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Bed roughness and grain sorting-an experimental study over fine to medium sand beds

International Journal of Sediment Research 2017年第3期32卷 384-400页

作者： barendra purkait Dipanjan Das Majumdar Rajat Mazumder Geological Studies Unit Indian Statistical Institute Department of Geology University of Calcutta Department of Remote Sensing& GIS Vidyasagar University School of Biology Earth and Environmental Sciences University of New South Wales Dept. of Applied Geology Faculty of Engineering and Science Curtin University malaysia

The relation between grain-size distribution of the bed and in suspension was critically examined under a uniform flow velocity of 50 cm/s over two beds: one of mainly fine sands and the other of medium sands. Two sections-one 2.85 m downstream and the other 6.35 m downstream in the experimental channel-were selected for sampling to study the grain-sorting pattern in the vertical direction along the direction of transport. The shape and type of the grain-size distribution pattern were critically studied with height above the bed. The change in the distribution pattern has been attributed to the change of local bed roughness causing scouring against the protruded relatively coarse grains on the bed. Such trends are important to predict the nature of river bed topography. The sand of Bed-1 initially exhibits a log-skew-Laplace distribution at different heights of suspension. The distribution pattern, however,changes but this changing pattern is not consistent along the upstream side. For Bed-2, which initially exhibits a log-normal distribution, the same pattern persists from the height of suspension at 5 cm up to20 cm. Such consistency in log-normality is also observed at the downstream points of measurement. It is generally expected that the mean grain-size would reduce with increases of suspension height but the results of the experiments, in some occasions, differ significantly from the gradual fining upward *** result has been attributed to local changes of bed roughness arising from the protruded relatively coarse grains causing eddies, scouring, and turbulent phenomena which moves coarse particles higher in suspension adding a coarse tail to the distribution increasing the mean grain-size.

关键词： Flume experiments Suspension height Uniform flow velocity Bed roughness Grain-size distribution Bed scouring Turbulent flow

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Arsenic in Groundwaters and River Sediments of West Bengal,India:A Geochemical and Spectroscopic Study

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地学前缘 2009年第S1期16卷 242-242页

作者： Andrew Neal Karen Johannesson Pradipta Sur Debjani Sarkar barendra purkait Saugata Datta Department of Geology Kansas State University Department of Earth and Environmental Science Tulane University Department of Geology University of Calcutta Geological Survey of India

The problem of arsenic(As)poisoning in the upper deltaic plains of the Ganges-Bhagirathi river system of West Bengal(WB),India,is terrifying. Elevated As(>50 ppb)in well water was observed within a depth range of 10-30 m in older grey terraces of abandoned fluvial channel deposits in the Murshidabad and Malda districts in *** surface and cored(2-20 m)sediment samples from banks of the river Ganges and along a north-south transect of the main tributary Bhagirathi-Hooghly river

关键词： arsenic geochemistry Bengal delta mobilization groundwater

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MORPHOLOGY AND GROWTH PATTERNS OF THE USRI RIVER POINT BARS, BIHAR, INDIA

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International Journal of Sediment Research 2000年第4期 445-457页

作者： barendra purkait ( This work was completed at the Flume Laboratory, Indian Statistical Institute, Calcutta, E-mail: bijoy@isical.ac.in Senior Geologist, Map & Cartography Division, Geological Survey of India, 29, J.L. Nehru Road, Calcutta 700 016, INDI

Morphology and growth patterns of four point bars of the river Usri, Bihar, India were studied by plane table mapping for two to three successive years before and after the monsoon. In plan, all these bars are crescentic in shape with downstream tail. The central parts of the bars are wider than their two ends. The bar represents aggregate of strips of sediments added laterally. The elevation contour lines demarcate the individual strips by abruptly cutting at the junction of two adjacent strips. In transverse profiles, the strips show up as series of steps. General slope of the bar is towards the adjacent channel though this slope is interrupted by these steps. In 3-dimensions, all these bars exhibit avalanche faces near the downstream end. The longitudinal profile of the bar over this avalanche face resembles the profile of an asymmetrical ripple, the lee face being this avalanche face. Trenches across the avalanche face reveal cross-stratification with gradation of grain sizes within individual cross-strata, similar to the features generated in front of migrating ripples. All these features suggest that the point bar is a mega ripple (macro bedform) which migrates downstream during the high flood when the entire bar is inundated. SEPM bedform classification also records that dunes are bedforms with wave length greater than 100m and height greater than sin. The equation of relative discharge as worked out by Sundborg (personal communication) from the Chezy and Manning equations, has been applied to establish a relationship between relative discharge and relative volume of sedimentation in point bars.

关键词： Morphology Growth pattern Point bar Longitudinal profile Avalanche face Crossstratification Bedform

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