Indian Journal of Marine SciencesVol. 5, June 1976, pp. 9:17

Circulation ~ Sedi.meI?tation Processes in & around the Aguada Bar

C. S. MU1UY, P. K. DAS, R. R. NAIR, M. VEERAYYA & V. V. R. VARADACHARINational Institute of Oceanography, Dona Paula

Received 26 August 1975; revised received 20 November 1975

An attempt is made to ~ai1!some insi~ht into the complex processes of the estuarine andbay circulation and sediment transport and deposition responsible for the formation andmaintenance of the Aguada bar - a navigational hazard near the mouth of the Mandovi estuary.A s.chematic pattern of water circulation in the bay and the estuary entrance is arrived atthrou~h studies on the float trajectories and the sediment distribution in and around the bar.It is su~~ested that the hi~her wave activity and the large volume of sediment laden river dis­charge during' the south-west monsoon 'result in truncation of the bar followed by a generalshallowing of the surrounding region. The flow pattern for the fair weather season suggests agradual building up of the bar- through 2 circular cells of water movement in the bay near themouth of the es'tuary. The interaction of refracted and diffracted waves with the flood and ebbcurrents; over the shoals, is also discussed.

THE Aguada Bay and the Mandovi estuarysustain a heavy barge traffic carrying iron, and manganese ores to the adjacent port of

Mormugao apart from the mechanised fishing boatsoperating from a base at Panaji and the coastalpassenger ships sailing between Panaji and Bombay.However, this waterway has to be dosed down duringthe southwest monsoon months as navigation isrendered hazardous by the shoals/bars (popularlyknown as Aguada and Reis Magos bars) near theentrance of Mandovi estuary. These shoals areknown to have been present here over severalcenturies. Manuel Godinho de Eredias's mapl of1610 sho\~s these as a single patch of shoal near theentrance of Mandovi estuary. Ballard2 writes"Noronha crossed the bar at high tide with three

light galleys sounding and buoying the channel ashe went, until he arrived at the deep waters of thelower roadstead under fort Panjim".

Looking at the present conditions, prevailing inthe Bay and in the light of the above, it would appearthat, so far as these shoals and the navigational chan­nel are concerned, not much has changed over a fewcenturies. That the shoals/bars have neither grownsignificantly in size nor have they disappeared com­pletely suggest that a kind of balance exists betweenthe supply and depletion of sediment whichmaintain the shoals/bars controlled by the inter­action of tides, waves, river runoff, etc., whichdetermine the circulation pattern in the bay andin the mouth of the estuary. Further, it is onlyduring monsoon that this region poses navigationalproblems while during the fair weather season thenavigational channel remains relatively deep and.well marked. It is, therefore, of interest to under­stand the mechanism of sediment transport in thisregion for solving the economically important prob­lems of navigation posed mainly by the Aguada bar.

Methods

. In the present study an attempt has been madeto understand the nature of circulation in the bay and

in the estuary entrance which determine" the move­ment pattern of sediment in and around the bar.Surface circulation during premonsoon months wasstudied by releasing surface drifters during differenttidal stages which were tracked and their positionsdetermined by sextants. Navigational difficultiesruled out the possibility of conducting any suchexperiment during monsoon. Bottom samples werecollected for size analysis to understand the distri­bution pattern of sediments. Movement of sandgrains in and around the bar was also studied usingdyed tracer sands.

Regional SettingThe Aguada Bay (parts of which are known as

Caranzalem Bay and Sinquerim Bay) in which theAguada bar is located, is formed by 2 promontoriesof Cabo Raj and Aguada headland (Figs. 1 and 2)composed of Precambrian sandstones and ferru­ginous quartzites which have been intensively late­ritized. An interesting feature of rocks of the head­lands is the extensive development of tafoniaccompanied by the occurrence of alveolar orhoneycomb structure on the crust of the rocks.These weathering features develop as a result ofprolonged exposure to sea spray3. The bay sidebeaches (terminology after King4) of Mira Mar andCaranzalem are situated on the left bank of theMandovi, while on the right bank the beaches areeither poorly developed or totally absent.

The navigational channel in the Mandovi estuaryis located close to the left bank and is flanked bytwo shoals - the Aguada bar and the Reis Magosbar. Of these, the Aguada bar is more prominentlydeveloped and projects seaward from Gaspar Dias(Mira Mar; Fig. 1). At the time of sedimentsampling the maximum width of the exposed partsof the bar Was20 m at the proximal end and 10-12 mat the distal end. These exposed parts showedwell developed asymmetric ripples on areas of coarsesediments and poorly developed ones where finesediments prevailed, as at the seaward end. A

Fig. 2 - Schematic representation of the water cirCulationi~ the Caranzalem Bay and the movement of sediment along

the beach and over the bar

Fig. 1 - Bathymetry of the region investigated [Bathymetric~ontours are in metres .• - Locations of sediment samples.

# A and B location of beach profiles]

HydrographyThe Mandovi estuary and the Aguada Bay have

an yearly cycle of variation in their hydrographicfeatures consisting of 3 distinct phases - the south­west monsoon and the preceding and following fairweather seasons - with varying wave climates pre­vailing in the bay and the adjacent sea coupled withwide fluctuations in the freshwater discharge throughthe estuary. It is thus a mixed estuary during thefair weather season (December to May) which, withthe onset of the south-west monsoon (June toSeptember), becomes progressively stratified innatures when the freshwater discharge is at itsmaximum6• Suspended sediment loads (surfacewater, midstream) are about 30 mgjlitre in fairweather in contrast to about 100 mg/litre duringmonsoon7•

The tides here are of semidiurnal type with arange of 2 m. In the absence of any significantfreshwater discharge, as in the premonsoon season,the currents in the estuary are mainly domin­ated by the flood and ebb tides. Typical floodand ebb velocities measured upstream of the barSare of the order of 160-108em/sec at the surfaceand 128-98 cm/sec near the bottom (Fig. 3). Thetidal currents in the bay would be of lesser magni­tude compared to the figures quoted above whichare for the constricted part of the estuary wherethe tidal currents are relatively stronger.

Wind waves and swells which enter the bay areof markedly different characteristics during south­west monsoon as compared to fair weather seasons.Low swells from W to NW predominate the latterperiod, whereas, during the south-west monsoonhigh wind waves from W to SW prevailS.

Wave heights measured by a wave recorder out­side the bay during the fair weather season indicatethe prevalence of waves of 6'8 see significant periodand 0·38 m significant wave height9• Visual esti­mates of breaker heights at Mira Mar and Caran­zalem in the bay indicate significant differences.In the former area the breaker heights are higherthan at caranzalem which may be due to the factthat the waves, as they enter the bay, first breakover the bar, reform and break again on MiraMar beach. Caranzalem beach on the o~herhand does not receive the swells directly but onlythe refracted and the diffracted portions of the wavefronts converging on. Cabo Raj. The net resultof the varying energy inputs is reflected in thenarrow, steeply sloping foreshore at Mira Mar withthe frequent presence of erosion scarps in contrastto the flat and rela.tively, wider foreshore at, Caran­zalem (Fig. 4).

4"

ROCK

ctGASPAR CIA

INDIAN J. MAR. SCI.• VOL. 5, JUNE 1976

tional channel while the southern side had a rathergentle slope.

Reis Magos is a shoal which is too ill-definedto be termed as a bar. Average depth in the bayis 4 m and maximum depth in the area of investi­gation is 2 m. A notable feature is the presenceof a ramp along which the navigational channel islocated. It has a gradient of 20° between the5 .and 1·5 m isobaths (Fig. 1) along the rampaXIS.

AGUADA BAY

3i 47

15"36N

15''28'

15'29'

runnel· like feature was present transverse to thebar at the southern side of the proximal end of thebar. The northern edge of the bar had a very steep.gradient (60°) sloping down towards thenaviga-

10

MURTY et al.: CIRCULATION & SEDIMENTATION IN AGUADA BAR

TIME (HOURS)

Fig. 3 - Variation of current with tide at station (A)[Murty and Das5J

Results and Discussion

The Aguada bar, strictly speaking, can neitherbe called a "bar" nor a "spit". The term'submerged bar' would be a closer description than'transverse bar '10. Following Oertalll both Aguadaand Reis Magos may also be termed as ' ramp marginshoals'. However, Hayes12 calls such accumula­tions near tidal inlets' ebb tidal delta' and, accord-

ing to this nomenclature, Aguada bar would bea 'linear sand bar' which is basically an ebbtidal delta. 'Ramp margin shoal' appears to bea more appropriate descriptive term for theseshoals, since they are actually situated on themargins of a ramp to the sea. However, in orderto avoid confusion, the popular names are usedin this paper. Moreover, a greater emphasis hasbeen laid on the Aguada bar for the obviousreasons that the navigational channel grazes thesteep northern edge of this bar and that the threatto navigation is posed mainly by this bar.

Float Trajectories

Fig. 5 shows the float trajectoriesin the channeland a part of the bay. This may be taken asrepresentative of fair weather conditions when theinteraction between the incoming wave and thetidal ebb and flood is of importance since there isnegligible fresh water discharge through the estuary.Along the ramp and the deep channel, the flowpattern is linear-either moving out with the ebb ormoving in with the flood. Beyond the distal endof the bar, however, the ebbing water encountersthe incoming wave transport and the circulationis modified during the transition from ebb to floodwhen the wave transport and the weak ebb currentinteract in such a way that the trajectories take a

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INDIAN J. MAR. seI., VOL. 5, JUNE 1976

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Fig. 5 - Float trajectories

'turn towflrds the sides of the ramp. A very similarcase in the Georgia estuary has been reported. by·Oertall1• With the flood tides slowly taking over

and theliWaves rounding the shoals, the circulationtakes a. gyral form on either side of the ramp(Fig. 6). In the Aguada bay, this gyral circulation

takes a w1ellmarked shape on the Mira Mar-Caranza­lem side, whereas on the Sinquerim side thegyarl­pattern cannot develop fully due to the unevenbottom tppography and the shape of the shoreline.MoreoveI;, with the flood water entering the bay

along with the waves, there is a slackening of thespeed along the central a:x;isof the bay as comparedto that through the sides of the bay. This wouldbe evident from: 'a cursory look at the isobaths inFig. 1. The refracted and the diffracted wavescause a wave transport along the Cabo Raj side ofthe bay which then moves alongshore towards MiraMar giving rise to a cyclonic cell. 'This is furtherevidenced from the fact that the farthest end of theCaranzalem bay near Cabo Raj has been undergoingerosion for quite a, few years. But this erosion

12

MURTY et at.: CIRCULATION & SEDIMENTATION IN AGUADA BAR

Gl GASPAR DIAS

FLOOD CURRENT

EBB CURRENT

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of 8%. Percentage decreases to <1 towards itsdistal end. The values lie between 3 and 4% atthe foreshore where the bar is connected, and, gradu­ally decrease to <1% towards Caranzalem withhigh concentration at the northern end of the rockoutcrop. In the shallow offshore region off MiraMar, the percentage of very coarse sand is negligible.

Coarse sand (Fig. 7b) - Highest percentage ofthis component (28) is confined to the middle andproximal end of the bar. Percentage decreasesfrom the bar as well as the beach towards theshallow offshore region « 1). The values decreasetowards the distal end of the bar and coincide withthe value of the shallow region. However, the nexthigher value is encountered in a sample off the rockoutcrop. On the. Caranzalem side, the percentagevalues decrease III the southern direction.

Medium sand (Fig. 7c) - Maximum percentage(54) of this sediment unit occurs at the proximalend of the bar with a decreasing trend towards theoffshore shallow region. Also a high percentage(49) is recorded on the southern side of the distalend ot the bar. There are 2 prominent cells - oneat the distal end and the other at the proximal end- with the minimum and maximum values occur­ring at these 2 ends respectively. On the Mira Marbeach the medium sand percentage is about 40 andit decreases towards the Caranzalem beach (3).

Fine sand (Fig. 7d) - Percentage of fine sand ismaximum (86) at the distal end of the bar. Twocens of low concentrations, one at the proximal endand the other at the northern flank ot the bar wereencountered. However, the percentage values in­crease from this region towards the centre of thebar. The next high percentage occurs in theshallow offshore region. An equal percentage offine sand component is found in the Caranzalembeach. From the Mira Mar beach to the shallowoffshore region the percentage of fine sand increases.

Very fine sand (Fig. 7e) - Its percenta;-e is lowestat the proximal end of the bar w;lereas the percent­age increases towards the distal end. The concen­tration of this component increases from north tosouth on the Mira Mar beach and maximum per­centage (40)is found in the low energy environment­Caranzalem.

Relative abundance of these components ofsamples of different environments of the beaches(dominantly composed of medium sand, fine sandand very fine sand) and the bar (consisting ofcoarse, medium and fine sands) has a strong bearingon the textural characteristics (mean grain size,standard deviation parameters) of sediments.

Grain Size Distribution The distribution and the probable factors givingSediment samples collected from the beach up rise to such differences are discussed below:

to the bar and along the beaches of Caranzalem and 111ean grain size and standard deviatio1~ (Figs. 7fMira Mar, before conducting the tracer experiments, and 7g) - Sediments of the beach foreshore at Mirahave been analysed for their grain size distribution. Mar are in medium to fine sand grade, moderatelyThe results reveal that the sediments of th.ese. well to poorly sorted, whereas those of Caranzalemenvironments' have considerable variations in beach are in fine sand grade and w~ll t() moderatelydifferent saud. siZe..class.materials and partide ~ite.~s9rte!i._ ~n .general,_ from_ thLupper to the lowercharacteristics. foreshore, the mean grain size increases (4- values

Very coarse sand (Fig. 7a) - Over the proximal decre<:se) and standard devi~tion. values increase.end'of the bar where it joins the beach, the very At Mua Mar, the mean gram Size of the beachcoarse sand component attains a maximum value foreshore sediments decreases (4- values increase)

proceeds at a slow rate and it may be accounted forby the same cyclonic circulation which would carrysediment with it from the sea side to the bay. Thealongshore flow, however, is modified by a rockoutcrop (Fig. 1) which during low waters acts as apartial barrier to the sediments moving fromCaranzalem side. The flat beach foreshore slopesof the Caranzalem beach as compared to the steepforeshore slopes of the Mira Mar beach (Fig. 4) pro­vide a further support to this view. On the northernside of this rock outcrop the presence of a baysuggests an alongshore drift in a northerly direction.The dimensions of this outcrop are not large enoughto alter the flow during high waters when the out­crop gets submerged. However, the most signi­ficant observation is that the cyclonic gyre in theCaranzalem Bay and the ill-developed anticyclonicflow in the Sinquerim Bay are maintained through­out the tidal cycles w;1ich sustain the general move­ment pattern of the sediments.

49'

Fig. 6 - Schematic representation of waves over shoals andflood, and ebb currents around the bar

13

INDIAN J. MAR. seI., VOL. 5, JUNE 1976

E

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Fig. 7 - Grain size distribution along the Mira Mar-Caranzalem beach and over the bar [A, Very coarse sand;B, coarse sand; C, medium sand; D, fine sand; E, very fine sand; F, mean grain size (Mz q,); and G, standard deviation

(alq,). Contour values in A to E are in per cent]

14

MURTY et al.: CIRCULATION & SEDIMENTATION IN AGUADA BAR

periods the bar is never exposed at lowest tidesbecause of the deepening of the water over thebar through the removal of material by Waves.

Circulation and Associated Sediment Transport

During high tide, the flow around the Aguadabar forms a separate loop with the flood water andthe wave surge rounding the shoal and joining thenavigational channel at the base (proximal end) ofthe bar (Fig. 6). This may be the reason for theformation of a runnel like feafure at the base of thebar where the isobaths curve inwards. It wouldalso mean that during this stage there would be anet sediment transport to the channel from thesouthern side of the bar. During the low tides,however, when the bar is either exposed or partiallyexposed at the base, the sediments would not beable to cross over the base to the channel and hencegive rise to partial accumulation on the southernside.

The northern edge of the bar (along the channel)is always subjected to tractional forces of a linearflow (either upstream with the flood or downstreamwith the ebb). No accumulation is therefore possiblehere. This is evidenced by the fact that the gradientis very steep on this side and only coarser sedimentsprevail. However, the flow of sediment across thebar towards the channel due to the circulationpattern described above and the continuous churningof the sediments over the bar caused by the diamondshaped wave crests surging over the bar make goodthe loss of material on the channel side of the bar.The deep ebb-flood channel thus maintains an equi­librium through this balance of removal andsupply of sediments.

During the monsoon this particular pattern otwater movement and sediment transport would notprevail as the wave activity over the bar thenincreases significantly. It would then be expectedthat increased wave disturbed sediment wouldtruncate the bar considerably and there would bea net loss of sediment from the bar to the channel,the latter shallowing up progressively. To this wemay now add considerable amount of sediment loadcarried by the monsoonal discharge of the Mandoviriver. The sudden widening of the estuary nearthe bar would then slacken the ebb and tend todeposit a part of the suspended sediment here. Thisprocess is altogether absent during the fair weatherseason as the river then carries negligible amountof suspended sediment. During the latter periodthen, through the cycles of ebb and flood thereis a progressive loss of material (fine sediment) fromthe edges of the bar as from its surface and from

the navigational channel. Thus, as the monsoonrecedes, the Aguada bar gradually regains itsoriginal shape and size of the fair weather months.This cycle is maintained in essence from year toyear. One thus concludes that monsoon is theperiod when removal of material is the prevailingfeature and that the compensating supply comesduring the fair weather season when long periodand low swells Push the sediments back into theestuary. Moreover, the gyral circulations in theCaranzalem and Sinquerim bays are of importancein keeping the bar nourished.

AcknowledgementThe authors are grateful to Dr S. Z. Qasim,

Director, and Prof. H. Postma, Nederlands InstituutVOJr Onderzoek der Zee, Texel, the Nederlands,for going through the manuscript and suggestingmodifications. They thank the Captain of Ports,Panaji, for providing boat facilities.

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17