THE COASTAL WIND FIELD OF THE SOUTHERN CAPE by. Ian Tyrrell HUNTER

Transcription

1 THE COASTAL WIND FIELD OF THE SOUTHERN CAPE by Ian Tyrrell HUNTER ABSTRACT The castal regin f the Suth Cape is presented as a typical semidevelped castal zne with a limited envirnmental dataset. The authr places himself in the psitin f a castal engineer requiring wind data fr design purpses. The varius surces f wind measurement are discussed. Time series are presented depicting respnses at different sites t the same large-scale synptic situatin. Spatial variatins, bth acrss the castal bundary and in the ffshre regin are emphasized. These variatins are brught t the attentin f thse engineers wh may have t extraplate wind cnditins t their site f interest. 1 INTRODUCTION The Cape suth cast extends frm Cape Agulhas t Cape Padrne at the eastern extremity f Alga Bay (Figure 1). The cntinental shelf widens cnsiderably in this regin t frm the Agulhas Bank which has been chsen as the site f a multidisciplinary ceangraphic research prject. As part f the meterlgical prgram fr the prject, existing surces f wind data were fully explred. They include: (i) Lng-term wind data frm weather ffices. (ii) Synptic reprts frm ships f pprtunity, (iii) Wind frm ffshre drilling platfrms. (iv) Estimated wind data frm lighthuses. An additinal surce was created in February 1982, when three autmatic weather statins were erected at suitable castal sites. A furth statin was peridically munted n the R V Mei-ving Naude and this prvided invaluable ffshre data. Figure 1 shws the distributin f the varius surces f wind data alng the cast. The varius types f wind bservatins are discussed in terms f their accuracy, representativeness and distributin. Justificatin fr such a study, frm a castal engineering pint f view, cmes frm the fact that a significant prprtin f the wave energy n this cast is generated by the high-frequency, lcally generated cmpnent. 2 WIND OBSERVATIONS AT FULLY FLEDGED WEATHER OFFICES There are tw weather ffices in the study area, ne at H F Verwerd Airprt, Prt Elizabeth, and the ther at Gerge. Bth have been Natinal Research Institute fr Oceanlgy, Cuncil fr Scientific and Industrial Research, Stellenbsch, Republic f Suth Africa. 2551

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3 COASTAL WIND FIELD 2553 recrding hurly wind speed since 1951, using Dines pressure tube anemmeters. H F Verwerd Airprt (see inset Figure 2), is lcated 3 km frm the cast with the anemmeter at a height f 60 m a.s.l. The general terrain surrunding the airprt is flat with n marked tpgraphical features. Yet it can be shwn that data cllected here are nt representative f castal cnditins. During the perid 1 t 3 September 1978, a series f intense lw pressure systems resulted in high seas n the Agulhas Bank. In Figure 2 hurly wind data frm H F Verwerd Airprt are cmpared with thse frm the il rig Sedc-K, rughly 40 km t the suth (See Sectin 5 fr details f wind reductin t standard height). It is evident that there was initially n cupling between air mvement at the airprt and that ffshre. Hwever, this situatin imprved as the bundary layer became mre mixed, especially vernight. Nevertheless, if ne cmpares the maximum values measured n the Sedc-K, with the 50-year return value f hurly wind speed fr the airprt (22,3 m/s), (Suth African Weather Bureau,1974), it is bvius that the airprt is underestimating ffshre cnditins. This particular series f strms resulted in a significant wave height f 8,6 m (Shillingtn and Britten, 1979), at the Sedc-K site while an estimate f up t 15m was received frm a tanker within the Agulhas current further up the cast. The nly ther surce f lng-term, accurate wind data is the weather ffice at Gerge. Hwever, since this statin is 221 m a.s.l. and mre than 10 km frm the cast it was cnsidered t be unsuitable as a surce f infrmatin n wind cnditins. Hsu (1980) has related mean mnthly ffshre wind cnditins t thse at a suitable castal site. Unfrtunately, insufficient data were available fr such a study f cnditins here. Hwever, it is bvius frm Figure 2 that the airprt is nt suitable fr measuring castal wind cnditins. 3 WIND FROM OFFSHORE DRILLING PLATFORMS The search fr il n the Agulhas Bank and the assciated demand fr envirnmental data, have prvided researchers with invaluable ffshre wind measurements. Hwever, even this dataset presents prblems: (i) Althugh measurements began in 1978, the recrd is by n means cntinuus, (ii) Measurements represent several well sites n the Agulhas Bank, (iii) The anemmeter is 73 m a.s.l. which pses a height reductin prblem. With regard t (ii), anther drilling rig, the Sedc-708, was temprarily n the Agulhas Bank, drilling t the suth f Sedc-K. While it was in a psitin rughly 78 km suthwest f Sedc-K, wind speeds registered at Sedc-708 were significantly higher fr much f the

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5 COASTAL WIND FIELD 2555 time (Figure 3). Hwever, the latter rig was later mved t a psitin rughly 74 km WSW f the Sedc-K and the cmparisn was greatly imprved (Figure 4). It was significant that Sedc-708 was clser t the warm Agulhas Current when it was in the first psitin. The reductin f wind speed frm 73 m t the standard 10 m a.s.l. is a functin f bundary layer stability, which may vary cnsiderably in time and space. Whereas the wind pwer law has previusly been applied with the expnent fr a neutrally stable atmsphere, i.e. 10 ' U3 U.0 = U 73 # subsequent wind measurements shwed that this reductin may be t severe, especially under unstable, high-wind cnditins. The hurly wind values pltted in Figure 2 were btained using an expnent f 0,1 which effectively reduced the 73 m wind by 18%. It is felt that this reductin will, if anything, underestimate the 10 m wind, under extreme cnditins. In rder t btain a 50-year return perid hurly wind fr the Agulhas Bank, the methd utlined by Katsiambirtas (Suth African Weather Bureau 1975), was fllwed. It shuld be nted that the chsen dataset f twelve mnths did nt include the September 1978 strm (Figure 2). A value f 25,8 m/s was btained. This value was exceeded n several ccasins prir t the used dataset. This illustrates the flly f applying statistical methds t a relatively shrt wind recrd. 4 SHIPS OF OPPORTUNITY Prir t the advent f ffshre drilling peratins, synptic reprts frm merchant ships ('SHIP' reprts) represented the nly significant surce f ffshre wind data n the Agulhas Bank. The castal engineer planning t utilise these data wuld have t bear in mind that : 1 Mst vessels are 30 km r mre frm the cast in the regin between Cape St Francis and Cape Agulhas (see Figure 5). 2 Reprts are usually sent nly n the main synptic hur, i.e. every 6 hurs. 3 Vessels will be trying t avid extreme cnditins if pssible. 4 Observatins are ften neglected when the ship is within sight f land. 5 The majrity f SHIP reprts cntain estimated wind speeds. With regard t this last pint, it must be brne in mind that mst deck fficers use state f sea t estimate wind speed. Hwever, sea state is nt purely a functin f wind. Bundary layer stability and surface currents play an imprtant rle. Als it must be expected that wind estimates will decrease in accuracy as wind speeds increase. There is n simple slutin t this prblem, since an anemmeter wuld measure relative wind speed and further errrs may be intrduced in the calculatin f true wind speed. A height reductin prblem als arises since anemmeter height is nt prvided in the standard message and this will vary greatly frm vessel t vessel.

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10 2560 COASTAL ENGINEERING ESTIMATES OF HOURLY WIND FROM LIGHTHOUSES This represents the nly surce f lng-term, truly castal bservatins. There are nw nly three fully manned lighthuses n the Cape suth cast (Figure 3). Of these three - 1 Cape St Francis is fitted with a pressure plate which is f n use under extreme cnditins as it is almst hrizntal at 15 m/s. 2 Cape St Blaize is badly psitined fr wind bservatins, being sheltered frm wind frm certain directins. Bearing in mind the limited accuracy f bservatins frm these lighthuses, it has nt been pssible t determine their relatinship t ffshre cnditins. 6 AUTOMATIC WEATHER STATIONS Three permanent autmatic weather statins were set up n the Cape suth cast in February At all three sites, the prevailing winds have very limited stretches f cast t crss befre reaching the anemmeters. The surrunding terrain is als reasnably flat. The anemmeters are all placed well abve the internal bundary layer (IBL). A furth autmatic weather statin is munted n the NRIO's research ship, R V Meiring Naude, fr thse perids which she spends ff the suth cast. These wind data are invaluable in the study f ffshre variatin. Figure 6 illustrates hw ncturnal cling may ttally decuple the air flw n shre frm that ffshre. Early n 9 August, the R V Meiring Naude was crssing Alga Bay in the directin f Bird Island. A land breeze f abut 5 m/s was blwing as the vessel passed within 15 km f H F Verwerd Airprt. At the latter site the establishment f the ncturnal inversin resulted in calm cnditins frm 22h00 until llhoo the fllwing day. In the vicinity f Bird Island the land breeze reached a peak f 9,2 m/s at 08h00, finally dying ff at llhoo. The third curve in Figure 6 represents cnditins at Cape St Francis (see Figure 1). The high wind speeds recrded there n 9 August are due t a frntal wave which was much weaker when passing the R V Me-iving Naude (the vessel was at that stage t the west f Cape St Francis). Nte that the anemmeter at Cape St Francis is abve the ncturnal inversin with the land breeze there reaching a peak speed at 04h00. 7 CONCLUSIONS This study has cmpared actual hurly wind averages as ppsed t the cmparisn f lng-term means. The fllwing cnclusins may be drawn: (1) Althugh a simple relatinship may, at times, exist between wind speed at a castal site and ne a few kilmetres inland, the inland recrd may becme cmpletely decupled due t excessive bundary layer stability. Shuld extreme return perid winds fr such an inland site be applied t the castal site, a severe underestimate may result.

11 COASTAL WIND FIELD 2561 (2) Extreme return perid winds fr an ffshre site, based n a relatively shrt dataset (less than 10 years), may als underestimate the true cnditin. (3) Offshre cnditins may vary ver relatively shrt distances (less than 100 km). This may be attributed, amngst ther things, t sea surface temperature distributin and preferred lcatins fr new synptic develpments. 8 ACKNOWLEDGEMENTS Appreciatin is expressed t the Suthern Oil Explratin Crpratin (Pty) Ltd., the Lighthuse Department f Suth African Transprt Services, and the Suth African Weather Bureau fr the valuable data which they prvided. REFERENCES 1 HSU, S A (1980) On the crrectin f land-based wind measurements fr ceangraphic applicatins. Prc. 17th Cnf. n Cast. Eng. Sydney, Australia. 2 SHILLINGTON, F A and BRITTEN-JONES, A (1979) Features f surface waves ff the suthern Cape cast. S. Afr. J. f Sci. 3 SOUTH AFRICAN WEATHER BUREAU (1974). Climate f Suth Africa, Part II, Pretria. 4 SOUTH AFRICAN WEATHER BUREAU (1975). The estimatin f extreme wind gusts frm inadequate data. Technical paper N. 2.