cyclone hazards in the arabian sea–a numerical modelling case study of cyclone … ·...

Cyclone hazards in the Arabian sea–A numerical modelling casestudy of Cyclone Nilofar

Mohammad Akhtaruzzaman Sarker

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

Keywords

numerical modelling; natural hazards; cyclone;

extreme waves; storm surge; port develop-

ment; Arabian Sea.

Correspondence

Dr Mohammad Akhtaruzzaman Sarker,

Principal Engineer; research fields: numerical

modelling of coastal processes including

waves, tides, sediment transport, cyclones,

tsunamis, sea ice and water quality (dredge

plume, outfall discharge and oil spills).

Email: [email protected]

doi:10.1111/wej.12214

Abstract

Cyclones cause significant loss of life and damage to properties, ecosystems and

marine facilities. To address such issues, Royal HaskoningDHV (RHDHV) has devel-

oped regional tidal hydrodynamic and wave models covering the Northern Arabian

Sea. A total of 29 major cyclones were identified in the Arabian Sea since 1945.

However, as less information is available on Cyclone Nilofar (2014), this paper has

concentrated on this event to illustrate the use of numerical modelling to simulate

waves and surge generated by cyclones. Sample results from the modelling study

are presented in this paper. The methodology described in this paper for modelling

cyclone waves and surges in the Arabian Sea could be applied to simulate such nat-

ural hazards at other sites around the world.

Introduction

Tropical cyclones are associated with high-pressure gra-

dients and consequent strong winds and storm surges. Very

strong winds may damage installations, dwellings, transpor-

tation and communication systems, trees etc. and cause

fires resulting in considerable loss of life and damage to

property. Destruction of transportation or communications

infrastructure hampers clean-up and rescue efforts. Heavy

and prolonged rains due to cyclones may cause floods and

submergence of low lying areas and can lead to mudslides

and landslides in mountainous areas causing loss of life and

property. Floods, standing water and coastal inundation due

to storm surges pollute drinking water sources and spread

diseases leading to outbreak of epidemics.

Cyclones also impose significant risks during construction

and operation of sea ports, oil terminals & jetties, offshore

exploratory drilling rigs and offshore oil extraction rigs. They

put lives and properties in coastal areas at greater risks and

cause significant loss of ecosystems and marine facilities.

The destruction from a tropical cyclone depends on its inten-

sity, its size, and its location.

During the last two centuries, tropical cyclones have been

responsible for the deaths of about 1.9 million people

worldwide (https://en.wikipedia.org/wiki/Effects_of_tropical_

cyclones). It is estimated that 10,000 people per year perish

due to tropical cyclones (https://en.wikipedia.org/wiki/

Effects_of_tropical_cyclones). For example, Bangladesh is

especially vulnerable to tropical cyclones with around

718,000 deaths from them in the past 50 years (Haque et al.,

2012). The deadliest tropical cyclone was the 1970 Bhola

Cyclone, which had a death toll of anywhere from 300,000 to

500,000 lives (https://en.wikipedia.org/wiki/Effects_of_tropi-

cal_cyclones).

Despite their devastating effects, tropical cyclones are

essential features of the Earth’s atmosphere as they bring

rain to dry areas and transfer heat and energy between the

equator and the cooler regions nearer the poles.

A large tidal hydrodynamic model is required to simulate

cyclone surge on a region whereas a large wave model is

required to simulate cyclone waves. Given the above risks,

RHDHV has developed regional tidal hydrodynamic and

wave models covering the Northern Arabian Sea to investi-

gate the natural hazards and to support their project work in

the region. The models have been used to assess cyclones

within this region.

Literature search on cyclones in the Arabian Sea was car-

ried out and a total of 29 major cyclones were identified

since 1945. However, the present study was focused to the

recent cyclone “Cyclone Nilofar” that occurred in October

2014. Cyclone waves and surge were modelled. Sample

results from these modelling studies are presented in this

paper for illustration purposes only.

The methodology described in this paper for modelling

cyclone waves and surges in the Arabian Sea could be

1Water and Environment Journal (2016) VC 2016 CIWEM.

Water and Environment Journal. Print ISSN 1747-6585

http://https://en.wikipedia.org/wiki/Effects_of_tropical_cyclones






applied to simulate such natural hazards at other sites

around the world.

Cyclones in the Arabian sea

As reported in [https://en.wikipedia.org/wiki/North_Indian_

Ocean_tropical_cyclone, the Arabian Sea is located in the

north-west of the Indian Ocean. Tropical cyclones in the

basin are abbreviated ARB by the India Meteorological

Department (IMD), the official Regional Specialized Meteoro-

logical Centre (RSMC) of the basin. The Arabian Sea’s coast is

shared among India, Yemen, Oman, Iran, Pakistan, Sri Lanka,

Maldives and Somalia. Monsoons are characteristic of the

Arabian Sea and responsible for the yearly cycling of its

waters. In summer, strong winds blow from the south-west

to the north-east, bringing rain to the Indian subcontinent.

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

the Northern Arabian Sea Region. These events usually occur

during the transition periods of the monsoons which are

between May and June and between October and November.

Cumulative track map of Tropical Cyclones in the Arabian Sea

from 1970 to 2005 is illustrated in Fig. 1 (https://upload.wikime-

dia.org/wikipedia/commons/c/c0/North_Indian_cyclone_tracks.

jpg). The Saffir-Simpson Scale classifying depression, tropical

storm and cyclone is given in Table 1.

A total of 29 cyclones were initially identified to have

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

proximity to the site of interest. Tracks and pressure fields of

the selected cyclones were obtained from the Joint Typhoon

Warning Center (JTWC), USA (The Joint Typhoon Warning Cen-

ter (JTWC), the U.S. Department of Defence Agency). Data of

these 29 selected cyclones are listed in Table 2.

In particular, the passage of Cyclone Gonu (in 2007),

Cyclone Phet (in 2010) and Cyclone Nilofar (in 2014) has

raised awareness of the risk of cyclonic events. Cyclones

Gonu and Phet had a significant effect on the Omani coast-

line and their impact is well documented. As less information

is available on Cyclone Nilofar, this paper has concentrated

on this event to illustrate the use of numerical modelling to

simulate waves and surge generated by cyclones.

Cyclone Nilofar (2014)

Formation of Cyclone Nilofar

Cyclone Nilofar was classified as an extremely Severe

Cyclonic Storm and was the strongest tropical cyclone of

2014 within the North Indian Ocean and the strongest storm

to form over the Arabian Sea since Cyclone Phet in 2010.

Nilofar originated from a low pressure area in the Arabian

Sea that intensified into a depression on 25 October 2014. It

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]

Cyclone hazards in the Arabian sea M. A. Sarker1

2 Water and Environment Journal (2016) VC 2016 CIWEM.

https://en.wikipedia.org/wiki/North_Indian_Ocean_tropical_cyclone


http://https://upload.wikimedia.org/wikipedia/commons/c/c0/North_Indian_cyclone_tracks.jpg



https://upload.wikimedia.org/wikipedia/commons/c/c0/North_Indian_cyclone_tracks.jpg



slowly consolidated and reached cyclonic storm strength

the following day. The system rapidly intensified in the fol-

lowing days, reaching a peak intensity of 950 mbar (28.05

inHg) on 28 October 2014. Over time the storm tracked

northeastwards towards an area of high vertical wind shear,

causing the storm to rapidly weaken. The name Nilofar,

referring to the water lily, was suggested by Pakistan

(https://en.wikipedia.org/wiki/Cyclone_Nilofar).

Track of Cyclone Nilofar

The track (route) of Cyclone Nilofar was obtained from JTWC and

is shown in Fig. 2 (The Joint Typhoon Warning Center (JTWC), the

U.S. Department of Defence Agency). The JTWC archived cyclone

data also contains 6 hourly information including date and time,

tracks (path), maximum sustained wind speeds, radius of maxi-

mum sustained wind speeds and the minimum central pres-

sures. Such data of Cyclone Nilofar is provided in Table 3.

Table 1 Saffir-Simpson cyclone classification

Storm type Category Pressure (hPa)

1-min peak wind

speed (knots)

1-min peak wind

speed (mph)

1-min peak wind

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

Table 2 Major cyclones in the Arabian Sea during 1945-2014 (The Joint Typhoon Warning Center (JTWC), the U.S. Department of Defence Agency)

No. Year

Codes &

Names

Distance from Ras

Markaz (miles)

Time & Date Max sustained

wind speeds

(knots)

Minimum central

pressure (mb)

Radius of max

winds (nm)Start End

1 1959 01 Unknown 18 May 18:00 24 May 00:00 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


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


18 1992 TC 06A 55 29 Sep 00:00 04 Oct 12:00 55 Unknown Unknown


20 1995 TC 02A 120 11 Oct 00:00 18 Oct 12:00 50 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

M. A. Sarker1 Cyclone hazards in the Arabian sea


http://https://en.wikipedia.org/wiki/Cyclone_Nilofar

Wind and pressure fields of Cyclone Nilofar

The MIKE21 Cyclone Wind Generation Tool of DHI (DHI 2016a)

was used to generate the cyclonic wind and pressure fields.

The tool allows users to compute wind and pressure data due

to tropical cyclone (hurricane or typhoon). Several cyclone

parametric models are included in the tool such as Young and

Sobey model (1981), Holland – single vortex model (1981),

Holland – double vortex model (1980) and Rankine vortex

model (DHI 2016a). All the six input parameters required by

the Young and Sobey model (i.e. time, track, radius of maxi-

mum wind speed, maximum wind speed, central pressure

and neutral pressure) were available for the study and this

was, therefore, used to generate the cyclonic wind and pres-

sure fields. The other models require some additional parame-

ters (such as Holland parameter B and Rankine parameter X)

that need to be calculated using empirical relationships which

add further uncertainty to the generated wind and pressure

fields and were, therefore, not used for the present study. Fig.

3 shows an example of wind and pressure fields of Cyclone

Nilofar. These wind and pressure fields were used to drive the

cyclone wave and surge models described later.

Arabian sea regional models developedby RHDHV

The regional tidal model

RHDHV has developed a two-dimensional Regional Tidal

Hydrodynamic Model for the Northern Arabian Sea using the

MIKE21/3 Flow Model FM software of DHI (DHI 2016b). The

model is based on the numerical solution of the two/three-

dimensional shallow water incompressible Reynolds aver-

aged Navier-Stokes equations invoking the assumptions of

Boussinesq and of hydrostatic pressure. Thus, the model

consists of continuity, momentum, temperature, salinity and

density equations.

The regional model covers the coastlines of six countries

i.e. Yemen, Oman, UAE, Iran, Pakistan and India (see Fig. 4).

The model has two open boundaries – one to the south and

the other to the north-west. The model was set up in such a

way that with a finer local mesh and more detailed bathyme-

try and land boundary data within a specified area, localized

water movement can be correctly modelled at a point of

interest without the need of introducing nested models.

With this unstructured flexible mesh, it is easy to refine the

mesh in an area of interest.

For the present study, the regional model was modified

by providing a high mesh resolution within the shallow water

areas and at the study site where changes in physical proc-

esses take place quickly within short distances. The model

bathymetry is shown in Fig. 4 was obtained from the C-Map

Global Database (C-Map JEPPESEN Commercial Marine,

2014). The model was driven by tide levels at these two

boundaries obtained from the Global Tidal Model Database

available within the MIKE21 Toolbox (DHI 2016a).

The model can be used in its own right to simulate tidal

movements and surges within the Northern Arabian Sea as

well as “building block” to drive a wide range of other mod-

els such as cyclone, tsunami, oil spill, water quality, sediment

transport and morphological models. The regional tidal

model was used to drive the cyclone surge model to assess

cyclone surge within the region.

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]



The regional wave model

RHDHV has also developed a two-dimensional Regional

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

growth by action of wind, non-linear wave-wave interaction,

dissipation due to white-capping, dissipation due to bottom

friction, dissipation due to depth-induced wave breaking,

wave diffraction, wave refraction, wave shoaling and wave-

current interaction. The fully spectral formulation of the

model is based on the wave action conservation equation,

where the directional-frequency wave action spectrum is the

dependent variable.

The model extent, mesh system and bathymetry are the

same as the regional tidal hydrodynamic model described

above. The regional wave model was used to drive the

cyclone wave model to assess cyclone wave conditions

within the region.

Cyclone Nilofar wave modelling

The model

The regional wave model developed by RHDHV based on

the MIKE21 Spectral Wave (SW) Model was used to simu-

late the cyclone waves. The model was used to simulate

the generation and propagation of cyclone waves. Fully

spectral formulation was used with in-stationary time for-

mulation. The higher order numerical scheme was used in

the study to improve accuracy in model results. Wave dif-

fraction, wave breaking, bottom friction and white

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








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










capping were included in the model simulations. Quadru-

plet wave interaction was also included in the simulations.

JONSWAP fetch growth empirical spectral formulation was

used.

Methodology

The cyclone wave model was driven by wind and pressure

fields as shown in Fig. 3. A constant water level of 12.6mCD

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



(5 2.1m MHHW 1 0.5m surge) was used. The model simula-

tions covered the entire passage of the Cyclone Nilofar

across the Arabian Sea.

Model validation

Formal observed wave data on Cyclone Nilofar was not avail-

able to validate the wave model. However, limited wave

data was available on Cyclone Gonu (2007) and Cyclone Phet

(2010). Therefore, the model validation was focused primar-

ily to these two cyclones. Wave heights, periods and direc-

tions were extracted from model results at selected

locations and were compared to those obtained from vari-

ous sources to validate the model and thereby to improve

confidence in model prediction. The model validation was

also described by Sarker and Sleigh (Sarker & Sleigh 2015).

Comparison for Cyclone Gonu (2007)

Most of the available wave information was for Cyclone

Gonu (2007) and, therefore, the model validation was

focused primarily to this cyclone. Table 4 compares the

model prediction with those from the Oman Meteorological

Office (Ministry of Transport and Communications, Civil

Aviation Affairs, General Directorate of Meteorological and

Air Navigation, Department of Forecasting and Monitoring,

Sultanate of Oman) and the World Meteorological Organiza-

tion (WMO) (World Meteorological Organisation 2009).

Table 4 compares the model results with those from other

sources. The modelled wave heights at Chabahar and in the

Gulf of Oman compared well with those reported by WMO

(World Meteorological Organisation 2009). However, the

modelled wave heights in the Arabian Sea are higher than

those reported by WMO (World Meteorological Organisation

2009). Similarly, the modelled wave heights in the Arabian

Sea are higher than those reported by the Omani Meteoro-

logical Office (Ministry of Transport and Communications,

Civil Aviation Affairs, General Directorate of Meteorological

and Air Navigation, Department of Forecasting and Monitor-

ing, Sultanate of Oman), however, the range of significant

wave heights reported by the Omani Meteorological Office

(Ministry of Transport and Communications, Civil Aviation

Affairs, General Directorate of Meteorological and Air Navi-

gation, Department of Forecasting and Monitoring, Sultan-

ate of Oman) is rather wide (6 – 12 m). Maximum significant

wave heights of over 11m in the Arabian Sea were reported

by WMO (World Meteorological Organisation 2009). On the

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

Table 4 Comparison of cyclone results for Cyclone Gonu (2007)

Locations

Maximum significant wave heights

World meteorological

organization

Oman meteorological

office Present study

Measurement point AW2 at

Chabahar in Iran at 30m depth

4.2 m - 4.5 m

Gulf of Oman 8 m - 9 m

Arabian Sea >11 m 6-12 m Up to 15 m



other hand, maximum wave height of up to 15m was found

in the present study. It should be noted that wave conditions

reported by WMO (World Meteorological Organisation 2009)

at Chabahar were measured using ADCP.

Although there are some differences in wave heights

reported by other organisations, it is concluded that there is

reasonable agreement in the pattern and magnitude of

waves (particularly in the coastal zone).

Comparison for Cyclone Phet (2010)

Some information was also obtained on Cyclone Phet (2010)

from literature search. Table 5 compares the model predic-

tion with those from the Oman Meteorological Office (Minis-

try of Transport and Communications, Civil Aviation Affairs,

General Directorate of Meteorological and Air Navigation,

Department of Forecasting and Monitoring, Sultanate of

Oman).

The modelled wave heights in the Gulf of Oman are similar

to those reported by the Omani Meteorological Office (Minis-

try of Transport and Communications, Civil Aviation Affairs,


Department of Forecasting and Monitoring, Sultanate of

Oman). However, in the Arabian Sea the modelled wave

heights are significantly higher than those reported by the

Omani Meteorological Office (Ministry of Transport and

Communications, Civil Aviation Affairs, General Directorate

of Meteorological and Air Navigation, Department of Fore-

casting and Monitoring, Sultanate of Oman).

Model results and discussions

The maximum significant wave height of approximately

12.8 m (with associated peak wave period of 13.3 s) was found

at a location of 61.788E, 17.98N on 28 October 2014 18:00:00.

The two-dimensional distribution of wave height contours

superimposed by wave directional vectors is shown in Fig. 5

for this time-step. The figure indicates that the maximum wave

height was found in the middle of the Arabian Sea. The tempo-

ral variation in significant wave height and peak wave period

at this location is shown in Fig. 6. The figure indicates that sig-

nificant wave heights higher than 8m were sustained only for

duration of about half a day.

Statistical analyses of model results were carried out

using the MIKE21 Tool to derive mean and maximum wave

conditions over the whole model domain during the entire

duration of Cyclone Nilofar. Fig. 7 shows the maximum sig-

nificant wave heights over the whole model domain during

the entire duration of the cyclone. This figure also shows

some selected points (points P1 to P27) along the cyclone

track where model results were extracted. Maximum signifi-

cant wave heights along the cyclone track during the entire

duration of the cyclone were provided in Fig. 8. Figs. 7 and 8

indicate that the maximum significant wave height was

found in the centre of the Arabian Sea and that in contrast to

Cyclones Gonu and Phet the height of waves reaching the

Omani coast was limited. The maximum significant wave

heights along the cyclone track shown in Fig. 8 were repro-

duced in Fig. 9 to provide clearer comparison of wave

heights at various points along the cyclone track. The

Table 5 Comparison of cyclone results for Cyclone Phet (2010)

Locations

Maximum significant wave heights

Oman meteorological office Present study

Gulf of Oman 4 m 4 m

Arabian Sea 7 to 8 m 13 m

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



Fig. 6. Time-series of significant wave

heights at 61.788E, 17.98N during the entire

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




horizontal axis in this figure shows the selected locations

along the track (a total of 27 points as illustrated in Fig. 7)

and the vertical axis shows the maximum significant wave

heights at these points. The line in this figure shows the max-

imum significant wave heights along the cyclone track and

the points in this figure show the maximum significant wave

heights (in metre) at selected 27 points along the cyclone

track.

Cyclone Nilofar surge modelling

A storm surge is an abnormal rise of sea level near the coast

caused by a severe tropical cyclone. As a result sea water

inundates low lying areas of coastal regions drowning

human beings and livestock, eroding beaches and embank-

ments, destroying vegetation and reducing soil fertility.

The model

The regional tidal hydrodynamic model developed by

RHDHV based on the MIKE21/3 Flow Model FM was used to

simulate the cyclone surge. The higher order numerical

scheme was used in the study to improve accuracy in model

results. Standard “Flood and Dry” were included in the

model to consider flooding and drying processes. Barotropic

density type and Smagorinsky eddy viscosity type were

used. Coriolis forcing was included in the model as varying in

domain. A constant bed resistance as Manning’s number

(n 5 1/44 m1/3/s) was used throughout the model domain.

Methodology

The cyclone surge model was driven by the cyclonic wind

and pressure fields as shown in Fig. 3. A constant water level

of 12.6 mCD was imposed at the open boundaries at the

south and north-west. An initial water level of 12.6 mCD was

maintained over the entire model domain.

Model validation

Limited data on storm surge along the coastlines was avail-

able to carry out model validation. The following quantitative

data has been extracted on storm surges from literature

search and previous project work carried out by RHDHV in

the region:

a) The 1 in 100 year storm surge as 0.2m for a project site

south of the Duqm Port in Oman

b) The surge at Fujairah in Oman did not exceed 0.5m during

the Cyclone Gonu (2007)

c) Limited qualitative information was found from a litera-

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

(2010) as 0.30m. It was, therefore, concluded that the surge

model should provide a reasonable prediction of storm

surge for Cyclone Nilofar. The model validation was also

described by Sarker and Sleigh (Sarker & Sleigh 2015).

Model results and discussions

Statistical analyses of model results were carried out using

the MIKE21 Tool to derive mean and maximum surge values

over the whole model domain during the entire duration of

Cyclone Nilofar. Fig. 10 shows the maximum surge values

over the whole model domain during the entire duration of

the cyclone. The figure indicates that the highest surges

occurred in the centre of the Arabian Sea and close to the

cyclone track. The highest surge was found at a location of

61.668E, 17.48N with reduced surge values towards the

Omani coast. The temporal variation of surge at this location

Fig. 9. Maximum significant wave heights along the cyclone track during the entire duration of Cyclone Nilofar [Colour figure can be viewed at




during the entire duration of the cyclone is shown in Fig. 11.

The maximum surge of approximately 0.8 m was found on

28 October 2014 16:00:00. Therefore, the highest surge and

the maximum significant wave height occurred almost simul-

taneously (only 2 hours apart).

Uncertainties in modelling results

A flexible mesh was used in the study which fits better with a

curved coastline and also allowed smaller grids in the areas of

higher importance to obtain better accuracy in model results.

Bathymetry is a major input parameter to the model which

was obtained from C-Map Database. The accuracy of this data

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.

On the tidal modelling it is important that the regional cir-

culations are understood so that these are captured within

the model.

Widely accepted and commonly used values of wave

breaking parameter, bed friction and white capping were

used in the wave modelling study. There are always some

uncertainties in the wind input although careful precaution

measures were taken to derive the wind conditions. It is diffi-

cult to quantify the uncertainty resulting from these input

parameters, however, RHDHV previous experience suggests

that the error will not be significant.

A numerical model is developed based on various

assumptions. Although the MIKE21 developer (DHI) carried

out calibration and validation as part of the development

process, local site specific calibration and validation are

required before applying the model. Good quality measured

data are required for model calibration and validation which

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

Fig. 11. Time-series of surge at 61.668E, 17.48N during the entire

duration of Cyclone Nilofar [Colour figure can be viewed at




were lacking for the present study. However, some meas-

ured data were obtained from public domain which was

used to reasonably calibrate/validate the model. Further-

more model results were extracted at deep waters and

hence no major errors are expected in the model results.

Although there are various uncertainties, numerical mod-

els are considered as useful tools by researchers and

practitioners.

Application of modelling results

The results from cyclone wave and surge models provide

valuable information at all stages of a project including plan-

ning, design, environmental impact assessment, construc-

tion, operation, and de-commissioning. The model results

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.

The general application of the models and the methodol-

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

range of other models such as cyclone, tsunami, oil spill,

water quality, sediment transport and morphological models.

b) The cyclone wave and surge models are key tools for deriving

robust design conditions for coastal and marine structures

and facilities. The models can also provide input conditions to

scale physical models for testing structural stability and over-

topping rates and input to coastal flood studies.

c) Although the emphasis has been on modelling the cyclone

wave and surge within the Arabian Sea, the methodology

outlined in the article could be applied to sites within other

regions that are affected by such natural hazards.

Findings and conclusions

This article illustrates how tidal hydrodynamic and wave

models can be used to simulate the impacts of cyclones on

coastal developments, facilities and communities. The find-

ings from the study are summarized below:

a) The maximum significant wave height of approximately

12.8 m (with associated peak wave period of 13.3 s) was

found;

b) The maximum wave height was found in the middle of

the Arabian Sea;

c) Significant wave heights higher than 8m were sustained

only for duration of about half a day;

d) In contrast to Cyclones Gonu and Phet the height of

waves reaching the Omani coast was limited;

e) The highest surges occurred in the middle of the Arabian

Sea and close to the cyclone track. The highest surge was

reduced towards the Omani coast; and

f) The highest surge and the maximum significant wave

height occurred almost simultaneously (only 2 hours

apart).

The methodology described in this paper for modelling

cyclone waves and surges in the Arabian Sea could be

applied to simulate such natural hazards at other sites

around the world.

Acknowledgements

The author would like to thank Royal HaskoningDHV (an

independent, international engineering and project

management consultancy company, www.royalhasko-

ningdhv.com) for giving permission to publish this article.

To submit a comment on this article please go to

http://mc.manuscriptcentral.com/wej. For further information please

see the Author Guidelines at wileyonlinelibrary.com

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