Wave Energy & Littoral Drift Off Krishnapatnam

Transcription

1 Indian Journal of Marine Sciences Vol. 11, March 1982, pp Wave Energy & Littoral Drift Off Krishnapatnam B S R REDDY, G VEKATA REDDY' & K HEMATHA KUMAR Department of Meteorology & Oceanography, Andhra University, Waitair Received 16 March 1981; revised received 16 May 1981 Deepwater wave energy off Krishnapatnam (14 15', 80007'E) is computed for different months in a year. The alongshore wave energy for the region is derived from which the littoral drift is calculated by means of an empirical relation. The littoral drift oftkrishnapatnam is compared with the estimated values of littoral drift near other ports along the east coast of India. The possibilities of maintainingsuflicient depths in the entrance channel are discussed. In an earlier study off Krishnapatnam (Jat ', long. 8000TE), a minor port on the east coast of India, Reddy et au observ~d that navigable depths near the mouth of the river Khandaleru can be maintained by occasional dredging. In this paper certain aspects of the sediment transport off Krishnapatnam, along a stretch of about 12 km, are presented, and this knowledge is essential for the dredging and maintenance operations. Materials and Methods The wave data reported by the ships plying off Krishnapatnam between lat. 12 to 17 and long. 800E to 8SOE during 1967 to 1976 have been collected. from the records of India Meteorological Department. The data are ana lysed statistically and the frequencies of their occurrence from 16 directions equally divided from through E to back are obtained for each month. Wave heights and periods in each direction have been classified into 10 ranges. Average distribution of wa ve heights and periods for different months are presented as Rose diagrams (Figs 1 and 2). The weighted mean values of the wave heights (Ho) for each month has been calculated using 10 yr wave data. Similarly the weighted mean values of the wave periods (To) for different months have been obtained from which the corresponding wave lengths in deep water have been calculated. The deep water wave energy (Eo) is calculated from the following relation given by Jhonson2. Eo = ploh~ [l4.98(h 0)2 /(Lo)] 8 where p is the density of seawater and Lo and Hoare the deep water wavelength and wave height respectively. Distribution of deepwat~r wave energy off Krishnapatnam in a year for different months is shown in Fig. 3. It is well known that only a percent of Eo Present address: ational Institute of Oceanography, Dona Paula, Goa which is called the alongshore wave energy Ea is effective in generating longshore currents which cause the littoral material transport. The alongshore wave energy Ea is obtained from the relation Ea = EOK sm. excosex 2 where Ea is alongshore energy in foot pounds per foot of beach. Eo is deep water wave energy in foot pounds per foot of crest length. K is the refraction coefficient of the breaking wave and exthe angle between wave ray and the shore normal at the breaker line. Wave refraction diagrams for the region published earlier by Reddy et at.l have been utilised to determine the K and exvalues for different months. Since the c.oastline to the south of Krishnapatnam is oriented in a ESW direction, the swell from SW do not reach the Krishnapatnam beach. As such an average K of unity and ex= I S obtained from the refraction diagrams for SW waves have been used for March to October while K =0.85 and ex=2 have been used for ovember to February. Distribution of Ea for different months for the region is presented in Fig. 4. Correlation of the transport rates with alongshore wave energy by different workers in the past36 indicates that these 2 parameters can be related by an empirical relation. Different relations established by different workers based on reliable field and lab studies are referred earlier6. The Coastal Engineering Research Centre, USA 6 after analysing the data obtained at Anaheim Bay and South lakeworth inlet and combining these with the data obtained by various laboratory investigations has suggested the following formula that holds good for sandy coasts. Q = 135 Ea where Q is the littoral drift and Ea is alongshore wave energy. This formula should be applicable for Krishnapatnam region, where the sediments are of medium sand size (0.25 to 0.5 mm diam). This formula 26

2 '. REDDY et af.: WAVE EERGY & LITTORAL DRIFT OFF KRISHAPATAM FEBRUARY JAUARY MARCH E O. If 10''_ co I'. 0, SEPTEMBER s Fig. laverage monthly wave roses {or wave direction and wave height 27.

3 ,, IDIA 1. MAR. SCL, VOL. II, MARCH 1982 JAUARY E w JUE s SEPTEMBER Fig. 2Average monthly wave roses for wave direction and wave period 28

4 29 g 100. ;;; 90 """ i!;80 """ III S i ~ """ >.ti : ~I II: U i ~!C ~ 40 S ~ &. ~ 30 III ~ If 0.. REDDY et al.: WAVE EERGY & LITTORAL DRIFT OFF KRISHAPATAM f o 1 0 ~ FEI MAR API' MAY JAM JUL AUG SEP OCT MOV DEe """ ) 25. 4" 705,56.05 """ lu I JAM FE8 MAR APR MAY JUE JULY AUG SEP OCT OV DEC G ~... o~ 0:0 «3 00 J::l..0 ee. ~O I/) IJ: l<: «..J ~ : «Ii: 3 0 J:... 0: :l J: 08 I/) en.., Z 9 «, o \ 0, ' f b.' '" ' , I J A FE8 MAR APR MAY JUE JULY AUG SEP OCT OV DEC Fig. 3Distribution of deepwater wave energy in a year. Fig. 4Monthly distribution of alongshore energy In a year. Fig. 5Monthly distribution of littoral drift in a year

5 Il..nA J. MAR. SCI., VOL. II, MARCH 1982 has also been tested by Reddy 7 for Kakinada region with success. A similar formula with a slightly different constant had been used by Sastry et al.8 for the computation of littoral drift in the environs of Tarapur Atomic Power Station. The littoral drift during Different months are shown in Fig. 5. Results and Discussion Wave characteristicsrose diagrams showing the percentage occurrence of a given range of heights and periods for different months are shown in Figs 1 and 2, while the average wave height, periods and lengths are presented in Table 1. During northeast monsoon season (ovember to February) an average of about 82 % of waves have directions between and E with E being the predominant wave direction. From May to September, when the southwest monsoon prevails a major percentage (87 %) of waves come between Sand W with the most predominant direction being the SW. In March, April and October, waves are found to propagate in all directions, since the months represent the transition periods. Wave heights during ovember and December range between 1 and 2.5 m for a large percentage of waves with an average wave height around 2m. In January slightly smaller wave heights ranging between 0.5 and 1.5 m are most frequently observed and in February to April minimum wave heights of < 1 mare observed for a large percentage of waves. During southwest monsoon period (May to September) higher waves (around 2 m) are more frequent but again during the transition month of October the average wave height decreases to about 1.5 m. Exceptionally high waves of the order of 4m and even more are also occasionally observed. But they are very rare (10 days in a year) and might be associated with the cyclones in Bay of BengaL Average wave periods show only slight variation from month to month. Wave periods between 6 and 8 see are more common during ovember to February while periods ranging between 5 and 7 see are more frequent during March to October. Waves around 6 see are more frequent during August to September. Of all the months February shows highest average periods, around 8 sec. Values of wave steepness HolLo are given in the 5th column of Table I, on which the deep water wave energy is directly dependent. The wave steepness is more (around 0.025) during June to August. Wave steepness is least during February (0.0086) when low waves with high periods occur. Wave energyin deepwater (Fig. 3) it is maximum during June and July while it is minimum in February to ApriL January, May and August to October show intermediate values while ovember and December show slightly higher values. Alongshore component of wave energy (Fig. 4) is directed southwards during ovember to February, while it is directed northwards for the other months (March to October). Maximum values of alongshore wave energy are found in ovember and December, since the refraction function (J. happens to be more for the waves approaching the coast from the directions between and E. June and July also show higher values since the deep water wave energy is more for these months. Littoral driftoff Krishnapatnam it (Fig. 5) is southward from ovember to February while it is northward from March to October. Rate of littoral drift is maximum in ovember and December. The total southerly drift during a year is computed as lakhs of cubic meters (4.394Iakhs of cubic yards) while the total northerly drift in a year is 4.42 lakhs of cubic meters (5.78 lakhs of cubic yards). Hence, the net littoral drift is only 1.06 lakhs of cubic meters (1.39 lakhs of cubic yards) and is towards. Ash and Rattenbury? based on data relating to the amounts of dredged material near ports have estimated a net northerly drift of 5 lakhs of cubic meters for Visakhapatnam, and Spring I 0 estimated a net drift of Table iaverage Wave Heights, Periods and Lengths Month Ho To Lo Ho/Lo ex Eo E, Q ft see ft Deg ft pounds/ft of ft pounds/ft of ortherly( + }/southerly( } crest length beach Lakhs of meter) Jan Feb March IOS April IOS May I S June I S July I S Aug IOS Sept IOS Oct I S ov l" Dec

6 REDDY el af.: WAVE EERGY & LITTORAL DRIFT OFF KRISHAPATAM around 5 lakhs cubic meters for Madras; both on the east coast of India. Reddy 7 also, based on similar data estimated a net northerly drift of 2.5 lakhs of cubic meters for Kakinada. Comparison of the computed values of littoral drift at Krishnapatnam with the estimated values at different locations along east coast of India reveals that the littoral drift is relatively much smaller at Krishnapatnam. This is because of the fact that the offshore topographic features and the particular orientation of the coast line also influence the littoral drift (through wave refraction) apart from the prevailing wave conditions. Smaller values of littoral drift near Krishnapatnam suggest that it should be possible to deepen the port entrance channel here and maintain the required depths 'around the year with minimum dredging requirements. References 1 Reddy B S R, Venkatareddy G & Durga Prasad, Indian} mar St;i, 8 (1979) Jhonson J W, Proceedings fifth hydrological conference (State Univ of Iowa, Stu of Engg Bull 34) Watts G M, Tech Memo 42, (Corps of Engrs US Dept of Army, Washington DC) Caldwell J M, Tech Memo 68 (Beach Erosion Board, Corps of Engrs US Army) Fairchild J C, Beach Erosion Board, Ann Bull o 12, Coastal Engineering Research Center, Tech Report, Reddy B S R, Oceanographic Studies in the neighbourhood of a barrier beach near Kakinada, Ph.D. Thesis, Andhra University, Dr J S Sastry & R S D'Souza, J Inst Eng, 53 (1973) Ash W C & Rattenbery 0 B,} Inst CiVil Eng, 1 (1935) Spring F J E, Proc Minutes Proc Insl Civil Engrs 210,