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Wave Characteristics during Severe Cyclonic Storm Jal, along east coast of India

Conference Paper · February 2012

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Sisir kumar Patra Basanta Kumar Jena Ocean Science & Surveying Pvt Ltd. National Institute of Ocean Technology

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The user has requested enhancement of the downloaded file. WAVE CHARACTERISTICS DURING SEVERE CYCLONIC STORM JAL, ALONG EAST

COAST OF INDIA

Sisir Kumar Patra* and B. K. Jena National Institute of Ocean Technology (NIOT), Coastal and Environmental Engineering Division, NIOT Campus, Pallikaranai, – 600100, Ph: 044 66783474, Fax: 044 66783336/3468. E-mail: [email protected]

Abstract

Wave characteristics during cyclone has great importance to life and property near to coasts, due to high wind speed, wind generated high wave height and storm surge resulted from severe flooding by cyclonic landfall. The cyclone Jal developed from a low pressure area in the on 28 October entered to on 4 November as a depression and became a severe cyclonic storm on 6 November, 2010. The peak of the storm was centered near on 7 November, with maximum sustained wind speed 30.87 m/s and wind gust 43.65 m/s, before the system made its landfall in the surrounding areas of Tamil Nadu and Andhra Pradesh.

During this cyclonic event the maximum and significant wave height (H max and H s) of 5.7 m and 2.7 m recorded from 12 km off Cuddalore; 6.3 m and 3.23 m from 10 km off Cheyyur by Datawell directional wave rider buoy deployed at 25 m water depth. Results of wave analysis by wave rider buoy shows that, the storm period was dominated by strong wind sea waves (75 %), compared to low intensity swells (25 %) arrived from East (E) and East South East (ESE) 2 directions. The maximum spectral energy density (S max ) of 10.2 m /Hz with Peak period (T p) 2 11.76 s and H s 2.16 m recorded from Cuddalore, and it was 18.2 m /Hz, 11.26 s and 2.95 m for Smax , T p and H s from Cheyyur on 7 November at 1200 hrs. Wave spectrums during peak of cyclone were single peaked with peak period varied from 6 to 11 seconds.

In this paper, the statistical analysis of wave characteristics measured from wave rider buoy measured at Cheyyur and spectral wave model results derived from MIKE 21 is presented. The simulated wave height reached maximum value of 7.3 and 3.9 m for H max and H s at 25 m depth off Ennore, the nearest point where cyclone passed before its landfall. The model simulated wave heights are in agreement with the observed buoy measured values. The storm induced surge by Jal cyclone and its severe flooding due to landfall killed 63 people, with loss of Rs. 2739.3 Crore property and more than 2 lakh houses in Andhra Pradesh and 2 lives in Tamil Nadu.

Key words: Jal, Wave characteristics, Sea and Swell, Mike 21 spectral wave model and east coast of India

1. Introduction

The evolution of tropical cyclone is a regular feature in pre-monsoon (May) and post- monsoon (October and November) over Bay of Bengal. The east coast of India experiences maximum number of low pressure systems. 5-6 tropical cyclones per year originating from Bay of Bengal and has increased the frequencies and intensities recently (Singh, 2001 and Mohapatra and Mohanty, 2004). The wave characteristics during cyclone along Indian coast and their assessment of storm disaster addressed by many authors in the recent past (Sanil et al., 2004, Rao et al., 2007, Black et al., 2008, Sanil et al., 2008, Mishra et al., 2011). Evaluation of wave characteristics during cyclone is essential to assess the risk of damages to off-shore structures or for Oceanographers and modelers to calculate trends in the past and future wave fields (Lionello et al., 2007). Tropical cyclones cause damage to properties near coasts due to high

152 ! wind speed and loss of life on account of severe flooding due to resulting storm surge (Satish Kumar et al., 2003). Storm generated wave, coupled with locally generated waves create complex characteristics in the near shore region. Keeping in view the importance of wave characteristics during tropical cyclone, the present study addresses the wave conditions of east coast of India, during passage of Jal super cyclone in November, 2010.

The wave climate of east coast of Indian varies from south-west monsoon (June to September) to the north-east monsoon (October and November) and fair weather (December to February) to summer monsoon (March to May) period (Chandramohan and Nayak, 1994). On a seasonal basis, the wave climate can be subdivided into four categories i.e., high energy waves occur during June, July, August and September; moderate energy are the months of October and November; Low energy are the months of March, April and May and very low energy occurs in December, January and February months ( Mishra, et al., 2011). Sea dominated double peaked wave spectra were seen during June to September because locally generated waves dominate the swells during the south-west monsoon period compared with other periods (Sanil Kumar et al., 2003). In general, the wave spectra are double or multi-peaked in Bay of Bengal (Aboobacker, et al., 2009), due to existence of wind seas along with ‘Young’ swells. The single peaked spectra are observed during cyclone events, wherein all the energies are concentrated in low frequency region.

Keeping in view the devastation of Jal cyclone along east coast of India, the present study attempts to understand the wave characteristics (sea, swell, wave height, period, energy and direction) from observed data and simulated model output.

2. Data and Methodology

2.1 Wave rider buoy Wave measurements were carried out using two Datawell directional wave rider buoy (Datawell, 2006) in 25 m of water depth, 12 km offshore at Cuddalore and 10 km offshore at Cheyyur, from a part of ongoing research project Technical Criteria Atlas (TCA) sponsored by the Ministry of Earth Sciences (MoES), Govt. of India. The data used here is from the period 4 to 10 November, 2010. The data were recorded for 30 minute duration at every one hour interval and sampled at a frequency of 1.28 Hz. In the present study the buoy data received from Cheyyur is only used for analysis and comparison.

The separation of wind sea and swell components from the wave spectra was done by wave steepness method of National Data Buoy Center (Gilhousen et al. 2001). Estimation of wind sea and swell are made by selecting a separation frequency (f s) that partitions the wave spectrum into its wind sea and swell parts and their components viz. significant wave height (H sw and H ss ), zero crossing period (T sw and T ss ), mean wave direction ( θsw and θss ).

2.2 Spectral wave model (Mike 21) Mike 21 Spectral wave model (DHI, 2007) is a new generation spectral wind wave model based on unstructured meshes. The model simulates the growth, decay and transformation of wind-genarated waves and swells in offshore and coastal areas. The model takes care of the effects of wave generation due to winds, refraction, shoaling due to varying depths and frictional resistances. It also includes the effect of interaction between waves with different frequencies.

For this model study, the reanalyzed NCEP winds available in the form of U and V velocity components for 2.5 ° × 2.5 ° grid spacing for every 6 hours interval is used for the period 4 to 10 November, 2010. The Etopo-1 bathymetry data (National Geophysical Data Centre, USA) is used for offshore with coarser resolution of 0.75 ° × 0.75 ° and MIKE CMAP of DHI is used for near shore region (upto 200 m water depth).

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The model domain is about 2775 × 2775 km with a maximum water depth of approximately 5002 m, extended from 0 ° S to 25 ° N and 86 ° E to 101 ° E. The boundary along 0 ° S latitude is considered as open boundary, and all other boundaries are closed. The model simulations are made by considering 162 time steps with an interval of 1 hour from the period 4 to 10 November, 2010.

3. Results and discussions

3.1 Wind Characteristics Severe cyclonic storm Jal is designated by BOB05 by JTWC (Joint typhoon warning center), is the fifth cyclonic storm of 2010, along north . The average atmospheric pressure during cyclone Jal was below normal (1004 hPa) with lowest low pressure of 988 hPa recorded on 6 November at 1200 hours and continued till 00 hours of 7 November (IMD, 2011). The cyclone crossed north Tamil Nadu and south Andhra Pradesh coast close to north Chennai near 13.3 ° N and 80.2 ° E around 1600 UTC. The peak of the storm was recorded near Chennai on 7 November before its landfall over Tamil Nadu and Andhra Pradesh. The maximum sustained wind speed and wind gust was 30.87 and 43.63 m/s respectively at 00 hours of 7 November.

3.2 Sea and Swell Characteristics The wind sea and swell components were separated from the measured data and variations of sea and swell significant wave height (H sw and H ss ), zero crossing period (T sw and Tss ), mean wave direction ( θsw and θss ) and percentage of sea and swell are plotted in the Fig 1(a-d). The minimum, maximum, average and standard deviation of wave parameters during 4 to 10 November, 2010 are given in the Table 1.

Table 1 - The minimum, maximum, average and standard deviation of wave parameters off Cheyyur during 4 to 10 November, 2010

Standard Parameter Minimum Maximum Average Deviation Significant wave height, H s (m) 0.63 3.23 1.21 0.57 Significant wave height of wind sea, Hsw (m) 0.42 1.82 0.89 0.34 Significant wave height of swell, Hss (m) 0.34 2.78 0.78 0.50 Maximum wave height, H max (m) 0.63 6.30 1.71 0.89 Zero Crossing period T z, (s) 3.54 7.67 5.06 0.98 Zero Crossing period of wind sea, T zw , (s) 2.86 5.59 4.02 0.69 Zero Crossing period of swell, T zs , (s) 9.01 11.12 10.12 0.47 Average wave period, T 02 (s) 3.94 7.96 5.69 0.98 Peak Wave period T p 3.85 14.29 9.72 3.28 Mean wave Direction, θ (Deg.) 18.30 344.50 123.25 37.69 Mean wave Direction of wind sea, θsw (Deg.) 20.26 356.07 96.59 46.54 Mean wave Direction of swell, θss (Deg.) 78.97 158.38 126.25 23.19 Percentage of wind sea (%) 47.93 93.76 75.54 9.63 Percentage of swell (%) 32.99 87.34 62.24 10.81

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Figure 1 (a-d) - Variations of (a) sea, swell and total significant wave height, (b) zero crossing periods, (c) mean wave direction and (d) percentage of sea and swell

The range of H ss and H sw is from 0.34 to 2.78 m and 0.42 to 1.82 respectively. The standard deviation of H sw is 0.34 which is less than that of H s and H ss indicating the range is narrow spread for Wind Sea. The range of T z is from 3.54 to 7.67s with average value of 5.06, mean T sw and T ss were 4.02 and 10.12 s respectively. The waves measured during cyclone Jal are predominantly sea waves from the local disturbances, combined with low intensity long swells from the East (E) and East-South-East (ESE) directions. The contribution of seas to the total wave height is 74.7 % and swell is 25.3 %. The Wind Sea and Swell were observed from the same directions, largely contributed in increasing the wave height. Regression analysis shows positive correlation (R = 0.97) between H s and H sw , compared to 0.92 between H s and H ss . The height of the swell shows constantly increasing trend during the cyclonic period and seas scattered due to complex sea state. The direction of swell wave is from SE to ESE and sea wave from E, except 7 November, when the system was nearer to coast.

3.3 Wave height, period, direction and energy Height of the waves during cyclone largely depends upon the intensity of wind force in generating the wave. During this cyclonic event the H max and H s of 6.3 and 3.23 m recorded from 10 km off Cheyyur; and it was 5.73 and 2.7 m from Cuddalore on 7 November at 1200 and 1500 hours respectively. The maximum minimum and average value of wave parameters are presented in Table 1. The present study shows that the observed H max is approximately 1.95 times H s with a correlation coefficient of 0.91, is higher than the study conducted by Sanil et al,

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(2004) for Vishakhapatnam coast (1.65) and Mishra et al, (2011) for Gopalpur coast (1.68) along east coast of India.

The variation of average wave period (T 02 ), zero crossing wave period (T z) and period corresponding to maximum spectral energy (T p) during the observation period is shown in the Fig 2(a). During cyclone the wave energy increases and hence wave height and average wave period, but peak period decreases (Mishra et al, 2011). The value of T 02 , T z and T p ranged between 3.94 to 7.96 s, 3.54 to 7.67 s, and 3.85 to14.29 s respectively (Table 1). In general, the normal sea state is characterised by average wave period and extreme sea state by peak wave period. In the present study, the peak wave period during the storm condition (5 to 9 November) was low (6-11 s) and average wave period was high. (Regression analysis between H s and T p are positively correlated with co-relation coefficient R = 0.39, compared to 0.33 between H s and T02 . The correlation values are 0.6 and 0.1 for T p and T 02 for Vishakhapatnam coast, when it was compared with H s (Sanil et al, 2004).

The variation of maximum spectral energy density (S max ) observed during 6 November 900 Hours to 7 November 2100 hours is presented in Fig. 2(b). The maximum value of S max 2 observed on 7 November, 2010 at 1200 hours is 18.2 m /Hz with T p 11.26 s and H s 2.95 m. Single peaked wave spectra are observed during 6-9 November, with peak period 6 to 11 s and it was double or multi peaked before and after the cyclonic event (T p = 11 to 15 s). The energy of the single spectra was concentrated in the frequency range of 0.07 to 0.14 Hz and wave direction 90 (easterly). The high energy wave spectra may be attributed, due to changes in wave conditions and cyclonic winds associated with low pressure system. Immediately, after passage of cyclone, the sea state became normal due to low wind and high pressure prevailed over there. Similar conclusion obtained during cyclone by Sanil et al (2004) and Mishra et al, (2011) for Vishakhapatnam and Gopalpur coast respectively. The waves became south-easterly (SE) direction after passage of cyclone (Fig. 3b) along Cheyyur coast.

Figure 2 (a-b) - The time series plot of (a) mean wave period, zero crossing period and peak period, and (b) the variation of maximum spectral energy density with frequency

3.4 Spectral wave model (MIKE 21) Simulations of wave characteristics for Jal severe cyclone along east coast of India was carried out using the third generation spectral wave model of MIKE 21. The most important wave parameters simulated are significant wave heights (H s), peak wave period (T p) and mean wave direction ( θ). The above wave characteristics at eight locations, viz. Cuddalore, Cheyyur, Ennore, Machilipatnam, Yanam, Vishakhapatnam, Gopalpur and Puri along east coast of India are extracted and presented in Table 2. 156 !

Table 2 - Model simulated minimum (m) and maximum (M) value of wave parameters along eight locations of east coast of India

Station Machilipa Vishakhap Cuddalore Cheyyur Ennore Yanam Gopalpur Puri Name tnam atnam Wave m M m M m M m M m M m M m M m M Parameter Hs (m) 0.1 2.9 0.2 3.3 0.2 3.9 0.2 3.2 0.3 3.2 0.4 3.1 0.3 2.1 0.2 1.7 Hmax (m) 0.2 5.4 0.3 6.2 0.4 7.3 0.4 6.1 0.5 5.5 0.8 6.0 0.7 4.1 0.4 3.2 Tp (s) 2.4 13.4 2.5 13.4 2.5 13.5 2.5 14 2.7 13 3.1 13.3 2.8 13.7 2.5 13.8 T02 (s) 2.0 11.8 2.1 11.1 2.1 11.7 2.1 9.7 2.3 11 2.6 9.8 2.4 10.7 2.1 11.1 Tz (s) 1.8 11.3 2.0 10.6 2.0 11.3 2.0 8.2 2.1 10.3 2.4 8.9 2.2 9.8 2.0 10.4 Tθ (Deg.). 52 225 50 225 47 228 45 141 45 155 45 155 45 161 45 176

The simulated wave height reached H max and H s of 7.3 and 3.9 m at 25 m depth off Ennore, the nearest point the where the cyclone passed before its landfall. The observed and modeled significant wave height, peak wave period, and mean wave direction of Cheyyur are shown in the Fig 3(a-c). The simulated wave height is in agreement with the observed buoy data off Cheyyur. The correlation coefficient between the measured and modeled significant wave height is 0.58. The correlation between measured and simulated peak wave period and mean wave direction are 0.32 and 0.28. The large discrepancy in the measured and modeled wave period and direction are mainly due to deficiency in spatial and temporal resolution of wind parameters as the local cyclonic wind effect is not incorporated in the simulation and winds used in the boundary have coarse resolution (2.5 ° × 2.5 °). However, the observed and simulated wave parameters are in close agreement during the peak of the storm (6 to 9 November). After immediate passage of cyclone, the observed wave height reduced to 1-1.5 m from 3 m, where as model result not reflected the same. The model results could be improved with fine spatial resolution of wind input data (Resolution of 0.25 ° × 0.25 ° and 1 hour data interval).

Figure 3(a-c) - The measured and modeled a) significant wave height, b) peak wave period, and c) mean wave direction of Cheyyur

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3.5 Impact Jal Cyclone Jal Cyclone produced substantial rainfall over many parts of Tamil Nadu and Andhra Pradesh and moderate rainfall in other parts of India. Most of damages of life and properly have been reported from the coastal districts of these Tamil Nadu and Andhra Pradesh, due to severe flooding and increase in surge level near to coasts. The cyclonic storm killed 63 people, with loss of INR 2739.3 Crore property and more than 2 lakh houses in Andhra Pradesh (www.disastermanagement.ap.gov.in) and 2 lives in Tamil Nadu. The storm also produced severe flooding, severity of damage and losses of life (63) in , Srilanka and due to heavy rainfall.

Acknowledgement

The present study is a part of the ongoing research project ‘Technical Criteria Atlas (TCA)’ sponsored by the Ministry of Earth Sciences (MoES), Govt. of India. We are grateful to Dr Sailesh Nayak, Secretary, MoES, Govt. of India, New Delhi and Dr M.A. Atmanand, Director and Dr K.M. Sivakholundu, Project Director, CEE Division, National Institute of Ocean Technology, Chennai for their guidance, encouragement and necessary funding support.

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