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141.
Staircases of gravelly river terrace deposits in southern England occupy valleys typically underlain by frost-susceptible and brecciated bedrocks. The valleys developed during the Quaternary by alternating episodes of (1) brecciation, incision and planation through the bedrock, forming wide low-relief erosion surfaces; and (2) aggradation in braidplains of gravel a few meters thick that bury the erosion surfaces. A conceptual model to account for some of the terraces proposes that brecciation resulted from ice segregation in the ice-rich layer in the upper meters of Pleistocene permafrost, making them vulnerable to fluvial thermal erosion and therefore predisposing the bedrock to planation. The low gradients of the valleys were adjusted such that rivers transferred fine materials out of the basins but lacked the competence to remove gravel, which therefore accumulated within floodplains. The model challenges the prevailing view of incision during climate transitions. It attributes incision and planation to very cold and arid permafrost conditions, when rivers had limited discharges and hillslopes supplied limited volumes of stony debris into valley bottoms. 相似文献
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地面核磁共振方法具有直接找水、提供水文地质参数、受地形影响小、垂向分辨率高、经济快速等优点,但它同时也具有易受电磁噪声干扰、横向分辨精度低、勘探深度小等缺点。通过核磁共振先探明地层中的地下水。再辅以钻探验证,说明该方法在层状基岩找水也是一种有效的方法。 相似文献
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采用秩评分方法,选取NCEP再分析资料作为“实测”依据,以10个统计特征值为基础,评估了21个GCMs对东南诸河流域17个气候要素的模拟效果.结果显示:气候变量表现出不同的统计特征,GCMs对不同气候变量的模拟效果并不一致.与地面气象站点实测值对比,CSIRO:MK30、GFDL:CM21、GFDL:CM20、INM:CM30对降水和地面气温的模拟效果相对较好;流域高空气候要素的模拟效果更佳的模式有CGCM2.3.2、GISS:EH、BCCR:BCM20、GFDL:CM20对;综合所有气候变量,BCCR:BCM20、GFDL:CM20、CGCM2.3.2、GISS:EH的表现更优.优选出的GCMs可以为东南诸河流域气候变化的进一步研究提供科学依据. 相似文献
146.
采用秩评分方法,选取NCEP再分析资料作为“实测”依据,以10个统计特征值为基础,评估了21个GCMs对东南诸河流域17个气候要素的模拟效果.结果显示:气候变量表现出不同的统计特征,GCMs对不同气候变量的模拟效果并不一致.与地面气象站点实测值对比,CSIRO:MK30、GFDL:CM21、GFDL:CM20、INM:... 相似文献
147.
The ability to predict bedform migration in rivers is critical for estimating bed material load, yet there is no relation for predicting bedform migration (downstream translation) that covers the full range of conditions under which subcritical bedforms develop. Here, the relation between bedform migration rates and transport stage is explored using a field and several flume data sets. Transport stage is defined as the non‐dimensional Shields stress divided by its value at the threshold for sediment entrainment. Statistically significant positive correlations between both ripple and dune migration rates and transport stage are found. Stratification of the data by the flow depth to grain‐size ratio improved the amount of variability in migration rates that was explained by transport stage to ca 70%. As transport stage increases for a given depth to grain‐size ratio, migration rates increase. For a given transport stage, the migration rate increases as the flow depth to grain‐size ratio gets smaller. In coarser sediment, bedforms move faster than in finer sediment at the same transport stage. Normalization of dune migration rates by the settling velocity of bed sediment partially collapses the data. Given the large amount of variability that arises from combining data sets from different sources, using different equipment, the partial collapse is remarkable and warrants further testing in the laboratory and field. 相似文献
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研究塔里木盆地周边河流对罗布泊盐湖的盐分补给,对于研究罗布泊盐湖钾盐物质来源具有重要意义。文章在分析前人对塔里木盆地气候、温度、径流量变化等研究的基础上,分别根据竺可桢近5000年的古温度曲线、青藏高原古里雅冰芯δ18O记录的古气候变化及六盘山朝那黄土剖面磁化率记录的古温度变化,得出了近5000年、12万年、200万年的温度变化,并根据温度-径流量变化模型,估算了塔里木河流域三源流在研究时段内的径流总量。根据径流总量和河水中的K+浓度,估算了第四纪200万年以来塔里木河流域河水对罗布泊钾离子的输运量。 相似文献
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Jim Pizzuto 《地球表面变化过程与地形》2009,34(9):1234-1244
The highly stochastic nature of riverbank erosion has driven the need for spatially explicit empirical models. Detailed bank profile surveys along a meander bend of the Brandywine Creek in Pennsylvania, USA, before and after 28 high flow events over a 2·5 year period are used to develop an empirical model of cohesive bank profile erosion. Two hundred and thirty‐six bank erosion observations are classified as hydraulic erosion or subaerial erosion. Threshold conditions required to initiate bank erosion cannot be defined based on field measurements. Using the near‐bank velocity and the number of freeze–thaw cycles as predictors, regression equations are derived for hydraulic erosion that specify the length, thickness, and location on the bank face of eroded blocks. An empirical discriminant function defines the critical geometry of overhang failures, and the volumes removed by overhang failures are computed using another regression equation. All the regression equations are significant, but have low correlation coefficients, suggesting that cohesive bank erosion has a strong stochastic component. Individual events typically remove small masses of soil (average volume 0·084 m3/m) a few centimeters thick (median = 0·057 m) and a few decimeters in length (median = 0·50 m) from the lower third of the bank. Hydraulic erosion is responsible for 87% of all erosion. When applied to three survey sites not used in its development, the profile model predicts the total volume of erosion with errors of 23%, 5% and 1%. Twenty‐four percent of computed erosion volumes for single events are within 50% of observed volumes at these three sites. Extending the approach to decadal timescales and to entire bends will require three‐dimensional observations of bank failure, and spatially and temporally explicit methods to account for the influence of individual large trees on bank failures and near‐bank hydraulic processes. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献