The results of the current study on the cyclone frequency and density highlighting the highly vulnerable areas and storm surge model results pertaining to these areas pertaining to West Bengal, Odisha and Andhra Pradesh were detailed in the following sections.
4.1 Analysis of historical cyclone tracks
The historical cyclone tracks recorded during 1842-2016 were assessed to calculate the cyclone density and frequency. The results reveal the varying density and the frequency of the cyclones along the east coast at grid levels (fig.2a&b) and at district level (fig.1a). It was observed that majority of cyclones enter India via Orissa – West Bengal coast and move ahead. The grid which is formed at the border of Orissa-West Bengal and extends towards the sea is seen to have highest cyclone count of 297. Which means about 297 cyclones have occurred in an area of 13,200 km2 (1×1degree) in the past 173 years. The highest density was recorded at Bhaleshwar district of Odisha (Fig.1a).
The map represents the distribution of frequency of cyclones (fig.2a) that has occurred in the past. The highest frequency observed was 1.8 per year in the northern Orissa at Bhaleshwar district and West Bengal covering Midnapore and South 24 Paraganas coasts. The grids with red color are the areas which experience more than one cyclone per year on an average (fig.2b). The orange colored grids experience up to one cyclone per year on an average. Some parts of Andhra Pradesh experience a cyclone in 2-4 years. This analysis clearly highlights Orissa and West Bengal coasts are experiencing the most frequent as well as dense cyclones.
The assessment of total cyclones recorded at each district along the east coast (fig.1a), Baleshwar district of the state of Orissa and few coastal districts of West Bengal (East Midnapur; South 24 Parganas and North 24 Parganas districts) was recorded more than 145, the highest number of cyclones in the past. Bhadrak; Kendrapara; Puri and Ganjam district of Orissa state were recorded number of cyclones between 90 to 144. Whereas, all Andhra Pradesh districts recorded cyclones count between 49 to 89.
4.2 Storm surge modeling Results of Orissa:
The cyclone track selected for Orissa to study was super cyclone of 1999 which was known worst event in the area. cyclonic rainfall spread throughout the coastal districts of Odisha state, and effectively covering the area more than 200 km2. The maximum intensity of cyclonic rainfall more than 200 mm was observed and as a result, these were more prone to floods and damages (Patel & Sahoo 2018). At least 13 million people, including 3.3 million children, 5 million women and nearly 3.5 million elderly people were affected and 23,129 houses washed away, (Source: UNDP, Odisha Disaster Management Authority) with each other (Fig. 4). The results depict that the cyclonic rainfall spread throughout the coastal districts of Odisha state, and effectively covering the area up to > 200 km from the location of landfall ‘Paradip’ (Jagatsinghpur district) during Super–cyclone 1999 (Fig. 4a). The cyclonic rainfall of > 100 mm over these 3 days (29–31 October 1999) was observed along the coast line comprising several coastal districts of Odisha including the landfall location. Due to cyclonic activities, the maximum intensity of cyclonic rainfall of >200 mm was observed in those districts, which were located near to the landfall. As a result, these districts (i.e., Baleshwar, Bhadrak, Ken- drapara, Jajpur, and Jagatsinghpur) were more prone to floods and damages (Fig S2), wherein these districts also received surplus water from reservoirs/dams and river basins. The spatial distribution of cyclonic rainfall during Phailin 2013 showed that lesser magnitude of rainfall amount was received during this period, compared to that of Super–cyclone 1999 (Fig. 4b). The spread-out of rainfall due to Phailin 2013 was more profound than 1999 event, and effectively spread up with each other (Fig. 4). The results depict that the cyclonic rainfall spread throughout the coastal districts of Odisha state, and effectively covering the area up to > 200 km from the location of landfall ‘Paradip (Jagatsinghpur district) during Super–cyclone 1999 (Fig. 4a). The cyclonic rainfall of > 100 mm over these 3 days (29–31 October 1999) was observed along the coast line comprising several coastal districts of Odisha including the landfall location. Due to cyclonic activities, the maximum intensity of cyclonic rainfall of >200 mm was observed in those districts, which were located near to the landfall. As a result, these districts (i.e., Baleshwar, Bhadrak, Kendrapara, Jajpur, and Jagatsinghpur) were more prone to floods and damages (Fig S2), wherein these districts also received surplus water from reservoirs/dams and river basins. The spatial distribution of cyclonic rainfall during Phailin 2013 showed that lesser magnitude of rainfall amount was received during this period, compared to that of Super–cyclone 1999 (Fig. 4b). The spread-out of rainfall due to Phailin 2013 was more profound than 1999 event, and effectively spread up istrict) along the coast.
The cyclone track of 1999 Orissa Super Cyclone was made landfall near Paradeep coast (Fig.3a). The resultant different inundation depth (water level above ground) was estimated using the storm surge modelling depicts maximum surge inundation up to 4m (fig.3b). Inundation level of 3-4m was covered an area of 5.9 km2 in a patch near the coast was recorded and 2-3m inundation level covered 143.3 km2 were extended up to hinterlands in the vicinity of river and creeks. However, majority class was 1-2m inundation level covered 1,146.3 km2. There are few areas of 833.2 km2 slightly elevated areas were recorded up to 1m inundation. It was also observed there are some areas not inundated but surrounded by inundati0n were also in vulnerable as these areas will get stranded during an event of cyclone.
Totally the large area of 2128.7 km2 was depicted under inundation end extended up to 45 km hinterland from the coast (fig. 3b). Jagatsinghpur district is observed to be affected the most among two other districts, Kendrapara and Cuttack. The larger inundation was observed at the cyclone landfall area and the river systems act as corridors for inundation and carried inundation deep into hinterlands.
4.3 Storm surge modeling Results of Andhra Pradesh:
Andhra Pradesh coast experienced more than 62 cyclones including depression, cyclone surge, and severe cyclone surges in the past 40 years. Among these 32 cyclones were affected the Krishna–Godavari region, comprising four districts and around 7% of the area observed high vulnerability (Basheer et al., 2019). The cyclone track selected for the Andhra Pradesh coast was the Very Severe Cyclone of 1996 made landfall near Mummidivaram of East Godavari District (Fig.4a).
The storm surge simulation results of Very Severe Cyclone of 1996 hit Andhra coastreveal a maximum inundation depth of 2-3m covering up to an area 12 km2, 48.9 km2 area was experienced an inundaton depth of 1-2m and Majority of the area of 376.1 km2 is was inundated up to1m of water from the ground (Fig.4b). A total of 437.6 km2 of coastal area is inundated by storm surge and Mummidivaram is the most affected taluk followed by Kakinada and some parts of Ramachandrapuram (Fig4b). Even Razole and Amalapuram taluks in the south also recorded coastal inundation.
4.4 Storm surge modeling Results of West Bengal:
The synthetic track with hypothetical wind speed value of 155 knots was used to run a storm surge model for parts of West Bengal (Fig.5a). The maximum inundation depth due to storm surge depicted was 5.5 m and maximum extent was up to 145 km inland (Fig.5b) from the coast. Large inundation was observed in the West Bengal and Bangladesh in the Sunderban area (Fig.5b).
Inundation was carried deep inland up to 145 km due to the strong event and creek systems of Ganga-Brahmaputra. The large amplitude of surge heights was observed along the coasts of Bangladesh. Maximum area inundated in the eastern parts of south 24 Parganas district and southern parts of North 24 Parganas district (Fig. 5b). Maximum level of inundation depth of 3-4m was observed in the vicinity of creeks in West Bengal. The maximum area (5439.67 km2) was under one meters of inundation. Followed by 1-2m inundation recorded 2674.79 km2. Inundation more than 4m was recorded 25 km2 is constrained along the banks of the major creeks and small islands at the distal parts A total 9036.92 km2 area was inundated due to this storm surge event (Fig.5b).
The analysis of the historical cyclones revealed the Orissa and West Bengal states are highly vulnerable to cyclones with high count and frequency. The storm surge modeling was carried out at each state considering worst event in the past for Orissa and Andhra Pradesh. In case of West Bengal synthetic worst case was considered. The results of these storm surge simulations suggest the large inundation in the Orissa and West Bengal. A moderate inundation was recorded in the Andhra Pradesh coast. This may be due to the strength of the event and the large low-lying coastal areas exposed to the cyclones in the Orissa and West Bengal whereas the strength of the cyclone that occurred in Andhra Coast was not as great as the ones that occurred at West Bengal and Orissa coast. And also, the presence of low-lying large delta in West Bengal coast may have facilitated inundation extent and inundation levels. The large parts of coastal zones in West Bengal and Odisha are highly exposed to inundation due to low lying areas resulted in high vulnerability due to storm surges.