The south coast of Iran bordering on the Oman Sea is subject to tropical cyclone influence on an infrequent basis; however, these cyclones can generate relatively large sea states. At many locations along this coastline, particularly in the central and eastern portions of the coast, tropical cyclone waves are the dominant coastal design wave condition as the wave heights can exceed those generated from other meteorological phenomena, such as the monsoon and local westerly winds. Recently, in early June 2007, Cyclone Gonu entered the Oman Sea and large waves were experienced along the Iranian coastline. This cyclone had an unusual path, travelling much further west and north than the typical cyclone. Significant wave heights in excess of 4 meters were measured at Chabahar during this event.
One of the challenges in evaluating tropical cyclone wave generation in the Arabian and Oman Seas is the limited availability of historical data. Although, the paths of historical cyclone events have been established through various data sources, the severity of the cyclones is not well known except for storms occurring over the past 30 years. As well, the relative infrequency and significant spatial variability of tropical cyclones in this region (particularly the Oman Sea) implies that a simple assessment of historical storm events is not sufficient for statistical evaluation of design wave conditions.
In the present study, a detailed investigation of tropical cyclone wave conditions within the Oman Sea was completed. This investigatizn has been comprised of a variety of activities in order to better understand the effects of tropical cyclone wave activity, including:
• Statistical analyses of historical tropical cyclone data from a variety of sources.
• Review of previous tropical cyclone investigations for this region.
• Numerical simulation of waves generated by important historical cyclone events, including Gonu 2007.
• Monte Carlo numerical simulations of tropical cyclone-generated waves.
• Statistical extreme value analyses of maximum wave heights.
This paper provides a brief summary of the methodology and findings of the tropical cyclone study.
Historical Tropical Cyclones
The JTWC “Best Track” data are based on a re-analysis of historical cyclone data to provide “best” estimates of each cyclone track and intensity on a six-hourly basis. The “Best Track” tropical cyclone database was searched for storms within radial distances ranging from 250 km to 1000 km from the eastern end of the Iranian coastline on the Oman Sea (Guater Bay) were considered, as shown in Figure 1. The figure shows that the majority of the cyclones have occurred south and east, and at a significant distance from the Iranian coastline. This is important in that the height associated with cyclone-generated wave conditions reduces significantly with distance. In general the cyclones tend to travel either due west towards Oman or re-curve north to strike Pakistan or India. Cyclones in close proximity to Iran (< 300km) are relatively infrequent. The dataset indicates that there are two distinct tropical cyclone periods: May to July; and September to November. As the southwest monsoon becomes more prominent during the summer months, the potential for tropical cyclones to develop is reduced.
Fig 1 Tropical Storm Tracks Within 1000 km of the Eastern End of the Oman Sea Coastline
The India Meteorological Department compiled an extensive summary of cyclonic storm tracks for the period from 1877 to 1970. Review of the historical cyclone tracks showed that a number of Severe Tropical Storms entered the Oman Sea that would potentially generate large waves that would impact the Iranian shoreline. These events included storms from the following time periods: June 1889, June 1890, May 1898 and April 1901.
Cyclone Gonu, which existed from June 1 to 7, 2007, was the most intense tropical cyclone on record in the Arabian Sea. It developed from an area of persistent convection in the eastern Arabian Sea, and intensified to a Category 5 cyclone with maximum wind speeds of 250 kph (140 knots) by June 4. The cyclone moved in a northwest direction, making landfall at the easternmost tip of the Arabian Peninsula in Oman, then proceeded into the Oman Sea. The storm decreased in intensity as it moved northward from Oman.
Wave measurements for this time period were available from an ADCP at Chabahar. Figure 2 provides a time series plot showing significant wave height and peak wave period measured during Gonu. The waves achieved a maximum significant wave height of approximately 4.2 m with an associated period of 10 seconds.
Fig 2 Time Series of waves at Chabahar ADCP during Gonu
Simulation of Cyclones
Numerical modelling of tropical cyclone wave generation was used in this study to both simulate historical storm events and to investigate potential design wave conditions. The WAVAD model was used for the tropical cyclone wave simulations carried out in this study. The WAVAD model was utilized for both purposes of computational efficiency its accuracy in rapidly changing wind fields, such as occur in a tropical cyclone. As will be shown later, over 200 model simulations were carried out for this investigation, and computational efficiency was important.
As an example, the results of numerical simulation of cyclone Gonu are presented here. Figure 3 and provides a snapshot of wave heights during the course of the cyclone passage, while Figure 4 shows the maximum significant wave height estimated at each grid point throughout the passage of Gonu. Figure 5 shows a time series comparison to the waves measured at Chabahar. Excellent agreement was achieved between the measured and hindcast waves. Note that background wave conditions in the Indian Ocean were not included in the simulation, which would have little effect on the peak of the storm event but could affect the magnitude of lesser wave conditions before and after the cyclone passage.
Monte Carlo Approach to Cyclone Simulation
Derivation of design wave conditions directly from numerical simulation of the historical tropical cyclones alone could lead to potential under- or over-estimation of the design waves due to the limited number of historical events with reliable wind speed information. In this study, to overcome some of the limitations with the historical data, a Monte Carlo methodology has been used to derive estimates of extreme wave conditions along the Oman Sea coast of Iran. This method involves the development of a synthetic tropical cyclone database using the properties of the existing historical tropical cyclone population. In total more than 200 cyclones were simulated. The maximum wave height calculated for each cyclone simulation was extracted from the model results at locations of interest along the Iranian coastline. Statistical extreme value analysis was then performed on the data to define design wave conditions.
Fig 3 Contours of Significant Wave Height – June 5, 2006 12:00 UTC
Fig 4 Maximum Significant Wave Height in Simulation
Fig 5 Time Comparisons to Chabahar ADCP (30 m depth)
Figure 6 provides a graphical representation of the spatial variation in the estimate for the 100-year wave height.
Fig 6 100-year design waves
In general, the selection of design wave heights along the Oman Sea shores of Iran depends largely on the risk to be assumed and on the reliance that is placed in older records of tropical cyclone events. Tropical cyclone data from the past 30 years, including Gonu, indicate maximum offshore design significant wave heights of up to 8.8 meters. If the cyclone track data from the late 1800s and early 1900s is considered reliable, the potential for even larger cyclone design wave heights is feasible. As this potential variability in design wave height has a significant impact on the costs associated with coastal infrastructure, we recommend that a more detailed investigation of these historical events be carried out, particularly given the recent occurrence of Gonu. Such an investigation would require research into historical records with a view to establishing the track and intensity of these events, including:
• Review and assessment of the background references and data utilized in preparing the India Meteorological Department Tropical Cyclone summary.
• Evaluation of historical ship observations.
• Review of historical meteorological records for the region.
• Investigation of archival newspaper and government records, etc. for the relevant time periods to identify potential damage reports