Hindcasting Waye Climatology Off J eddah Coast

J.K.A. V:: Mar. Sci., Vol. 4, pp. 3-16 (1413 A.H./1993 A.D.)

Hindcasting Waye Climatology Off J eddah Coast

SAAD MESBAH ABDELRAHMAN

Marine PhysicsDept., Faculty ofMarine Science, King Abdulaziz University, Jeddah, Saudi Arabia

ABSTRACf. Wave characteristics off Jeddah coast are hjndcasted using an extensive set of hourly meteorological data extending over 12 months. A preliminary surveyshows that there are qnly limited information available regarding the wave characteristics in front of Jeddah, which appear to be basedon generalterms and not well founded on scientific basis. Method of Pierson, Neumann and James (PNJ) is applied to the adjusted wind data to hindcast the wave generation spectrum. To verify the results of the hindcasted scheme,waves are measured in front of Jeddah coast (4 miles west of Ubhur Creek entrance). Analysis of the observed wave records shows an energetic band of waves at frequencies around 0.135 Hz, (period = 7.4 sec)and significant wave h~ight around 1.3 m, during the period of measurements. Comparison between the hindcasted and observed waves shows a number of qualitative agreement with physical basis for the quantitative variations. Resultsdemonstratethe limitation of the standard procedure recQmmendedbySPM (1984)to adjust the over-land wind to over-sea especially at calm wind conditions. However, the comparison with fiell! data demonstrates the ability of PNJmethod to reasonablyhindcast wave climatology off Jeddahcoast.

Introduction To establish a sound coastal management for the conservation of coastal environment, it is essentialto have a thorough understanding of the wave dynamics. Sea wavesare generatedby wind and have irregular and complexshapebecauseof the interaction _bet~een individual waves. Propagating s4ore-ward, waves dessipate a 3

Hindcasting Wave ClimatologyOffJeddah Coast

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GeneralWind Pattern and Data Collection The Arabian Peninsulais mainly characterized by seasonalweather nature related to the evolution and development of large pressuresystemscoming from outside the peninsula. Over the whole area, the year may be divided roughly into two seasons: cool and hot corresponding to the conventional northern hemisphere winter and

summer. The cool season(Winter) may extend from mid-October to mid-April with large scale prominent features. The climatic pattern produce two distinguished wind systerns over the Red Sea. The northern part is affected by North- West.to North winds, while the southern part is subjected to South and South-Eastwinds as a part of the North-East Monsoon (Alasdair and Head, 1987). Therefore, in winter, a convergence belt of the two wind systemsoccurs at Lat. 18-200Nwhich is characterized by light and variable winds with mostly calm conditions. With the moving depressions, disturbance to this rather settled regime may occur causingrainfaU and high seaconditions. The hot season(Summer) may extend along sixmonths with wind blowing over the entire Red Sea basin mainly from North-West and North. In summary, North western wind are prevailing allover the year over the entire Red Sea except over the southern part where the wind becomesSouth-easterliesin Winter (Alasdair and Head, 1987). Mountains along the Red Seacoastsensure that the main wind systemsblow predominantly along its axis with only localized effects such as land-sea breeze. As a result, the prevailing wind directions are mostly un-

changeable. Atmospheric data collected at Jeddah airport station (Lat. 21°40'42"N, Long. 39°08'54"E, Z = 3.58 m) are made available by MEPA (Meteorological Environmental Protection Administration). Hourly values of the 12-month wind data are processedto obtain the 1990 frequency of occurrence of wind speed and direction (Table 1). Sixteen wind directions are considered with 8-correspondingwind speed ranges following the standard Beaufort scale. During 1990,wind speed at Jeddahis found to lie between wind force zero to six with rare occurrence of gales (Table 1). Most of the wind force occurrencelies betweenforce 2 and 4 on Beaufort scale(wind speedsfrom 4 to 16 knots) which agreeswith the generalclimatology of this area (e.g. Hydrographer of the Navy, 1980). Calm wind condition (where wind speed is defined to be equal or lessthan 1 knot) lastsmore than 16% all over the year. Graphical representations of monthly wind direction are shown in Fig. 2, demonstrating a NNNW predominant wind direction throughout mostof the year which agreeswith the prevailing wind pattern asdescribed earlier. However, deviation from this direction do occur especially during October to December. Becauseof the limited variabilities and constancyof the wind field around Jeddah, the variation in seaclimate is expected to be highly sensitive to the wind speedvariabilities. A "fully arisen sea" condition most frequently occurs at low wind speeds

Hindcasting Wave Climatology Off Jeddah Coast

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Hindcasting Wave Climatology OffJeddah Coast

where Z= height of the station above sealevel in meters, 3.58 mfor .1eddahAirport Station. A constraint on station location requires that both over-land and over water stations are governed by the samepressure gradient systemand the only major difference between them is the surface roughness.This condition is satisfied sincethe distance betweenthe .1eddahstation and the location of interest in the seaare about 15 km (Fig. 1). The location factor (RL), defined as the ratio between & whereUw = wind speed over water and UL = over-land, was found to vary from 0.9 to 2.10 according to Resio and Vincent (1977). Generally speaking,the wind speedover water is subjectedto reduction when reachingthe land especiallyat lower wind speed«~O knots) (Resio and Vincent, 1977). The stability of the boundary layer over the seasurface should also be considered for the fluxes transferred at the air-seainterface. The stability correction factor(Rr) depends on the thermal differences between air and sea. Atmospheric boundary layer has a neutral stability condition and no correction is needed when no thermal difference exists. If the difference (~T as= T air- Tsea) is negative, the boundary layer is unstable and the wind speed is more effective in causingwave growth. If ~ T asis positive, the boundary layer is stable and the wind speedis lesseffective. The mean monthly air and seatemperature for Jeddahgiven in Alasdias and Head (1987) are considered for stability calculations. Mean sea temperature seemsto be higher than air temperature all over the year exceptduring April to August. Monthly thermal differences (~T as)are ,within-3.0°C to + 1.0°C.These thermal differences around Jeddah in conjunction with the work of Resio and Vincent (1977)are consideredto estimate the stability factor for each month. A computer program is developed to correct the wind field using the above SPM procedure. Hourly adjusted wind data are then processedto hindcastthe wave field usingthe adopted PNJscheme. Monthly statistical summaryof the 12-monthis given for both wave height and period in Tables 2 and 3. The monthly averagewave height is found TABLE2. Statisticalsummaryfor h,indcasted wavefield.

.

Hindcasting Wave Climatology OffJeddah Coast

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July 1991 at 4 miles west ofUbhur Creek entrance (bearing 256°), (Fig. 1). This location is off the coral reef barrier at EI-Qita Alkabira facing the open sea.~he gauge was bottom mounted at depth 6.50 m after which, depth changesdirectly to multiples of hundred meters. The wave gauge consists mainly of a rubber tire connected to metal elastic bellows which are connectedto a group of levers to record the mechanical displacement. Therefore, any pressure fluctuation ~ p due to wave passage,will changethe balanced pressureinside the tire and therefore the metal bellows will expand or contract in responseto the passageof wave crest or trough respectively. The movements of the bellows are enlarged and recorded onto a strip chart. The instrument was set for a 10 minutes recording interval every 2 hours. The obtained time series representing the wave-induced dynamical pressure is known to be correlated to the sea surface elevation through a known relationship (Earle and Bishop, 1984). Analysis of the 10 minutes time series are done first to identify the individual waves using the zero-up crossingmethod. More than twenty files are analyzed and the individual wave period and height are tabulated for scale corrections. The wave length of eachindividual wave is computed from the dispersionrelationship following Wu and Thornton (1986)scheme.Then, a transfer function deduced from linear wave theory is applied to compute the wave height for each individual wave. Narrow banded files with sufficient wave cycles are selected for detailed analysis. For each file, significant wave height and period are calculated after sorting the individual waves in descendingorder. Average wave height is found to vary from 0.7 to 1.20 m and significant wave height is of order 1.60 m while significant wave period is 7.4sec. During the time of wave measurement, simultaneous wind data are acquired at Jeddah Airport Station. Over-land wind data are then adjusted to describe over water wind data accordingto the SPM proced~re describedearlier. Then, PNJ hindcasting method is applied to the adjusted wind values to hindcast the hourly wave characteristics. Hindcasted values are compared to the correspondingwave values. Figure 3 shows the wind speed and direction during the measuringtime and the results of the observed and hindcasted wave heights and periods. Wind speedshowsa daily cycle that increaseswith the increase in air temperature (Fig. 3-a). This indicates that the wind systemis governed by the local effects of differential heating of land-sea. However, wind direction is still consistent with the major wind pattern along Red Sea; i.e. from NNW (Fig. 3-b). Observed significant wave heights are shownto vary accordingly with wind variation and providing a daily pattern (Fig. 3-c). Comparison between hindcasted and measuredsignificant wave height showsthat hindcasted H1/3seemsto agreewith observed heights at some hours while it is underestimated at others. Qualitative agreement occurs at non-vanishing wind speed but underestimation is noticed at lower

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wind variabilities especially -at low wind speed where large discrepancies might occur. However, comparison gives number of qualitative and quantitative agreement at non-vanishing wind speed. In addition, a slight phasedifference betweenthe two wave sets is observed implying the closenessof the atmospheric station to the gaugelocation (around 15 km) and therefore, no need to adjust for the associatedlag between wavesand wind. Confirming our physical interpretation, significant wave heights are computed which by definition exclude the lower wavesand only considerthe highestone-third of any wave record. Hindcasted H1/3 is found to vary from 0.93 to 1.43 m and T 1/3 ranging from 6.64 sec to7 .81sec(Table 3). These results show a quantitative agreement when compared with the measuredwave characteristics. Also, improvements are achieved when compared with the averagehindcastedwaves.Table 3 may represent wave climate in front of Jeddah which showsseasonablevariabilities along the year. Summary and Conclusions Investigation of the prevailing wave field off Jeddah coastis the main objective of this study. Despite the rapid developments along the Saudi coast on the Red Sea,limited information are available regarding the wave climatology around Jeddah. Wave information are essential for improving our understanding of the associated nearshore processes.Therefore, prior to any marine construction or nearshore processesstudies, availability of wave data comesfirst. The present study can be summarized as follows: 1 -Meteorological data on hourly basis for Jeddaharea during 1990are corrected for elevation, location and stability of the atmospheric boundary layer. Based on the processedwind speed and direction, "fully arisen sea" condition is assumedto describe the wave generation condition. Then, the adjusted wind data are utilized to hindcast the waves using PNJ method. 2 -Statistical analysis of the 12-month data showshindcastedwaves with the following general characteristics: monthly averagewave height is varying from 0.45 to 1.04 m and averageperiod is varying from 3.14 secto 5.29 sec. 3 -A wave gauge is deployed in July 1991 to record waves off Jeddah coast. Analysis of the measureddata showsthe spectral peak at period 7.4 secwith significant wave height around 1.60 m. 4 -Verification to the PNJ hindcastingschemegave a reasonablequalitative and quantitative agreement when compared with the measured waves except at lower wind speed~ 5 -It is worth mentioning that the procedure suggestedby SPM (1984), to adjust wind information collected at land station to represent the marine environment, is not adequate for calm wind conditions. The underestimated wave values at hours low wind speed may be due to the limitation of the recommended (SPM) correction procedure. Calm wind conditions occurs at Jeddahas much as16% all over the year.

Hindcasting Wave Climatology Offleddah Coast

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Thornton, E.B. and Abdelrahman, S.M. (1991) Sediment Transport in the SwashDue to Obliquely Incident Wind Waves Modulated by Infragravity Waves, Proceedingsof the Coastal Sediments,91 Conference,ASCE, Seattle, Washington, U.S.A, pp. 100-113. Wu, C.S..and Thornton, E~B. (1986)Wave Numbers of Linear ProgressiveWaves,Journal of Waterway, Port, Coastaland Ocean Engineering, 112,No.4: 536-540.