P.C.Sinha, P.Chittibabu and S.K.Dube
Centre for Atmospheric SciencesIndian Institute of Technology
New Delhi –110 016, India
Tel:91-11-6591319, Fax: 91-11-6862037, Email: pcsinha@cas.iitd.ernet.in
Abstract: Storm surges associated with severe tropical cyclonic storms are
of common occurrences along the east and west coast of India. The coastal states
of Orissa, Andhra Pradesh and Gujarat have experienced major surges in the past.
The recent 1999 Paradip super cyclone and 1998 Kandla cyclone are severe most
cyclones that caused extensive damage to the property and loss of lives. Extreme
storm surges are major causes for coastal flooding in these regions. Damages can be minimized if the surges are
forecasted well in advance. The accuracy of the forecast depends upon the grid
resolution and offshore topography of the region of interest. Keeping this in
view we developed fine resolution location specific models for three coastal
states of India which are vulnerable to storm surges and associated inundation.
An attempt has been made to simulate extreme sea levels along the Andhra, Orissa
and Gujarat coasts using the data of past severe cyclones. The model results
reported in the present study are in good agreement with available
observations/estimates.
Keywords: storm surge, numerical model, tropical cyclone, indian coast
Coastal flooding associated with storm surges is a serious concern along 7500 km of Indian coastline (Figure 1). About 60 % of all deaths due to storm surges have occurred in the low-lying arable coastal areas of the countries bordering Bay of Bengal and the Arabian Sea (Murty, 1986; Dube et al., 1997). The frequency of cyclones and severe cyclones in the Bay of Bengal and the Arabian Sea during the 109-year period from 1891 to 1999 is shown in Figures 2a and 2b. The coastal districts of Andhra Pradesh, Orissa and Gujarat (Figure 1) have experienced major surges in the past. The recent 1996 Kakinada cyclone, 1998 Gujarat cyclone and 1999 Orissa super cyclone have caused extensive damage to life and property.
Using nomograms of storm surges Rao (1968) classified the Indian coasts into three types of vulnerability zones to storm surges (Figure 3). For type A coastline, the maximum total water level is less than or equal to 2 m during storm surge events, for type B the amplitude is between 2m and 5m, and for type C the amplitude is greater than 5 m. It can be seen from the figure that certain coastal regions of Andhra Pradesh, Orissa and Gujarat experienced surges greater than 5m.
The steady increase of population along the
coastal belt continuously increases the potential of the disaster. Even though
few cyclones form over the north Indian ocean the destruction potential is very
high because of the presence of shallow basins in this region. Keeping the above factors in view location specific
storm surge models are developed for Andhra , Orissa and Gujarat coasts. The
detailed mathematical description of the models are given in Chittibabu (1999).
Fig. 1 Map showing India and three costal regions of interest
Fig. 2a Landfall of cyclones on the east coast of India (1891-1999)
Fig. 2b Landfall of cyclones on the
west coast of India (1891-1999)
Fig. 3 Vulnerability of coastal regions to storm surges
The model is fully non‑linear and is forced by a dynamic storm model of Jelesnianski and Taylor (1973). The treatment of the coastal boundaries in the model involve a procedure leading to a realistic curvilinear representation for Andhra and Orissa coast models. This coastal representation has added advantage of taking automatically into account the finer resolutions in the shallow regions. This has been achieved by using a variable grid, which leads to a substantial refinement of resolution near the coastline and a coarser resolution in the deeper waters. To represent the two gulfs of Gujarat coast more realistically the coastline boundaries are represented by orthogonal stair steps.
The Andhra coast model covers an
analysis area from 12.2 N to 18.2 N and 79.8 E to 83.7 E (Figure 4). The
computational domain is divided into a rectangular mesh of 49×20 grid-points
having a grid distance of 2 to 15 km along the x-axis and 12.8 km along the
y-axis. Using this model the storm surge associated with 1977 Andhra cyclone is
simulated. The track of the cyclone is also shown in Figure 4. Numerical
experiments are performed with 80 hPa pressure drop and 40 km radius of maximum
winds.
The peak surge contours associated with this cyclone are shown in Figure 4. It can be seen that a maximum
surge of about 5m occurred close to the landfall point near Divi which agrees well with the post - storm survey
report. The coastal region between Ongole and Kakinada is inundated with a surge
more than 2 m.
Fig. 4 Peak surge contours(m) for 1977 Andhra cyclone
The analysis region extends from 18.25 N to 22 N and 84 E to 90 E ( Figure 5). The model is integrated with a pressure drop of 98 hPa and 40 km radius of maximum winds. Figure 5 shows the peak surge contours. The model predicted a maximum surge of 7.8m close to the landfall point. The coastal region between Konark and Chandbali is affected by a surge of more than 5m. Post- storm survey reports also show that the surge is more than 7-8 m, near Paradip.
Fig.
5 Peak surge contours (m) for 1999 Orissa Cyclone
The analysis
region of this model extends from 19.2 N to 23.2 N and 67.8 E to 73.0 E (Figure
6). The grid distance is 5km along both x and y directions. Using this model the
surge generated by 1998 Kandla cyclone is computed with a pressure drop of 40
hPa and 30 km radius of maximum winds. The model is integrated ahead in time
upto 48 hours. Figure 6 shows the model computed peak surge contours and a
maximum surge of about 5 m is predicted to the south of the landfall point. It
may also be seen that at Porbandar and Okha the computed surges are about 3.5 m
and 2 m, respectively. The maximum computed surge at Kandla was 2.1m, which is
in agreement with the reported surge (IMD Report, Feb. 1999).
Fig. 6 Peak surge contours (m) for 1998 Kandla Cyclone
Fine resolution storm surge models have been described for Andhra, Orissa and Gujarat coasts of India. The model results reported in this study are in good agreement with the available peak surge observations/estimates. The results emphasise the suitability of a fine resolution location specific model for a reasonable prediction of surges along the above-mentioned coastal states.
References
Chittibabu. P.
1999. Development of storm surge prediction models for
the bay of Bengal and the Arabia Sea. Ph.D Thesis. Indian Institute of
Technology, Delhi, India. 263 pp.
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Rao, P.C. Sinha ,T.S.Murty and N. Bahulayan. 1997. Storm surge in the Bay of Bengal and
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and A.D. Taylor. 1973. NOAA Technical Memorandum, ERL, WMPO‑3, 33 pp.
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