Flood Analysis Using Integrated HEC-RAS and GIS: A Case Study of Nashik city, India

Abstract

The developmental activities in the urban areas are severely affecting the river flow pattern of the rivers; especially the rivers which pass through the urban habitation are causing heavy floods. The effect of encroachment in the city limits due to urbanization reduces the flow path of rivers causing flooding in adjacent areas. The available data indicates that causalities due to increased frequency of occurrence of flood have increased from 1000 per year in 1965-75 decade to 1700 per year in 2005-15 decade. The available data also indicates from 2005 to 2015 the cumulative economical losses due to floods are about 2% of current GDP of India. Under these circumstances it’s necessary to asses flood risk of the cities which are located on the banks of major rivers. This paper deals with the study of Nashik, a Holy city located on the banks of river Godavari in Maharashtra State, India. The city is subjected to heavy floods frequently due to intense rainfall in the catchment, release from the Gangapur Dam located upstream of the city and increased construction activities resulting into the reduction of river flow path. Gangapur dam is the storage dam located at 15 km upstream of Nashik city with a gross storage of 215.88 MCM. During monsoon the dam gets filled up and the excess water is released in the Godavari River, this additional discharge in the river causes heavy flood like situations in the city. Considering the rainfall pattern, capacity of the Dam it is difficult to control floods however the losses due to floods can be reduced by analysing and taking precaution. In this study the floods occurring in the river are analysed by using the HEC-RAS software with RAS Mapper. A typical case of 2006 flood event is analysed with HEC-RAS. The results obtained from the analysis indicate that the cross-sections 13 to 29 are critical which are located in the parts of Nashik city and among these the cross-section 27(near Panchvati area) shown the maximum values of all parameters. The model can also be used for estimating the total submergence for maximum rainfall of various return periods. From the results it is concluded that the severity of the flood and the losses in Nashik city and in the nearby areas can be estimated and reduced by adopting few management techniques such as river cleaning, providing a safe passage to excess water from Gangapur Dam to the Darna river which is on the downstream side of the Nashik city, construction of gabion wall along the river at critical locations etc.