Simulating the hydraulic characteristics of the lower Yellow River by the finite‐volume technique

The finite‐volume technique is used to solve the two‐dimensional shallow‐water equations on unstructured mesh consisting of quadrilateral elements. In this paper the algorithm of the finite‐volume method is discussed in detail and particular attention is paid to accurately representing the complex irregular computational domain. The lower Yellow River reach from Huayuankou to Jiahetan is a typical meandering river. The generation of the computational mesh, which is used to simulate the flood, is affected by the distribution of water works in the river channel. The spatial information about the two Yellow River levee, the protecting dykes, and those roads that are obviously higher than the ground, need to be used to generate the computational mesh. As a result these dykes and roads locate the element interfaces of the computational mesh. In the model the finite‐volume method is used to solve the shallow‐wave equations, and the Osher scheme of the empirical function is used to calculate the flux through the interface between the neighbouring elements. The finite‐volume method has the advantage of using computational domain with complex geometry, and the Osher scheme is a method based on characteristic theory and is a monotone upwind numerical scheme with high resolution. The flood event with peak discharge of 15 300 m³/s, occurring in the period from 30 July to 10 August 1982, is simulated. The estimated result indicates that the simulation method is good for routing the flood in a region with complex geometry. Copyright © 2002 John Wiley & Sons, Ltd.     

Monitoring Flood Extent and Forecasting Excess Runoff Risk with RADARSAT-1 Data

Earth observation from active microwave satellites such as RADARSAT-1 is an excellent tool to monitor and forecast floods. Two complementary approaches are described in this paper: (a) real time or near-real time monitoring of flood extent and (b) mapping of hydrological properties of drainage basins. Since it can penetrate through clouds, which usually occur during precipitation periods, and due to the fact that it can be programmed with different incidence angles, RADARSAT-1 enables frequent coverage over specific areas of interest. It has been used successfully to monitor a major flood of the Red River in Manitoba in 1997, by providing frequent coverage of the flood during its progression and decrease. Resulting data and images have been useful in planning the emergency measures and in assessing flood damage. RADARSAT has also the ability to characterize hydrological properties of watersheds. It has been used in agricultural catchments in Europe for mapping soil surface roughness, which affects runoff coefficients, concentration time and resistance to erosion processes. Used to complement optical data, RADARSAT has provided information on the status of land use and soil protective cover in drainage basins. This information can then be translated into parameters and coefficients that hydrological models can use for runoff and flood forecasting     

Flood hydrograph attenuation induced by a reservoir system: analysis with a distributed rainfall‐runoff model

Spatially distributed hydrologic models can be effectively utilized for flood event simulation over basins where a complex system of reservoirs affecting the natural flow regime is present. Flood peak attenuation through mountain reservoirs can, in fact, mitigate the impact of major floods in flood‐prone areas of the lower river valley. Assessment of this effect for a complex reservoir system is performed with a spatially distributed hydrologic model where the surface runoff formation and the hydraulic routing through each reservoir and the river system are performed at a fine spatial and time resolution. The Toce River basin is presented as a case study, because of the presence of 14 active hydroelectric dams that affect the natural flow regime. A recent extreme flood event is simulated using a multi‐realization kriging method for modelling the spatial distribution of rainfall. A sensitivity analysis of the key elements of the distributed hydrologic model is also performed. The flood hydrograph attenuation is assessed. Several possible reservoir storage conditions are used to characterize the initial condition of each reservoir. The results demonstrate how a distributed hydrologic model can contribute to defining strategies for reservoir management in flood mitigation. Copyright © 2004 John Wiley & Sons, Ltd.     

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism     

Scaling and regionalization of flood flows in British Columbia,Canada

A regionalization of flood data in British Columbia reveals a common scaling with drainage area over the range 0·5×10²<A_d<10⁴ km². This scaling is not a function of flood return period, which implies that simple scaling—consistent with a snowmelt‐dominated flow regime—applies to the province. The observed scale relation takes the form , similar to values reported in previous studies. The scaling relation identified was used to define the regional pattern of hydroclimatic variability for flood flows in British Columbia after discounting the effect of drainage area. The pattern was determined by kriging a scale‐independent runoff factor k for the mean annual flood, 5 year flood and 20 year flood. The analysis permits quantification of uncertainty of the estimates, which can be used in conjunction with the mapped k‐fields to calculate a mean and range for floods with the identified return period for ungauged basins. Owing to the sparsity of data, the precision is relatively poor. The standard error is generally less than 75% of the estimate in the southern half of the province, whereas in the northern half it is often between 75 and 100%. Examination of the relative increase in flood magnitude with increasing return period reveals spatially consistent but statistically insignificant differences. Flood magnitude tends to increase more rapidly in the western regions, where rain events may contribute to flood generation. The relative increase in flood magnitude with return period is consistently lower in the eastern mountain ranges, where snowmelt dominates the flood flow regime. Copyright © 2002 John Wiley & Sons, Ltd.     

海温对辽宁省主汛期降水异常影响的统计分析

在对辽宁省主汛期降水异常的时变特征进行分析的基础上，研究了前期海温场对辽宁主汛期(7—8月)降水影响的统计特征。结果表明：1)辽宁省汛期降水异常有明显的年际、年代际变化特征，近44a降水呈减少趋势。小波分析表明存在3～5a的年际周期和10～12a的年代际周期。2)存在2个影响辽宁汛期降水异常的海温关键区及相应的关键时段，它们分别是南大西洋(1—5月)和西太平洋(3—4月)，综合考虑2个关键区可更好地预示辽宁汛期降水的异常。     

A Short-Term Climate Prediction Model Based on a Modular Fuzzy Neural Network

In terms of the modular fuzzy neural network (MFNN) combining fuzzy c-mean (FCM) cluster and single-layer neural network, a short-term climate prediction model is developed. It is found from modeling results that the MFNN model for short-term climate prediction has advantages of simple structure, no hidden layer and stable network parameters because of the assembling of sound functions of the selfadaptive learning, association and fuzzy information processing of fuzzy mathematics and neural network methods. The case computational results of Guangxi flood season (JJA) rainfall show that the mean absolute error (MAE) and mean relative error (MRE) of the prediction during 1998 2002 are 68.8 mm and 9.78%, and in comparison with the regression method, under the conditions of the same predictors and period they are 97.8 mm and 12.28% respectively. Furthermore, it is also found from the stability analysis of the modular model that the change of the prediction results of independent samples with training times in the stably convergent interval of the model is less than 1.3 mm. The obvious oscillation phenomenon of prediction results with training times, such as in the common back-propagation neural network (BPNN) model, does not occur, indicating a better practical application potential of the MFNN model.     

三峡水库汛期分期的变点分析方法研究

介绍了变点分析理论，结合三峡水库汛期的分期，阐述了均值变点、概率变点的理论与分析方法。采用宜昌站1882-2001年实测日流量资料，用均值变点分析方法对汛期每日最大洪峰构成的时间序列进行分期；同时选择一定的阈值，在假定发生概率服从二项分布的条件下，应用概率变点分析方法进行分期，最后给出了三峡水库汛期的分期方式。经比较表明，变点分析理论应用于汛期分期中，能反映来水的基本规律，具有一定的应用价值；但从理论上以及防洪的角度来讲，概率变点分析方法较均值变点方法更适于水库汛期的分期计算。     

西江下游高要水文站洪水预报方法的改进研究

西江下游河床逐年下切，使高要水文站洪水预报方案的预报误差增大。本文对有关情况进行了分析，并对如何改进高要水文站洪水预报方法进行了改进研究，取得了比较满意的成果。     

长江上游暴雨对1998年长江洪峰影响的分析

通过对１９９８ 年长江上游四川境内暴雨、雨量水文资料、卫星云图等分析,归纳出了１９９８ 年长江上游暴雨的基本特点,认识到长江上游暴雨、高原天气系统与四川云团的活动对１９９８ 年长江洪峰的形成有直接影响。