ABSTRACT
The recent advances in the geographical information system (GIS) and remote sensing (RS) have opened new vistas on flood assessment and forecasting. GIS together with RS has been a useful tool to integrate and manipulate not only technical information but also a communication tool for interaction with the public and decision-makers. Risk analysis is a judgment of probability and severity of consequences based on the history of previous incidents, local experience, and the best available current technological information. The main reasons for flooding in the lower Tapi basin (LTB) are heavy rainfall and discharge due to high water levels from the Ukai Dam. Various issues related to flooding in 106 km stretch of LTB are inundation due to overflowing of the banks, inadequate drainage capacity of the river, congestion at the point of confluence, excessive silt load factor in river discharge and high tide due to proximity to the Gulf of Khambhat. GIS-based weighted overlay analysis was performed using the multicriteria method for calculating flood vulnerability. Three classes of vulnerability were ascertained, viz., low, moderate and high. It is observed that the Tapi flood plain near Surat is highly vulnerable, and some of the villages along the banks are also highly vulnerable to flooding. Flood frequency analysis applied on river discharge data of 30 years (1978–2007) using six distribution methods viz., (i) extreme value type I distribution (EVI), (ii) generalized extreme value distribution (GEV), (iii) log-normal distribution parameter two (LN2), (iv) log-normal distribution parameter three (LN3), (v) Pearson type III distribution (P3) and (vi) log Pearson type III distribution (LP3) showing the LP3 distribution method as most reliable. It shows the highest correlation with simulated and observed data (R 2 = 0.98). The size of the flood with a return period of 100 years or an annual exceedance probability of 1% is 73,860 m3/s from the LP3 distribution method. In this study, 59 river cross-sections were prepared using SRTM DEM data of 30 m resolution for computing channel geometry characteristics using HEC-GeoRAS. In addition to the usage of this outcome in the HEC-RAS hydrodynamic model, river cross-sections were also examined for understanding any anomaly along and across the river. Tapi River has reduced water carrying capacity drastically at Surat. Field observations indicate that high siltation and encroachment are the two major factors for this reduction. Flood risk analysis was carried out based on vulnerability, land use and population density maps. Land use maps were prepared using multisensor satellite data for LTB. The risk-prone areas were classified into four groups, i.e., very high, high, moderate and low based on GIS-based weighted overlay analysis performed using the multicriteria method. It is observed that agriculture and residential areas are under the very high-risk category in LTB. Overall, the salient findings can be utilized by the concerned flood protection authorities in various state government departments such as State Disaster Management Authority, Irrigation Department, Surat Municipal Corporation, Rural development departments, Town and Country Planning, Public Works Department and other concerned.
