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Challenges in development of river valley and hydel projects in karst terrains - Broad issues. Yamuna Singh Centre for Earth, Ocean and Atmospheric Sciences, University of Hyderabad, Dr. C.R. Rao Road, P.O. Central University, Gachibowli , Hyderabad - 500046
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Challenges in development of river valley and hydel projects in karst terrains - Broad issues Yamuna Singh Centre for Earth, Ocean and Atmospheric Sciences, University of Hyderabad, Dr. C.R. Rao Road, P.O. Central University, Gachibowli, Hyderabad - 500046 E-mail:yamunasingh1957@gmail.com Keoti Fall in Rewa district, Madhya Pradesh, India
The Term “Karst” • Karst: Geomorphic landscapes formed by solution and corrosive actions of water. • CO2-rich / Acidic waters infiltrates along joints/fractures/bedding planes – forms secondary and tertiary porosities – leads to development of surface and sub-surface solution openings including vertical and horizontal drainage. • Thus dissolution and attendant karstification results in various types of landscapes – Some of them are known for World Geotourism. • Globally: Karst topography forms at all latitudes and elevations. • Covers about 20% of the earth surface. • Best developed karst regions of the world are in tropical (South China, Vietnam, Jamaica) and temperate (Yugoslavia) regions. • Karstifiable Rocks: Biogenic, biochemical and chemical sedimentary rocks, e.g., limestone, dolomite, chalk, anhydrite, evaporite, etc. • Quartzite, Granite, etc., karsts are also known.
Karst Landforms • Solution-enlarged fractures and channels (karren) and closed depressions of various dimensions (sinkholes/ dolines, poljes), caves with speleothems (cave deposits, e.g., stalactites, stalagmites, dripstones and flowstones). • In monsoonal tropics, speleothems form significant archives of past-monsoonal changes. • Earth’s most diverse scenic and resource-rich terrains – for underground wealth, like minerals, oil, natural gas, good aquifer, etc.
Main Causes of Karst Developments • Soluble nature of rocks • Joints/Fractures/Bedding planes/Differential solubility of constituent minerals • Relief • Rainfall • Vegetation • Climatic conditions • Anthropogenic activity
(a) Tunnel with solution channels and conduits (b) stalactites and stalagmites
Karstified limestone beds within shale sequence: Potential aquifers
Karstification along bedding planes and vertical joints (horizontally bedded quartzite)
Characteristics of karst terrains • Calcareous rocks show karstification due to corrosive actions of meteoric, surface and subsurface waters. In fact the corrosive action of the water is responsible for the development of various types of karstic features both on the surface and subsurface levels. Furthermore, pronounced solution action of water caused formation of dolines, which lead to development of caves passages and underground streams in karst terrains.
Characteristics of karst terrains (contd.) • The development of karst is intense around weak points such as intersections of joints, fractures, interstices, etc. Interconnectivity of such secondary pore system exercises significant role in geotechnical issues in development of river valley and hydel projects in karst terrains.
Geotechnical Challenges Due to interconnected network of secondary pore systems, karst terrains pose diversified geotechnical challenges in development of river valley and hydel projects including pressure tunnels, bridges, highways, etc. Among others, the pronounced problems include leakage and discharge in distal places and attendant degradation, which need to be addressed adequately. Choice of depth and spacing of drilling and grouting is normally dependent up on the depth, nature and degree of karstification.
Geotechnical assessments • Systematic and meticulous geotechnical assessments of karst terrains are thus imperative before development of river valley and hydel projects in them. • Such investigations obviously involve careful study of nature, depth, distribution and frequency of secondary pore systems and their interconnectivity by close-spaced drilling, geomorphological and geophysical surveys and geohydrological evaluation. • Sealing of pore systems through comprehensive pressure grouting by drilling closely-spaced bore holes is essential for checking distal leakages and discharges.
Challenges in Karst Terrains • Mapping of karstified and assiciated rocks • Structural mapping • Agents of karstification • Depth of karstification • Degree of karstification • Mode of developments of (secondary/tertiary) pore systems • Interconnectivity of pore systems • Identification of recharge/discharge sites • Delineation of catchments and actual source areas • Aquifer mapping • Geohydrological studies • Geophysical studies • Environmental issues
Environmental Issues in Karst Terrains • Sinking streams, solution hollows or dolines, provide direct entry points to groundwater with contaminants – threats to sensitive karst environments and water quality. • Accordingly, karst groundwater needs suitable protection against contaminants and pollutants. • Dumping of solid / municipal wastes / effluents at recharge sites – confronting environmental issues. • Delineating catchments and actual source areas that feed aquifer to prevent contaminants.
Direct discharge of effluents at various places along stream course draining karst terrain
Google image: Conversion of stream course in karst terrain into sewage discharge systems
Conclusions • Systematic and meticulous geotechnical assessments of karst terrains are thus imperative before development of river and hydel projects in them. • Such investigations should include careful study of nature, depth, distribution and frequency of pore systems and their interconnectivity by close-spaced drilling, geomorphological and geophysical surveys and geohydrological evaluation. • Sealing of secondary pore systems through comprehensive pressure grouting by drilling closely-spaced bore holes is essential for checking distal leakages and discharges and attendant degradations. • Environmental concerns: Sinkholes and other secondary openings, which are present along the stream courses and in their vicinity, being interconnected with each other through an intricate network of karstified joints and fractures, gradually carry pollutants to groundwater reservoirs in karst aquifers. • Hales Gar Dam in Tennessee (USA) failed as cavities in limestone at the foundation were not adequately treated. Therefore, to ensure projected life time functioning of river and hydel projects, proper geotechnical assessments during prefeasibility, feasibility and detailed feasibility stages must be critically done and adequately addressed.
Acknowledgements I thank profusely Prof. D.P. Dubey, Govt. Science College Rewa, M.P. for guidance and to Dr. P.S. Mishra and Dr. I.B. Chhibbar, Geological Survey of India for taking me to Bansagar Dam site and small Hydel Projects in and around Rewa and for explaining geotechnical aspects in field during late 1970s and early 1980s.
