陕西省山洪灾害特征及防治对策

界定了山洪灾害的概念．分析了陕西省山洪灾害的形成条件和影响因素。在时域上,具有多期性和同期群发性,与泥石流、滑坡等灾害形成灾害链,每年7～9月是山洪灾害的高发季节,4～6月偶有发生;在地域上．受暴雨中心和流域地形控制,具有相对集中性和成片成带性。划分了山洪灾害易发区,分区提出了相应的防洪减灾措施。     

Hydrometeorological Aspects of Floods in India

The Indian sub-continent being located in the heart of the summermonsoon belt, receives in most parts more than 75% of its annual rainfall during the fourmonsoon months of June to September. As the bulk of summer monsoon rainfall occurs withina period of four months, naturally majority of floods occur in Indian rivers during thisseason only. The ground conditions also help in generating high percentage of run-offbecause of the antecedent wet conditions caused by rainy spells occurring within the monsoonperiod itself. Besides mentioning different weather systems, which cause heavy rainfall and consequentfloods, a detailed discussion of 15 years' floods in different river systems has alsobeen given in the article. This study has shown that the flood problem in India is mostly confinedto the states located in the Indo-Gangetic plains, northeast India and occasionally in therivers of Central India.     

Recurring middle Pleistocene outburst floods in east-central Alaska

Recurring glacial outburst floods from the Yukon-Tanana Upland are inferred from sediments exposed along the Yukon River near the mouth of Charley River in east-central Alaska. Deposits range from imbricate gravel and granules indicating flow locally extending up the Yukon valley, to more distal sediments consisting of at least 10 couplets of planar sands, granules, and climbing ripples with up-valley paleocurrent indicators overlain by massive silt. An interglacial organic silt, occurring within the sequence, indicates at least two flood events are associated with an earlier glaciation, and at least three flood events are associated with a later glaciation which postdates the organic silt. A minimum age for the floods is provided by a glass fission track age of 560,000 ± 80,000 yr on the GI tephra, which occurs 8 m above the flood beds. A maximum age of 780,000 yr for the floods is based on normal magnetic polarity of the sediments. These age constraints allow us to correlate the flood events to the early-middle Pleistocene. And further, the outburst floods indicate extensive glaciation of the Yukon-Tanana Upland during the early-middle Pleistocene, likely representing the most extensive Pleistocene glaciation of the area.     

Geomorphic Effects of Monsoon Floods on Indian Rivers

The southwest summer monsoon contributesthe bulk of India's rainfall. Consequently,almost all the geomorphic work by the rivers is carried out during the monsoonseason in general and the monsoon floods in particular. Indian rivers arecharacterized by high average flood discharges and large temporal variability. Thereis also significant spatial variation in the magnitude, frequency and power of floods, on account of regional variations in monsoon rainfall, basin characteristics andchannel geometry. As a result, the channel responses and the geomorphic effects also varyspatially. This paper describes the hydrological and geomorphological aspects, as well asthe geomorphic effects of monsoon floods in the Indian rivers. The geomorphic effects of floods are most impressive only in certainareas – the Himalaya, the Thar Desert, and the Indus-Ganga-Brahmaputra Plains. There are numerous instances of flood-induced changes in the channel dimension,position and pattern in these areas. In the Ganga-Brahmaputra Plains, the annualfloods appear to be geomorphologically more effective than the occasional large floods.In comparison, the rivers of the Indian Peninsula are, by and large, stable and thegeomorphic effects of floods are modest. Only large-magnitude floods that occur at aninterval of several years to decades are competent to modify the channel morphology in asignificant way. A synthesis of the various case studies available from the Indianregion indicates that often the absolute magnitude of a flood is not as important withrespect to the geomorphic effects as the flow stress and competence.     

Three Recent Extreme Floods in Bangladesh: A Hydro-Meteorological Analysis

Bangladesh is highly vulnerableto floods due to its geographical location at the deltas of the Ganges, Brahmaputra and Meghna (GBM) rivers. About 92.5 per cent of the area of three basins lies outside the boundaries of the country. More than 80 per cent of the annual precipitation of Bangladesh occurs in the monsoon period between June and September. The hydro-meteorological characteristics of the three river basins are unique and they often cause large to extremely large floods in Bangladesh. It is possible that these floods could inundate 70 per cent of the country and the physical damage could be very serious for the economy of Bangladesh with its low gross domestic product (GDP). In 1987, 1988 and 1998, Bangladesh experienced three extreme floods, leaving trails of devastation and human misery. In this article it is demonstrated that these floods differed in terms of magnitude, extent, depth and duration. The external and internal hydro-meteorological dynamics were also different.     

Caractérisation des variations lithologiques saisonnières des sédiments de la baie de Seine orientale ; impact des crues de la Seine (France)Characterisation of seasonal lithological variations of the superficial sediment in the eastern part of the Bay of Seine: impact of floods Seine River (France)

This paper presents the first results obtained during cruises since 1995 in the southeastern part of the Bay of Seine. New sedimentological data concerning sediment samples, box cores and water column measurements have been obtained recently in the framework of the Research Programme PNEC (French National Coastal Environment Programme). Fine sediments are deposited on form of temporary veneers of fluid-to-soft mud formed during high floods episodes. In the subtidal area (<10 m water depth), mobility of fine sediments is important and temporary mud deposited during high discharge period composes part of the distal estuarine system of the Seine River. To cite this article: S. Garnaud et al., C. R. Geoscience 334 (2002) 323–330.     

黄河下游漫滩洪水冲淤规律

滩地的淤积层分布记录着以往漫滩洪水的特征,即反映漫滩洪水的量级、频率和持续时间等,同时河漫滩也是预估河流泥沙、洪水灾害防治和湿地生态系统保护等的重要组成部分。根据黄河下游水文年鉴资料,分析滩地的淤积与漫滩洪水的定量关系,为未来河流泥沙预估提供依据。经分析得到大漫滩洪水在来沙系数S/Q<0.030 kg·s/m6时,主槽冲刷而滩地淤积,反之则滩槽同淤。当S/Q<0.030 kg·s/m6时,大漫滩洪水滩地的淤积量主要与漫滩系数Q_max/Q_p、上滩水量W₀和含沙量S有关;大漫滩洪水的主槽冲刷量则除了与洪水期水量W和沙量W_s有关外,还与滩地的淤积量有关。一般漫滩洪水,当来沙系数S/Q<0.023 kg·s/m6时,主槽冲刷而滩地淤积,反之则滩槽同淤。一般漫滩洪水主槽冲刷量与来沙系数S/Q和洪水期水量W有关,而滩地淤积量仅与含沙量S有关。黄河下游漫滩洪水滩地的淤积和主槽的冲刷主要发生在孙口以上河段,而孙口以下河段主槽冲刷和滩地淤积量均较少。     

Sr and Nd isotopes as tracers of clastic sources in Lake Le Bourget sediment (NW Alps, France) during the Little Ice Age: Palaeohydrology implications

Geochemical methods (major elements and Sr, Nd isotopes) have been used to (1) characterize Lake Le Bourget sediments in the French Alps, (2) identify the current sources of the clastic sediments and estimate the source variability over the last 600 years. Major element results indicate that Lake Le Bourget sediments consist of 45% clastic component and 55% endogenic calcite. In addition, several individual flood levels have been identified during the Little Ice Age (LIA) on the basis of their higher clastic content (> 70%).Potential sources of Lake Le Bourget clastic sediments have been investigated from Sr and Nd isotope compositions. The sediments from the Sierroz River and Leysse River which are mainly derived from the Mesozoic Calcareous Massifs are characterised by lower ⁸⁷Sr/⁸⁶Sr ratios and slightly lower ?_Nd(0) ratios than the Arve River sediments which are derived from the Palaeozoic Mont-Blanc External Crystalline Massifs. The Rhône River appears to have been the main source of clastic sediments into the lake for the last 600 years, as evidenced by a similar Sr and Nd isotopic compositions analyzed in core B16 sediments (⁸⁷Sr/⁸⁶Sr = 0.719, ?_Nd(0) = − 10) and in the sediments of the Rhône River (⁸⁷Sr/⁸⁶Sr = 0.719, ?_Nd(0) = − 9.6).The isotopic signatures of flood events and background samples from core B16 in Lake Le Bourget are also similar. This indicates that prior to ∼ 1800, the inputs into the lake have remained relatively homogeneous with the proportion of clastic component mainly being a function of the palaeohydrology of the Rhone River. Early human modification (deforestation and agriculture) of the lake catchment before the 1800s appears to have had little influence on the source of clastic sediments.     

Complexity Theory of Natural Disasters; Boundaries of Self-Structured Domains

Disasters are often represented as complete breakdowns of quasi-stationary states in a landscape, but may also be part of the normal evolution of such states. A landscape is, in fact, an open, nonlinear, dynamic system where the tectonic uplift and the seismic activity represent the input, the mass wastage and the relief degradation the output. The apparent 'stability' is due to the fact that open, nonlinear dynamic systems tend to develop into relatively stable, self-organized ordered states 'at the edge of chaos', with a fractal attractor. Short of complete breakdown, such systems re-establish order in steps of various magnitudes which have a power-law distribution. Because of the fractal structure of the basic attractor, all subsets follow a power law which accounts for the distribution of the steps of recovery. As the domains of quasi-stationarity at the edge of chaos are represented by finite windows, the power-law does not cover all magnitudes. The stationarity windows are not only limited in range, but also in space and time. This should be taken into account in the assessment of hazards. Examples are given from seismology (earthquake frequency), volcanology (eruption frequency), river hydrology (flood frequency) and geomorphology (landslides).     

延河流域雨洪特性及洪水预报

延河是黄河的一级支流，流经安塞、延安、延长等陕北南部地区的重要城镇，是引发该地区城镇洪水的主要河流，因此，延河洪水预报对这些城镇的防洪安全具有重要意义。在分析延河流域雨洪特性的基础上，以提高大洪水的预报精度为主、适当兼顾中小洪水的原则，采用具有成因概念的系统模型和相关图法，建立了以延安市为重点、包括安塞及延长县城的延河区段洪水预报方案，可供作业预报试用。