The effect of expansion ratio on the critical Reynolds number in single fracture flow with sudden expansion

Flow in a single fracture (SF) is an important research subject in groundwater hydrology, hydraulic engineering, radioactive nuclear waste repository and geotechnical engineering. An abruptly changing aperture is a unique type of SF. This study discusses the relation between the values of the critical Reynolds number (Re_c) for the onset of symmetry breaking of flow and the expansion ratio (E) of SF, which is defined as the ratio between the outlet (D) and inlet (d) apertures. This study also investigates the effect of inlet aperture d on Re_c for flow in an SF with abruptly changing apertures (SF‐ACA) using the finite volume method. Earlier numerical and experimental results showed that flow is symmetric in respect to the central plane of the SF‐ACA at small Reynolds number (Re) but becomes asymmetric when Re is sufficiently large. Our simulations show that the value of Re_c decreases with the increasing E, and the relationship between the logarithm of Re_c and E can be described accurately using either a quadratic polynomial function or a logarithmic function. However, the relationship of Re_c and d for a given E value is vague, and Re_c becomes even less sensitive to d when E increases. This study also reveals that the hydraulic gradient (J) and flow velocity (v) follow a super‐linear relationship that can be fitted almost perfectly by the Forchheimer equation. The inertial component (J_i) of J increases monotonically with Re, whereas the viscous component (J_v) of J decreases monotonically with Re. The Re value corresponding to equal inertial and viscous components of J (named as the transitional point Re) decreases when E increases, and such a transitional point Re should be closely related to the critical Reynolds number Re_c, although a rigorous theoretical proof is not yet available. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessing the effectiveness of imperviousness on stormwater runoff in micro urban catchments by model simulation

Imperviousness, considered as a critical indicator of the hydrologic impacts of urbanization, has gained increasing attention both in the research field and in practice. However, the effectiveness of imperviousness on rainfall–runoff dynamics has not been fully determined in a fine spatiotemporal scale. In this study, 69 drainage subareas <1 ha of a typical residential catchment in Beijing were selected to evaluate the hydrologic impacts of imperviousness, under a typical storm event with a 3‐year return period. Two metrics, total impervious area (TIA) and effective impervious area (EIA), were identified to represent the impervious characteristics of the selected subareas. Three runoff variables, total runoff depth (TR), peak runoff depth (PR), and lag time (LT), were simulated by using a validated hydrologic model. Regression analyses were developed to explore the quantitative associations between imperviousness and runoff variables. Then, three scenarios were established to test the applicability of the results in considering the different infiltration conditions. Our results showed that runoff variables are significantly related to imperviousness. However, the hydrologic performances of TIA and EIA were scale dependent. Specifically, with finer spatial scale and the condition heavy rainfall, TIA rather than EIA was found to contribute more to TR and PR. EIA tended to have a greater impact on LT and showed a negative relationship. Moreover, the relative significance of TIA and EIA was maintained under the different infiltration conditions. These findings may provide potential implications for landscape and drainage design in urban areas, which help to mitigate the runoff risk. Copyright © 2015 John Wiley & Sons, Ltd.     

Frequency analysis of nonstationary annual maximum flood series using the time‐varying two‐component mixture distributions

The most popular practice for analysing nonstationarity of flood series is to use a fixed single‐type probability distribution incorporated with the time‐varying moments. However, the type of probability distribution could be both complex because of distinct flood populations and time‐varying under changing environments. To allow the investigation of this complex nature, the time‐varying two‐component mixture distributions (TTMD) method is proposed in this study by considering the time variations of not only the moments of its component distributions but also the weighting coefficients. Having identified the existence of mixed flood populations based on circular statistics, the proposed TTMD was applied to model the annual maximum flood series of two stations in the Weihe River basin, with the model parameters calibrated by the meta‐heuristic maximum likelihood method. The performance of TTMD was evaluated by different diagnostic plots and indexes and compared with stationary single‐type distributions, stationary mixture distributions and time‐varying single‐type distributions. The results highlighted the advantages of TTMD with physically‐based covariates for both stations. Besides, the optimal TTMD models were considered to be capable of settling the issue of nonstationarity and capturing the mixed flood populations satisfactorily. Copyright © 2016 John Wiley & Sons, Ltd.     

Synchronous pattern of moisture availability on the southern Tibetan Plateau since 17.5 cal. ka BP – the Tangra Yumco lake sediment record

A possible asynchronicity of the spatial and temporal moisture availability on the Tibetan Plateau has been a controversial subject of discussion in recent years. Here we present the first attempt to systematically investigate possible spatial and temporal variations in moisture availability by examining two lakes, Tangra Yumco and Nam Co, on an east–west transect on the southern Tibetan Plateau using identical proxies for palaeoenvironmental reconstruction. In this study, an independent record from Tangra Yumco was analysed applying a multi‐proxy approach to reconstruct variations in moisture availability since the Lateglacial. Results were subsequently compared with previously published records from Nam Co and additional records from Tso Moriri (northwestern Himalaya) and Naleng Co (southeastern Tibetan Plateau). Our results show that Tangra Yumco was at least partially ice covered prior to 17.1 cal. ka BP. A temperature rise after 17.1 cal. ka BP probably resulted in thawing of the permafrost. At 16.0 cal. ka BP moisture availability increased, representing an initial monsoonal intensification. Warmer conditions between 13.0 and 12.4 cal. ka BP and cooler conditions between 12.4 cal. ka BP and the onset of the Holocene reflect the Bølling‐Allerød and Younger Dryas. At the onset of the Holocene moisture availability rapidly increased, with moisture highest prior to 8.5 cal. ka BP when temperatures were also highest. After 8.5 cal. ka BP the moisture availability gradually decreased and showed only minor amplitude variations. These findings are consistent with the records from large lakes like Nam Co, Tso Moriri, and Naleng Co, revealing a synchronous pattern of moisture availability on the southern Tibetan Plateau.     

利用双线偏振雷达分析人工防雹作业效果

以北京市平谷区2011年6月11日防雹作业为例,利用双线偏振雷达资料,选取与作业区条件十分接近的云块为对比区,根据回波移动方向和速度,跟踪分析在不同高度上防雹作业前后云体宏观结构特征和粒子相态等微物理变化过程。作业后云体所呈现特征为:1作业区的云顶高度、强回波中心高度迅速下降,对比区变化不明显;2作业区水平反射率Zh减小,差分反射率Zdr、零相关系数ρhv增大,单位差分传播相移Kdp小范围内波动,对比区Zh、Zdr、Kdp变化不明显;ρhv增大;3作业区对流减弱,高层较大冰雹粒子、大雨滴下沉明显,最终以霰粒子为主;而对比区域则对流仍然旺盛,冰雹粒子有增多趋势。以上特征表明防雹作业可有效抑制冰雹胚胎成长为冰雹的过程,通过偏振雷达观测参量可对防雹作业效果进行较好的验证。     

线性相关“可传递性”的几何意义

从几何角度对随机变量显著相关可传递性质进行了分析,得出了样本容量为n(n≥3)随机变量显著相关可传递性质的概率的几何表达式。用Monte-Carlo方法验证了几何分析的合理性,并对此问题展开了一定的讨论。     

华北陆块晋冀Al- Fe- Au- Pb- Zn- Ag- Cu- 煤成矿带主要地质成矿特征及潜力分析

华北陆块晋冀成矿带是我国重要的金属和能源矿产资源基地,近年来新的找矿进展和预测成果不断涌现,使之成为我国重点找矿地区之一,需要对其开展系统的成矿区划和部署研究工作。本文以新的研究成果和找矿突破为基础,通过综合分析,在该地区划分新的重要成矿部署区带,厘定边界并将其命名为"华北陆块晋冀Al-Fe-Au-Pb-Zn-Ag-Cu-煤成矿带"。结合区域成矿地质背景的综合研究,在区内划分了10个成矿亚带,初步建立了Al-Fe-Au-Pb-Zn-Ag-Cu-煤成矿带成矿谱系,其成矿作用具有多期次发育和多系统控制的特点,在时间演化上表现为多旋回性、继承性、新生性和叠加性特点,主要成矿期有前寒武纪,寒武纪-晚二叠世,晚三叠世-晚白垩世和新生代,其中晚侏罗-早白垩世是内生金属矿床成矿大爆发期。该带产出的矿床类型复杂多样,与岩浆有关成矿作用有岩浆岩型、斑岩型、矽卡岩型、海(陆)相火山岩型、岩浆热液型和石英脉型,变质成矿作用有沉积变质型、绿岩建造型,层控成因矿床有碳酸盐岩中热液型,沉积成矿作用有海(陆)相沉积型、砂矿型、风化壳型矿床等。结合区域找矿进展和潜力评价最新成果,认为本区铝土矿、铁、金、锰、钼、铜、铅锌、煤矿潜力巨大,为下一步勘查部署的主攻矿种,同时在本区划分了26个重点远景区,对研究区下一步矿产勘查部署工作有一定指导意义。