CS(AR)-36/96-97 : Space-time distribution of rainfall in the catchment of Yeleru river downstream of Yeleswaram

Abstract

Tropical cyclones affect India's coast during October-December and April-May seasons and a majority of these hit Andhra Pradesh coast. Cyclones cause damage due to storm surges, high winds and extensive rainfall. As most of the regions in the East Coast are low lying flat areas, flooding is the natural disaster occurring in these areas. Although floods do not claim as many lives as do storm surges, they affect the residential areas, transportation, telecommunication and agricultural areas to a very large extent. Floods created by tropical cyclones can reach catastrophic proportions when aggravated by wind induced surges along the coastline.

The magnitude of peak flood and shape of the flood hydrograph depends not only on the magnitude of the total storm rainfall but also on its distribution in space and time. Improvements of the accuracy and timeliness of hydrological forecasting would thus largely depend on the prediction of rainfall distribution in space and time As the rainstorms associated with the cyclonic storms are likely to produce a somewhat different picture, which generally exceeds 24-hr and covering vast areas, it was found necessary to study this aspect in the study area, which is prone to regular cyclonic storms. An attempt has been made to study the space and time distribution of rainstorms associated with the tropical cyclones in the Bay of Bengal over a period of 20 years from 1976 - 96,

The study broadly indicated that • From the Spatial variation analysis of the selected heavy rainstorms occurred over the study area during 1976-96, it reveal that the rainstorms of 23 July, 1989 (1- day), 9-11 May 1990 (2-day and 3-day), have contributed the highest average rainfall depths of 254.8 mm, 483 8 mm and 682.6 mm for 1,2,3 day duration respectively • From the Temporal variation analysis of the selected rain storms occurred over the study area, it has been observed that 50 percentage of the rainfall in a storm spell occurred in the 25 percentage of the storm duration and the remaining 50 percentage of the rainfall occurred in the remaining 75 percentage of the storm duration. • From the lsohyetal maps it has been observed that rainfall is concentrated in lower part of the study area and decreasing towards the land, i.e., towards the upstream of the river catchment. • The flood/inundation problem is coupled with the favourable antecedent soil moisture condition, as most of the storms occurred in post-monsoon season. • The heavy storms of duration more than 24 hours concentrated over the study area are very conducive for causing floods and consequent inundation. • The study area experienced heavy rainfall after the cyclonic storm crosses the coast.