Water and Environment Journal. Print ISSN 1747-6585

Cyclone hazards in the –A numerical modelling case study of Nilofar Mohammad Akhtaruzzaman Sarker

Royal HaskoningDHV, Rightwell House, Bretton, Peterborough, PE3 8DW, UK

Keywords Abstract numerical modelling; natural hazards; cyclone; extreme waves; ; port develop- cause significant loss of life and damage to properties, ecosystems and ment; Arabian Sea. marine facilities. To address such issues, Royal HaskoningDHV (RHDHV) has devel- oped regional tidal hydrodynamic and wave models covering the Northern Arabian Correspondence Sea. A total of 29 major cyclones were identified in the Arabian Sea since 1945. Dr Mohammad Akhtaruzzaman Sarker, However, as less information is available on (2014), this paper has Principal Engineer; research fields: numerical modelling of coastal processes including concentrated on this event to illustrate the use of numerical modelling to simulate waves, tides, sediment transport, cyclones, waves and surge generated by cyclones. Sample results from the modelling study tsunamis, sea ice and water quality (dredge are presented in this paper. The methodology described in this paper for modelling plume, outfall discharge and oil spills). cyclone waves and surges in the Arabian Sea could be applied to simulate such nat- Email: [email protected] ural hazards at other sites around the world. doi:10.1111/wej.12214

Introduction especially vulnerable to tropical cyclones with around Tropical cyclones are associated with high-pressure gra- 718,000 deaths from them in the past 50 years (Haque et al., dients and consequent strong winds and storm surges. Very 2012). The deadliest was the 1970 Bhola strong winds may damage installations, dwellings, transpor- Cyclone, which had a death toll of anywhere from 300,000 to tation and communication systems, trees etc. and cause 500,000 lives (https://en.wikipedia.org/wiki/Effects_of_tropi- fires resulting in considerable loss of life and damage to cal_cyclones). property. Destruction of transportation or communications Despite their devastating effects, tropical cyclones are infrastructure hampers clean-up and rescue efforts. Heavy essential features of the Earth’s atmosphere as they bring and prolonged rains due to cyclones may cause floods and rain to dry areas and transfer heat and energy between the submergence of low lying areas and can lead to mudslides equator and the cooler regions nearer the poles. and landslides in mountainous areas causing loss of life and A large tidal hydrodynamic model is required to simulate property. Floods, standing water and coastal inundation due cyclone surge on a region whereas a large wave model is to storm surges pollute drinking water sources and spread required to simulate cyclone waves. Given the above risks, diseases leading to outbreak of epidemics. RHDHV has developed regional tidal hydrodynamic and Cyclones also impose significant risks during construction wave models covering the Northern Arabian Sea to investi- and operation of sea ports, oil terminals & jetties, offshore gate the natural hazards and to support their project work in exploratory drilling rigs and offshore oil extraction rigs. They the region. The models have been used to assess cyclones put lives and properties in coastal areas at greater risks and within this region. cause significant loss of ecosystems and marine facilities. Literature search on cyclones in the Arabian Sea was car- The destruction from a tropical cyclone depends on its inten- ried out and a total of 29 major cyclones were identified sity, its size, and its location. since 1945. However, the present study was focused to the During the last two centuries, tropical cyclones have been recent cyclone “Cyclone Nilofar” that occurred in October responsible for the deaths of about 1.9 million people 2014. Cyclone waves and surge were modelled. Sample worldwide (https://en.wikipedia.org/wiki/Effects_of_tropical_ results from these modelling studies are presented in this cyclones). It is estimated that 10,000 people per year perish paper for illustration purposes only. due to tropical cyclones (https://en.wikipedia.org/wiki/ The methodology described in this paper for modelling Effects_of_tropical_cyclones). For example, Bangladesh is cyclone waves and surges in the Arabian Sea could be

Water and Environment Journal (2016) VC 2016 CIWEM. 1 Cyclone hazards in the Arabian sea M. A. Sarker1

Fig. 1. Cumulative track map of Tropical Cyclones in the Arabian Sea from 1970 to 2005 [https:// upload.wikimedia.org/wikipedia/commons/c/c0/ North_Indian_cyclone_tracks.jpg]. [Colour figure can be viewed at wileyonlinelibrary.com] applied to simulate such natural hazards at other sites A total of 29 cyclones were initially identified to have around the world. crossed the Arabian Sea since 1945 and affected the Omani coastline. The events were selected based on a combination of severity (wind speed, central pressure and diameter) and the Cyclones in the Arabian sea proximity to the site of interest. Tracks and pressure fields of the selected cyclones were obtained from the Joint Typhoon As reported in [https://en.wikipedia.org/wiki/North_Indian_ Warning Center (JTWC), USA (The Joint Typhoon Warning Cen- Ocean_tropical_cyclone, the Arabian Sea is located in the ter (JTWC), the U.S. Department of Defence Agency). Data of north-west of the Indian Ocean. Tropical cyclones in the these 29 selected cyclones are listed in Table 2. basin are abbreviated ARB by the India Meteorological In particular, the passage of (in 2007), Department (IMD), the official Regional Specialized Meteoro- (in 2010) and Cyclone Nilofar (in 2014) has logical Centre (RSMC) of the basin. The Arabian Sea’s coast is raised awareness of the risk of cyclonic events. Cyclones shared among India, Yemen, , , Pakistan, Sri Lanka, Gonu and Phet had a significant effect on the Omani coast- Maldives and Somalia. are characteristic of the line and their impact is well documented. As less information Arabian Sea and responsible for the yearly cycling of its is available on Cyclone Nilofar, this paper has concentrated waters. In summer, strong winds blow from the south-west on this event to illustrate the use of numerical modelling to to the north-east, bringing rain to the Indian subcontinent. simulate waves and surge generated by cyclones. During the winter, the winds are milder and blow in the opposite direction, from the north-east to the south-west. Cyclones occur frequently in the Arabian Sea and can affect Cyclone Nilofar (2014) the Northern Arabian Sea Region. These events usually occur during the transition periods of the monsoons which are Formation of Cyclone Nilofar between May and June and between October and November. Cyclone Nilofar was classified as an extremely Severe Cumulative track map of Tropical Cyclones in the Arabian Sea Cyclonic Storm and was the strongest tropical cyclone of from 1970 to 2005 is illustrated in Fig. 1 (https://upload.wikime- 2014 within the North Indian Ocean and the strongest storm dia.org/wikipedia/commons/c/c0/North_Indian_cyclone_tracks. to form over the Arabian Sea since Cyclone Phet in 2010. jpg). The Saffir-Simpson Scale classifying depression, tropical Nilofar originated from a low pressure area in the Arabian storm and cyclone is given in Table 1. Sea that intensified into a depression on 25 October 2014. It

2 Water and Environment Journal (2016) VC 2016 CIWEM. M. A. Sarker1 Cyclone hazards in the Arabian sea

Table 1 Saffir-Simpson cyclone classification 1-min peak wind 1-min peak wind 1-min peak wind Storm type Category Pressure (hPa) speed (knots) speed (mph) speed (km/h) Depression TD - < 34 <39 < 63 Tropical Storm TS - 34 – 63 39 – 73 63 – 118 Hurricane 1 > 980 64 – 82 74 – 95 119 – 153 Hurricane 2 965 – 980 83 – 95 96 – 110 154 – 177 Hurricane 3 945 – 965 96 – 113 111 – 130 178 – 210 Hurricane 4 920 – 945 114 – 135 131 - 155 211 - 250 Hurricane 5 < 920 > 135 > 155 > 250

slowly consolidated and reached cyclonic storm strength Track of Cyclone Nilofar the following day. The system rapidly intensified in the fol- The track (route) of Cyclone Nilofar was obtained from JTWC and lowing days, reaching a peak intensity of 950 mbar (28.05 is shown in Fig. 2 (The Joint Typhoon Warning Center (JTWC), the inHg) on 28 October 2014. Over time the storm tracked U.S. Department of Defence Agency). The JTWC archived cyclone northeastwards towards an area of high vertical wind shear, data also contains 6 hourly information including date and time, causing the storm to rapidly weaken. The name Nilofar, tracks (path), maximum sustained wind speeds, radius of maxi- referring to the water lily, was suggested by Pakistan mum sustained wind speeds and the minimum central pres- (https://en.wikipedia.org/wiki/Cyclone_Nilofar). sures. Such data of Cyclone Nilofar is provided in Table 3.

Table 2 Major cyclones in the Arabian Sea during 1945-2014 (The Joint Typhoon Warning Center (JTWC), the U.S. Department of Defence Agency) Time & Date Max sustained Codes & Distance from Ras wind speeds Minimum central Radius of max No. Year Names Markaz (miles) Start End (knots) pressure (mb) winds (nm) 1 1959 01 Unknown 18 May 18:00 24 May 00:00 Unknown Unknown Unknown 2 1962 01 Unknown 27 May 18:00 30 May 00:00 Unknown Unknown Unknown 3 1963 02 Unknown 17 May 18:00 26 May 12:00 Unknown Unknown Unknown 4 1966 13 Unknown 31 Oct 18:00 11 Nov 12:00 Unknown Unknown Unknown 5 1970 01 Unknown 28 May 06:00 02 Jun 12 :00 Unknown Unknown Unknown 6 1970 11 Unknown 10 Oct 18:00 13 Oct 00:00 Unknown Unknown Unknown 7 1971 19 Unknown 14 Dec 06:00 21 Dec 00:00 Unknown Unknown Unknown 8 1972 02 330 25 Jun 06:00 27 Jun 00:00 Unknown Unknown Unknown 9 1972 03 280 01 Jul 06:00 02 Jul 00:00 Unknown Unknown Unknown 10 1976 06 265 27 Aug 00:00 09 Sep 00:00 Unknown Unknown Unknown 11 1977 TC 02A 140 09 Jun 00:00 13 Jun 00:00 60 Unknown Unknown 12 1977 TC 04B 240 27 Oct 00:00 04 Nov 12:00 40 Unknown Unknown 13 1978 TC 03A 290 03 Nov 00:00 13 Nov 00:00 80 Unknown Unknown 14 1979 TC 02A 25 16 Jun 00:00 20 Jun 00:00 50 Unknown Unknown 15 1979 TC 04A 30 16 Sep 00:00 25 Sep 00:00 55 Unknown Unknown 16 1983 TC 01A 90 09 Aug 00:00 10 Aug 12:00 45 Unknown Unknown 17 1987 TC 03A 280 04 Jun 06:00 12 Jun 00:00 50 Unknown Unknown 18 1992 TC 06A 55 29 Sep 00:00 04 Oct 12:00 55 Unknown Unknown 19 1994 TC 03A 90 05 Jun 12:00 09 Jun 18:00 45 Unknown Unknown 20 1995 TC 02A 120 11 Oct 00:00 18 Oct 12:00 50 Unknown Unknown 21 1996 TC 02A 15 09 Jun 00:00 12 Jun 12:00 40 Unknown Unknown 22 1998 TC 08A 40 11 Dec 18:00 17 Dec 18:00 65 Unknown Unknown 23 2001 TC 02A 215 24 Sep 00:00 28 Sep 12:00 35 997 30. 55 24 2007 TC 02A (Gonu) 270 31 May 06:00 08 Jun 00:00 145 898 10 25 2010 TC 03A (Phet) 120 30 May 12:00 07 Jun 06:00 125 929 15 26 2011 TC 03A 215 01 Nov 00:00 05 Nov 06:00 55 982 30 27 2011 TC 04A 205 07 Nov 12:00 11 Nov 12:00 35 996 25, 35, 40 28 2011 TC 05A 295 25 Nov 12:00 01 Dec 06:00 35 996 40 29 2014 TC 04A (Nilofar) 280 23 Oct 12:00 01 Nov 12:00 115 937 10

Water and Environment Journal (2016) VC 2016 CIWEM. 3 Cyclone hazards in the Arabian sea M. A. Sarker1

Fig. 2. Observed Track of Cyclone Nilofar, 2014 (The Joint Typhoon Warning Center (JTWC), the U.S. Department of Defence Agency). [Colour figure can be viewed at wileyonlinelibrary.com]

Wind and pressure fields of Cyclone Nilofar dimensional shallow water incompressible Reynolds aver- aged Navier-Stokes equations invoking the assumptions of The MIKE21 Cyclone Wind Generation Tool of DHI (DHI 2016a) Boussinesq and of hydrostatic pressure. Thus, the model was used to generate the cyclonic wind and pressure fields. consists of continuity, momentum, temperature, salinity and The tool allows users to compute wind and pressure data due density equations. to tropical cyclone (hurricane or typhoon). Several cyclone The regional model covers the coastlines of six countries parametric models are included in the tool such as Young and i.e. Yemen, Oman, UAE, Iran, Pakistan and India (see Fig. 4). Sobey model (1981), Holland – single vortex model (1981), The model has two open boundaries – one to the south and Holland – double vortex model (1980) and Rankine vortex the other to the north-west. The model was set up in such a model (DHI 2016a). All the six input parameters required by way that with a finer local mesh and more detailed bathyme- the Young and Sobey model (i.e. time, track, radius of maxi- try and land boundary data within a specified area, localized mum wind speed, maximum wind speed, central pressure water movement can be correctly modelled at a point of and neutral pressure) were available for the study and this interest without the need of introducing nested models. was, therefore, used to generate the cyclonic wind and pres- With this unstructured flexible mesh, it is easy to refine the sure fields. The other models require some additional parame- mesh in an area of interest. ters (such as Holland parameter B and Rankine parameter X) For the present study, the regional model was modified that need to be calculated using empirical relationships which by providing a high mesh resolution within the shallow water add further uncertainty to the generated wind and pressure areas and at the study site where changes in physical proc- fields and were, therefore, not used for the present study. Fig. esses take place quickly within short distances. The model 3 shows an example of wind and pressure fields of Cyclone bathymetry is shown in Fig. 4 was obtained from the C-Map Nilofar. These wind and pressure fields were used to drive the Global Database (C-Map JEPPESEN Commercial Marine, cyclone wave and surge models described later. 2014). The model was driven by tide levels at these two Arabian sea regional models developed boundaries obtained from the Global Tidal Model Database available within the MIKE21 Toolbox (DHI 2016a). by RHDHV The model can be used in its own right to simulate tidal movements and surges within the Northern Arabian Sea as The regional tidal model well as “building block” to drive a wide range of other mod- RHDHV has developed a two-dimensional Regional Tidal els such as cyclone, tsunami, oil spill, water quality, sediment Hydrodynamic Model for the Northern Arabian Sea using the transport and morphological models. The regional tidal MIKE21/3 Flow Model FM software of DHI (DHI 2016b). The model was used to drive the cyclone surge model to assess model is based on the numerical solution of the two/three- cyclone surge within the region.

4 Water and Environment Journal (2016) VC 2016 CIWEM. M. A. Sarker1 Cyclone hazards in the Arabian sea

Table 3 Cyclone Nilofar data (The Joint Typhoon Warning Center (JTWC), the U.S. Department of Defence Agency) Date and Time Latitude (8N) Longitude (8E) Max wind speed (knots) Central pressure (hPa) Radius (nm) Category 23/10/2014 12:00 10.8 62.1 15 1010 60 Tropical Depression 23/10/2014 18:00 11.1 61.8 15 1010 60 Tropical Depression 24/10/2014 00:00 11.3 61.6 20 1007 60 Tropical Depression 24/10/2014 06:00 11.6 61.5 20 1007 50 Tropical Depression 24/10/2014 12:00 11.8 61.7 20 1007 60 Tropical Depression 24/10/2014 18:00 12.0 62.0 25 1004 60 Tropical Depression 25/10/2014 00:00 12.3 62.3 30 1000 60 Tropical Depression 25/10/2014 06:00 12.9 62.4 30 1000 80 Tropical Depression 25/10/2014 12:00 13.5 62.8 35 996 60 Tropical Storm 25/10/2014 18:00 13.9 63.0 35 996 60 Tropical Storm 26/10/2014 00:00 14.1 63.0 40 993 60 Tropical Storm 26/10/2014 06:00 14.2 63.0 45 989 40 Tropical Storm 26/10/2014 12:00 14.3 63.0 55 982 15 Tropical Storm 26/10/2014 18:00 14.5 62.9 55 982 20 Tropical Storm 27/10/2014 00:00 14.7 62.8 65 974 20 Cyclone 1 27/10/2014 06:00 14.8 62.5 70 970 10 Cyclone 1 27/10/2014 12:00 15.0 62.3 80 963 10 Cyclone 1 27/10/2014 18:00 15.1 62.1 90 956 10 Cyclone 2 28/10/2014 00:00 15.5 61.7 95 952 10 Cyclone 2 28/10/2014 06:00 16.0 61.7 100 948 10 Cyclone 3 28/10/2014 12:00 16.8 61.8 115 937 10 Cyclone 4 28/10/2014 18:00 17.7 61.6 115 937 10 Cyclone 4 29/10/2014 00:00 18.0 61.6 105 944 7 Cyclone 3 29/10/2014 06:00 18.5 61.8 95 952 10 Cyclone 2 29/10/2014 12:00 18.8 62.2 80 963 20 Cyclone 1 29/10/2014 18:00 19.1 62.9 70 970 25 Cyclone 1 30/10/2014 00:00 19.6 63.5 60 978 30 Tropical Storm 30/10/2014 06:00 19.9 64.0 50 985 40 Tropical Storm 30/10/2014 12:00 20.0 64.5 45 989 40 Tropical Storm 30/10/2014 18:00 20.4 64.8 35 996 40 Tropical Storm 31/10/2014 00:00 20.7 65.0 30 1000 40 Tropical Depression 31/10/2014 06:00 21.0 65.1 25 1004 50 Tropical Depression 31/10/2014 12:00 21.4 65.0 25 1004 60 Tropical Depression 31/10/2014 18:00 21.7 64.7 20 1007 60 Tropical Depression 01/11/2014 00:00 21.7 64.2 20 1007 60 Tropical Depression 01/11/2014 06:00 21.7 63.8 20 1007 60 Tropical Depression 01/11/2014 12:00 21.7 63.3 15 1010 60 Tropical Depression

The regional wave model above. The regional wave model was used to drive the cyclone wave model to assess cyclone wave conditions RHDHV has also developed a two-dimensional Regional within the region. Wave Model for the Northern Arabian Sea using the MIKE21 Spectral Wave (SW) software of DHI (DHI 2016c). The model considers various physical phenomena, for example, wave Cyclone Nilofar wave modelling growth by action of wind, non-linear wave-wave interaction, The model dissipation due to white-capping, dissipation due to bottom friction, dissipation due to depth-induced wave breaking, The regional wave model developed by RHDHV based on wave diffraction, wave refraction, wave shoaling and wave- the MIKE21 Spectral Wave (SW) Model was used to simu- current interaction. The fully spectral formulation of the late the cyclone waves. The model was used to simulate model is based on the wave action conservation equation, the generation and propagation of cyclone waves. Fully where the directional-frequency wave action spectrum is the spectral formulation was used with in-stationary time for- dependent variable. mulation. The higher order numerical scheme was used in The model extent, mesh system and bathymetry are the the study to improve accuracy in model results. Wave dif- same as the regional tidal hydrodynamic model described fraction, wave breaking, bottom friction and white

Water and Environment Journal (2016) VC 2016 CIWEM. 5 Cyclone hazards in the Arabian sea M. A. Sarker1

Fig. 3. Wind and pressure fields of Cyclone Nilofar. [Colour figure can be viewed at wileyonlinelibrary.com]

capping were included in the model simulations. Quadru- Methodology plet wave interaction was also included in the simulations. JONSWAP fetch growth empirical spectral formulation was The cyclone wave model was driven by wind and pressure used. fields as shown in Fig. 3. A constant water level of 12.6mCD

6 Water and Environment Journal (2016) VC 2016 CIWEM. M. A. Sarker1 Cyclone hazards in the Arabian sea

Fig. 4. Model extent and bathymetry. [Colour figure can be viewed at wileyonlinelibrary.com]

Table 4 Comparison of cyclone results for Cyclone Gonu (2007) Maximum significant wave heights

World meteorological Oman meteorological Locations organization office Present study Measurement point AW2 at 4.2 m - 4.5 m in Iran at 30m depth 8 m - 9 m Arabian Sea >11 m 6-12 m Up to 15 m

(5 2.1m MHHW 1 0.5m surge) was used. The model simula- Aviation Affairs, General Directorate of Meteorological and tions covered the entire passage of the Cyclone Nilofar Air Navigation, Department of Forecasting and Monitoring, across the Arabian Sea. Sultanate of Oman) and the World Meteorological Organiza- tion (WMO) (World Meteorological Organisation 2009). Model validation Table 4 compares the model results with those from other sources. The modelled wave heights at Chabahar and in the Formal observed wave data on Cyclone Nilofar was not avail- Gulf of Oman compared well with those reported by WMO able to validate the wave model. However, limited wave (World Meteorological Organisation 2009). However, the data was available on Cyclone Gonu (2007) and Cyclone Phet modelled wave heights in the Arabian Sea are higher than (2010). Therefore, the model validation was focused primar- those reported by WMO (World Meteorological Organisation ily to these two cyclones. Wave heights, periods and direc- 2009). Similarly, the modelled wave heights in the Arabian tions were extracted from model results at selected Sea are higher than those reported by the Omani Meteoro- locations and were compared to those obtained from vari- logical Office (Ministry of Transport and Communications, ous sources to validate the model and thereby to improve Civil Aviation Affairs, General Directorate of Meteorological confidence in model prediction. The model validation was and Air Navigation, Department of Forecasting and Monitor- also described by Sarker and Sleigh (Sarker & Sleigh 2015). ing, Sultanate of Oman), however, the range of significant wave heights reported by the Omani Meteorological Office Comparison for Cyclone Gonu (2007) (Ministry of Transport and Communications, Civil Aviation Most of the available wave information was for Cyclone Affairs, General Directorate of Meteorological and Air Navi- Gonu (2007) and, therefore, the model validation was gation, Department of Forecasting and Monitoring, Sultan- focused primarily to this cyclone. Table 4 compares the ate of Oman) is rather wide (6 – 12 m). Maximum significant model prediction with those from the Oman Meteorological wave heights of over 11m in the Arabian Sea were reported Office (Ministry of Transport and Communications, Civil by WMO (World Meteorological Organisation 2009). On the

Water and Environment Journal (2016) VC 2016 CIWEM. 7 Cyclone hazards in the Arabian sea M. A. Sarker1 other hand, maximum wave height of up to 15m was found Communications, Civil Aviation Affairs, General Directorate in the present study. It should be noted that wave conditions of Meteorological and Air Navigation, Department of Fore- reported by WMO (World Meteorological Organisation 2009) casting and Monitoring, Sultanate of Oman). at Chabahar were measured using ADCP. Although there are some differences in wave heights Model results and discussions reported by other organisations, it is concluded that there is The maximum significant wave height of approximately reasonable agreement in the pattern and magnitude of 12.8 m (with associated peak wave period of 13.3 s) was found waves (particularly in the coastal zone). at a location of 61.788E, 17.98N on 28 October 2014 18:00:00. The two-dimensional distribution of wave height contours Comparison for Cyclone Phet (2010) superimposed by wave directional vectors is shown in Fig. 5 Some information was also obtained on Cyclone Phet (2010) for this time-step. The figure indicates that the maximum wave from literature search. Table 5 compares the model predic- height was found in the middle of the Arabian Sea. The tempo- tion with those from the Oman Meteorological Office (Minis- ral variation in significant wave height and peak wave period try of Transport and Communications, Civil Aviation Affairs, at this location is shown in Fig. 6. The figure indicates that sig- General Directorate of Meteorological and Air Navigation, nificant wave heights higher than 8m were sustained only for Department of Forecasting and Monitoring, Sultanate of duration of about half a day. Oman). Statistical analyses of model results were carried out The modelled wave heights in the Gulf of Oman are similar using the MIKE21 Tool to derive mean and maximum wave to those reported by the Omani Meteorological Office (Minis- conditions over the whole model domain during the entire try of Transport and Communications, Civil Aviation Affairs, duration of Cyclone Nilofar. Fig. 7 shows the maximum sig- General Directorate of Meteorological and Air Navigation, nificant wave heights over the whole model domain during Department of Forecasting and Monitoring, Sultanate of the entire duration of the cyclone. This figure also shows Oman). However, in the Arabian Sea the modelled wave some selected points (points P1 to P27) along the cyclone heights are significantly higher than those reported by the track where model results were extracted. Maximum signifi- Omani Meteorological Office (Ministry of Transport and cant wave heights along the cyclone track during the entire duration of the cyclone were provided in Fig. 8. Figs. 7 and 8 Table 5 Comparison of cyclone results for Cyclone Phet (2010) indicate that the maximum significant wave height was Maximum significant wave heights found in the centre of the Arabian Sea and that in contrast to Cyclones Gonu and Phet the height of waves reaching the Locations Oman meteorological office Present study Omani coast was limited. The maximum significant wave Gulf of Oman 4 m 4 m heights along the cyclone track shown in Fig. 8 were repro- Arabian Sea 7 to 8 m 13 m duced in Fig. 9 to provide clearer comparison of wave heights at various points along the cyclone track. The

Fig. 5. Highest significant wave height and its location during Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com]

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Fig. 6. Time-series of significant wave heights at 61.788E, 17.98Nduringtheentire duration of Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com]

Fig. 7. Maximum significant wave heights over the model domain during the entire duration of Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com]

Fig. 8. Maximum significant wave heights along the track during the entire duration of Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com]

Water and Environment Journal (2016) VC 2016 CIWEM. 9 Cyclone hazards in the Arabian sea M. A. Sarker1

Fig. 9. Maximum significant wave heights along the cyclone track during the entire duration of Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com] horizontal axis in this figure shows the selected locations south and north-west. An initial water level of 12.6 mCD was along the track (a total of 27 points as illustrated in Fig. 7) maintained over the entire model domain. and the vertical axis shows the maximum significant wave heights at these points. The line in this figure shows the max- Model validation imum significant wave heights along the cyclone track and Limited data on storm surge along the coastlines was avail- the points in this figure show the maximum significant wave able to carry out model validation. The following quantitative heights (in metre) at selected 27 points along the cyclone data has been extracted on storm surges from literature track. search and previous project work carried out by RHDHV in the region: Cyclone Nilofar surge modelling a) The 1 in 100 year storm surge as 0.2m for a project site A storm surge is an abnormal rise of sea level near the coast south of the Duqm Port in Oman caused by a severe tropical cyclone. As a result sea water b) The surge at Fujairah in Oman did not exceed 0.5m during inundates low lying areas of coastal regions drowning the Cyclone Gonu (2007) human beings and livestock, eroding beaches and embank- c) Limited qualitative information was found from a litera- ments, destroying vegetation and reducing soil fertility. ture search but suggests that the storm surges particu- larly along the southern Omani coastline are not large. The model predicted maximum surge for Cyclone Phet The model (2010) as 0.30m. It was, therefore, concluded that the surge The regional tidal hydrodynamic model developed by model should provide a reasonable prediction of storm RHDHV based on the MIKE21/3 Flow Model FM was used to surge for Cyclone Nilofar. The model validation was also simulate the cyclone surge. The higher order numerical described by Sarker and Sleigh (Sarker & Sleigh 2015). scheme was used in the study to improve accuracy in model results. Standard “Flood and Dry” were included in the Model results and discussions model to consider flooding and drying processes. Barotropic Statistical analyses of model results were carried out using density type and Smagorinsky eddy viscosity type were the MIKE21 Tool to derive mean and maximum surge values used. Coriolis forcing was included in the model as varying in over the whole model domain during the entire duration of domain. A constant bed resistance as Manning’s number Cyclone Nilofar. Fig. 10 shows the maximum surge values (n 5 1/44 m1/3/s) was used throughout the model domain. over the whole model domain during the entire duration of the cyclone. The figure indicates that the highest surges Methodology occurred in the centre of the Arabian Sea and close to the The cyclone surge model was driven by the cyclonic wind cyclone track. The highest surge was found at a location of and pressure fields as shown in Fig. 3. A constant water level 61.668E, 17.48N with reduced surge values towards the of 12.6 mCD was imposed at the open boundaries at the Omani coast. The temporal variation of surge at this location

10 Water and Environment Journal (2016) VC 2016 CIWEM. M. A. Sarker1 Cyclone hazards in the Arabian sea

Fig. 10. Maximum surge along the track during the entire duration of Cyclone Nilofar [Colour figure can be viewed at wileyonlinelibrary.com]

is the same as if extracted directly from Admiralty Chart data at the various scales available. Admiralty Chart data is based on surveys carried out in the past and some changes in the seabed particularly at shallow waters are expected over time. There- fore, there are uncertainties in model results at shallow waters due to expected seabed changes over time. However, the model results were extracted at deep waters and hence no effect of discrepancy in bathymetry data is expected. Input water levels to drive the tidal model were obtained from the Global Tidal Model of DHI. All models including this one have some uncertainties and limitations. Fig. 11. Time-series of surge at 61.668E, 17.48N during the entire On the tidal modelling it is important that the regional cir- duration of Cyclone Nilofar [Colour figure can be viewed at culations are understood so that these are captured within wileyonlinelibrary.com] the model. Widely accepted and commonly used values of wave during the entire duration of the cyclone is shown in Fig. 11. breaking parameter, bed friction and white capping were The maximum surge of approximately 0.8 m was found on used in the wave modelling study. There are always some 28 October 2014 16:00:00. Therefore, the highest surge and uncertainties in the wind input although careful precaution the maximum significant wave height occurred almost simul- measures were taken to derive the wind conditions. It is diffi- taneously (only 2 hours apart). cult to quantify the uncertainty resulting from these input parameters, however, RHDHV previous experience suggests Uncertainties in modelling results that the error will not be significant. A numerical model is developed based on various A flexible mesh was used in the study which fits better with a assumptions. Although the MIKE21 developer (DHI) carried curved coastline and also allowed smaller grids in the areas of out calibration and validation as part of the development higher importance to obtain better accuracy in model results. process, local site specific calibration and validation are Bathymetry is a major input parameter to the model which required before applying the model. Good quality measured was obtained from C-Map Database. The accuracy of this data data are required for model calibration and validation which

Water and Environment Journal (2016) VC 2016 CIWEM. 11 Cyclone hazards in the Arabian sea M. A. Sarker1 were lacking for the present study. However, some meas- f) The highest surge and the maximum significant wave ured data were obtained from public domain which was height occurred almost simultaneously (only 2 hours used to reasonably calibrate/validate the model. Further- apart). more model results were extracted at deep waters and The methodology described in this paper for modelling hence no major errors are expected in the model results. cyclone waves and surges in the Arabian Sea could be Although there are various uncertainties, numerical mod- applied to simulate such natural hazards at other sites els are considered as useful tools by researchers and around the world. practitioners.

Application of modelling results Acknowledgements The results from cyclone wave and surge models provide The author would like to thank Royal HaskoningDHV (an valuable information at all stages of a project including plan- independent, international engineering and project ning, design, environmental impact assessment, construc- management consultancy company, www.royalhasko- tion, operation, and de-commissioning. The model results ningdhv.com) for giving permission to publish this article. can also be used in emergency planning and decision- making to estimate potential loss of life, damage to proper- ties and marine facilities and to develop rescue and mitiga- tion measures and plan clean-up operations. To submit a comment on this article please go to http://mc.manuscriptcentral.com/wej. For further information please The general application of the models and the methodol- see the Author Guidelines at wileyonlinelibrary.com ogy are: a) The regional tidal hydrodynamic model can be used to simu- late tidal movements and surges as well as to drive a wide References range of other models such as cyclone, tsunami, oil spill, https://en.wikipedia.org/wiki/Effects_of_tropical_cyclones (R. Fadler, water quality, sediment transport and morphological models. 2005) b) The cyclone wave and surge models are key tools for deriving Haque, U., Hashizume, M., Kolivras, K.N., Overgaard, H.J., Das, B. robust design conditions for coastal and marine structures and Yamamotoa, T. (2012) Reduced death rates from cyclones and facilities. The models can also provide input conditions to in Bangladesh: What more needs to be done? Bull World scale physical models for testing structural stability and over- Health Organization, 90, 150–156. topping rates and input to coastal flood studies. https://en.wikipedia.org/wiki/North_Indian_Ocean_tropical_cyclone c) Although the emphasis has been on modelling the cyclone https://upload.wikimedia.org/wikipedia/commons/c/c0/North_ wave and surge within the Arabian Sea, the methodology Indian_cyclone_tracks.jpg outlined in the article could be applied to sites within other The Joint Typhoon Warning Center (JTWC), the U.S. Department regions that are affected by such natural hazards. of Defence Agency, http://www.usno.navy.mil/JTWC. https://en.wikipedia.org/wiki/Cyclone_Nilofar Findings and conclusions DHI. (2016a) MIKE21 Toolbox User Guide, DK-2970, Hørsholm, Denmark. This article illustrates how tidal hydrodynamic and wave DHI. (2016b) MIKE21 Flow Model FM User Guide, DK-2970, models can be used to simulate the impacts of cyclones on Hørsholm, Denmark. coastal developments, facilities and communities. The find- DHI. (2016c) MIKE21 SW User Guide, DK-2970, Hørsholm, Denmark. ings from the study are summarized below: C-Map JEPPESEN Commercial Marine. (2014) Hovlandsveien 52, a) The maximum significant wave height of approximately Egersund, Postal Code 4370, Norway. http://www.jeppesen. 12.8 m (with associated peak wave period of 13.3 s) was com/index.jsp. found; Ministry of Transport and Communications, Civil Aviation Affairs, General Directorate of Meteorological and Air Navigation, b) The maximum wave height was found in the middle of Department of Forecasting and Monitoring, Sultanate of Oman. the Arabian Sea; Report adverse weather conditions for the special Tropical c) Significant wave heights higher than 8m were sustained Cyclone (bit) (during the period May 31 to June 6, 2010). only for duration of about half a day; Sarker, M.A. and Sleigh, A.J. (2015) Cyclone and tsunami hazards d) In contrast to Cyclones Gonu and Phet the height of in the Arabian Sea–A numerical modelling case study by Royal waves reaching the Omani coast was limited; Haskoning DHV. J Shipping Ocean Eng., 5, 242–254. e) The highest surges occurred in the middle of the Arabian World Meteorological Organisation, WWRP 2010–2,1st WMO Sea and close to the cyclone track. The highest surge was International Conference on Indian Ocean Tropical Cyclones and reduced towards the Omani coast; and Climate Change, , Sultanate of Oman, 8-11 March 2009.

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