Causes, concerns and hazards of sinkhole formation in Brengi stream catchment of Upper Jhelum basin, Kashmir Himalaya

On February 11, 2022, the Brengi streambed caved in, and a sizable sinkhole (33.57 N and 75.33 E) was developed. The Brengi stream is a major tributary of the Upper Jhelum River in Kashmir Himalaya. The sinkhole swallowed the entire stream flow and caused mass mortality of fish and other aquatic creatures, besides disrupting completely the local water supplies. The present study analyzed the ancillary data reports, geological and topographic maps, satellite and Google Earth images, and climatic data in conjunction with the field, geophysical and chemical tracer studies to understand the causes of sinkhole formation, the outlet of captured flow and its associated hazards. The study indicated the sinkhole as a “Collapse Sinkhole” formed due to the collapse of the limestone (Triassic) roof cap of an existing deep-seated underground cavern through a prolonged dissolution and physical erosion by the weakly acidic stream and groundwater circulation under suitable climatic conditions. In this area, the limestone bedrock covered under a thick pile of overburden alluvial material comprises large structural discontinuities (bedding planes, fractures, joints and shear zones) and karst features. Before the collapse, a large water pool consisting ~ 80 m3 of the water column was estimated to be at the sinkhole site. Proton Precession Magnetometer survey around the sinkhole indicated that the underlying caverns are about ~ 100 m long downstream. A stream flow of ~ 15.03 m3/s was estimated to be swallowed by the sinkhole continuously for a couple of weeks till its closure. As a consequence, an estimated area of ~ 77.15 km2 comprising 58% agricultural/horticultural land, 25% vegetation cover and 10% built-up area was at high risk of catastrophe downstream of the sinkhole. From the tracer study, the outlet of the sinking water was found located at 16 km downstream at Achabal springs with an average flow velocity of ~ 1.3 km/h from the sinkhole site to the outlet. The sinkhole is a part and characteristic feature of karst topography of the area. Therefore, a detailed study on sinkhole susceptibility and hazard assessment is required. Although the sinkhole formation is an event of local significance in terms of the scale of the disaster, it may be of extreme importance in planning a given investment and management of local groundwater resources and karst springs.