Indian Journal of Marine Sciences Vol. 12, December 1983, pp.209-212

Littoral Drift along Shoreline between Munambam & Anthakaranazhi, Coast

S PRASANNA KUMAR, S S C SHENG) & P G KURUP* Department of Marine Sciences, University of Cochin, Cochin 682016 Received 7 October 1982; revised received 3 October 1983

Probable volume of littoral sediment transport at 2 m depth contour along the shoreline between Munambam and Anthakaranazhi has been computed for waves approaching the shore from directions varying from 220' _3000 TN w,ithperiods varying from 6 to 14sec. It is estimated that about 9 x 106 m3 of material drifts towards south annually between Munambam and while about 7 x 106 m3 of material drifts towards south between Fort Cochin and Anthakaranazhi. The drifts are discussed in relation to the availability of sediments and the instability of beaches.

Information on sediment movement, on a long term influencing the beach processes along this part of the basis, in the nearshore environs is a pre-requisite for coast. Off Malipuram and Narakkal, the shore is the design of shore protection structures and for usually protected by the presence ofmudbanks6 active navigational purposes. It is known that this movement during southwest monsoon season. The sediments, in of sediment, particularly in the surf zone, is governed general, beyond 3-4 m depth up to about 140 mare by complex interplay of waves and the nature of silty-clays or clayey-silts 7. Sandy sediments in fine to prevailing sediments in addition to morphological medium size limits prevail in the beach and surfzone8• features of adjoining beach sections. Broadly, the Data on wave climate for the region from lat. 8c30' alongshore component of wave energy together with to 11030'N and between long. 73°30'E and the the intense turbulence associated with breaking waves coastline presented in the earlier investigation 9 were determine the sediment movement quantitatively. This utilized to compute the deep water wave energy and to takes place in 2 ways: in a zig-zag motion on the beach face in swash-bltckwash zone as beach drift and as determine the direction function () at 2 m depth through construction of wave refraction diagrams. suspendedjbed-load movement in the surf zone. Orientation of shoreline along the Kerala coast is in Studies on sediment drift based on empirical NNW -SSE direction and so waves approaching the relationship developed from quantitative estimates of coast between 220° and 3000 are of great significance in littoral flows through wave refraction studies and field the littoral processes along the shore under study. and laboratory experiments on the nature of Percentages of different periods and heights were movement of sediment in the vicinity of natural calculated for each direction of deep water wave barriers have been made along various coastlines of the approach to compute the alongshore component of the world 1• Along the coastlines ofIndia such quantitative available wave energy 1O(E),assuming that waves from determinations have been made only at few the above directions only occur on any day in any localities2 -s. The present. study deals with the quantitative estimations of the sediment drift along the particular month. Here, E = ),H~~o cos ()sin ()where ~' coast of Kerala between Munambam and is the specific weight of sea water, Ho the deepwater Anthakaranazhi which has been experiencing severe erosion since few decades. wave height, Cothe deep water wave velocity and (J the angle subtended by the breaking waves with shoreline. Materials and Methods The alongshore component of wave energy E (kg• m.m -1 wave) calculated at all stations (A to S; Fig.I) The region under investigation is about 54 km utilizing (J values from the earlier study9 are presented stretch of low, sandy shore, extending from in Table 1. While computing E when Ho exceeds 2 m Munambam to Anthakaranazhi (Fig. I) and forms part and the wave further gets amplified due to refraction of a barrier beach system. River opens into the during its shoreward propagation, a limiting wave Arabian Sea and discharges fresh water along with height of 2 m only was considered to prevail, for the sediments during southwest monsoon season, simple reason Hb = 0.83 d (ref. 11)at the final break point at or close to 2 m depth contour.

*For correspondence Evaluation of the probable littoral sediment drift

209 INDIAN J. MAR. SCI .. VOL. 12. DECEMBER 1983

10' Iff 115' lis' tN • 10

Fig. l--Location of sts A to S

Q(m3) has been made, usil!g the empirical relation12 west coast of which are subjected to wave conditions similar to those along the stretch of coast Q = 210.7 E under study is 1.5-2 m3.d -I (ref. 13). The computed values in Table 2 (column 1) closely correspond to where E is expressed in 106 kg-m.m -1 of the wave. these values. The navigational channel of the Cochin harbour Results and Discussion consists of 6 km long outer channel and 2 inner Longshore component of wave energy (Table I) channels (inside the backwater system). The approach shows considerable variation from month to month at channel was constructed by cutting the offshore sand each of the 19 stations. The entire stretch of the shore bar at a distance of 1.6 km west of the coast. Silting is a experiences 2 energy maxima -one during April, and severe problem in the harbour's outer channel and the oth~r during Dec. On an average the longshore nearly 2 x 106 m 3 of the material is dredged every year. energy has a southerly (down coast) component during Of this quantity about 70% is from the approach all the months except November, when the direction channel14. Present study shows that littoral drift reverses to northerly (up coast) at almost all stations. during rough weather season north of Co chin harbour entrance channel is southerly while it is northerly south The quantity of littoral sediments subjected to of the channel. These opposing sediment drifts are alongshore drift during rough weather season (April• likely to be carried into the channel and may, thus, be Sept.) shows an irregular trend (Table 2). The quantity responsible for the high amount of deposition taking of material in motion is high at st A (377.16 m3) and st place in the outer channel. The calculated drift is about F (380.07 m3) and low at st 0 (l95.61 m3). From sts A 0.3 x 106 m3 during rough weather season and 0.33 to D the drift value shows a continuous decrease x 106 m3 during fair weather season. The remainder whereas from sts E to F and further on these values probably reaches the channel either from the offshore show alternate increase and decrease. During the fair or the backwater system by the relatively strong flows weather season (Oct.-March) the trend remained more associated with the flood and ebb currents. These flows or less the same but with lesser magnitude. act in a way similar to that of a dynamic barrier The total drift during both seasons is higher north of preventing any possible by-passing of sediments from Fort Cochin than on the south. The observed rate of one side to the other. sedimen.t transport on the mainland beaches along the The above computed drift values are to be viewed in

210

I 'I' PRASANNA KUMAR et at.: LITTORAL DRIFT ALONG SHORELINE

Table I-Alongshore Component of Wave Energy E (kg-m'm-1) o(Wave per Day

St2903.041883.526143.0418316.802116.802090.881356.485711.0413582.082479.688475.8420545.905063.041848.962885.7610307.5226040.961416.961460.165097.6012173.761658.886039.3616104.962237.761961.281641.606480.0017418.24846.721572.481589.76717.12924.48665.283283.2011335.68777.60 Feb.March2522.885702.403144.963758.403611.52-1304.64-3127.6810532.1610100.16-751.68-1399.683274.56-1598.409460.80-267.84--259.20-388.804717.444743.366419.528441.289028.807620.484760.645495.042246.405883.84AprilMay2021.768484.481969.92259.202669.7612553.921736.64155.52-86.40.~207.682410.56699.842116.80751.685676.483283.202047.689564.48June2712.964276.804060.801235.521736.642574.722583.364484.165520.964717.442816.642064.969426.247266.249218.887810.562462.40967.682782.085106.242462.403525.122522.884034.88-2998.089694.08-1235.52-2177.28-1183.68-164.16-2090.88423.36-397.446030.725944.322401.922496.961105.924510.086549.125149.446713.282980.80-1218.24-1200.965011.206799.685901.\23533.761321.927577.283957.126013.443585.601598.405590.083611.521105.924026.24-768.96466.561909.442471.041581.\24268.16-864.07JulyAug.Sept.Oct.Nov.Dec.5762.88 665.286013.44466.56656.6417.28293.763620.16527.04 172&8.6410454.47 20252.162773.4410152.0011370.2419742.4010860.48-15621.\216588.801391.04-1477.4414627.522548.8021997.445028.4811275.20 16692.4819198.0819224.007205.76 -1356.488553.60 9892.809607.688069.76-1753.92-4173.12755\.364354.562384.642626.56 1779.841140.485235.848873.282860.642039.041034.08267.84803.52 -2427.84 -2289.60-1546.56-1252.80-673.92 9469:448069.76 11370.24 8873.28 0KELFJ CHBGMNSD RI A - = northerly; QP+ = southerly

Table 2-Littoral Drift along the Stretch of Beach from the light of availability of sandy sediments in the surf Munambam to Anthakaranazhi zone and the adjacent beach sector. Considering the [Littoral drift expressed in m3 per season] mass balance, a total of about 9.5 x 106 m3 during rough weather season and 7 x 106 m 3 during fair Sts Littoral drift * weather season moving south should be compensated by equal amounts from the north. As sandy sediments April-Sept. Oct.-March in the shore regions are limited to only 3-4 m depth (Rough weather) (Fair weather) contour and the beaches being low, it is improbable A 377.16 217.63 that this quantum of sediment is actually under transit. B 289.99 176.40 Moreover, the backwater system extending over the 181.59 C 217.56 length of the study area traps the otherwise available D 195.61 122.33 E 206.11 136.81 sand size sediments carried by way ofland runoff and. F 380.07 215.72 hence, deplete the barrier beach. The computed values G 340.76 194.15 reflect only the potential of available energy for H 206.44 164.66 sediment drift. In the absence of such large quantities I -8.12 57.62 of sediment supply from the north, the littoral flows 43.58 92.37 J associated with the wave activity are likely to be K 18.35 58.44 L 230.03 132.44 undersaturated and give rise to erosion of existing M 201.52 134.35 material in the shore zone. The reported failure of t~e N 162.20 126.70 shore structures constructed along this stretch of the o 149.42 129.98 coast may partly be due to this imbalance in the sand P 112.39 90.66 budget. Q 15\.39 117.69 R 108.13 119.88 Detailed examination of the bathymetric charts S 189.40 149.64 shows a progression of shoreline north of Cochin with flat inshore bottom contours and recession of shoreline south of Cochin with steep and close *Total drift (m3) for bathymetric contours. This feature indicates de• sts A to H: April to Sept. = 5.8147 x 106 and Oct. to March positional nature of the stretch of the beach north of = 3.6987 x 106• Cochin harbour entrance channel. This progression of sts I to S: April to Sept. = 3.7057 x 106 and Oct. to March the shore corresponds roughly to stations from G to H =3.3005 x 106. (Fig. I). The net southerly drift estimated in the present study indicates that the growth of shoreline near Vypin .

211 INDIAN J. MAR. SCI., VOL. 12, DECEMBER 1983 takes place at the expense ofthe sediments eroded from 2 Sastry J S & D'Souza R S, J Instn Engrs India, S3 (1973) 170. those reaches situated north of st G. The material 3 Antony M K, Indian J Mar Sci, S (1976) 8. drifting towards south from st H is blocked at the 4 Lalithananda Prasad A, Hanumantha Rao K, Veenadevi Y & Harbour entrance and gets either deposited or Lakshmana Rao G R, Indian J Mar Sci, 10 (1981) 15. 5 Reddy B S R, Venkata Reddy G & Durga Prasad N, Indian J transported to other reaches depending on the flow Mar Sci, 8 (1979) 67. conditions. In the region south of Cochin the absence 6 Kurup P G, Bull Dept Mar Sci Univ Cochin, 8 (1977) I. of mass balance could be inferred from this study for 7 Nair R R & Pylee A, Bull Nat Inst Sci India, 38, Part I (1968)41 I. the simple reason that 7 x 106 m3 of sediment is not 8 Murthy P S N, Nair R R, Pylee A & Varadachari V V R, HOE available for transport towards south. Since there is no Newsletter, 4 (1966) 9. supply of sediment to this stretch from the north, the 9 Shenoi S S C & Prasanna Kumar S, Indian J Mar Sci, 11 (1982) existing material is eroded from this stretch. The 201. eroded material might be getting deposited further 10 Johnson J W & Eagleson P S, in Estuary and coastline south of the area of the present study. hydrodynamics, edited by AT Ippen(MeGraw-HiII Book Co Ine, New York) 1966,404. Acknowledgement I I Longuet-Higgins M S,in Waves on beaches, edited byRE Mayer The authors are grateful to Dr G S Sharma, for (Academic Press, New York) 1972; 413. providing facility. S P Kumar and S S C Shenoi are 12 Inman D L& Bagnold RA, in The sea, Vol 3,edittd by M N Hill thankful to CSIR for providing Junior Research (John Willey & Sons, New York) 1963,529. Fellowship. 13 Veerayya M, Studies on the geological aspects of the beaches of Goa, in relation to some meteorological and physical References oceanographic factors, Ph.D. thesis, Andhra University, I Horikawa K, in Coastal engineering: An introduction to ocean 1978. engineering, (University of Tokyo Press, Tokyo) 1978,273. 14 Gopinathan CK & Qasim SZ,J Mar Bioi Ass India, 13(1971) 14.

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