A TENTATIVE ANALYSIS OF WAVE DATA FOR DESIGN WAVE CRITERIA AROUND TAIWAIN

Transcription

1 ACTA OCEANOGRAPHICA TAIWANICA SCIENCE REPORTS OF THE NATIONAL TAIWAN UNIVERSITY NO.3. PP. -4, 9 TABLES, II FIGS, DECEMBER. 973 A TENTATIVE ANALYSIS OF WAVE DATA FOR DESIGN WAVE CRITERIA AROUND TAIWAIN C. L. BRETSCHNEIDER \ ABSTRACT The island of Taiwan is situated climatically between short winter northeast monsoons and long summer western Pacific and South China Sea typhoons. The oceanic areas of Taiwan, as well as the waters to the north and east of Taiwan, are rough during the monsoon season. During the typhoon season normal sea states are much less than during the monsoons, except when a typhoon actually affects the area; then wind and wave conditions are experienced. Thus, wave conditions around Taiwan come from two different statistical tamilies, namely the monsoon and the typhoon families, although there is an overlap between the two. There is an immediate need for design wave criteria for coastal and offshore structures, underwater pipelines, mooring lines, and the like, for the waters around Taiwan. Some wind and wave data have already been gathered for a few locations in coastal waters for both monsoon and typhoon conditions. These data have been compared with wave hindcasting techniques, and have been presented in an excellent paper by Tang (970), following the procedures given by Ijima (960). Other work is continuing, but this only the beginning of a much needed larger research program required to establish better statistical sea state and design criteria. The work of Tang (970) was of a specific nature. The present report is of a general nature, and we consider only extreme conditions required for design wave criteria. Two approaches, different from that used by Tang (970) are used to obtain reasonable estimates for design wave criteria for the general area of Taiwan. Fisrt, we used data of Hogben and Lumb (966), based on voluntary ship wave obserations for the 8-year period 953 to 96. These data are from both monsoon and typhoon conditions, but no attempt was made to separate the data. A simple approach was used to obtain significent wave height vs, recurrence intervals. Second, we used the hurricane wave model of Bretschneider (97) and typhoon information reported by Chin (97). Of 33 significent typhoons affecting the Taiwan area during the!o-year period 96 to 970, 0 of the typhoons have nearly adequate meteorological data to use in the wave model, although certain assumptions had to be made. Results of the analysis of typhoon conditions were compared with our statistical analysis of ship wave observations. The typhoon data are not from the same time period as the ship wave observation. Finally, we made recommendations for selection of the design waves to be used, but with caution, until additional field data, research and office studies can be undertaken, including wave hindcasting. In the meantime, this report should be used for design waves from deep water shoreward to the coastline. After the research program has been completed and the results have been printed, then this present report should be updated and expanded, revised where necesary or else replaced with an entirely new but similar report. INTRODUCTION There is an immediate need for design wave criteria for coastal and offshore structures, under water pipelines, moorings and the likes for the waters around Taiwan. This report is intended to help ease the frustration situations that the engineers have when they seek design wave criteria to calculate corresponding pressures, forces and loads on structures, in order to determine the stresses, responses and motions of the structures.

2 C. L. Bretschneider This report makes use of twq separate approaches to the needs of the design engineer. First, we have used the data of Hogben and Lumb (966), based on voluntary ship wave observations, corrected to significant wave height and period. A very simple statistical method has been used to extrapolate these data to 00 year recurrence interval. Second, we have used with certain assumptions the hurricane wave model of Bretschneider (97) and actual typhoons that have adequate meterological data for those typhoons having significant effect on the area around Taiwan. The typhoon information is given in Vol., Royal Observatory Hong Kong by Chin (97). There is a great need for wave gaging stations around Taiwan for both coastal and offshore structures in order to arrive at better design criteria, both from the point of view of economics and safety. The results in this report seem conservative, but one can never be sure until there is a direct comparison and correlation between recorded wave data and those wave observations and wave hindcast data used in this report. In the meantime there is very little choice for the Design Engineer, to use either this report or some other reports or means for arriving at design wave criteria. This report should be used only as a temporary means to determine design wave criteria. After a thorough field wave gaging program has been 'completed, then this report should either be revised, brought up to date, or else discarded for an entirely new report. If the field wave gaging program agrees substantially with this report, then this report can continue to be used, otherwise it should be replaced with a new report. All calculations made in this report are slide-rule calculations and are subject to some error, but such errors are probably far less important than the errors of interpretation of the results, because of some of the assumptions used may be questionable. Wave heights given in this report are deep water significant wave heights. Fig. shows the location map of Taiwan and typical paths of typhoons. Fig.. Location Map of Taiwan anrj Typical Paths of Typhoons.

3 A Tentative Analysis of Wave Data Jor Design Wave Criteria Around Taiwain 3 AN EVALUATION AND INTERPRETATION OF WAVE DATA BY HOGBEN AND LUMB (966) Hogben and Lumb (966) compiled ship observations for all the world, for the yeass 953 to 96 (8 years). They grouped the Marsden squares into areas, because there seemed to be not adequate wave data for Marsden squares. The areas given by Hogben and Lumb (966) see Fig. are of regional interest and are therefore of special general area interest. It can be seen from Fig., that Taiwan is located within toe general area No.!. The key to wave height and period is given in Table. Table for all seasons and all directions observations wave height and period are from Hogben and Lumb (966), who report the wave height and wave period to points of the compass, i. e. every 30 degrees. They also give the data by seasons and to points of the compass. Table only represents a summary. For more detail the reader is referred to Hogben and Lumb (966). Fig. 3 shows the number of observations versus direction, area, which is for Taiwan. It is seen from Fig. 3 that most of the observations are from the North and Northeast, corresponding to the Northeast Monsoon season. The maximum wave heights do not necessarily coincide in direction with the maximum number of observations. The cold fronts that come off mainland China and the typhoons that occur during the non-monson are responsible for this difference between maximum wave height and maximum number of observations according to direction. Hogben and Lumb (966) give statistical relationships between voluntary ship wave reports ship wave reports and weather ship wave observations reports. They have also correlated ship borne wave recorded data with weather ship wave observation reports. In terms of significant wave height, based on spectral analysis of ship borne wave recorded data, one can determine the following empirical equation. H/s= H v o () Fig. 4 shows the statistical relationship based on eq., but when observational wave height is zero eq. () predicts significant wave height equal to meters. This can not be. This comes about only because they used no data for low wave heights and used linear regression analysis. If they had use a polynominal expression instead and used the condition that when H o w =0 so does H/s=O. The dashed line going through the origin (H o w =H/s=O) is the interpretation of the author of this report. The analysis of the data by Hogben and Lumb (966) considered first the conversion of the code, Table to significant wave heights. Since we are interested in extrapolation into the future the proceedure was to tabulate the number of observations beginning with the maximum value of the significant wave height, and then accumulate the number of observations with decreasing wave height. In this manner we define the recurrence interval for wave heights equal to less than by the following equation: y R.I.=N () where R. I. = recurrence interval in years Y = number of years of record (8 years) N=number of wave observations accumulated from the highest downward to the lowest Hl/ s = significant wave height beginning with the largest value. We then plot R. I. (recurrence interval) versus H/S significant wave height on semi-log graph paper (this could have been done by statistical least squares techniques). The exact use of R. I.-recurrence interval in this case is not entirely clear because the data itself is not entirely clear with respect to location and time. The ships pass certain preferred routes, and when the weather gets bad, they tend to steare off course. However, for the sake of

4 YAJlSDEN SQUAR.E CHAltT ". 50 Fig.. Grouping of Marsden squares Into Areas (After Hogben and Lumb 966)

5 A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain Table. Key to Wave Height and Wave period codes Wave Height Code Feet Wave Height Metres Wave Pepiod Code X o / W~ve Period // Seconds Calm or period undetermined 50' less 6 or 7 8 or 9 0 or 6 or 3 4 or 5 6 or 7 8 or 9 0 or... _..._-_.....Dver;L.

6 C. L. Bretschneider ~~I ~ j o ~ t3: Totals 9~ X Table. Wave Heights and Wave Periods Area All Seasons Direction Class-Alldirections Wave Period Code o 4 7 _ 33cf \ 3~~:: 60 IN 36cf (f 3et 69 7cf :6~ /90 69~. ~~//:3 69\ "'~ 0" 5d' Bd' Fig 3. Number of Wave Observations Versus Direction, Area.

7 A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain 7 a / /!).'l7' 7. ~~. ~~V.: V -:,4z... ItITERPOtATI II / / / o o 4 6 j 8 0 Fig. 4. OBSERVED WAVE HEIGHT METERS Relationship between Recorded Significant Wave Height and Observed Wave Height Hl/s= Hv.o. B based on Hogcen and Lumb (967) presentation of the data we will use the term recurrence interval. With this explanation we should all understand what we mean. In this manner we then plotted the data for various directions, all seasons, and all directions all seasons on semilog graph paper. The results of these plots are shown in Fig. 5, 6 and 7 for the various directions of the observed data. It will be noted in these figures that there is generally very little scather except. at the very upper most extremes where the deviations of the eye-balled line are extremely great, this is probable due, and obviously likely due, to the fact that the data includes typhoon waves which are very large and extremely rare when compared with the many days of observations of Monsoon waves. The ideal thing would be to separate the Monsoon wave data from the typhoon wave data and statistically analyse the data separately. This is hardly practically since it would be difficult to go back to the actual situations at hand, but it could be done if enough of the information from ship logs is available in the achieves. In the development of Fig. 5, 6 and 7, the curves drawn neglected these extreme points as can be seen. Since typhoon data are of such a radical nature, it might be supposed for example that these data come from another sample similar to the sample of a comet, that is in these graphs the data for typhoons for ten year recurrence interval might actually be 0, 50 or 00 year typhoon data. One must be very careful how one handels wave statistics when the waves themselves are of two entirely different families, normal Monsoon, and typhoons. We have extrapolated the data on Fig. 5, 6 and 7 to the 00year recurrence interval. Based on the results of these extrapolations we have prepared Table 3 predicted deep water significant wave height versus our definition of recurrence interval, for,5, 0,5,50 and 00 years. From the results of Table 3 we have prepared Fig. 8, one, ten and 00 year expected wave observation. It is of great interest to note the comparison between Fig. 3, number of wave observations

8 c.l. Bretschneider AREA o o. ALL SEASONS x 30 o 60" A 90 of ALL DIRECTiONS 0. Fig Significant Wave Hesght sn Meters Significant Wave Height vs Recurrence Interval of Observations in Years. Directions Between North and East. 8 0.'

9 A Tentative Analysis 0/ Wave Data lor DeliS" Wave Criteria Around Taiwain 9 j.5 5- J ~Q)...0 *:-_---;!;:--_-+- ;!-_----;!;:--_-±-_---:~-_:_! o Significant Wave Height in Meters Fig. 6. Significant Wave Height vs Recurrence Interval of Observations in Years.

10 0 c.l.: Bretschneider AREA ALL SEASONS X 330 ~ ;ALL DIRECTIONS 0... ) Fig Significant Wave Height in Meters Significant Wave Height vs Recurrence Interval of Observations in Years, Directions Between West and North.

11 Direction A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain Table 3. Predicted Deep Waetr Significant Wave Height Observations Verus Recurrence Interval in Years for Area (Taiwan) Significant Wave Height in Meters Recurrence Interval in Years O' or 360' ' ' ' ' ' ' ' ' ' ' ' ALL Dir, Hu s Hmax=.6 H/s' o - Years -0 years -00 years 0 80 Fig. 8. One, Ten, and 00 Years Waves Based on Figures 5, 6. 7.

12 C. L. Bretschneider versus wave direction and Fig. 8, one, ten and 00 year expected maximum wave observation versus wave direction. The most number of observations come from the North and Northeast, typical of Mossoon waves. The largest waves come from the Northwest and the Southwest. The Southwest waves are very infrequent, but when they do occur they are very large. HINDCAST OF TYPHOON HIGHEST SIGNIFICANT WAVES A model hurricane wave field corresponding to a model hurricane wind field has been proposed by Bretschneider (97). Some typhoon wind and pressure data can be used, listed in the report of the Royal Observatory Hong Kong, by Chin (97). The model hurricane has been applied to 0 typhoons affecting the area around Taiwan for the period There were 33 typhoons and tropical storms of importance during , and they are listed in Tables 4, 5 and 6. Of these 33 typhoons and tropical storms, 0 had almost sufficient meteorological data so that one might apply the model hurricane wave field. None of the typhoons prior to 96 had adequate data so that the model hurricane could be applied. Note that this period of years follows the period of observations by Hogben & Lumb (966). The 0 typhoons had fixes on location central pressure and maximum winds. These fixes were obtained by U. S. A. aircraft (airforce and navy). Fig. 9a and Fig. 9b shows these 0 typhoons. Table 4. Typhoons and Tropical Storms Taiwan (96-970) Period % 96 % Total Apr. -5 (Violet) May -5 Betty Judy May 3-June 4 Babe June 0-4 Dinah * June 5-9 Dorisl July 5-9 Doris July 0-4 Elsie Wendy July 5-9 Kate Nina July 0-4 Harriet Nadine July 30-Aug 3 June Opal Aug. 4-8 Betty Aug. 4-8 Mary Aug. 0-3 Lorna Aug. 4-8 Wendy Aug. 9-Sept Amy Cora Sept. 3-7 Gloria Fran Sept. 8- Pamela * Georgia 3 Sept. 3-7 * Sept. 8- Essie Sept. 3-7 Sally Dinah Sept. 8- Oct. Flossy Nov. 7- Gilda ---"_.--_ _._----_ _._._-~---~---~._._ _._._._..~.- Total * No Name

13 .._ ~_ A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain 3 Table 5. Typhoons Near or Over Taiwan of Major Importance (96-970) No. ~._-_._ _ Year and Designation (5 day period) Closest to Coast Max Min.. --~----_ --~._.._._--"--- Month Day Hrs. Us (Knots) mb Us (Knots) mb 96 Betty (May -5) May 6 (6-) June (July 30-Aug. 3) Aug ) (8) 959 (8) Lorna (Aug. 9-3) Aug ) Pamela (Sept. 8-) Sept. 0-8) Sally (Sept. 3-7) Sept. 8 (0-6) (5) (983) Amy (Aug. 9-Sept. ) Sept. 5 (0-6) (53) (94) Wendy (July 0-4) July 6 (0-6) Gloria (Sept. 3-7) Sept ) (46) Babe (May 3-June 4) June ) (0-6) 53 MSG (67) Dinah (June 0-4) June 9 (8-4) Mary (Aug. 4-8) Aug. 8 (0-8) 4 MSG 946 (58) Judy (May (-5) May 30 (6-) (7) 97 3 Elsie (Sept 8-) Sept ) Violet (April -5) April ) (40) Clora (July 5-9) July (0-6) (84) 960 (84) Nora (Aug. 4-8) Aug ) Gilda (Nov. 7-0 Nov. 8 (0-6) (4) (955) Sept. 5 <8-4) 8 Wendy (Aug. 4-8) Sept. 6 (0-) Betty (Aug. 4-8) Aug ) (57) (57) Elsie (Sept. 8-) Sept ) (8) (90) and 970 (None) mb=millibars

14 4 C. L. Bretschneider Table 6. Remarks on Tracks, Typhoon Near or Over Taiwan of Major Importance (96-970) NO. Remarks on Track 96 SSE-S over SW Taiwan, 300/4=.5 K (KNOTS) moved from S to N enter E. coast Taiwan middle then NNW S of Taipei, 70/48=5.6 K 3 moved SE to NW few rniland S. tip Taiwan, 40/4=0K 4 moved E. to WWNWover NII3 Taiwan, 4OO/4=6.6K 5 moved from E to W touch S. tip Taiwan, 0/=7.5 K 96 6 moved from SE to NW over N. Taiwan <3/4 from S), 0/4=8.7 K moved from SE to NW /3 up Taiwan, 0/4=9. K 8 moved from SE to WNW over N. tip Taiwan. 80/4=7.5 K moved from China Sea W to E over Central Taiwan aad died 0.6=0 K 0 moved from S to N over Central Taiwan, 360/4=5 K moved from ESE, E across N. Taiwan, just S. Taipei tm WNW, *\'4=7.5 K 966 moved from China Sea W. to E over Southern Taiwan then NE, 300/4=I.SIt, 3 moved from S. China Sea WSW to ENE accoss S. tip Taiwan, 300/36=8.3 K moved West to East 0 miles S. of Taiwan, 50/8=8.3K 5 moved from SSE to WEW over Northern half </3 up) Taiwan, 80/4=7.5 K 6 moved from E to W across Center Taiwan <0.6 from S. tip), 360/4=5 K 7 moved from SE to NW over Center Taiwan, 40/30=8 K moved E to W few miles S. of S. tip Taiwan, 00.4=4. K moved WNW from SE touching N. tip Taiwan, 360/4=5K 0 moved WWNW from E across Cantral Taiwan <0.6 from S. tip), 360/=5 K The model hurricane requires the following parameters: (J) latitude IlP reduction in central pressure from normal R. radius of maximum wind VF forward speed of the hurricane Three of the above parameters, (J), IlP, and V F can be obtained from Chin (97). The radius of maximum wind, R, is not available and is not easily determined, except by a very careful and detailed analysis of the weather charts, and this is not always possible, because R is very small from about 4 to 0 nautical miles. The radius R is pretty close to the full diameter of the hurricane. Some estimates of the diameter of the eyes of a typhoon can be found in reports by ship captains, present radar, aircraft, and satillite photographs. Northern Taiwan has approximately the same latitude as Southern Florida, U. S. A. the U. S. Weather Bureau has determine for Southern Florida, as well as the rest of U. S. A. East and Gulf Coasts, radius of maximum wind, R, for standard project and probable maximus hurricanes. For

15 A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain 5 the latitude of South Florida, U. S. Weather Bureau give values of R, R=4 nautical miles for small diameter hurricane, R = 7 nautical miles for medium diameter, and R = nautical miles for maximum diameter. The radius of maximum wind R increases with increase of latitude and can approach 40 nautical miles at 40 latitude.. Based upon this information, we have made estimates of possible values of R for Taiwan. The range of values we have used are R=4, 8,, 6 and 0 nautical miles. Without knowing the actual value of R for anyone of the typhoons we have calculated highest significant wave height of each of the typhoons, using each R=4,8,, 6 and 0 nautical miles, we believe that this range of R values more than brackets the true R values. We used the following formula: where HR=KIVRAP~ H R is in feet Kl is obtained from the graph of Bretschneider (97) R is radius of maximum wind in nautical miles AP is central pressure reduction from normal inches mercury /' / /, / Fig, 9a. Paths of Taiwan Typhoons For Year (March-August)

16 6 C.L. IJrltscJmeider Fig. 9b. Paths of Taiwan Typhoos For Year % (August-November) The above formula is for a stationary hurricane. A small COrrection is required for the moving hurricane. The actual highest significant wave height HA, is given by: I V ) HA=HB( + U: s where H = actual highest significant wave height HB=highest significant wave height stationary hurricane VF=forward speed of hurricane URS=sUrface wind speed, ten minute average 0 meter elevation at radius of maximum wind R The results of these calculations are given in Tables 8 and 9 for R=4 and 8 nautical Fig. 0, shows a comparison of these calculations with those given previously in Fig. 5,6, and 7, for all seasons and all directions. Fig. 0 gives the highest significant wave heights calculated for the 0 typhoons using R=4,8, and nautical miles. The calculations for R=6 and 0 nautical miles have not been used, because the values of H B and H a approved to be unreasonably high. It appears from the two methods of approach Hogben and Lumb (966) Data and Bretschneider (97) Model Hurricane Calculations that there is every reason to expect such large deep

17 Table 7. A Tentative Analysis of Wave Data for Duign Wave Criteria Around Taiwain 7 Chntral Pressure and Maximum Winds of Typhoons Near or Over Taiwan of Major Importance (96-970) Off Coast Taiwan Average Conditions Values of U No. Pn Po Po AP= V R in Knots r In Hg mb In Hg Pn-Po Knots f R=4 R=8 R= R=6 R=0 ---._-_..~~-" % (988) MSG :~ (955) % % % f=ao sin!/> Ua=KVAP-0.5 fr K=67 water waves off the coast of Taiwan. The 00 year deep water significant wave height, of 6 meters does not seem unreasonable. We have not calculated the significant wave period, but it can be calculated using the following formula: T. 40H ra U=0.4 tan h{.07(arc tan h ~ } where T = significant wave period in second U&.wind speed in knots H=significant wave height in feet

18 Table 8. Deep Water Wave Height and Period Off Coast Taiwan Stationary Typhoons and Moving at Actual Forwa 0;; rd Speed for R=4Nautical Mile No. AP f U RS H fr/ur K' RAP R U R 40H T R H a T, (IN H g ) (Knots) (feet> (Knots) (U RS ) T/URS '(sec) (feet> (sec) ! r- 965 f ~ a H ~., { ' U Rs= 866UR Hr=KVRAY R=4 N. miles

19 A Tentattse Analysis of Wave Data for Design Wave Criteria AroumJ Taiwain 9 Table 9. Deep Water Wave Hgights and Periods Off Coast of Taiwan for Stationary Typhoons and Moving Atactual Forward Speed for R =8 Nautical Miles No. FR/Ua K' n, Vas 40H a feet knots (Uas)if T a sec H a feet I i ' i : ; S ~ R=8 N. miles Since the equation is general the subscripts H.A' HR, VRs have been left off. The necessary information for calculating the significant wave period (T = T/3) is given in the Tables. However, the wave heights are subject to some type of error, and we do not know how much, it will be sufficiently good to use the following simple formula: T / 3 =.3YH/3 THE SUGGESTED DEEP WATER. DESIGN WAVE FOR 00 YEARS The 00 year significant wave height in deep water is probably close to H/3 = 6 meters or H/3 = 5.5 feet. The significant wave period will be very nearly given by

20 C. L. 8I'etsehneid" , 0 j.s j 8 j OJ FROM OBSERVED SHIP REPORTS (SEE FIG. 7) Fig Significant Wave Height in Meteos Predictions of Typhoon Waves With Comparison With Ship Observations Versus Recurrence Interval in Years.

21 A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwain T/B=.l3Y5.5 = 5.4seconds The significant wave height is the average of the highest one third or the waves. This mean about 6 per cent of the waves will be higher than 6 meters. For a structure to be safe, Forces and overturning moments must be calculated based upon the maximum probable wave height and not the significant wave height. The most probable maximum wave height can be estimated from the following formula: H m ax=hl/ 8 ~ In N <:.8Ill/ 8 (In is natunal log) Where N is the number of waves during the storm under steady state wind conditions of we took the upper limit of.78 then H m ax == 6x.78 =8.5 meters=93.5 feet, and this is not an unreasonable value, because maximum waves in typhoond and hurricanes have been estimated by observation to have exceeded 30 meters. However Taiwan location seems somewhat protected as compared to the Phillipine Island and Japan, and so ihe waves should be somewhat less. Perhaps Hmax = 6x.6= 5.6 meters is more reasonable. Perhaps also H/8 = 6meters is too high or may be too low. For example if H/3= 7 meters, then this ratio of.6 leads to H m ax = 7. meters or 89.feet. This is a game of numbers, mainly because we do not have adequate wave measurements. Based upon experience elsewhere in the world, I would like to temporarity recommend that no offshore structure in deep water be designed for less than H m ax =7.5 meters or 90 feet, and significant wave period, T/3=5.4 seconds for deep water in the immediate vicinity of Taiwan. Anything less than H m ax=7.5 meters would be temptation with safety of life and property.. The waves over the continental shelf or shallow water in the Taiwan Straits will be less, but we do not know how much less. Certainly in 50 or 60 feet of water or less the design wave will be governed by the breaking wave criteria where and H max=h,,=o.78d Hb=breaking wave height d=depth of water Then one must also taken into account bottom friction and refraction. IUs certain that if one use H m ax =90 feet in 00feet of water, the estimated loads and forces On a structure would be over estimated' In this case there would be a waste,of economy. AN ESTIMATION OF WAVES FROM DEEP WATER TO THE COASTLINE Since many strctures will be located in water less than deep, it is important to make some kind of an attempt to bring the deep water waves shoreward. As the waves propagate shoreward various modifications take place. These modifications are due to shoaling bottom friction, percolation in the permeable sea bed, refraction and finally breaking in the surf zone. We consider here only shoaling until the waves reach the breaking wave limit given. by tbe following equation: Hb=O.78d where Hb= breaking wave height d=depth of breaking

22 C. L. Bretscltneider Refraction may cause the wayes to break sooner or -Iater than otherwise, depending on each,different bottom topography situation. Bottom friction will alwayscause the wavesto break later or closer to the coast, tha~ if no bottom friction were present. For very steep bottom profiles such as off the East Coast Coast of Taiwan bottom friction is not too important. For the West Coast of Taiwan where there is a lot of shallow water, bottom friction becomes important, and in some cases can cause considerable reduction in wave height so that the waves do. not break until they are close to the Coast. The analysis in this report does consider bottom friction; as this indeed would be a lengthly study. Fig. shows how the deep water waves changed by shoaling until they reach the corresponding breaking depth. Sixteen per cent of the waves will break between the depths corresponding to H m ax and H/s' Fig. has very practical uses until more detailed studies are made. Fig. should be used for the design wave for 00 year recurrence interval for all water depth. The total water depth includes mean low water, plus maximum spring tide, plus stormsurge. 30r ~ , 6 VI 0:: w!j:j3 ~ z -4 :::c C> ~ wlo ~~ ~. 6 Fig TOTAL WATER DEPTH IN METERS Tentative Design Waves For Area Taiwan 00 Year Recurrence Interval. In general the actual 00 year wave on the West Coast of Taiwan should be less than that on the East Coast because of bottom friction, and fetch length and width limitations. How much less, one does not really know until wave measurements are made and correlated with the theory. It is true that the structures will be over designed, but how much is difficult to say. Some reduction in the design wave can be made for economics, but too much reduction in design wave can cause failure disaster loss of life and property, and of course excessive reconstruction and maintenance costs.

23 A Tentative Analysis of Wave Data for Design Wave Criteria Around Taiwaln SUMMARY CONCLUSIONS AND RECOMMENDATIONS There is an immediate need for design wave criteria for coastal and offshore structures, under water pipelines, morings and the like, for the 'waters around Taiwan. This is more of an engineering report than a scientific report. It has been found that the available data and information required of such a report is very much limited and certainly not very adequate. Netherthess, a best. effort was made to present a reasonably satisfactory report that can be used by the design engineer. The analysis of the results given in the report ends with Fig., a very useful graph for design engineers to use as design wave criteria from deep water to the coastline. It is believed that the values of wave height given are conservative, but pretty close to that which one would experience off the East Coast of Taiwan. In the Straits of Taiwan it is expected that wave heights will be less than off the East Coast of Taiwan, but how much less can only be determined by more study. It is therefore recommend that a research program on waves be initiated for the waters around Taiwan. This should include theory, a reasonable wave gaging program, analysis of data and report preparation. This should include wave hindcasting for ordinary Monsoon conditions, other storms, as well as for typhoons and tropical storms. In the meantime this report, particularly Fig., should be used for the 00 year design waves. The wave period is Tl/s=5.4 seconds, and the wave heights are as shown in Fig. from deep water to the breaking wave zone, after which shoreward breaking wave criteria govern. After the research program has been completed and the reports have been printed, then this report should either be brought up to date, revised where necessary, or else be replaced with an entirely new but similar report, ACKNOWLEDGEMENTS I would like to extend my gratetude to the Institute of Oceanography, National Taiwan University who permitted the time for writting the report, and in particular Mr. Yin Fuh who helped with the drafting and saw to it the report was completed after I left Taipei. REFERENCES CITED HOGBEN, N. and F. E. LUMB U%6) Ocean Wave Statistics, National Physical Laboratory, Teddington, Middlesex, England. CHIN, P.C. (97) Tropical Cyclone Climatology for the China Seas and Western Pacific from 884 to 970 Vol. ; Basic Data, Royal Observatory Hong Kong. R. O. TECHMEM No. MARC DECHEVRENS, S. S. (88) the typhoons of the Chinese Seas in the year 88, ZI-KA-WEI Observatory near Shanghai China. BERGHOLZ, PAUL (899) <English translation by Scott, Robert H.) Meteorological Observatory in Bremen, 7 pp, BRETSCHNEIDER, CHARLES L. (97) a non-dimensional stationary Hurricane wave model, proceeding, offshore technology conference, Houston Texas paper OTC 57. BRETSCHNEIDER, CHARLES L. (97) Revisios to Hurricane Design Wave Procticer, Proceeding 3th Coastal Engineering Conference, Vancouver Chp. 7 pp, TANG, FREDERICK L. W. (970) Researches on Calculations of Waves on Long Shorline Beaches Joutal of civil Hyitanh Engineering Cheng Kung University Taina, Taiwan China, : 0-64.

24 4 c. L. Bretschneider 一個臺灣附近風浪分析的嘗試以 作為設計波浪之準則 布萊特斯奈德 摘要 臺灣氣候受較起期的東北季風及較長期的太平洋及南中國海颱風影響, 在季節風期間臺灣海峽及 臺灣東面和北面近海地區經常有巨大之風浪, 除了當颱風實際侵襲此區棋附近外, 在颱風季節臺灣附 近之海面狀況反而較季風期間平靜 因此, 在臺灣之法浪情形, 可以分成兩頭 ; ~P 季風浪浪及颱鼠 按 浪, 而此二者有時會同時出現 為了海岸或海洋結構物, 水下管線, 碼頭等工程, 吾人迫切需要一個設計說浪的準則 一些風及投浪資料在少數地點日經蒐集到了 這些資料已經用來和設浪推算結果作一比較, 它們登在揚購置是 (970) 根據并島 (Ijima, 960) 的方法推算的研究報告中 其它的研究工作正在進行, 但對於一個為了要找出更好的投浪統計資料及設計渡浪, 如此大的研究計劃而言, 這僅是一個開端 揚麟武 (970) 的研究是屬於較專門性的 : 本報告則屬一般性的, 而僅考慮找出設計混浪準則之 極端情況, 本人以兩種不同於湯麟武 (970) JiJf 用的方式 : 其一用 Hoghen 及 Lumb (966) 的 953 年到 96 年船船自願觀測資料 遺些資料包括在季風及颺風時, 說浪的情況, 可是本報告並沒有將 此二種資料分開 僅用一簡單的指示波高興再發期間的近位關係 其二, 以作者的颱風模型 (97) 及 CHIN (97) 的颱風資料報告, 從 96 年到的 70 年的 0 年間有 33 個具有代表性的影響到臺灣的颱風為基礎其中取 0 個具有適當氣象資料的颱風用來作設浪分析, 其中有很多假設在內 分析結果再與船船觀測相比較 不過要注意的, 此二資料不是來自相伺時間的區間 最後, 害人推薦使用研選擇的設計說浪, 但要小心需要另加現場混浪實測資料 包草書說浪推算, 此項工作可由研究所及官方機構共同承擔 同時, 這份報告應用於由深水向海岸推進前的設計混浪 在前述之研究計劃完成及其結果出版之後, 然後將新資料加入及攘充此報告, 修正需要修正之處或全部更換作出類蝕的報告