峡蓄水拦沙后,荆江将发生较大冲刷,这是否导致城陵矶以下河道淤积是问题的关键。有关研究认为,本河段将会有较大冲刷,从而大量增加洞庭湖的防洪能力。但分析发现这些成果存在很多矛盾。根据其城陵矶以上大量冲刷的结果,城陵矶以下应该淤积。这对长江中游防洪形势将产生不利影响。本文主要提出了问题,望得到更多关注和研究,以利于科学规划长江中游的防洪建设。 相似文献
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三峡水库运行后城汉河段会只淤不冲吗?——对"关于三峡工程对城陵矶防洪能力影响有关研究的讨论"的讨论 总被引:2,自引:1,他引:1
本文对“关于三峡工程对城陵矶防洪能力影响有关研究的讨论”一文进行回应和讨论。文中首先分析了城汉河段在一段时期淤积的机理,同时指出从近50年来该段并不是如文献[1]所说的只淤不冲,而是也发生了冲刷,其中1992年至1998年就冲刷0.868×108m3。冲刷的原因就是因为宜昌水文站年来沙减少了1×108t。更雄辩的事实是,在三峡水库蓄水后该段也发生了冲刷,这些说明文献[1]断言,三峡建成后在100年内城汉河段都处于淤积状态是完全不符合实际的。其次,指出文献[1]采用的分析方法在理论上根据不足,而且无法证明城汉河段长期淤积。本文接着从非均匀沙挟沙能力的理论,通过水量百分数调整、粗细泥沙交换、以及挟沙能力负荷等多方面论证了城汉河段在三峡水库蓄水后即会发生冲刷,而汉口以下在一些年以后也会发生冲刷,从而推翻了文献[1]的结论。最后本文在上述讨论的基础上质疑了文献[1]认为三峡建库对城陵矶防洪不利的观点。 相似文献
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通过溪洛渡水电站与三峡水库之间,400余km河道冲淤变形的多种途径研究,认识到坝下游宽级配卵石河床清水冲刷的冲刷量是有限的。悬移质基本不与床沙交换,上游水库淤积多少悬移质泥沙量,下游水库入库输沙量就几乎减少同样多的量;在上游水库推移质未出库前,下游水库入库推移质输沙量随时间增长呈递减。两库之间的河床变形不明显,河槽冲刷深度、水位下降幅度很有限,对环境影响不大。在不影响溪洛渡水电站的功能和效益的条件下,溪洛渡水库泥沙淤积后,对三峡水库的减沙效益是显著的。与三峡单库运行相比,不但减少了三峡水库淤积量,增加了词节库容,防洪库容,还降低了水库尾部洪水位,减小水库淹没损失,改善了通航条件。随着三峡上游若干大型水库不断修建,上游诸水库与三峡水库进行联合调水词沙,三峡泥沙问题将能延缓到大坝设计基准期以后出现。其前提是主要产沙区河流上拦沙能力很强的水库要尽早修建,其中最重要的就是金沙江上的白鹤滩水电站,必须加快设计,尽早建设。 相似文献
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三峡水库蓄水改变了长江中下游水沙条件,进而影响洲滩演变,两者均会对航道产生影响。基于实测资料分析表明:三峡建库后,水沙过程变化有利于航道条件好转,具体表现为,最小流量增加、最枯水位抬高,浅滩冲刷时间延长,沙量减少加快浅滩冲刷,增加了航槽水深;三峡建库后,崩岸、切滩更为频繁,低滩呈较为明显的萎缩趋势,但并非所有的洲滩变形均会导致水道碍航,只有当归槽河宽超过某一临界河宽时,洲滩变形才有可能导致浅滩碍航,长江中下游临界归槽河宽为900~1900m,随设计水位及航道尺度不同而有所变化。 相似文献
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Martin J McKeown Z Jane Wang Rafeef Abugharbieh Todd C Handy 《IEEE engineering in medicine and biology magazine》2006,25(2):91-101
Independent component analysis (ICA) has proved to be a powerful method for exploratory analysis of functional magnetic resonance imaging (fMRI) data. It has been used to uncover unexpected activations in fMRI data derived from brain activation. ICA has been used to characterize other sources of variability in the fMRI signal besides task-related activity, as well as challenging some of the assumptions inherent in other fMRI analysis methods. As a data-driven fMRI analysis technique, the philosophy of ICA is often in disagreement with hypothesis-driven methods. By exploiting the fact that much of fMRI data has deterministic spatial-temporal structure, a scheme employing ICA denoising and least squares (LS) estimation of the evoked hemodynamic response (HDR) is proposed. Simulations suggest that the method is more robust to different noise models compared to naive application of LS. The result is a considerably increased level of significance of activation for a given voxel but still qualitatively similar spatial distribution of activations over all voxels. This suggests that the proposed method has the potential to substantially reduce total scanning time requirements to achieve the same level of statistically significant activation. 相似文献
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In this study, we made single chamber gas sensor which had two SnO2 electrodes pasted onto either side of zirconia electrolyte. Transition metal (TM) was added in one of two SnO2 electrodes to induce and thus confirm the catalytic effect of transition-metal. Since 1 mol% TM-doped SnO2 showed the negligible change in microstructure, the effect of microstructure change could be ignored. The Co addition increased
the e.m.f. of working electrode (TM-added SnO2) in air, implying the enhanced oxygen adsorption. The addition of TM to SnO2 was also effective in changing the e.m.f. values in H2 balanced by air. Fe and Ni addition exhibited decreased e.m.f. in H2 from that in air. Thus, Fe and Ni addition improved the catalytic activity for H2 oxidation. Cu addition slightly improved the catalytic activity for CO oxidation. The results show that Co is expected to
be a good additive for the cathode of solid-oxide-fuel-cell (SOFC) at low temperature. On the other hand, Fe, Cu, or Ni is
good additives for anode. 相似文献
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Jun Ding Wexler A.S. Binder-Macleod S.A. 《IEEE transactions on neural systems and rehabilitation engineering》2002,10(1):59-67
For pt. I see ibid., vol. 10, no. 1, p. 48-58 (2002). We have recently developed a force- and fatigue-model system that accurately predicted the effect of stimulation frequency on muscle fatigue (see pt. I). The data used to test the model were produced by stimulation trains with resting times of 500 ms. Because the resting times between stimulation trains affect muscle fatigue, this study tested the model's ability to predict the effect of resting times on fatigue. In addition, because this study included different subjects than those used to develop the model, the validity of the model could be tested. Data were collected from human quadriceps femoris muscles using fatigue protocols that included resting times of 500, 750, or 1000 ms. Our results showed that the model predicted fatigue as being a decreasing function of resting time, which was consistent with experimental data. Reliability tests between the experimental data and predictions showed interclass correlation coefficients of 0.97, 0.95, and 0.81 for the initial, final, and percentage decline in peak forces, respectively, suggesting strong agreement between the experimental data and the predictions by the model. The success of our current force- and fatigue-model system helps to validate the model and suggests its potential use in identifying the optimal activation pattern during clinical application of functional electrical stimulation. 相似文献
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A predictive fatigue model. I. Predicting the effect of stimulation frequency and pattern on fatigue
Jun Ding Wexler A.S. Binder-Macleod S.A. 《IEEE transactions on neural systems and rehabilitation engineering》2002,10(1):48-58
Previously we developed a mathematical force- and fatigue-model system that could predict fatigue produced by a wide range of frequencies and pulse patterns. However, the models tended to overestimate the forces produced by higher frequency trains. This paper presents modifications to our previously developed force- and fatigue-model system to improve the accuracy in predicting forces during repetitive activation of human skeletal muscle. By comparing the predictions produced by the modified force and fatigue models to those by our previous models, the modification appears to be successful. The current force- and fatigue-model system accounts for about 93% variance in experimental data produced by fatigue protocols consisting of trains with a wide range of frequencies and pulse patterns. In addition, the present models successfully predict the effect of stimulation frequency and pulse pattern on muscle fatigue. The success of our current force- and fatigue-model system suggests its potential use in helping to identify the optimal activation pattern to use during the clinical application of functional electrical stimulation. 相似文献
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Geoffrey M Graham T Adam Thrasher Milos R Popovic 《IEEE transactions on neural systems and rehabilitation engineering》2006,14(1):38-45
Muscle contractions induced by functional electrical stimulation (FES) tend to result in rapid muscle fatigue, which greatly limits activities such as FES-assisted standing and walking. It was hypothesized that muscle fatigue caused by FES could be reduced by randomly modulating parameters of the electrical stimulus. Seven paraplegic subjects participated in this study. While subjects were seated, FES was applied to quadriceps and tibialis anterior muscles bilaterally using surface electrodes. The isometric force was measured, and the time for the force to drop by 3 dB (fatigue time) and the normalized force-time integral (FTI) were determined. Four different modes of FES were applied in random order: constant stimulation, randomized frequency (mean 40 Hz), randomized current amplitude, and randomized pulsewidth (mean 250 micros). In randomized trials, stimulation parameters were stochastically modulated every 100 ms in a range of +/-15% using a uniform probability distribution. There was no significant difference between the fatigue time measurements for the four modes of stimulation. There was also no significant difference in the FTI measurements. Therefore, our particular method of stochastic modulation of the stimulation parameters, which involved moderate (15%) variations updated every 100 ms and centered around 40 Hz, appeared to have no effect on muscle fatigue. There was a strong correlation between maximum force measurements and stimulation order, which was not apparent in the fatigue time or FTI measurements. It was concluded that a 10-min rest period between stimulation trials was insufficient to allow full recovery of muscle strength. 相似文献
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为节能减排,大唐淮北发电厂对漏风大、换热效率低的5号炉老旧预热器进行了换型改造。通过对改造后风机电耗、锅炉效率和粉烟尘排放等多项运行数据分析,改造后预热器漏风率大幅度降低,风机电耗下降;换热效率增强,热风温度提高,锅炉效率提高,粉尘及其它污染物排放减少,达到了预计的技术改造效果。 相似文献
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Spatial disorientation (SD) is a major cause of aircraft mishaps in military as well as civilian aviation. Despite steady improvements in flight safety, SD-related losses remain unacceptably high and warrant special attention. SD occurring at low altitude is a particular problem, as there is little time for the pilot to recognize the problem and to take corrective action. Inevitably, under such circumstances, mishaps are common and aircrew rarely survive. To illustrate the problem faced by aircrew in overcoming disorienting illusions, this article looks at one possible cause for SD in the low-level environment-the G-excess effect-and describes a recent Royal Air Force mishap in which this illusion may have played a part 相似文献
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《Engineering & Technology》2009,4(7):64-67
Is life on the Internet having a disastrous effect on our children's minds? Or are they developing into the ultimate multi-taskers? Juan Pablo Conti finds out. 相似文献