ESTIMATION OF 50-YR MAXIMUM RETURN WAVE CONDITIONS FROM LAND-BASED WIND MEASUREMENTS IN KUWAIT

Jen-Men Lo and N. Al-Amer

Hydraulics and Coastal Eng. Dept., Environmental and Earth Sciences Division

Kuwait Institute for Scientific Research, P.O. Box 24885

13109-Safat-Kuwait

E-mail: jlo@safat.kisr.edu.kw

Abstract: There is a need to determine the 50-yr maximum return wave conditions for one of the coastal development projects in Kuwait’s southern coastal area.  Due to the absence of the long-term wind measurement for the coast of Kuwait, the 50-yr maximum return wind conditions for the coast of Kuwait were determined using the 34-yr land-based (about 16 km in land) wind measurements.  The Gumbel extreme value distribution method was first used to determine the relation of the maximum wind speed with the return period (extreme value equation) for different wind directions at the land station using the land-based wind measurement.  Then, the wind data recorded at the coastal station are assumed to have the same slope in the extreme value equation for each wind direction as the extreme value equation slope determined for the same direction from the land-based wind measurement.  The constant term of the extreme value equation for each wind direction at the coastal station was later determined using the 5-yr maximum return wind data for each direction recorded at the coastal wind station.  Then the 50-yr maximum return wind speed for each direction at the coastal area was calculated using the predetermined extreme value equations for the coastal station. The revised Sverdrup-Munk-Bretschneider (SMB) method was used to determine the 50-yr maximum deep-water wave conditions using the predicted coastal maximum return wind conditions. A wave refraction numerical model was used to predict the 50-yr maximum return wave conditions near the proposed coastal development area from the given 50-yr maximum deep-water wave conditions. 

Keywords: coastal structure, shallow-water wave, deep-water wave, wave prediction, wind measurement

1    INTRODUCTION

Fifty-year maximum return wave conditions are required for designing the coastal structures of one of the coastal development projects in Kuwait’s southern coastal area. There has been no long-term continuous wave time serious measurement along Kuwait’s coastal area; the longest continuous deep-water wave time serious data was recorded by Marmoush et al. (1994) during the period from January to December 1993. It is not reliable to predict 50-yr maximum return wave conditions using only one-year of continuous recorded wave data.

Deep-water wave conditions can be determined by the local wind conditions. There are no long-term recorded wind data for Kuwait’s coastal area; however, winds have been monitored at Kuwait International Airport (KIA) continuously since 1958. The KIA station is an inland station; the shortest distance from the KIA wind station to Kuwait’s coastal line is about 16 km (Fig. 1). Ras Al-Khafji is a coastal city belongs to the Kingdom of Saudi Arabia, it is near Kuwait’s southern border (Fig. 1). The Arabian Oil Company had a wind station at Ras Al-Khafji to measure the coastal wind conditions from 1967 to1984; however, only the data recorded from 1976 to1980 are valid for the present study. The wind data recorded at KIA cannot be applied to represent the wind conditions along Kuwait’s coastal area directly (Lo et al. 1994). However, the KIA recorded wind data strongly correlate with the data recorded at Ras Al-Khafji (Lo et al. 1994). The data recorded at Ras Al-Khafji was previously applied to represent the coastal wind conditions along Kuwait’s coastal area (Lo et al. 1987). Therefore, it can be assumed that the 50-yr maximum return wind conditions along Kuwait’s coastal area could be determined by determining the 50-yr maximum return wind conditions at the KIA station.

2    DETERMINATION OF THE 50-YR MAXIMUM RETURN WIND SPEED

The Gumbel extreme value distribution method was first used to determine the relations of the maximum wind speed with the return period (extreme value equation) for different wind directions based on the KIA wind data (from 1962 to 1995). A graphical technique was used to determine the best-fit extreme value equation based on the recorded annual maximum wind speed for each wind direction. For each wind direction, the yearly maximum wind speeds are ranked according to magnitude, and the recurrence interval is estimated using the equation:

                                          (1)

In Eq. 1, n is the number of years of records (for the present study, n = 34); m is the order number in the rank; and T_r is the recurrence interval in years. Then, the rank data and the computed recurrence interval are plotted on a semi-log (nature log) paper.  Table 1 presents a sample data table for ENE winds at KIA from 1962 to 1995. A best-fit equation was determined from the plotted data points based on the least square method. Fig. 2 presents the best fit for the maximum return wind speeds at KIA from the ENE. Then, we assume that the wind data recorded at Ras Al-Khafji has the same slope in the extreme value equation for each wind direction as the extreme value equation slope determined for the same direction at KIA. The constant term of the extreme value equation for each direction at Ras Al-Khafji was calculated by comparing the 5-yr (1976-1980) maxim wind data recorded at Ras Al-Khafji. Then, the 50-yr maximum return wind speeds for each direction at Ras Al-Khafji can be determined. Figure 2 presents a typical extreme value equation determined using the Gumbel extreme value distribution method, and the calculation for determining the 50-yr maximum return wind speed at KIA and Ras Al-Khafji. Table 2 presents the predicted 50-yr maximum wind speed at KIA and Ras Al-Khafji.

   Table 1    A sample data table for the ENE wind direction at KIA

KIA = Kuwait International Airport

   Table 2    Predicted 50-yr wind speed at KIA and ras Al-Khafji

3    DETERMINATION OF THE 50-YR MAXIMUM RETURN WAVE CONDITIONS

A deep-water wave forecast for the waves within Kuwait's coastal area was studied by Lo et al. (1987). The predicted deep-water waves were determined using the revised Sverdrup-Munk-Bretschneider (SMB) method (U.S. Army Coastal Engineering Research Center 1984) and the wind data recorded at Ras Al-Khafji during the period from 1976 to 1980. Due to the limited size of the Arabian Gulf (the maximum length is about 900 km, and the maximum width is about 360 km), they concluded that, in general, the deep-water waves generated within the Arabian Gulf are fetch limited. Therefore, assuming the fully developed wave conditions were fetch limited, then the SMB method was used to determine the 50-yr maximum deep-water wave conditions using the newly predicated 50-yr maximum wind conditions at Ras Al-Khafji. Table 3 presents the predicted 50-yr maximum deep-water wave conditions for Kuwait’s coastal area.

             Table 3    Predicted 50-yr deep-water wave conditions along Kuwait’s territorial waters

Waves are transformed when they travel into a different water depth; their height, length, and celerity change. A wave refraction numerical model developed by the Kuwait Institute for Scientifin Research (KISR) (Lo et al. 1987) was used to predict the 50-yr maximum return wave conditions near Kuwait’s Mangaf coastal area. The deep-water wave conditions for the input of the numerical model are based on the data in Table 3. Table 4 presents the predicted 50-yr maximum return wave conditions near the Mangaf coastal area (water depth of about 9 m). The maximum wave height is 5.31 m, with a wave period of 8.56 s, 76.19 degrees clockwise from the north.

         Table 4    Predicted 50-yr shallow-water wave conditions along the proposed

             project’s coast

* clockwise from north direction

4    CONCLUSIONS

Due to the lack of reliable long-term wave and coastal wind field measurements, a simplified approach was adopted to predict the 50-yr maximum return wave conditions for one of the coastal development projects in Kuwait’s southern coastal area based on long-term land-based wind measurements. The Gumbel extreme value distribution method was first used to determine the relation of the maximum wind speed with the return period (extreme value equation) for different wind directions at the land-based wind measurement location. Then, the wind data recorded at the coastal station was assumed to have the same slope in the extreme value equation for each wind direction as the extreme value equation slope determined for the same direction from the long-term land-based wind measurement. The constant term of the extreme value equation for each wind direction at the coastal station was later determined using the 5-yr maximum return wind data for each direction recorded at the coastal wind station. Then the 50-yr maximum return wind speeds for each direction at the coastal area were calculated using the predetermined extreme value equations for the coastal station. The SMB method was used to determine the 50-yr maximum deep-water wave conditions using the predicted coastal maximum return wind conditions. A wave refraction numerical model was used to predict the 50-yr maximum return wave conditions near the proposed coastal development area from the given 50-yr maximum deep-water wave conditions. When there is a lack of long-term wave measurement, this study demonstrates a simple approach that could be applied in predicting the design wave height based on long-term inland and short-term wind measurements.

Acknowledgements

This paper is part of a study sponsored by Pan Arab Consulting Engineers, Kuwait under the title “Environmental Impact Assessment for the Mangaf Resort Hotel Project”.

References

[1]    Lo, Jen-Men, A. Tayfun, and L. Ra'ad. 1987. Wave refraction shoaling maps for Kuwait coastal area. Kuwait Institute for Scientific Research, Report No. KISR 2221, Kuwait.

[2]    Lo, Jen-Men, N. Al-Madani, M.A. Tayfun, and D. Al-Ajmi. 1994. Estimation of winds over the sea from land-based measurements. Ocean Engineering, Vol. 21, No. 7, pp. 607-628.

[3]    Marmoush, Y., J.-M. Lo, M.A. Tayfun, L. Ra'ad, and K. Al-Salem. 1994.  Analysis of wave-buoy data recorded offshore Fintas, Kuwait. Kuwait Institute for Scientific Research, Report No. KISR 4399, Kuwait.

[4]    U.S. Army Coastal Engineering  Research Center. 1984. Shore Protection Manual. Department of the Army, Corps of Engineers, USA.

Fig. 1    Location map of the meteorological stations.

p

*5-yr maximum return wind speed at KIA=11.4m/s

*50-yr maximum return wind speed at KIA=19.2m/s

*5-yr maximum return wind speed at RAS Al-Khafji=17.5m/s

(from the field data)

*50-yr maximum return wind speed at Ras Al-Khafji=

3.403466 ln(50)+12.0223=25.3m/s (The constant value of 12.0223 was derived from the 5-yr maximum return wind speed at Ras Al-Khafji of 17.5m/s, and the KIA best-fit equation.)

Fig. 2    A sample determination of the 50-yr maximum wind speed at Kuwait International Airport (KIA)and Ras Al-Khafji.