Waves at Varne Light Vessel Dover Strait

Transcription

1 L NATIONAL INSTITUTE OF OCEANOGRAPHY WORMLEY, GODALMING, SURREY Waves at Varne Light Vessel Dover Strait by L. DRAPER and R. GRAVES N.I.O. INTERNAL REPORT NO. A.4 SEPTEMBER 1968

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4 NA2I0HAL imstirute OF UCEAKOGRAPHY Vormley, Gcdalming, Surrey. A'aves at Vame Light Vessel, Dover Strait by L. Draper and R. Graves N.1,0. Internal Report No. A. 4 September 1968

5 GONITBNrS Page Description of the investigation Discussion of results Acknowledgements References Fave Height icxceedance Winter Spring Summer Autumn Figure 1 2 h- VifavePeriod Occurrence Winter Spring Summer Autumn Spectral Width Parameter Occurrence Whole Year 9 Scatter Diagram IWiole Year 10 Persistence Diagram \7hole Year 11 'Lifetime' Wave Prediction Graph 12

6 u Waves have been recorded by a Shipbome Wave Recorder (Tucker, 1956) placed on the Vame Light Vessel which is stationed in 1$ fathoms of water south west of the Vame Bank in the Dover Strait. The records from the first year of operation, from February 196$, have been analyzed, mainly following the method of analysis developed by Tucker (1961) from theoretioal studies by Cartwright and Longuet-Higgins (1956). The method of presentation is that recently recommended for data for engineering purposes (Draper, 1966). Records were taken at three-hourly intervals, and the analysis yields the following parameters: (a) = The sum of the distances of the highest crest and the lowest trough from the mean water level, (b) Hg = The sum of the distances of the second highest crest and the second lowest trough from the mean water level. (o) Tg = The mean zero-crossing period. (d) T^ = The mean crest period. From these measured parameters the following parameters have been calculated, after allowing for instrumental response: (e) H = The significant wave height (mean height of the highest one-third of the waves): this is calculated separately from both and Hg, and an average taken. The relationship between the parameters is

7 2. where f is a faotor related, to the number of zero-crossings in the reoord. (Tucker, 196^) A similar relationship is used for the oaloulation of H from H*. s 2 (f) H, 'x = The most probable value of the ^ ' maxl hours; ^ height of the highest wave whloh occurred in the recording inteival (Draper, 196^). (g) = The spectral width parameter, which is calculated from T and T (Tuoker, 1961 )i = 1 / I,) The results of these measurements are expressed, graphically* divided into seasons thus: Winter: Januaiy Februaiy March Spring: April May June Summer: July August September Autumn: October November December For each season a graph (Figures 1-2*.)shows the cumulative distribution of significant wave height H, and of the most probable value of the height of the highest wave in the recording interval, hours)' The distribution of zero-orossing period is given for eaoh season (Figures 5-8). The distribution of the spectral width parameter is given for the whole year (Figure 9). Figure 10 is a scatter diagram relating significant wave height to zero-crossing period.

8 . Figure 11 is a persiatenoe diagram for the whole year. Figure 12 is a plot of hours) Probability paper, for the whole year. Discussion of results From Figures 1-4 may be determined the proportion of time for which H or H. \ exceeded any eiven s maxlj hours; " height. For example, in the Winter the significant height exceeded 4 feet for 28 per cent of the time. ^^ve heights are generally higher in the winter months, but it is worthy of note that the highest measured wave of 25 feet in height and with a zero-crossing period of 6*4 seconds occurred on 18 September, and a wave 19 feet in height was measured on 29 July. There is little seasonal variation in either the wave period or spectral width parameter. The scatter diagram of Figure 10 relates the significant wave height to zero-crossing period, with the numbers of occurrences ezpressed In parts per thousand; for example, the most oommon wave conditions were those with a significant height of between 2 and feet and a zero-crossing period of between 4*5 and 5 seconds, which occurred for 65 thousandths, or 6*5 per cent, of the time. The rapid attenuation of the shorter waves with depth means that the pressure units, which are necessarily situated at about 4'9 feet below mean water level, do not record waves which have a period of less than about *5 seconds; this is the cause of the cut-off below that period. A parameter which is sometimes of interest is the wave steepness, expressed as wave height : wave length; it may also be expressed as a decimal number. It should be noted that the steepness of a wave is not the same as the maximum slope of the water surface during the passage of a wave. linesof constant steepness of 1 : 20 and 1 : 40 are drawn on Figure 10.

9 4e (in thia oase, steepness relates to significant wave height : wave length calculated from the zero-crossing period.) important feature of this analysis is the number of waves i%ith high values of steepness, which would appear to result in the occurrence of individual waves steeper than that theoretically possible. The reason for this is almost certainly the presence of strong tidal currents, which o(ui reach 2"6 knots at spring tides, according to the Admiralty chart. As the vessel is anchored, when the current la flowing strongly in the same direction as the waves, the vessel behaves as though it were travelling through the water at a speed equal to that of the current, resulting in an encounter peirlod shorter than the true wave period. The effect of this on the subsequent analysis is that the apparent period, and therefore the apparent wavelength, is shoirter, giving increased steepness, The converse situation also applies when the tidal flow is reversed, resulting in a longer encounter period. The overall result of this effect is that there is a spread of apparent wave period introduced by the method of recording, but this is more important in its effect at shorter periods and nearly inegligible at longer periods. With a current of 2*6 knots travelling in the same direction as the waves, a real period of abou^ 4 seconds will appear as one of about * seconds, 5 seconds appears as about 4' seconds, 6 seconds appears as atcmt 5«1 seconds, 7 seconds appears as about 6*2 seconds, 8 seocwk^ appears as about 7*2 seconds, From the persistence diagram, Figure 11, may be deduced the number and duration of the occasions in 1 year on which waves persisted at or above a given height. For example, if the limit for a particular operation of a vessel is a significant height of 6 feet, it would have been unable to operate for spells in excess cf 10 hours on 5 occasions, or spells in excess of 24 hours on 14 occasions.

10 A 'Lifetime' wave can often be predicted by use of the presentation used in Figure 12, but in this case the pronounced curvature makes reliable extrapolation rather unreliable. The authors wish to thank the Corporation of Trinity House for permission to install the equipment on their vessel and the masters and ore* for operating it. The efforts of their colleague Mr. J.W. Cherrimaa, which resulted in the reliable operation of the instrument, are very much appreciated.

11 6. TUCElilR, M.J A Shipbome,,'ave %eoorder. Trans. Instn. nav. Arohit. Lond, TUCKER, M.J Simple measurement of wave records. Proc. Gonf. wave recording for civ. Engra. (N.I.O.) 22-. TUCKER, M.J. 196 Analysis of records of sea waves. Proc. Instn. civ. Engrs C^ar.mGHT, D.E. and LON&UET-HIG.&INS, M.S. 19^6 The statistical distribution of the maxima of a random function. Proc. roy. Soc. A 27, DRAPER, L The analysis and presentation of wave data - a plea for uniformity, Proc. 10 Conf. on Coastal Engineering, Tokyo, Chapters 1 and 2. DRAPER, L. 196 The derivation of a 'design-wave' from instrumental measurements of sea waves. Proc. Instn. civ. Engrs *

12 PERCENTAGE EXCEEDANCE W

13 I I ' 1 FIG. 2. SPRING max( hrs) 2 U WAVE HEIGHT IN FEET

14 FIG. SUMMER maxohrs) WAVE HEIGHT IN FEET VNE

15 I T I I I FIG. 4 AUTUMN maxohrs) A WAVE HEIGHT IN FEET

16 FIG. 5. WINTER # 1, IS ^^0 I ZERO CROSSING PERIOD Tz IN SECONDS VNE

17 SPRING ZERO CROSSING PERIOD IN SECONDS VNE

18 25 SUMMER FIG HI Of Qi: U u o O10+ UJ L) (L ZERO-CROSSING PERIOD IN SECONDS 9 10 VNE

19 25 FIG. 8, AUTUMN ZERO CROSSING PERIOD Tj IN SECONDS 10 VNE

20 I I FIG. 9 WHOLE YEAR Oa SPECTRAL WIDTH PARAMETER

21 20 FIG. 10. WHOLE YEAR Uj15- % o LUlO" " 65 CALMS ZERO-CROSSING PERIOD 7% IN SECONDS VNE 10

22 J L FIG Hs IN FEET WHOLE YEAR DURATION IN HOURS VNE

23 PERCENTAGE OF TIME THAT A WAVE IS LIKELY TO EXCEED A GIVEN VALUE ONCE IN A -HOUR INTERVAL (O to (O o o o hj o g g g o yi ho C O > c m % m 5 % 1 z O" -n m «m 1 g- -n P O" < z m o ' ' I I

24 ;, w-ijs-' - % # _.il m... ii-'a'i, #$ 44K