Tektural Characteristics of Calangute Beach Sediments, Goa CoastM. VEERAYYA

National Institute of Oceanography, Panaji

ManuscriPt received 18 May 1972

Sediments of Calangute beach (between Baga and Aguada headlands), collected along

5 Ictions during September 1969-August 1970,have been investigated for their textural charac­

te istics, viz. mean grain size, sorting and skewness along with the beach topographic changes.T e study indicates a distinct seasonal variation in these parameters at all the profile sectionsan the results of Student's t test reveal that the variations are highly significant. During thefa' weather season, the sediments are in medium to fine sand grade and are moderate to well

ed, with negative to very negatively skewed class, whereas during the monsoon season theyin coarse to medium sand grade (often very coarse sands) in moderate to well sorted and

sy metrically skewed class. In general, the sediments of the beach are represented by granuleto fine sand size classes. The mean size and sortin~ values of the sediments tend to increasefr m the upper to lower foreshore, the increase ~enerally bein~ proportional to the ~radient ofth beach. The coarsest ~rain size is encountered at the' step formation ' durin~ the monsoon.T e berm is composed of material coarser than the foreshore represented by medium sandsding the fair weather season. The breaker zone is composed of medium to fine sands, in

derate to poorly sorted class throu~hout the study period. The tests of significance forerences in the textural parameters between the stations durin~ the fair weather seasonicate that the sediments on the southern side (near CandoIim) are coarser and less well sortedn at the other stations on the northern side. The sediments at the northernmost part, i.e.

B a, are finer and very well sorted in nature. However, the variations in textural parametersof the sediments noticed during the fair weather season are absent durin~ the monsoon. Theseva iations have been discussed in relation to the topographic chan~es, wave climate and theIi oral drift.

BEA studies have gained considerableim 01 tance in India in view of the pre8enceof conomic placer mineral deposits on the

one han and also of the severe erosion experiencedin some arts of the west and east coa,sts of Indiaon the 0 u. As part of the coastal and nearshoreoceanogr phy programme of the Institute studieson the dimentological aspects of Calangute andColva be ches are undertaken. The plesent reportdeals w' h the investigations on, the sedimentsamples ollected during the period 1969-70 flOmCalangut beach. The principal aim of the inves­tigations is to undelstand the seasonal variationsin the t tural characteristics of the sediments ofthe beac , in relation to beach topography, meteo­rological and oceanographic factOls affecting thechanges 'n the environment. This study subse­quently elps i.n understanding the stability of thebeach as the latter constitutes a natural rt'SOUlCt'of recrea ional value.

Excep for the studies on the (i) geology of Goaby Oelt 11 and Sinha2, (ii) geomorphology byD'Souza3 and (iii) Foraminifera of Baga bEach byRocha a d Ubaldo4, no infOlmation is availableon the d stribution, composition and origin of therecent u consolidated sediments of this beach.

Descript on of AreaRegion 1 setting, PhysiograPhy and bathymetry­

The coas of Goa trending NNW-SSE more or lessin a stla 'ht line, facing the Arabian Sea, forms apart of he central west coa8t of India. It is acoast of ubmelgence and the remarkably straight

28

alignment has been attributed to the faulting, per­haps in Plio-Pleistocene tilllt'S5.6. Most of thecoastline consists of beautiful sandy beaches sepa­rated from one another by rocky headlands andriver mouths.

The area of study, known as Calangute beach,is one of the two sandy beaches en the northern partof ~his coast and is 8ituated between two rock pro­montories - Baga in the north and Aguada inthe south, at a longitude of 73°46'E and latitudeof 15°32'N (Fig. 1). On the landward side, theshore is flinged by coastal sand dunes and on theother by the continental shelf. A series of sanddunes parallel the entire backshore of the study area(beach terminology aHel Shepard7) and extendapproximately 0,50 km inland and merge with thelateritic soil of the coastal plain. The dunes varyin height from 0·50 to 10 m and their spacingvaries from a few metres to 15-20 m along the shore­line. The distribution of unconsolidated sedimentsalong the beach is asymmetrical; the accumulationis much thicket to the south than to the north. Thebeach at Baga is narrow with a backshore of fewmetres wide and a moderately steep foreshore, thelatter merging with the uniformly sloping offshOletopography. A continuous backshore with an averagewidth of 35 to 40 m extends 0·50 km south ofBaga up to Station .3, and thereafter up to Station1 its width varies between 25 and 35 m. Ingeneral, the backshore slopes seaward while theregion behind the new berm crest slopes landward.The lOcky headlands, capped by 30 to 40 mthick laterite, reach an elevation of 75 to 100 m. A

VEERAYYA: TEXTURAL CHARACTERISTICS OF CALANGUTE BEACH SEDIMENTS

of the rocky headlands on the beach, the rest ofthe stations are exposed to the open sea, Samplesof sediment Were collected along the profile sectionsat fortnightly intervals during low tide period, fromthe dunes, backshore, berm and foreshore-breakerzone of the beach, along with the measurementof the beach profiles. The main aim was to studythe grain size distribution of the sediments in Fela­tion to the topographic changes of the beach, andhence uniform samples from the upper few centi­metres of the beach were obtained. Concurrentwith sediment sampling, periods, heights and direc­tions of waves, direction and speed of wind, littoralcurrent velocities, foreshOle gradients and groundwater tables were recorded during each survey.

Laboratory methoas - Although the samples werecollected at fortnightly intervals, 585 samples be­longing to one series of collections at monthly in­ter vals were analysed in the pre£ent s~udy,

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few laterite formations crop out at Baga and areexposed during low tide period. The river Man­dovi, south of Aguada, the river Chapora, northof Baga and a tidal creek at Baga drain into theArabian Sea, apart from the small streams fromcoastal plains, which cut across the beach duringthe south-west monsoon season. The submarinecontours (Fig. I), as depicted in the Admiralty ChartNo. 2022, are approximately parallel to the shore­line. The offshore region in the area of study hasan average slope of 1,50 m per km up to 30fathom contour (shown up to 12 fathoms only).MethodS

Fiela methods - Five beach stations Were moni­tored along this strip of coast, during September1969-August 1970 (Fig. 1). Stations 1 and 5near Candclim and Baga respectively were estab-,lished to delineate the influence of the proximity

1

29

INDIAN J. MAR. seI., VOL. 1, JUNE 1972

In th laboratory, all the sediment samples werewashed free of salt content, oven-dried, represen­tative ortions selected by coning and quarteringand the subjected to mechanical analysis. Sievingwas pc formed for 15 min on an ILM Laborsieve s akel using ~. <p interval Endecotts testsieVeS. All the fractions were weighed on a Mettlerbalance. The results Were plotted as cumulativefrequen cmves on arithmetic probability paperusing scale on the abscissa and cumulativepercent ge coarser on the ordinate.

The rain size distributiom of the sedimentswere de cIibE:dby evaluating the descliptive statis­tical pa ameters, viz. graphic mean (Mz), inc1U5ivegraphic standard deviation (O'r), ir,clusive graphicskewnes (Ski) proposed by Folk and WardS.Results

Varia~on of the textural parameter:" viz. meansize, st ,dard deviation and skewness, of the "edi­ments a ong one of the typical pr ofile sections, i.e.

at Station 3, for different mon~hs is shown in Figs.2-4. The monthly distribution of the maximum,mean and minimum values of textural parametersof the foreshore sediments at all the stations isshown in Figs. 5-7. Frequency distribution curveshave been drawn to show the percentage abundanceof occurrence of the parameter values for fore­shore region at each station separately duringthe pe,iod October-April. Class intervals of 0·20 <p.

0·15 <p and 0·10 Ski for mean size, standard devia­tion and skewness values respectively have beenchosen while drawing the curves. The curves fOlthe individual station and for the combined valuesof all the staticns in respect of each parameter areshown in Figs. 8 to 10. Frequency distributioncurves for the monsoon season could not be drawn,since the number of samples Were inadequate.

A close examination of the monthly distributionof the textural parameter values for the differentstations (Figs. 5-7) suggests that they exhibit dis­tinct differences in regard to their distribution

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riation in mean grain size (Mz ¢) of sediments aeross the beach at Station 3 for different months [BB­EE- October, GG-November, II- December, JJ- January, LL- February, NN- April (i), 00­Q - May, SS - June, UU - July and WW - August. September to December 1969 and January to

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30

VEERAYYA: TEXTURAL CHARACTERISTICS OFCALANGUTE BEACH SEDIMENTS

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31

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INDIAN J. MAR. SCI., VOL. 1. JUNE 1972

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INDIAN J. MAR. SCI., VOL. 1, JUNE 1972

patter during the fair weather sEason (Octoberto A il) and monsoon season (June to August)with ay and September as transition periodsbetwe the seasons.

As s ch, for a comparison of the values betweenthe se sons, all the individual values in respect ofone pa ameter in the fair weather season were com­bined nd the same was done for the data for themonso n season. The mean and standmd devia­tion f values in the two seasons Were calculatedfor th three gfain size parameters and are pre­sented in Table 1 for each of the stations.

Test of significance fOl the differences betweenthe tw seasons for the same station and betweenthe st tions during the same ~eason have beencarried out by Student's t test9 and the results pre­sented in Tables 2 and 3.

Variati ns in Textural Parameters

The ariations in the textural parameters of thesedime ts are briefly described below.

Grap ic mean (Mz) - The mean grain dze acrossthe be ch showed an increasing trend from theupper lower foreshore during thF period of study(Fig. ). However, the inconsistencies present,at tim s, in this trend were due to the variationin bea h slopes. The sediments of the backshorewere 0 served to be coarser than those of the fO!e-

shore (except during the monsoon season). Thebreaker zone in contrast to the foreshore is repre­sented by a mixture of granule to fine sand classmaterial during all the surveys. The mean grainsize of the adjoining dunes was in medium to finesand grade.

As indicated in Fig. 5, the average values of meansize from October to March and for a part of Aprilfell in fine sand grade, while from.. the end of Aprilthey showed an increase and were in coarse tomedium sands and often very coarse sand alsowas enccuntered. The t test results 5how that thedifferences in mean size between the two seasons(Table 2) are highly significant at all the stations(P < 0·001) thus indicating a distinct seasonalvarh~ion.

There is an apparent variation in mean size fromstation to station during the fair weather season.The frequency distribution curves (Fig. 8) showthat the maximum percentage of Mz values at Baga(Station 5) occur between 2·00 and 2·60 ~, withpeak percentage between 2·40 and 2·60 ~ indicatingthe abundance of finer end of fine sand class. AtStation 4, south of Baga, the sediments arecomposed of medium to fine sands with clusteringof Mz values alOund 1·60 to 1·80 ~ and 2·20 to 2·40~. Fig. 8 reveals that the Mz values at Station 3further south are predominantly medium and to

TABLE 11- MEAN AND STANDARD DEVIATIONS OF GRAIN SIZE PARAMETERS (MEAN SrZE, STANDARD DEVIATION,SKEWNESS) DURING FAIR WEATHER SEASON AND MONSOON SEASON FOR ALL THE STATIONS

Grai~ sizeparameters

Mean sizil (Mz 4»

St. deviJtion (al)

Skewnesi'i (Ski)

Mean sizil (Mz 4»

St. deviation (al)SkewnesJ (Ski)

(Values are mean ± SD)

Stations,;2

345

Fair weather season----

------No. of samples: (n)

5234373159

1·7501

2·10252·05242'11272·3177±0·4683

±0'1432±0'2838±0·2708±0'19050,6000

0'56380,59680'59120'5258±0·1803

±0'1600±0'1911±0'1980±0·1814-0·2950

-0,3368-0'3700-0,3533-0,3476;-±0'1144

±0'1194±0·1251±0'1120±0'1592

Monsoon seasonNo. of samples: (n)

1211141011

0·7306

0'69280,70900'95220'7838,±0'6990±0·3808± 0'3572±0'4401±0'6446

0·73510'61250'73500·71110'7843

±0·0588±0'1102±0'2816±0·1187±0'1581

-0·0705-0'01370·05160,01850,0683

±0·1644±0·2160±0'1093±0·0899±0'0014

TABLE 21- VALUES OF STUDENT'S t TEST AND THEIR SIGNIFICANCE FOR THE DIFFERENCES IN GRAIN SIZE PARAMETERSBETWEEN FAIR WEATHER SEASON AND MONSOON SEASON AT EACH STATION

Grai~iZeStations

para ters 12345

DF:

62 43493968

Mean sizl (Mz 4»

6,155*18,260*13,020*10,040*15'390*St. devia ion (al)

2-550t0'938t2'013tl'803t4'416*Skewness (Ski)

5,622*6·308*11'140*9'521 *7'628*

*p < 0·01.

tP < 0,05.;tNot significant.

36

VEERAYYA: TEXTURAL CHARACTERISTICS OF tALANGUTE BEACH SEDIMENTS

a lesser extent fine sands and shows the presenceof a greater percentage of mediwn sand. At Station 2further south. the percentage of Mz values isclustered around 2·0 to 2·20 cp and indicates agreater percentage of fine sand size material thanat the previous two stations. In contra5t to theabove, the sediments at Station 1 are predominantlycomposed of finer division of mediwn sand clas5 withminor percentage of Mz values in fine sand class, thusindicating the existence of coarser size material at theextreme southern end than at the other stationstowards north. When grouped together, the valuesclustered around 2·0 to 2·50 cp, indicating the per­centage abundance of fine sand during the fairweather season.

There is a significant difference between the meansof Mz values at Station 1 and those at other stationsduring October-April. The sediments at Station5 are relatively finer than at the other stations,the differenCes in means being highly significant(Table 3).

However, the variations in mean size along thebeach, found during OctOber-April, were absentin the monsoon season (Table 3).

Inclusive g1'aPhic standard deviation (ar) - Thesorting values (Fig. 3) show an increasing trendfrom upper to lower foreshcre corresponding toan increase if. the mean size. The backshore is COm­posed of moderately sorted sediments while sedi­ments of poor sorting were encountered at thebreaker zone. The dunes are represented by mode­rately to well sorted sediments.

Besides mean size, the standard deviation valuesindicate a clear-cut seasonal variation (Fig. 6). Thevalues increase from the end of April throughAugust broadly corresponding to the distinct changein mean size. They tend to decrease from Septem­ber through October to March and later the increas­ing trend is noticed. The lowest standard deviationvalues representing very well sorted class wereencountered during October to April, while thesediments in monsoon were dominantly well tomoderately sorted in nature. The differences instandard deviation values between the seasons

mentioned above were significant in all the stationsexcept at Station 2 where they Were not signi­ficant and at Station 4 they were significant at 10%level only (Table 2).

The means of standard deviation values broadlyindicate variation along the beach similar to thatseen in the mean size during fair weather season(Table 1). These variations have been broughtout clearly by the frequency distribution curves(Fig. 9). At Station 5 the values show a bimodaldistribution with clusteling around very welland moderately well sorted classes. This groupingapproximately represents the finer and coarser sub­divisions of fine sand class. South of Station 5,i.e. at Station 4, the sediments are well to mode­rately sorted in nature, dominated by moderatelywell sorted class. Beyond this station towardssouth and up to Station 1, the sorting values comeunder well to poorly sorted class, majority ofthem being in well to moderately sorted classes.Though this is the general pattern from Station 4­

to Station 1, the sorting values at Station 2 in parti­cular are distributed in well to moderatelysorted classes. The poorly sorted class has lesspercentage of representation throughout the fore­shore dur ing the above period. When the standarddeviation values of the foreshore are groupedtogether, the distribution is peaked around 0·50 to0·65 cp, thus showing the dominance of well tomoderately sorted sediments. However, a tail cor­responding to poor sorting of some of the sedimentsis also evident from Fig. 9.

Inclusive graPhic skewness (Sk~)- The skewnessvalues of the foreshore sediments showed an increas­ing trend towards low water line correspondingto an increase in mean grain size. The backshoresediments are positively skewed or nearly sym­metrical throughout the study period.

Monthly distribution of the average values ofskewness also indicates a distinct seasonal variationat each of the stations (Fig. 7). The valuesapparently decrease from May-July and later onshow an increasing trend from August onwards.The study reveals that the sediments have negative

37

INDIAN J. MAR. SCI., VOL. 1, JUNE 1972

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Fig. 8 - ~;'requency distribution curves for mean grain size (Mz q,) values of the foreshore sediments during the fairweather season (1969-70)

38

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VEERAYYA: TEXTURAL CHARACTERISTICS OF CALANGUTE BEACH SEDIMENTS

TOTAL

#5

#=4

#=3

30

20

10

'0

#=2

39

Fig. 9 - Frequency distribution curves for standard deviation (al) values of the foreshore sediments during the fairweather season (1969-70)

#1

1.1 1.2 1."?l 1.40.1 0.2 0.3

5 TAN DA ROD E V I AT ION \61 J

oo

10

20

30

INDIAN J. MAR. seI., VOL. 1, JUNE 1972

30

20

10

metrical pattern at Station 3 and a minor percentageof values at Stations 2 a.nd 1 represent a tail

TOTAL towards lessee skewness value.;. When groupedtogether the values were clustered around -0·20to -0·60 Ski showing negative to very negativeskewed c1as~.

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Variations in Beach Topographic Features

It was found that the changes in sedimentcharactefistics mentioned above were associatedwith corresponding variations in width and eleva~

tion of the beach (backshore and foreshore regions)from time to time. In order to get a relativelybetter picture of the variations in sediment charac~teristics, a few of the beach profiles are describedarJd discussed below.

The monthly profiles for Station 3 shown inFig. 11 reveal that the beach can be delineated intobackshore and foreshore. In general, most of the ero­sion took place during the months of May, June, Julyand in the first half of Ahgust with a reduction inthe berm widths, and a correspcnding increase inmean grain size (i.e. lesser rp values) and less wellsorting of the sediments (Figs. 5 and 6). Anotherfeature of importance was the formation of 'step'just at the lower foreshore during monsoon sea~on.

In contrast to the rapid ('hanges and large varia­tions in the foreshore topography during the monsoonseason, the recovery of the beach sediments neededlonger period after the monsoon season. A maxi­mum build up of 1·7 to 2·0 m at Stations 2 and 5while a build up of the Older of 1 m at Station 1and of a les,er order at Station 4 was recordedduring fair weather season. This build up wasaccompanied by a decrease in mean grain size aswell as in sorting values (Figs. 5 and 6).

Discussion

The above seasonal changes in sediment para­meters as well as in dimensions of the beach arepos,ibly due to the corresponding changes inoceangraphic conditions. It is known that winds,waves and tides are the important factors respon­sible for the movcmmt of the material on the beachand the resultant form of the beach profile10•Among these, the force of wind-genuated surfacewaves is the primary oceanoglaphic factor affectingthe changes in the beach. Although no completestatistical wave data are available for this strip ofcoast, the wave heights and periods observed duringtht; surveys and the compiled swell wave statistics forthe areall arc in good agreement. The predomi­nant directions from which severe wave~ were ex­perienced are between west and south-west (duringSW monsoon season) generated by west and south­west winds, the wave periods langing from 5 to14 see, and waves approaching from west and WNWdirections reach the shoreline during the rest ofthe year.

These seasonal changf~sin textural characteristicsof the sediments as well as in dimem'ions of theforeshore reflect the changes in oceanographic Con­ditions as outlined by Trask12. The coarser grainsize in conjunction with the erosional featuresduring May to August is due to the high and steepwaves associated with the south-west monsoon.Field observations show that the prevailing as wellas dominant winds with velocities of 18-20 knots

#5

#2.

#4

#1

o +0.' +0.2 ~.l

SKEWNESS (SKI)

-g.8 -8.7 -0.6o

10

20

30

20

10

Fig. 10 -4 Frequency distribution curves for skewness (Ski)values od the foreshore sediments during the fair weatherseason (1969-70)

egative skewness and negative to positiveskewnes during fair weather and monsoon seasonsrespecti ely. However, in the latter season theyare sy etrically distributed witbin the negativeto posit ve skewness classes.

Frequ ncy distribution curves (Fig. 10) indicatethat at tation 5 the values are clustered aroundtwo clas es, viz. negative to very negative skewness.This clu tering shows a biomodality and a similarphenom non was observed in the case of standarddeviatio values at this station. The nearly sym­metrical distribution class also has about 6% ofthe valu s at this station. South of Station 5, up toStation ,the skewness values have approximatelya range om 0·00to -0·65 Ski, thus representing thenearly s metrical to very negatively skewed dis-tributio The occurrence of peak percentage ofvalues oes show an apparent variation fromstation 0 station, viz. at Stations 3 and 2 thevalues e concentrated around -0,30 to -0·40whileaL tation 1 they are between -0,20 and -0·30.The val s are approximately distributed in a sym-

'",

40

'I"

-2

130120110

HW

FORESHORE

MONTHLY PROFILES ACROSS STATION W 3

DISTANCE SEAWARD IN METRES

~ ~ ~ ~ 00 ~ ~

HW

3020

PROFILES DATE OF OBSERVATION

26; 9:69---- 28,10:69-.-.-.- 29,11 '69

31:12'69----- 14' 1'~-"-"- 25' 2 '70LW-'''-'''- 12' 3'70

29: 4:70•....-.-.- 29: 5:70•

26' 6:70+--+--.

30' 7:70----- "31: 1!'70

VEERA YY A: TEXTURAL CHARACTERISTICS OF CALANGUTE BEACH SEDIMENTS

6

4

2

5

o

-1

z

HW - HIGH WATER

L W - LOW WATER

Fig. 11 - Monthly beach profiles for Station 3

and at times gusty winds with high velocities ofabout 28-30 knots blow onshme during the monsoonseason. These prevailing onshore winds causeshoreward transport of surface water and com-'pensatory offshore flow at depth13• BH.aker heightswere of the ordec of 2·50 to 3·50 m at Stations 1and 2, and slightly less in height beyond Station 2up to Station 4, while 1·00 to 1·50 m breakerswere observed at Station 5 during the SW monsoonseason. The turbulent swash of these breakerswould keep thE fine sediments of the beach in sus­pension. However, prior to the prevalence of severemonsoon ccnditions, the concentrated wave actionon the beach developed a vertical scal p just belowthe new berm edge. The development of thisscalp gave rise to steep slopes of the Older of 15to 17° at the upper fOleshore which d('creased to3 to 4° towards lower foreshore. The return flowof water in the fOlm of backwash, aided by the steepslopes, transports the sediment seaward, thus givingrise to coarser grain size on the foreshole. Thisfeature in the month of May at Station 3 may beo b.,erved flOm Fig. 2. In this process the bermis cut back and it further undergoes reduction inwidth as soon as the monsoon intensifies. The

result of this is an accumulation of sediment belowthe low water line (and also outside the breakers)at the expense of the beach in the form of anoffshore bar. Further, a part of the suspendedsediment is being carried offshore by th(' rip currentSprevailing in this region during the monsoon season.

Although the variations in sediment parametersduring January-February (ariothel period of ero':'sion) were not very pl.Onounced, the correspondingchanges in elevations of the foreshore, in general,were relatively noteworthy. These can ie attri­buted to the changes in wave regime in this partof the year. Except for these minor changes, whichmay be of local nature, the beach has undergoneaccretion till April aided by the relatively long-periodswell waves. Th('se swell waves cause net shore­ward transport. In this process, the bottom sedi­ment is moved shoreward and it thus re-enters thelittoral zone13. Further, these low flequency wavesbleak in mch a way that the swash is more effectivethan the backwash, thus moving the material upthe beach10 and replenishing the beach material.Hence, the sediments are repref'ented by fine sandsize class in majority of the cases during this period.The monsoon high waves, while aiding the cut at

41

INDIAN J. MAR. SCI., VOL. 1, JUNE 1972

the sea ard side of the berm during spring tides,push b ck some of the material landward, therebybuildin up the berm from that end. The sedimentwhich as pushed back thm contributes to a fur­ther b 'ld up of the berm or double belm. Sub­sequen y as the SW monsoon conditions recede,the be ms start building up and grow seawardduring neap tide conditions and the ~ediment ispushed back by the succ,-;eding5pring tide swashes14and thi feature was observed during the tidal cycleobserva ions on this beach, the details of whichwill be described separately elsewhere. The mate­rial de osited in thi, way on the landward sidesloping pal t of the new belm was left out as lagdeposit This washovel material was 15enerallycompos d of ~hell" shell fragments, well-roundedrock a d quartz pebbles of granule to pebblesize cIa s. This may be one of the reasons for thecoarser ean dze of the new berm than that of thefOlesho e and backshore of the beach (Fig. 2). Thiswashov r material is present at all the stations,except t Station 5 and is one of the characteristicfeature of this beach. If similar sediments werepreserv d in the stratigraphic columns of theancient eaches, they would aid in the identificationand del [,eation of regions of high wave activitiesalong t e past shorelines and the results of thisstudy orrobolate the statement that 'washovermateria present interpretative problems of paleo­environ ents'15.16. Thus the unique ptesentationof wide range of size of material, i.e. representedby gran Ie (often pebble size) to fine sand clas;:,es,represe ts a special case17. Besides this feature,fragme s of tar of about 5 to 15 mm and of lesserdimensi ns were concentrated at swash marks andalso at he upper limits of swashes of the monsoonhigh w ves. A similar feature was observed onthe Me itetranean beaches of Israep8. The detailedinvestig tions about the nature and origin of thetar ma erial in the area of study may, perhaps,reveal s me interesting results.

Besid s the general increase in the mean sizefrom t upper to lower foreshore the study alsoindicate maximum grain size material at the' -tepformati n' during June to August correspondingto high ave energies as predicted under the modelp,'opose by Miller and Zeigler19. In genelal, themean s e increases cOlfesponding to an increasein the reshore gradient. The presence of coars,:size ma erial at the lower foreshOle is due to thefact th t it receives mOle energy than the upperforeshor. However, at times the upper foreshorewas al0 rep;'esented by coarse size materialand thO is the result of the winnowing awayof fine aterial by wave action and wind. Breakerzone is he other region where coarser size materialwas en untered. This phenomenon was due tothe act ve hydrodyrtamic processes in this zonewhich k ep the fine particles continually in suspen­sion20,21

The elatively good sorting of the sedimentsduring ir weather season indicates the dominanceof one 'zc cIas;;, i.e. fine sand; whereas the poorsorting 1 monsoon is due to tbe presence of a com­binati( n of several grades dominated by COalSE'to medi m sands. However, the transition periodsare cha cterized by high sorting values, Decauseof the p esence of coarse and fine sands which con­tribute to high deviation values. Subsequently

42

as the monsoon condition5 recede, the finer sedi­ments are being brought back on to the beachand the mean ~ize shows decreadng trend, thusgiving rise to lesser standard deviation values.The significant differences in these value~ betweenthe two extreme stations (Stations 1 and 5) aredue to the fact that the sediments at Station 5are dominated by fine sands wherea3 at the otherend they are mainly composed of coarse to mediumsands, and to a lesser extent the fine sands presentgive rise to a better sorting at Station 5 and a l€~swell sorting at Station 1.

In general, the sorting values increa~e from theberm crest to seaward side of it en the foreshore.This increase is due to the corresponding increasein the SiZE;of the sedimEnts (Fig. 3). In contrastto the very well to moderately sorted sedimentsof the foreshore (fair weather Eeason). the breakerzone is characterized by poorly SOl ted sediments.This poor sorting i" due to the presence of widerange of material sjze~ pn:.sent22. The winnowingaction of waves transports the fine material fromthe beach face and subsequently a part of it wouldbe deposited below the low water line. The poorsorting is the result of mixing of this fine materialwith the coarse matel jal already pre~ent in thisregion. Similar pherlomenon was observed byInman and Chamberlain23 in the surf zone off LaJolla, California, USA.

Most of the time the sediments are havirlg theirmean diameters in medium to fine sand grades andalso in coarse to very coarse sand grades, i.e. theyare beyond the best sorted range of Inman24 andare her<celess well sorted in nature. The frequencycurves for the individual samples show thatthey are represented by more than one popu­lation, i.e. from granule to fine sand grade, andexhibit, at tim~s, a tendency towards bimodality.The mixing of these populations led to decreaseof level of sorting:?; while the sediments withdominantly one population becoming well sorted.Although it has been stated in the literatUle thatcertain size fractions are relatively scarce amongterrigenous sediments, the sediments of this beachare compo:,ed of granule to coar~e sand materialfairly in good percer.tage (particularly in the mon­soon season). The dominance of this ~izeof materialin the area of study may reflect the dominance ofthis size material in the sourCE;area26. Hence, thedegree of sorting is dependent not only upon theeffects of wave energy reaching the coast, but alsoupon the source from w;lich the sediments werederived:?'7.

However, plots of sorting versus mean size forthe foreshore sediments (Fig. 12) indicate broadlya sinusoidal pattern as noticed by Folk and WardSand fmther reveal that the best sorted sedimentscoincide on the graph with the lower deflectionson the sinusoidal trend. It also shows that an in­crease in standard deviation occurs from fine tomedium sand sizes as reported by McKinney andFriedman26. This shows that the measures of sized;stribution are somewhat interdependent and thata physical relationship exist: between the meandiameter and the standard deviation for most water­transported sediments24.>8.

Of the three measures of size distril::ution underconsideration, the skewness parameter has shownsignificant seasonal variations~o next to mean size

(

VEERAYYA: TEXTURAL CHARACTERISTICS OF CALANGUTE BEACH SEDIMENTS

---

III0..

tIII~

t "~ t ' ",.

"

f III

~>

..

zo•...4: 1.0>wooQ::4:oz4:•.•• 0.5Vl

0.2

0,1

o-1 o

MEAN

:

1

SI ZE (M:r.~J

....

. ,"

:. ..

2

..

,.•

3

Fig. 12 - Scatter plot of mean grain size (Mz ¢) versus standard deviation (a,) of the foreshore sediments [VWS­very well sorted, WS - well sorted, MWS- moderately well sorted, MS - moderately sorted, and PS - poorly sorted]

(Table 2). As mentioned earlier all the foreshoresamples, during the fair weather season, are nega­tively skewed and this negative skewness is dt:eto the presence of fine size material in greater per­centage with a coarse tail thereby skewing thefrequency curve to the coarser side, The positiveskewness or the nearly symmetrical distribution ofthe foreshore sediments during monsoon season maybe due to the subtraction of fine size materialleaving behind the coarse mode with fine tails(relative to the coarse sizes), The positive skewnessor nearly symmetrical nature of the backshore andthe dune sediment,; wculd result from the incor­poration of fines by onshore winds from the l:'xposedparts of the foreshore and berm crest.

Apart from winds, waves and tides, the direc­tion and amount of littoral drift and the configu­ration of the shoreline are also important in deter­mining the distribution of sedin/cnts of the beach29•This is due to the reason tl1at waves breaking atan angle with the shoreline set up a longshore ferlittoral cuuent which may reach a velocity ofseveral knOt2 and which is capable of carryinglargt> amounts of sediments3o. Since the shorelineunder investigation trends approximately in NNW­SSE direction, the longshore drift would be in eitherof these directions, depmding upon the prevailingdirection of onshore wave travel due to ~(a andswell. The longshore current velocities determinedby observing float movements varied from a fewcentimetres to about 80 em/sec. During the monsoonseason they Were directed towards north and in

f.

the lest of the year they were partly north, andalso in offshore-onshore directions, and at timessouth.

The observed variations in mEan size alongthe beach from Station 1 to Station 5 during fairweather season reveaied that the mean sizedecreases in that direction. The sorting valuesalso showed a similar pattern. It is surmised thatthese differencfs would arise due to the prevailingnortherly littolal currents. Further it has beenobserved do/ing the surveys that on the southernpart of the beach the waves break closer to theshoreline, while on the northern side they brEak alittle away from the shoreline. This may also bepartly responsibl£ for the gmeral decr£ase in themean size flom south to nOlth. This indicatesthat the change in mean grain size along thebeach reflects the chanl'es in the nearshore energyenvironment3l.

However, lack of significant variatior.s in texturalparamders of the sediments along the beach duringthe monsoon season may indic",te the intemity ofoceanographic conditions prevailing OVl:'rthl:' areaduring this season.

Acknowledgement

The author wishes to express his sincere thanks toDr N. K. Panikkar, Director, for his keen int('l'Estand encouragement. He is indebted to Dr V. V. R.Varadachari, Head of Physical OceanographyDivision and Leader of the Coastal and ,Near&horeOceanography Project, for constant guidance and

43

INDIAN J. MAR. SCI., VOL. 1, JUNE 1972

uggestions and to Shri C. S. Murty for con­help and advice. Help rendered by Shrio of Goa Medical College during the statis­lysis of the data is highly appreciated.

1. OER L, G., A Geologia Do Distrito De Goa, ComunicaGo dos Servicos de Portugal, Tomo XL, (1958),155

2. SINH, B. N., Gen. Rep. Geol. Surv. India, Report, 1962.3. D'So ZA, NOEL G., J. geol. Soc. (Univ. of Saugar),

1 ( 965), 13.4. Roc ,A. T. & UBALDO,M. L., Gracia de Orta, Lisboa,

12 1964), 645.5. GUH , S. Ie, MicroscoPic evidence of a major fault

zon in the Arabian Sea parallel to the west coast ofInd a (International Oceanographic Congress, AAAS,Wa hington), 1959, 26.

6. KRIS NAN, M. S., History of the Indian Ocean (Inter­nat nal Oceanographic Congress, AAAS, Washington),195 , 34.

7. SHEP RD, F. P., Submarine geology (Harper & Row,Ne York), 1963, 557.

8. FOLK R. L. & WARD, W. C.,]. sedim. Petrot., 27 (1957),3.9. GRIF ITHS, J. C., Scientific method in analysis of sediments

(Me raw-Hill Book Co. Inc., New York), 1967, 508.10. KING C. A. M., Beaches and coasts (Arnold, London),

195 , 403.11. RED ,M. P. M., Mahasagar, 1 (1970), 28.12. TRAS , P. D., Tech. Memo. Beach Eros. Bd U.S., 91

(19 ), 49.

44

13. COOK, D.O., Sand transport by shoaling waves, paperpresented at GSA Convention, 1969.

14. SHEPARD, F. P., Tech. Memo. Beach Eros. Bd U.S .. 20(1950), 26.

15. HAYES, M. 0., Rep. Bur. econ. Geol. Univ. Tex., 61 (1967),54.

16. HAILS, J. R. & HoYT, J. H., J. sedim. Petrol., 39 (1969),559.

17. ZENKOVICH,V. P., Processes of coastal development (Oliver& Boyd), 1967, 738.

18. EMERY, K. O. & NEEV, V., Bull. geol. 5urv. Israel, 26(1959), 1.

19. MILLER, R. L. & ZEIGLER, J. M., J. Geol., 66 (1958),417.

20. INMAN,D. L., Tech. Memo. Beach Eros. Bd U.S., 39 (1953),34.

21. BOYNAGRYAN,V. R., Oceanology, 9 (1969), 537.22. KING, C. A. M. & BARNES, F. A., Z. GeomorPh., 8 (1964),

105.23. INMAN, D. L. & CHAMBERLAIN,T. K., Finding ancient

shorelines, SEPM Pub!., 1955, 106.24. INMAN, D. L., J. sedim. Petrol., 19 (1949), 51.25. INMAN,D. L., GAYMAN,W. R. & Cox, D. C., Pacif. Sci., 17

(1963), 106.26. McKINNEY, T. F. & FRIEDMAN, G. M., ]. sedim. Petrol.,

40 (1970), 213.27. HAILS, J. R., ]. sedim. Petrol., 37 (1967), 1059.28. GRIFFITHS, J. c., J. Geol., 59 (1951), 21t.29. SHUYSKIY, Yu. D., Oceanology, 10 (1970), 89.30. PUTNAM, J. A., MUNK, W. H. & TAYLER, M. A., Trans.

Am. geoPhys. Un., 30 (1949), 337.31. FELIX, D. W. & GORSLINE,D. S., Marine Geol., 10 (1971),

177.

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