Physical Vulnerability Analysis for Coastal Flooding in Godavari River Delta, Andhra Pradesh, India

摘要

Tropical cyclones cause a variety of damages. The major causes of damage are due to Storm surge, fierce winds, and torrential rains. Storm surge is the greatest potential threat to life, property, and coastal environment. Strong winds cause structural damages and heavy insitu /catchment's rainfall amplifies the menace of flooding. In the present study, it is attempted to study the physical vulnerability of Andhra Pradesh (AP) coast with reference to coastal flooding. The Godavari river delta region is selected as a case study for vulnerability analysis. All the 33 mandals in East Godavari (EG) district and 27 mandals in West Godavari (WG) district within Godavari delta are studied. The main objective of the study is to identify the inundated areas for most severe event in the last 50 years, if occurred at different locations along the coast of Godavari delta. Weightages are assigned to different depth of inundations depending on their capacity to affect the resources in the region. Based on the extent of inundation in different mandals, mandals are classified into different vulnerability classes. This is achieved by simulated cyclone tracks resembling the severe cyclone to cross the Godavari delta front at different locations spaced at an interval of 20 kms. The landfall time and intensity is so selected that the storm gains maximum intensity just before crossing. The landfall time is also made to coincide the spring tide to get maximum inundation. Wind fields are generated at a regular 6 hourly interval from 14th April 1999 1200UTC till 17th April 1999 0000UTC. Inundations are calculated using Delft3D flow model, in each cell of a curvilinear grid. The maximum depth of inundation for Godavari delta region is derived for each cell obtained by different storm runs. The highest maximum depth of inundation of 5.2 m. is observed in Thalleravu mandal of EG district and 3.698 in Narasapur mandal of WG district. The lowest maximum depth of inundation of 1.762 m. is observed in Kadiam mandal of EG district and 0.634 in Kalla mandal of WG district. Maximum depth of inundation is grouped into five physical vulnerability classes: 0-0.5m, 0.5-1m, 1-2m, 2-4m and 4-6m. The maximum inland penetration across Kakinada region in Eastern Godavari delta is observed to be extending up to 45 kms, and across Amalapuram region in Central Godavari delta up to 27 kms, and across Narasapur region in Western Godavari delta up to 30 kms. Suitable weightages ranging from 0.125 to 1.0 are given to the above five physical vulnerable classes based on depth of inundation. Higher the weightage is given for greater depth of ranges. The weighted vulnerable area is arrived from the product of weightage and the varying grid cell resolution. Finally, the relative vulnerability is tabulated based on the ratio of weighted vulnerable area and the total geographical area. The six classes of mandal level physical vulnerability are: Very High Vulnerable, High Vulnerable, Moderate Vulnerable, Low Vulnerable, Very Low Vulnerable, and non-vulnerable deltaic mandals.