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Indian Journal of Marine Sciences Vol. 18, June 1989,pp.1I3-1I7

Characteristics of monsoon waves off U ran, west coast of India

B U Nayak, P Chandramohan &S Mandai

National Institute of Oceanography, Dona Paula, Goa 403 004, India Received 27 May 1'988; revised 6.March 1989

Waves were measured off Uran in the Dharamtar creek (lat.18°5(")'and long.72°55') during monsoon months from 26June to20 September 1984 using a Datawell wave rider buoy. The wave records were analysed using Tucker's and the spectral methods for determining various wave parameters. Monsoon wave climate was stronger with the occurrence of the highest significant wave height of2.45 m and the corresponding maximum wave height of3.9m in July. Significant wave"heightvaried from0.45 to2.45 m, maximum wave height from 0.6 to 3.9 m and the zero crossing wave period from 5 to 18.5 s during the study period.

I '

The information on wave characteristics is extreme- ly important for any project related to coastal and offshore development. The availability of instru- mentally measured waves around the Indian coast is very limited1 - 3 and it often becomes necessary to use the ship reported visual data for engineering ap- plications. It would therefore, be of great value to carry out an instrumental wave measurement pro- gramme spreading over the vast coastal zone of In- dia. In the present study, wave measurements have been made using wave rider buoy off Uran in the Dharamtar creek in the vicinity of Bombay metro- polis. The study is intended to understand the mon- soon wave characteristics which can help in the de- sign of a marine outfall for discharging the effluents from the L.P.G. plant at Uran.

General oceanographic condition in the study area was dominated by the monsoon seasons with high waves throughout southwest monsoon period and comparativeiy calm sea during the rest of the year. The current pattern in the area was influenced by tides and the average spring and neap tidal ranges were about 4 and 1 m respectively.

Methods

Time series records on waves were obtained from 26 June to 20September 1984.The depth of water at the wave rider location was about 10 m at mean sea level and it was located on the eastern limit of the Bombay harbour (Fig. 1). The waves were re- corded for 20 min at 3h intervals in analog form on a chart paper roll and in digital form on magnetic cassettes. The significant wave height, maximum wave height, zero-crossing wave period, wave peri-

od corresponding to the maximum wave height and spectral width parameters were computed from the analog records using the Tucker's method4,5.

The wave steepness (S) is estimated as S=H/L, where Hs is significant wave height and L is wave- length at measured location with 10 m depth of wa- ter for the wave period of Tz using linear wave theo- ry6.

Digital data were analyzed using the statistical and spectral methods. Fast Fourier Transform (FFT) algorithm was used for computing the raw spectrum of the wave record and a moving average of 6 raw spectral values was used for obtaining a smooth spectra7•8

Fig. 1""7Location map

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NAYAK ef al.:CHARACTERISTICS OF MONSOON WAVES

Fig. 3- Percentage distribution ofHs. HmaX' TzandTHmax

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and August and about 50% of the time in Septem- ber. The percentage occurrence of spectral width parameter (f) and the wave steepness are shown in Fig. 4. The spectral width parameter mostly varied between 0.8 and 0.9 in July and between 0.7 and 0.8 in August and between 0.6 and 0.7 in September.

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Results and Discussion

The daily variations of significant wave height (H.), maximum wave height (Hmax), zero-crossing wave period (Tz)and wave period corresponding. to the maximum wave height (THmax) are shown in Fig. 2.

Significant wave heights varied from 0.63 to 2:45 m in July, 0.45 to 2.28 m in August and 0.62 to 1.83 m in September 1984. Maximum wave height varied .from 0.8 to 3.9 m in July, 0.6 to 3.82 m in August and 0.9 to 3.08 m in September 1984. The highest significant wave height of 2.45 m and the highest maximum wave height of 3.9 m were observed on 2 July 1984 .

The zero crossing wave periods varied from 6 to 13.5 s in July, 5.5 to 18.5 s in August and 5 to 9 sin September. Wave period corresponding to maxi- mum wave height varied from 4 to 14 s in July, 5 to 13 s in August and 5 to 14 s in September. DattatriI reported the maximum wave height varying from 0.4 to 5.6 m during the southwest monsoon off Mangalore along the west coast.

Monthwise percentage distribution ofHs, Hmax, Tz and THmax are shown in Fig. 3. Most of the time, the significant wave heights persisted between 1 and 1.5 m. Significant wave heights in this range occurred about 40% of the time in July, 50% of the time in August and 45% of the time in September. The maximum wave heights between 1 and 2.5 m oc- curred about 90% of the time in August and Sep- tember and about 75% of the time in July. Maxi- mum wave height of 2.5 to 3 m occurred 20% of the time in July. The zero crossing wave periods persist- ed between 8 and 9 s for about 27% of time in July

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115

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FREQUENCY IN S-'

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INDIAN J MAR seI, VOL. 18, JUNE 1989

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Fig. 6-Representative sample bycomputed wave spectra

The high values of spectral width parameters indi- cated that the monsoon waves in July and August were distributed in a broad frequency band.

The joint distribution of significant wave height and zero crossing wave period for different months are plotted in the form of scatter diagram in Fig. 5.

The line of constant wave steepness (H/L) superim- posed on the scatter diagram indicates that the mon- soon waves have the steepness mostly lying between 0.015 and 0.03. Similar observations were also made in earlier studies off Mangalore coast! and off Kerala coast'!.

Typical wave spectra representing the monsoon wave climate pertaining to July and August are pre- sented in Fig. 6. In general, the wave spectra Were single peaked but a few records with the occurrence of 2 peaks were observed in July indicating the suo perposition of 2 independent wave trains at the wave measurement location. This phenomenon was also observed at other parts of the Indian west coastl,lo.

In general, the wave climate on the west coast of India is dominated by 2 different seasons distin- guished by the southwest monsoon period from June to September and the non-monsoon period from October to May. High waves with compara- tively short periods have been observed during the southwest monsoon. The results of the wave para- meters obtained from the present study are based on the limited period of observation of about 3 months. Though measurements for 1 year would be essential for evaluating the variation of wave para- meters over an annual cycle, the wave climate of the monsoon which covers the roughest season of the year, reported in the present study, will be of value in design applications.

Acknowledgement

The authors are thankful to the Director for the encouragement. Financial assistance provided by the Oil and Natural Gas Commission, India is grate- fully acknowledged.

116

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NAYAK et al.:CHARACTERISTICS OF MONSOON WAVES

References

1 DattatriJ,] Waterways Harbour Div ASCE, 99 (1973) 39.

2 BabaM &Harish CM, Indian] MarSc~ 14 (1985) 1.

3 Dattatri J &Nampoothiri S M, I Nat Conf on Dock and Harb Engg (Indian Institute of Technology, Bombay), 1985, E87.

4 Tucker M J, Proc Inst Civil Engrg London. 1 (1963) 305.

5 Tucker M J, Ocean Wave Spectra, (Prentice-Hall, New Jer- sev)1963,219.

6 Weigel R L, Oceanographical Engineering (Prentice-Hall, New Jersey) 1963.

7 Torsethaugen K&Krogstad H E, NEPTUN-A computer program for the analysis of ocean wave records, Report (Continental ShelfInstitute, Trondheim), 1979.

8 Cooley J W&Tuckey J W, Math Comp, 19 (1965) 297.

9 Hameed T S S&Baba M, Ocean Engrg, 12 (1985) 309.

10 Harish C M&Baba M. Ocean Engrg, 13 (1986) 239 .

..

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