Wave Characteristics Along Visakhapatnam Coast

Transcription

1 Indian Journal of Marine Sciences Vol. 3, December pp. 1974, Wave Characteristics Along Visakhapatnam Coast R. VARADARAJULU & G. SATYANARAYANA RAJU Department of Meteorology & Oceanography, Andhra University, Waltair Received 15 October 1973; revised received 28 October 1974 Wave characteristics along Visakhapatnam coast have been studied with a view to finding out the property of high storm waves responsible for erosion along this coast, and their effect on navigational operations along Visakhapatnam coast. Analysis of wave height, direction and period in different months shows that the predominant wave directions are southwest and northeast, and high amplitudes are associated with stormy weather conditions in the Bay of Bengal. Wave characteristics are explained in relation to the climate over India and its neighbourhood. An attempt has been made to hindcast the significant waves along Visakhapatnam coast during OctoberDecember when severe tropical storms of Pacific origin enter into the Bay of Bengal and intensify. PNJ method adopted appears to give satisfactory results for this region when compared with the ship observations. Wave characteristics along the coast significantly differ from deep sea wave characteristics and the deviations are attributed to the refraction while propagating over irregular bottom topography in shallow water. CEAN waves along various coasts have been studied since the advancement of navigation and development of harbours. Wave characteristics in the open ocean and in the basins are statisticall y analysed from data collected over a long period and Atlases for heights and periods have been prepared, monthwise, for all regions navigationally important. But, forecasting of waves along ship tracks and around the harbours will be very useful for safe ravigation and to prevent damage to the sailing vessels and to the harbours. In view of these, attempts are being made to correlate different meteorological phenomena, primarily the wind characteristics with wave characteristics and the generating area, and to estimate the probable change in wave characteristics at different distances from the generating reigon, since the waves are considerably influenced by local winds, frictional losses accross the airsea boundary and especially bottom friction in the shallow waters. Wave spectra and statistical distribution of wave characteristics have been studied 18 andempericaland statistical relations between wave characteristics and wind characteristics have been established. Munk andarthur r 3 and Pierson et a l 9 have attempted forecasting the ocean waves from wind data in the generating area and the neighbourhood. In the present investigation, an attempt to forecast ocean waves in the neighbourhood of Visakhapatnam following the method of Pierson et al. 9 has been made. PNJ (Pierson, Neumann and James) method has been taken up due to its reliable statistical approach. Further, the energy distribution of wave spectra predicted by PNJ method is more reliable than the one predicted by other methods, which are also attempted with similar data over the Bay of Bengal. The hindcasted values of wave characteristics for few severe cyclonic storms agree well with the reported values from ships around Visakhapatnam. Hence, the PNJ method seems to be quite suitable for forecasting ocean waves in the Bay of Bengal. Wave characteristics available in the Atlases for various regions in the Bay of Bengal represent the 120 average conditions since they are prepared from observations collected by ships under fair weather conditions. So far no attempts have been made to investigate the influence of cyclones in the Bay of Bengal on the state of the sea, andthe study of waves during cyclones in the Bay of Bengal will be of immerse value for navigational operations in the sea and also in the harbours. In addition to the hindcasting of waves, characteristics of waves reaching Visakhapatnam coast during storms in the Bay of Bengal are also presented in this paper. Methods Wave heights and periods along Visakhapatram coast visually recorded by the Visakhapatnam port for the last 30 yr were analysed for the wave characteristics in shallow water. Wave characteristics were collected from the daily weather reports published by India Meteorological Department for the past 15 years. Data on wave characteristics were collected from the ship observations published in the Indian daily weather report for an unselected station during storm periods. Wave characteristics for the storm and calm weather were seperately processed in order to distinguish the nature of waves under normal and disturbed weather conditions in the Bay of Bengal. Forecast of waves primarily requires irformation on the prevailing wind conditions in the generating area and its neighbourhood and the state of the sea and wind characteristics during at least the preceding cyclonic day. Data on wind characteristics in the generating area and its neighbourhood were obtained from the Indian daily weather reports. Information of wind speed and direction will be rarely available in the storm region since mercantile vessels avoid the bad weather regions and hence wind characteristics in the selected fetch were derived from the sea level atmospheric pressure distribution. Gradien t winds were calculated using the following relation, between gradient wind and pressure gradient : l/2 Vg = f.lr sin C$ I

2 VARADARAJULU & RAJU: WAVE CHARACTERISTICS ALONG VISAKHAPATNAM COAST where ap/ar is pressure gradient; V,, gradient wind speed; R, radius of curvature of isobar; P, density of air; 51, angular velocity of rotation of earth; and #, latitude of the station. Sixty per cent of gradient wind has been used as the actual wind as indicated by Ramanadham and Varadarajulu 10 for the Bay of Bengal. The use of 0.6 as the ratio between the actual wind and the gradient wind also agrees well with the conclusion of Johnson l1 obtained from the airsea temperature difference. However, during cyclones, the above ratio seems to be higher having a value of 0.75 approximately and this may be related to the stability criteria in the lower atmosphere. Forecasting of wave characteristics for few storms has been made following the method of Pierson et al 9. The method requires careful selection of fetches responsible for the swell at the forecasting spot. The fetch has been located on the weather map suitably from the distribution of isobars in the storm area such that the forecasting region is along the direction of progress of waves or in its neighbourhood. Results and Discussion Wave characteristics Results on wave characteristics along Visakhapatnam coast are according to the visual observation conducted by the Visakhapatnam Port Authority in the shallow waters. Hence wave characteristics may be slightly different because of the refraction effects leading to charge in the direction and in amplitude. Wave heights during calm weather have a range of 210 ft. During the northeast monsoon season, low waves with heights rarely exceeding 3 ft prevail. During April to October, wave heights range from 4 to 10 ft. The corresponding periods are 6 to 12 sec. The most frequent waves have periods between 8 and 10 sec. In general, the wave periods increase with the increase of wave height. The direction of approach is from east and east southeast during the northeast monsoon season, west during southwest monsoon and premonsoon. Characteristics during fair weather are safe for ravigation and are of no consequence in producing rough sea or strong littoral currents rear the coast. Study of waves during storm periods for 30 years (the magnitudes of which are given in Table 1) indicate widely varying amplitudes in the same month for different years or different months of the same year, since the waves originated from different storm of different intersities located at different positions in the Bay of Bengal. Waves as high as 40 ft are reported during October A careful analysis of storm waves reported during 1936 to 1964 shows that high waves have been reported prior to 1949 and that the heights of storm waves rarely exceeded 8 ft. After 1949, the waves were highest in 1961 and the average distribution of wave heights is about 56 ft even for the severe storm. The forecasts prepared for 15 storms during 1955 and 1965 for September and October indicate the approach of significant waves of about 18 ft whereas the corresponding observed wave height is only 6 ft. The predicted values for the period 1936 to 1946 are fairly in good agreement with observed amplitudes. Wave heights reported by the Visakhapatram Port Authority during storm periods discussed above indicate that the wave heights in shallow water are largely modified after 1949 and the wave heights are signficantly reduced. For the coastal investigation along the Visakhapatnam region the average and maximum wave characteristics are essential TABLE 1 WAVE HEIGHTS (FT,IN.) ALONG THE VISAKHAPATNAM COAST DURING STORMS IN THE BAY OF BENGAL FROM 1936 TO 1963 Year :z:; Jan. Feb. c 3,o March April 7, 3 8, 0 3x3 May June 15, 3 16, 1 i6 9,lO YO 9,o 13, ,6 10,s 8,6 6,2 10; 6 4, 6 7, 9 July Aug. Sept. Oct. 11, 4 12, 6 10, 0 17, 6 9, 0 8, 4 10, 0 12, : t : : 77: 0 t : : 9: C 6 1:: ; 102 7,4 Nov. Dec. 12, 6 10, 5 12, 6 5, 3 13, 2 12 : 333 7: 2 183: ii,; :; 5 I_2 4:10 fo? 2:, T Fl 4, 6 6,o 67 : 5, ,9 3, 0 4,o 4,6 C = calm.

3 INDIAN J. MAR. SCI., VOL. 3, DECEMBER 1974 and hence the frequency distribution. of heights ar,d periods of deep water waves approaching Visakhapatnam coast, from different directiocs have been analysed and the results presented in the form of Rose diagrams. Mean annual frequency distribution of average wave heights from differentt directions (Fig. 1) indicates that for about 20% of the days of the year, waves approach from southwest and wave heights rarge from 4 to 8 ft. For about 5% of the days, the heights exceed. 8 ft. About 80% of the waves from southwest have periods less than 7 sec and only about 3% have periods greater than 13 sec and the remaining fall between the two. Next to southwest the predomirart direction of the wind is northeast from which 12.5% of the waves propagate and 70% of the waves coming from this direction have heights less than 4 ft and rarely exceed. 8 Significart ft. numbers of waves come from south southwest, east northeast and west southwest. Mean. annual frequency distribution of average wave heights and maximum wave heights (Fig. 2) has a distinct south southeast direction and very TOTAL NO. OF OBSERVATIONS 3*71m &ABOVE SCALE: km?/. Fig. 1 Mean annual Rose diagram for wave heights 6.1 TO 12ft N Fig. 2 Mean annual Rose diagram for average and maximum wave heights [Numerator in the centre of each diagram indicates percentage of calm period and denominator indicates total number of observations] rarely has a direction away from the coast. On an average, the sea is calm for about 33% of the days and moderately high waves prevail durirg 25% of the days. Wave periods mostly rarge from 7 to 13 sec and only occasionally waves of lower and higher period prevail. The preceding discussion on. wave characteristics of deep and shallow water waves shows significant difference on all wave characteristics, viz. direction, height and periods, while waves travel from deep to shallow water. Wave periods show considerable increase while waves enter into shallow water from deep sea. Average wave heights in shallow water are comparable with wave heights in deep water but the maximum heights reported in shallow water are relatively higher than those occurring in the deep water. Significant differences of wave characteristics in deep ard shallow water may be attributed to the irregularities of the bottom topography of the shallow water which produces shrinking or stretching of wave fronts. High waves reported in the shallow water are of the storm period and corresponding observations in deep water may be missirg since ships rarely ply in the vicinity of the severe storms. The change in direction of waves is due to the refraction since there is a charge in velocity as the waves enter into the shallow waters. Wave heights must certainly have been affected by the bottom topography as they approach the coast. Findings on bottom topography variation in the vicinity of the Visakhapatnam harbour 12 show a shoaling of about 9 ft near the 3 fm line and an erosion of 600 ft wide beach to the north of the Visakhapatnam harbour entrance channel. Thus the movenent of waves over longer distance in shallow waters play an important role in damping of waves and hence in the reduction of wave heights. Considering the above modification of wave characteristics in the shallow waters of Visakhapatnam coast further studies on hindcasting of waves have been made out for various spots in deep sea in the Bay of Bengal where ships have reported wave characteristics during fair weather period and cyclonic weather in the Bay of Bengal. Wave forecasts About 17 cyclones during 1955 to 65 have been selected for forecasting of wave characteristics off Visakhapatnam and the results are compared with the observations made by ships plying very near Visakhapatnam coast. The average height of the significant wave is around 10 ft for storms of low intensity and occasionally exceeded 20 ft for severe cyclonic storms. The results of the forecasts for the 3 storms in different months of 1960 and two fair weather situations in generating area with high winds during 1973 are discussed. The pressure distribution of cyclone on 31 Oct is shown in Fig, 3 and the fetch effective to Visakhapatnam coast is towards northeast, since the waves generated in. this region are directed towards Visakhapatnam. Similarly for the cyclones of 18 Nov. and 2 Dec suitable fetches are determined. Surface pressure distributions for the above two storms are shown in Fig. 3. The fetches are so chosen by trial and error so that reasonable values of wave characteristics are obtained. In the cyclone of 31 Oct., 75% of the computed gradient wind speed is 28 knots. For the cyclone on 18 NOV., 75% of the computed gradient wind speed is 24 knots and for the cyclone

4 VARADARAJULU & RAJU: WAVE CHARACTERISTICS ALONG VISAKHAPATNAM COAST of 2 Dec. 1960, 75% of computed gradient wind For the cyclone of 31 Oct. significant wave speed is 20 knots. It is assumed that the sea is fully developed under these conditions. height reported is 6.3 ft value which of 6.5 is in ft. agreement with the Significant wave height given as a forecast after 30 hr after 0830 IST on 18 Nov. is 4.2 ft. No corresponding observed value is available. But this can be taken as a fair guess since the wind speed is 24 knots. Similarly the value of significant wave height given by forecast, 27 hr after 0830 IST on 2 Dec., is 4.3 ft. This is due to the nearness of the storm to the point of forecast. The forecasted values for the 3 storm are shown in Table 2. Average periods of waves for storms on 31 Oct., 18 Nov. and 2 Dec are 8, 7 and 6 set respectively. But from number of observations the value is found to be between 5 and 26 sec. Two fair weather situations are discussed with 31 OCT. 60% of geostrophic wind speed. The first was on 18 June Sixty per cent of geostrophic wind is 29 knots ard the reported wind is 34 knots. The average wave height is 12 ft and significant value is 19.6 ft. For the other storm on 19 June % of geostrophic wind is 23 knots and the observed wind is 28 knots. The average wave height is 7 ft and significant value is 11 ft. The average periods are 8.3 and 6.5 sec respectively. The reported value of the sea on 19 June 1973 is 10 ft. On 18 June only swell is reported. The reported value of the swell is 13 ft. Corresponding 25' 8 NOV. Fig. 3 Surface Fressure.distribution of cyclones during different days at 0830 IST in 1960 TABLE ~FORECAST OF WIVES GENERATES BY STORMS IN THE BAY OF BENGAL Hours Date Av. wave IST height ft SignifiLant wave height ft lo60a l do do l 16Ob do do do ~ do do d Av. period: a, 8 set, b, 7 xc, c, 6 set TABLE 3 AVER.~GE AND SIGNIFICANT W.~VE HEIGHT AT DIFFERENT INTERVALS OF TIME Date and IST (311060) 2330 (l1160) 0230 (l1160) (l1160) (l1160) (l1160) % Elf,) :; E(fa) 41. zz.5 Difference of E(f,) and E(fe) E = 21 o/o of difference /E ,/Z av. height /E significant height :.ii 19 ;:

5 INDIAN J. MAR. SCI., VOL. 3, DECEMBER ' 75' 80 85" 90" Fig. 4 Fetch used for computing wave heights periods of swell on 18 and 19 June are 8 and 10 sec respectively. An example for the workirg out the forecast is given. below: Av. period = 0285x V = 8.3 sec Observed height is 10 ft Observed period is 10 sec. Calculation of wave heights based on PNJ. method. Acknowledgement. (31 Oct. 1960) The input data and calculated values are given in Table 3 and Fig. 4. Computed wind speed, 37 knots; 75% of computed wind speed, 28 knots; F = fetch lergth = nautical miles c: 200 nautical miles R0 = 243 nautical miles 2: 250 nautical miles t& =7 ; 04. = 120 angular spreading for e3 = 57.5% and for e4 = 36.5%; difference = 21% So angular spreading factor = 21% From cocumulative spectra, the minimum important frequency present is From the dispersion. diagram it takes 12 hr for the first component of the spectrum to reach the station.. So it is sufficient to forecast at 15, 18, 21, 24, 27 and 30 hr. f2 and f6 are determined from the dispersion diagram usirg the value R0 = 250 and R0+F = 450. Then from the cocumulative spectra E(f2) and E(f6) are determired. The av. period T = 0.285X V(V in knots) Av. period = 8 sec Forecast for the fair weather situation (19 June 73) E = (V/10)5 V in knots Dimensions of E are ft 2 Av. wave height = 7 ft Significant wave height = E = 11 ft 124 Authors are thankful to Prof. K. Kamanadham for his encouragement and able guidance. Thanks are due to India Meteorological Department, Government of India and Visakhapatnam Port Authorities for the supply of data. References 1. JOHNSON, J. W., Trans. Am. Geophys. Union, 31 (1950), MUNK, W. H. & ARTHUR, R. S., Forecasting ocean waves (Compendium of Meteorology Boston American Meteorological Society, Washington), 1951, & JOHNSON, J. W. & RICE, E. K., Trans. Am. Geophys. Union, 33 (1952), PUTZ, R. R., Trans. Am. Geophys. Union, 33 (1952), LOUNGETHIGGINS, M. S., J. mar. Res.. 11 (1952), PIERSON, W. J. (JR) & MARKS, W., Trans. Am. Geophys. Union, 33 (1952), DARLINGTON, C. R., Q. Jl R. met. Soc., 80 (1954), PHILIPS, P. M., J. Fluid Mech., 2 (1957), PIERSON, W. J. (JR), NEUMANN, G. & JAMES, R. W., Observing and forecasting ocean waves by means of wave spectra and statistics (US Navy Hydrographic Office H.O.), Publ. No. 603, RAMANADHAM, R. & VARADARAJULU, R., Indian J. pure appl. Phys., 3 (1965), JOHNSON, P. W., Proceedings of 1st conference on ships and waves (Berkeley Council of Wave Research, USA), 1955, 509:. 12. RBMANADHAM, R. & VARADARAJULU, R., J. Inst. Engg (India), 47 (1967), 229.