Indian Journal of Marine SciencesVol. 8, June 1979, pp.78-84

Hydrographic Characteristics & Tidal Prism at the Cochin Harbour Mouth*

V S RAMA RAJU, P UDA YA VARMA & ABRAHAM PYLEE

National Institute of Oceanography, Regional Centre. Cochin 682018

Received 28 August 1978

Diurnal and seasonal variations (May 1975 to April 1976) in hydrographic characteristics at the inlet of the CochinHarbour are presented. During the SW monsoon season under the predominant influence offresh water influx into the estuaryno appreciable variations in the temperature and salinity values of the thin surface layer ( <4 m) are observed in relation to tide;whereas the subsurface water (4 m to bottom) shows close relation with the tide. Significant vertical gradients in temperatureand salinity are noticeable between surface and bottom layers during this season. Prominent saline wedge is formed with theflood current in this season and during the ebb current homogeneous water of low salinity ( < 100

/00) pervades the entire depthof the harbour mouth. Mean vertical salinity profiles for the monsoon, post and premonsoon seasons indicate that the highlystratified conditions that exist during the monsoon season gradually change to partially mixed and homogeneous conditions ofthe post and premonsoon periods apparently due to decreasing river flows and increasing tidal influence. Maximum velocity ofthe current observed is 174 ern/see which occurs during the ebb current of the spring tide of the posunonsoon season. Tidalprism in the pre monsoon season during which time the fresh water addition to backwaters is least is found to be 31.5 x IObm3.

During the other seasons, the values of tidal prism vary between 9.5 x IOOm·I and 132 x 100m .1. Factors effecting the stability ofthe inlet are discussed.

The Cochin backwaters estuarine system is formed byan offshore bar built along the shore extending fromCranganore in north to Alleppey in the south coveringa length of about 90 km. The estuary is wide (16 km) inthe Vembanad Lake area and several narrow canalsdrain water into the backwaters. Major perennialrivers-the Periyar, the Muvattupuzha and thePamba-discharge fresh water directly into thisestuarine system. The Cochin Harbour mouth is themajor inlet connecting the backwaters to the ArabianSea and the 2nd inlet at Azhikode is relatively shallow.A 3rd opening exists near Thuravur, but it remainsclosed most part of the year except during floods in themonsoon season. This 3rd opening has no or negligibleinfluence on the backwaters.

Although hydrographic characteristics of thebackwaters have been described 1 - 11, no detailedinformation is available on the intensity of the currentsat the surface and subsurface levels and the associatedexchange and mixing of the estuarine water with theArabian Sea coastal waters. It is, therefore, importantto study in detail hydrographic characteristicsincluding volume transport across the harbour mouthto know the problem of sedimentation in the harbour,the stability of the inlet, and discharge of pollutantsintroduced into the backwaters, for model studies andfor various harbour development works.

* Paper presented at the symposium on Environmental modellingof physical oceanographic features as applied to Indian Ocean,NPOL, Cochin. February 1978

78,

In this paper, hydrographic conditions at theharbour inlet along with volume transports arepresented. Factors effecting the stability of the inlet arealso briefly discussed.

Materials and MethodsAs part of the studies on hydrographic conditions in

the inner harbour of Cochin Port for harbourdevelopment works undertaken by N 10. extensivedata have been collected during May 1975 to April1976 on temperature, salinity, suspended sedimentload and current from 30 stations in the inner harbourregion.

The semi diurnal tide at Cochin has a maximumrange of I m. The diurnal mean sea level is normallylow during the monsoon season compared to post andpremonsoon periods.

A 30 ft motor boat was moored to anchored buoysfor observations. Three stations, A, Band C (Fig. I)were covered along the cross section at the inletbetween Fort Cochin and Vypeen Island.Observations were made at or around Full and NewMoon days to represent spring tide conditions andabout a week after that to represent neap tideconditions. A total of 6 days observations were madein different months. On each day, continuous hourlyobservations from surface (0.5 m below) to bottom (Im above) were made at intervals of2 m depth. Currentswere measured at all the 3 stations and thetemperature, salinity and suspended sediment loadobservations were made at mid station B. Bottom

RAMA RAJU et al ..· HYDROGRAPHIC CHARACTERISTICS & TIDAL PRISM

soundings were also taken using a lead line. Theseobservations covered a total of 3 spring and 3 neap tideconditions. Some practical difficulties were en-countered at the inlet section partly due to drifting ofthe anchors or snapping of the anchor ropes under theeffect of strong currents at mid tide timings.

Surface samples were collected by a plastic sampler,and subsurface samples by a specially designed glassstoppered bottle or an indigenously fabricated plasticsampler. Salinity and temperature of most of thesamples were determined using a T -S bridge and thevalues were read up to first decimal. A few sampleswere analysed for salinity by normal titration method.

Suspended sediment load was determined byfiltration technique using Whatman filter paper No.42. The currents were measured using a direct readingPlessey current meter which gave integrated values ofthe current over a period of 28 sec and the directionwith reference to magnetic north.

Results and DiscussionBottom profile of inlet-Cross sectional bottom

profile of the inlet between Fort Cochin and Vypeen isshown in Fig. I. The maximum depth of the inlet is 17m below the chart datum occurring within a distance of40 m from the Fort Cochin bank. The inlet bottomslopes steeply with a gradient of 1:2 on the Fort Cochinside and less steeply with a gradient of 1:6 on theVypeen side. The central part of the inlet bottomgradually slopes to its maximum depth with a gradientof I:36. This bottom profile represents the condition ofdynamic equilibrium as a result of adjustment intopography under the influence of natural factors andno dredging is done to maintain the required depths forsafe navigational purpose.

Temperature-Diurnal variation of temperature forthe spring tide on 24-25 July 1975 (monsoon season) isshown in Fig. 2A. Variation of temperature is (Porn 24°to 29°C, the surface water usually recording highervalue. While the difference in surface watertemperature with the tide is only 1.5°C, the subsurface

VYPEEN 403· 8m FORTC AV[RAQf TIOE LEVEL 8 A COCHiN----·----0-·-----·· . 0~S-·-·-· 0

\... CHART DATUM

COCHIN HARBOUR INLET6

1!·

8 •.10 ~

12

14

16~--------------------------~~~~18Fig. 1- Cross sectional bottom profile of Cochin Harbour inlet[Lat. 9°58'N, long. 76° 14'30"E. Cross sectional area (m2) withreference to average tide level (April 1976): 0-4 m, 1567; 4-8, 1469; 8.

12, 1106; 12-16,675; and> 16,80]

water from 4 m to bottom shows a difference of 4cCwith the tide. Vertical temperature gradients vary from0.36° to O.loC/m between fldod and ebb tidesrespectively. The increasing subsurface temperaturesare associated with the progress of ebb flow and thedecreasing temperatures are associated with theprogress of flood flow. Similar variation oftemperature with tide is noticed during the neap tidealso. Low subsurface temperatures are attributed tothe incursion of cool upwelled water from the shelfregion into the backwaters+. The lowest annualtemperature of the Cochin backwaters usually occursduring the monsoon season coinciding with the lowtemperature period of the coastal waters I o· 12. Thetemperature 24° to 25°C of the bottom water at theinlet during flood tide seems to be due to the inflow ofmixed subsurface nearshore water of 20 m depth(temp. 20°-23°C) with that of the estuarine water.

Spring and neap tide observations in December 1975and April 1976 falling in the post and premonsoonseasons respectively do not show any clear relationshipbetween the surface or subsurface temperature withthe tide. Also vertical temperature gradients are small(O.07°C/m) and the temperature varies between 28°and 31°C.

Salinity--Typical diurnal variation of salinityduring the same period as for temperature is given inFig. 2B. The thin surface layer ( < 4 m) shows salinityvariation from almost freshwater to 9%0, but thevariation has no relationship with the tide. Thisindicates the predominant role of the freshwater flowover the tidal flow in the surface layer at the inlet which

•

4

23

36 ..,

32 J B28

24

°~20>-

:~ 16'012(j)

6

4

i 0

1FLOOD-_ Ebb_FLOOD __ Ebb

f I Iii, I I i I I Iii I i I I

0.7 09 II 13 15 17 19 21 23 01 03 05 07hrs

I' 24-7-1975-----=----_1'<--25-7·1975~

Fig. 2-Diurnal variation of temperature (A) and salinity (B) atstation B [Depth (m): a, 0; b, 4; c, 8; d, 12; and e, bottom]

79

INDIAN J. MAR. sci, VOL. 8, JUNE 1979

•

may prevail up to some distance from the coast alongthe approach channel. The subsurface layer showswide variation from 2 to 340100and the variation showsclose relation with the tidal flow, the higher salinityvalues occurring at HHW or HW and the lower valuesat LL W or LW timings. Except during the slackperiods, marked variations occur in subsurface salinitywith the tide leading to steep salinity gradients in thevertical and fast changing salinity conditions, whichare typical of this estuary during monsoon season.Similar conditions are observed with the neap tide inJuly 1975.

Diurnal variations of salinity at the spring tide ofDecember 1975 indicates somewhat similar conditionsas those of monsoon, but with reduced salinityvariations (12 to 32%0) between surface and bottomlayers. Also, during this season, surface and subsurfacesalinity shows close relation with the tidal flow unlikeduring monsoon period indicating reduced role of thefreshwater discharges over the tidal flow in the surfacelayer ( <4 m).

Premonsoon spring and neap tide diurnal variationsof salinity, observed during 12 to 13 April 1976 and 21to 22 April 1976 respectively, show salinity variation of30.5 and 35%0' which has no relationship with the tidalflow.

Mean vertical salinity profiles drawn for all the daysof observations are presented in Fig. 3. Steep salinitygradient (of the order of 1.6%0/m), even in the meanvalues during monsoon season, coupled with lowtemperatures of bottom layers signify highly stratifiedconditions. High salinity, low temperature (thereforehigh density) bottom water moves through the inlet asa saline wedge prominently during flood tides andspreads into the inner channels of the harbour at thebottom.

During post monsoon season the vertical salinitygradient (0.6%0Im) is smaller indicating partially

Salinity ok.,

2 6 10 14 18 22 26 30 34o

2 '""~ \ ~4 ~,,\ edl

6 "\ I '\ \~-8 .0 ,.

110 i\, !

:: ~--~18 .L- --l

Fig. 3-Mean vertical salinity profile at station B during SWmonsoon, pre and post SW monsoon seasons [a, spring-24 to 25July 1975; b, neap--31 July to I August 1975;c, spring-2 to 3December 1975;d, neap-5 to 6 February 1976;e, spring-I 2 to 13

April 1976; f, neap--21 to 22 April 1976]

80

mixed conditions. In premonsoon season, the meanvertical salinity gradient is least indicating well mixedconditions. Thus from the mean vertical salinitydistribution, it can be inferred that the hydrographicconditions at the inlet change gradually from highlystratified to partially mixed and well mixed state as theseasons advance from monsoon to post andpremonsoon periods respectively. A salinity change of< 50 % between surface and bottom water is taken asan indication for well mixed condition and 50 to 75 %for partially mixed and> 75 % for stratified condition.

Saline wedge-Vertical salinity variation at hourlyintervals (Fig. 4) shows that a thin surface layer of lowsalinity ( < 10%0) up to a depth of 2 m overrides thehigh salinity (25-30%0) bottom water (6 to 16 m)during the flood flow and a marked saline wedge isformed at the inlet. But the saline wedge disappearsduring the low tide when the entire inlet is occupied bylow salinity « 10%0) water under the combinedinfluence of the ebb current and the outflow of thefreshwater through the inlet. One or 2 hr after the floodtide begins, intrusion of the high salinity water can benoticed first near the bottom and later pushing up thelow saline water or freshwater to the surface. Similarfeatures of saline wedge are noticed during the neaptide observations in July 1975. The intrusion of salinewedge and its characteristics are important for modelstudies, improvement in navigation, flood control anddisposal of waste material through the inlet.

During the postmonsoon season (December 1975)saline wedge is noticeable for a short duration duringthe flood flow at certain favourable conditions whenfresh water discharges are high, but the salinitydifference (10%0) between surtace and bottom layer ismuch smaller compared to that of the monsoonseason. During the premonsoon season (April 1975),homogeneous water of salinity around 3SO100is noticedat the inlet from surface to bottom and it isindependent of the tidal flow.

Suspended sediment load-Hour:y values ofsuspended sediment load, observed at the inlet, showvery high fluctuations at short intervals both in spaceand time as they did at other stations in the im . ..:rharbour. Diurnal variations of suspended sedimentload are studied in relation to tide and current but nodefinite relationship is noticed. Gopinathan andQasimf have reported that suspended material in theestuary varies markedly with the state of the tide whichis not noticed in the present observations. Therefore,an attempt has been made to average the hourly valuesand study them separately for spring and neap andflood and ebb flow during the 3 seasons to know thegeneral trend (Fig. 5). A general increase in thesediment load from the monsoon season to post andpremonsoon seasons can be seen, the highest values

RAMA RAJU et al.: HYDROGRAPHIC CHARACTERISTICS & TIDAL PRISM

occurring during the postmonsoon season. Alsosuspended sediment load is higher with increasingdepth. Higher sediment load during post andpremonsoon seasons than in monsoon season may bepartly due to the dredging operations being carried outin the outer channel of the harbour at that time.

Vertical variation of annual mean suspendedsediment load for ebb and flood is shown in Fig. 6. Theannual mean values vary between 25 and 50 mg/litre.The mean sediment load during flood tide is higherthan at ebb tide indicating that during an average tidalperiod of equal flood and ebb, more suspendedsediment moves into the harbour with the flood

. current.

Current-Diurnal variation of current at spring tideon 24-25 July 1975 at the inlet is shown in Fig. 7. In themonsoon season, ebb currents predominate inintensity and duration at all depths. At times, duringthe rising tide, the surface flow shows high ebbvelocities while the tide rises solely due to the floodcurrent in the subsurface layers. Similar dominant ebbcurrents are observed during the neap tide of themonsoon season. In the other 2 seasons, mostlyrythemic variations of flood and ebb currents takeplace throughout the depths with rising and fallingtides.

Highest velocity observed in the inlet is 174 em/sec.It occurs during the ebb current of spring tide in the

Fig. 4-Diumal vertical salinity variation during spring tide 24 to 25 July 1975 at station B [Time (hrs): a, 0930; b, 1010; c, 1115; d, 1200; e,1330; f, 1410; s. 1515; h, 1610; i, 1705;j, 1800; k, 1900; I, 2100; m, 2200; n, 2325; 0, 0010; p, 0200; q, 0300; r, 0405; s, 0655]

80 40 0 40 80 40 0 40 60::-r 20 30

Suspended aedlment,mg/litre

eo 60

Sill, mg/litre

505

:l: -ng

-p

..

Ebb ~12

E 8.s="~0,2

PreSWM

16 Ebb Flood

PostSW M

Fig.5

40160 ...,-------------:l,---,

24--7- 7!1-t-2!!-7- 75110 -'--"-----------------J

Fig. 5-Average silt content at different depths for ebb and flood of spring and neap tides during SW monsoon, pre and post sw monsoonseasons at stauon B. Fig. 6-Vertical variation of the annual mean suspended sediment concentration at station B. Fig. 7-Diumal

variation of current at station B [Depth (m): a, 0; b, 4; c, 8; d, 12; e, bottom]

81

INDIAN J. MAR. set, VOL. 8, JUNE 1979

postmonsoon season at the surface but in general thesurface current velocities are higher during themonsoon season compared to post and premonsoonperiods. Highest velocities observed in the monsoonand premonsoon seasons are 157 and 128 em/seerespectively.

Diurnal variations of current in the vertical athourly intervals during the spring tide 24-25 July 1975(monsoon) and at neap tide on 5-6 February 1976(premonsoon) are shown in Figs 8 and 9 respectively.Monsoon observations show that the surface andbottom currents are in opposite direction persisting fora few hours during rising or falling tides and themagnitudes of this 2 layer flow at particular times areas much as 120em/see ebb current near surface and 40ern/see flood current near bottom. At the end of theflood current, ebb current can be seen throughout theentire depth of the inlet, whereas at the end of the ebbcurrent, flood current begins first near the bottom ofthe inlet and then gradually extends up to a depth ofabout 4 m from the surface. Fast changes in the speedof the current with time are also evident.

Sequence of neap tide current changes in thepremonsoon season does not show 2 layer flow withrising or falling tides. It also shows that the reversal ofcurrent occurs as the tide changes in the entire layerfrom surface to bottom. The velocity gradients of thecurrent with depth are small compared to those ofmonsoon season.

Per cent total flow down stream-Percentage distri-bution of flow down stream with depth in the 3seasons during spring and neap tides is presented inFig. 10. Above 50% down stream flow indicatespredominance of ebb flow over flood flow and below

Currlnt, em ,1,-1

50% indicates the predominance of flood flow over ebbflow. Percentage distribution of flow down streamdepends on the intensity and duration of flood and ebbcurrents which in turn are effected by the tides andfreshwater discharge into the estuary. The flow duringthe days of observation (Fig. 10) in the monsoonseason is predominantly down stream (ebb current).During the postmonsoon season in December 1975atspring tide, the ebb flow is predominant whereas inFebruary 1976 at neap tide, the flood flow ispredominant. In the premonsoon season, both atspring and neap tides, up stream flow is predominantthroughout the depth of the inlet. Predominant downstream flow in the monsoon observations is evidentlydue to the dominant influence of freshwater dischargeover the tidal flow. In the premonsoon season thepredominant up stream flow is due to dominantinfluence of the tidal flow.

Tidal prism-Tidal prism for the inlet, i.e. totalvolume of water exchanged, has been calculated usingthe average current velocities of the layers, 0-4, 4-8,

Per cent tota' flow down .,.,.."

o30 40 !l0 60 70 80 90 100

:. 7!5

100 i~----~------------~Fig. 10-Vertical distribution offtow in the 3 seasons [a, spring-24 to 2§ July 1975; b, neap-31 July to I August 1975; c. spring-2 to3 December 1975: d, neap-Ji to 6 February 1976; e, spring-I 2 to 13

April 1976; and f, neap- 21 to 22 April 1976J.\-

Fig. 8-Diurnal vertical variation of current during spring tide 24to 25July 1975 at station B[Time (hrs): a, 0930; b, 1010; c, l l l S; d, 1200; c,1330; f, 1410; g, 1515; h, 1610; i, 1705; j, 1800; k, 1900; I, 2100; m, 2135; n, 2200; 0, 2345; p, 00 15; q, 0200; r, 0300; s, 0405; t, 0500; u, 0600; v,

0655]. Fig. 9- Diurnal vertical variation of current during neap tide 5 to 6 February 1976 at station B [Time (hrs): a, 1015; b, 1110; c, 1200;d, 1310; e, 1410; f, 1500; g, 1610; h, 1700; i, 1800; j, 1900; k, 2015; I, 21t10; m, 2200; n, 2300; 0, 0000; p, 0100; q, 0210; r, 0300; s, 0400; t, 0500; u,

0600; v, 0700; W, O9OOJ

82

RAMA RAJU et 01.: HYDROGRAPHIC CHARACTERISTICS & TIDAL PRISM

8-12 and 12 m to bottom and the respective crosssectional areas. Spring tide periods are chosen to getthe maximum possible values of tidal prism for floodand ebb tides in the 3 seasons. The average tidal prismfor the spring. tide. in the premonsoon season is31.5 x 106m3 (35.3 x 106 for flood and 27.7 x 106 forebb). The freshwater inflow into the backwaters is leastin this season and consequently the down stream flowof low salinity water in the inlet is small. The effect offreshwater inflow on the tidal prism during monsoonand postmonsoon seasons is very significant. Tidalprism values range between 9.5 x 106 and 83 x 106m3

for flood and 27.7 x 106 and 132.6 x 106m 3 for ebbtide. These values represent the order of magnitudeand considerable variations occur depending on thetidal range and duration, quantity of freshwaterdischarge into the estuary, intrusion of saline wedge,etc.

Stability of the inlet-While considering the stabilityof the inlet, changes in the location of the inlet itselfand changes in its cross sectional area are to beconsidered. If N represents total rate of transport ofsediment through the bed load as well as suspendedsediment load across the inlet, taking the direction ofebb flow as positive X direction, the condition forstability of the inlet cross section over a period of time tis

taNJ -a dt= 0o x

taNirJ -a dt> 0 erosion of inlet occurs, and

o x

taNif J -a dt < 0 deposition at inlet occurs.

o x

As seen from the survey charts of the Cochin Port,the inlet has remained practically in its present locationfor the last few decades and so appears to have locationstability. As the location stability of an inlet is likely tobe altered mainly through major changes in thechannel orientation, the inlet may have locationstability as long as the orientation of the outer andinner approach channels is not tampered with.

The inlet has not been dredged as the need for it inthe context of safe navigation has not arisen so far. Thehigh tidal current velocities near the bottom appear tosweep the bottom and no net sediment accumulation istaking place at the central part of the inlet.

According to Bruun and Gerritsen 13, cross sectionalstability of an inlet may be altered by any or acombination of the factors: (i) prolongation of inletchannel; (ii) abnormal deposition of littoral driftmaterial during severe cyclones; (iii) splitting of maininlet channel into 2 or more channels; and (iv) changes

in the bay area from which water flows into the inlet-construction of dams or reclamation of land byconstructing bunds, etc.

In respect of the Cochin Harbour inlet, items i to iiiare not important as the prolongation of the inletchannel or bifurcation of it can be ruled out forobvious reasons and the abnormal deposition oflittoral drift material also may not be expected as thefew cyclones that reach this part of the coastlineusually loose their severity. Therefore, the mostimportant factor to be considered is the man-madecauses like construction of dams and reclamation ofland from backwaters. The reduction in bay volumeleads to reduced volume exchange through the inletover a tidal cycle and consequent reduced maximumvelocities of the tidal current.

The Thanneermukkam Bund construction has beencompleted a few years back separating the VembanadLake with the rest of the backwaters. The flow throughthe bund is controlledand the intrusion of saline waterinto the Vembanad Lake during the premonsoonseason is prevented, thereby reducing the bay area fortidal flow. In the monsoon and postmonsoon seasonsthe flood water of the Vembanad Lake is allowed topass through the bund. Therefore, the effect ofThanneermukkam Bund on the Cochin Harbour ismainly on the reduction of flow through the inlet in thepremonsoon season which results in reducedmaximum tidal velocities that can cause increasedsedimentation rates in the harbour area. Considerableamount of siltation occurring in the outer channel at adistance of 1800 m from the inlet has been reportedearlier 14. It is important for the Cochin Harbour toassess the sedimentation rates in the harbour, study thebed load movement at the inlet and watch for shoalingarea so as to connect the cause and effect, if any.

Acknowledgement

The authors are grateful to the authorities of Co chinPort Trust for sponsoring the Cochin Harbour Projectsurvey during which these data are collected. Theyexpress their sincere gratitude to Dr S. Z. Qasim,Director and to Dr T.S.S. Rao, Scientist-in-Charge,for their interest and encouragement. Thanks are dueto their colleagues at the Regional Centre forcollection of part of the hydrographic data. Thanks arealso due to Dr J.S. Sastry for going through themanuscript and offering valuable suggestions.

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INDIAN J. MAR.SCl., VOL. 8, JUNE 1979

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