Theme-5-2-Flash floods and their forecasting - A few case studies.

Abstract

Flash floods occur from heavy rainfall and sudden overflowing of rivulets and streams into the low lying areas. They occur (i) from the movement of depressions/cyclonic storms (ii) the shift of the axis of the monsoon trough close to the foot hills during monsoon (iii) the incursion of southerly current from the Arabian Sea and the Bay of Bengal into the mountaineous regions of the Himalayas (iv) the development of off-shore vortices along the west coast during monsoon under the active equatorial trough. On satellite imageries, the areas of intense convection coincide with the equatorial trough and the embedded cyclonic circulations at 500 hPa during the monsoon season and the cyclonic circulations at 700 hPa during the post monsoon season. During the pre-monsoon and sometimes during the post-monsoon seasons these coincide with the cyclonic circulations close to the ground (0.9 Km asl). Sometimes, there are no circulations associated with the intense convection but the ascent is provided to the southerly moist current by the mountaneous.