RN-30 : Flash flood studies

Abstract

Flash floods occurs suddenly and it is usually difficult forecast because of short time available between their causative event and their actual occurrence. Flash floods generally result from very intense rain and often happen on catchments of moderate size. it is obvious that the physiography of the catchment, especially its slope and the type of vegetation also have a great influence on its capability to generate such flash floods. In addition to these causative factors, flash floods may also occur due to dam failure, landslide, sudden obstruction on the river channel due to failure of bridges etc. Since the occurrence of flash floods is not predictable, it is generally treated as a random phenomena and the risk associated with such floods could be higher. In flashy streams since the flow is of high velocity and transport huge quantity of sediment in suspension, current meters cannot be deployed for the measurement. Slope area method, by adopting artificial control, and radar measurement of rainfall may be used for studying flash floods. In this review note the causative factors, measurement and forecasting of flash floods are discussed in details along with a number of flash flood examples which occurred in India. The Indian literature review reveals that a number of case study reports are available, but a systematic study of flash floods leading to a technique of forecast and flash flood warning is not available. In view of this fact, this note elaborates the strategy followed by U.S. National Weather Service in combating the problem of flash floods.