Hazards of mining activities downstream of Khanpur Dam and safety-evaluation

Downstream of Khanpur Dam, large-scale mining activities, comprising excavation of gravel from Haro River-bed and blasting in limestone hills, for obtaining aggregate and lime, have greatly changed the flow-regime of Haro River. On the basis of field survey, a number of cross-sections have been developed and compared with the original river-bed, which indicate lowering of river-bed by 25, 20, 19, 15, 14, 16, 13 feet at RD 1500, 2000, 2500, 3000, 4000, 5000, and 6000, respectively, due to excavation of grovels since construction of the dam in 1985. Lowering of the river-bed has increased the head across the dam, which has directly influenced and increased the rate of seepage. The probability of sediment-migration from spillway surrounding, to refill the excavated depressions, has greatly increased, which is very serious from Dam-safety viewpoint. The limestone hills are the extensions of right abutment of the main dam. Shock waves generated due to heavy blasting do not allow the natural healing process of joints and fractures in the Dam-abutments and riverbed alluvium through sedimentation, which are the major seepage carriers. The waste material of crushers, comprising discarded fines sediments and shale, are being dumped in the river channel, which is responsible for induced meandering of river during spillway operation. As a result, the Mora Gutta village and some other adjoining areas on the left bank of the river has come under direct hit of river flows. The massive intake-structure of Punjkatha Channel, constructed during 1905 at Mora Gutta on the left bank of Haro river, has also been damaged by induced meandering action of the river in 1992. Hence, during high flood, the risk of erosion of these areas has greatly increased. Combined effect of the blasting and excavation of gravel from riverbed downstream of the dam site has affected the safety of the dam. Due to these activities, average seepage from the dam has increased from 30 cusecs (during 1990) to 47 cusecs (during 1998), which is a significant economic loss of Rs.10.747 million per year, as the water is being utilized for municipal as well as for industrial purposes. Suggestions for rectification of the situation have been made in this paper in order to save the precious national asset.