• Dipankar Saha

      Articles written in Journal of Earth System Science

    • Geomorphologic, stratigraphic and sedimentologic evidences of tectonic activity in Sone–Ganga alluvial tract in Middle Ganga Plain, India

      Sudarsan Sahu Dipankar Saha

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      The basement of the Ganga basin in the Himalayan foreland is criss-crossed by several faults, dividing the basin into several sub-blocks forming horsts, grabens, or half-grabens. Tectonic perturbations along basement faults have affected the fluvial regime and extent of sediment fill in different parts of the basin during Late Quaternary. The East Patna Fault (EPF) and the West Patna Fault (WPF), located in Sone–Ganga alluvial tract in the southern marginal parts of Middle Ganga Plain (MGP), have remained tectonically active. The EPF particularly has acted significantly and influenced in evolving the geomorphological landscape and the stratigraphic architecture of the area. The block bounded by the two faults has earlier been considered as a single entity, constituting a half-graben. The present investigation (by morpho-stratigraphic and sedimentologic means) has revealed the existence of yet another fault within the half-graben, referred to as Bishunpur–Khagaul Fault (BKF). Many of the long profile morphological characters (e.g., knick-zone, low width–depth ratio) of the Sone River at its lower reaches can be ascribed to local structural deformation along BKF. These basement faults in MGP lie parallel to each other in NE–SW direction.

    • Harvesting of water by tunnelling: A case study from lateritic terrains of Western Ghats, India


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      Harvesting of water by tunnelling in the Western Ghats lateritic terrains of India known as surangam (tunnel well) is practiced since ages. It is common even today in the northern parts of Kerala state in India. Surangams are featured by horizontal tunnel wells, dug manually within the laterite, where the tunnel wall collects the seepage water, flows out under gravity and get accumulated in open tanks or dug wells for further use. The tunnels are laid horizontally or slightly inclined, placed just over the lithomarge clay so that maximum seepage can be accumulated through the porous laterites (generally of 3.95–30.20 m thick). A study is done on surangams from the Kasaragod district of Kerala, where it is still popularly used. They represent different designs; single tunnel, criss-cross tunnels and diversion tunnels, depending upon the terrain conditions, the permeability of the laterites and thickness of the saturated zones within laterites, demand of water and location of delivery points, etc. Surangam system may start from a dug well, end in a dug well and there can be an open shaft or dug well in between. Total 32 surangams are inventoried in Bedadka Panchayath, of which 24 are investigated in detail. The discharge varies from 0.078 to 13.29 m3/day during pre-monsoon while it remains between 172.80 and 691.20 m$^3$/day during monsoon period in 2019. In 2019 summer, 12 inventoried surangams remained dry. The hydrochemical facies of water from the surangams generally varies from Ca–HCO$_3$ type to Ca–Mg–Cl type indicating rock–water interaction. Because of lack of proper maintenance caving of walls occur during the rainy season. Lack of maintenance renders the priceless traditional groundwater abstraction structures disappearing fast. Surangams with good discharge are to be protected with proper concrete reinforcing of the outlet of the tunnel and proper storage options and arrangement of delivery systems up to the user point. Besides, water harvesting measures in the catchment of surangams are required for round the year sustainability of yield. A plan has been proposed for the revival of the surangams along with design specifcations.

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