Adv.SpaceRes.Vol. 13,No. 5, pp.(5)75—(5)78 1993 0273—1177/93$24.00Printed in Great Britain. All rights reserved. Copyright0 1993COSPAR

STUDY OF SEDIMENTDYNAMICS USINGSATELLITE REMOTESENSING

B. Manikiam, H. HonneGowda,P. Manavalan,V. Jayaramanand M. G. Chandrasekhar

Indian SpaceResearchOrganisation,Bangalore-560094,india

ABSTRACT

Variationsof opticalpropertiesof seawateraredueto thepresenceof dissolvedand suspendedmaterialswithdifferentabsorptionandscatteringcharacteristics.Satellitebasedobservationshavebeenfoundto providevaluableinferencesaboutthe sedimentlevelsof oceanicwaters. TheWestcoastof India is highly pronetoerosionespeciallyduringthe monsoonseasondueto the wave actionassociatedwith the south westerlycurrents. Theheavyrun-off from westflowing rivers in thepeninsularregionalso contributeto thehigh levelsedimentationin the offshoreregions. In thepresentstudy, themultidatedatafrom IndianRemoteSensingSatellite,IRS-lA hasbeenutilisedto analysethesedimentationlevelsanddispersalpatternsfor thecoastalregionof Mangalorein SouthernIndia. Digital techniqueshavebeenemployedto bringout the variationsinthesedimentationpatterns(upto 14 levels)for thepre andpost-monsoonseasons.Theavailableinsituobservationson wind, wavesandtides havebeenusedto understandtheseasonaldispersalcharacteristics.Thestudyhasindicatedhigh level of sedimentationin thepost-monsoonperiod, revealinghighly erosionpronenatureof the coast. A semi-quantificationof thesedimentationhasalsobeenattemptedthroughcorrelationwith seatruth data.

INTRODUCTION

Satelliteremotesensingby providing realtime, qualitativeandquantitativeinformationon the suspendedsediments,hasbecomean importanttool for monitoring thedynamicsof coastalwaters. Knowledgeon thetransportof coastalsedimentis importantandalwayshasbeenof interestto coastalgeomorphologistsandnear-shoreoceanographers,as it is basic solution for various problemssuch asthe modificationof harbourbasins,beacherosion/accretion,thesedimentbalanceof enclosedbaysor theuseof suspendedsedimentsas a tracerfor studying the circulation in coastalandnear-shoreenvironment. Severalattemptshavebeenmadetoestimatesuspendedsedimentconcentrationfromtheremotelysensedreflectanceof water /1-10/. In thepresentstudy an attempthasbeenmadeto utilise multidatesatellitedatafor assessingthe sedimentdynamicsoffMangaloreCoastin SouthernIndia.

STUDY AREA AND DATA

ThestudyareaaroundMangalore(lat.12°52’N,long. 74°51‘E) is an importantregion alongthewestcoastofIndia andoccupiesapre-eminentposition in the KarnatakaStateas amajor fishing harbouras well asanimportantport. The entranceto thefishing harbouris via themouth of NethravathiandGurpurrivers. Anarrow sandspit separatestheGurpur river from the Arabiansea,which is severalmiles long. TheNethravathiand Gurpurriversoriginatein theWesternGhats wherethe averagerainfall is about330 cm withmaximumdischargeof approximately7,100cu rn/secand 1700 cu m/secrespectively/11/. The NethravathiandGurpurrivers areestimatedto carryabout3 million cu m of sedimentload peryear/12/ . This coastaltractis also subjectedto strongwinds andresultantcurrentsduringthe south-westmonsoonseason. Onaccountof thesefactors,causinglargescaledischargeof sedimentloadby thesetwo rivers into theArabianSeaoff Mangalorecoast,thenavigationalchannelnearthemouth of theestuaryandseaareunderconstantthreatofsiltation anddevelopmentof barsleadingto navigationalhazardsparticularlyduringthe startof fishing season(Septemberonwards),which is immediatelyafter thesouth-westmonsoonseason.

IndianRemoteSensingSatellite- IRS-lA data(LISS II sensor)havinga spatial resolutionof 36.25 meterscovering thestudyareawas utilised. Multidate dataof December1988, November1989 and February 1991coveringthepostandpre-monsoonseasonswerechoosento understandthesedimentdynamicsvis-a-visthenearshorecurrentpatternsduringthefair weatherseasons.Satellitedatafor themonsoonseason(June-Sept)couldnot beobtaineddueto persistentcloudcover. Ancillary datasuch as Surveyof India topomapsforMangaloreregionon 1:50,000scale,bathymetricchartsavailablefrom NationalHydrographicInstituteanddataon seatruth collectedthroughrecentsurveysand alsoby earlierworkers/13-16/ wereutilisedfor thestudy.

METHODOLOGY

Digital analysisof thesatellitedatawascarriedout usingtheVAX 11/780basedimageprocessingsystem

availableat theRegional RemoteSensingServiceCentre,Bangalore. The spectralbands4,3 and2 were

(5)75

(5)76 B. Maniltiamet al.

selectedfor the analysisto getthebestresultsby wayof qualitativediscriminationof thesedimentpatterns.Basedon thetonal variationseenon theimage,trainingsetswereassigned.Detailsfrom thetopographicmapsand bathymetricchartswereusedas partof thegroundtruth information. Classificationwas performedon allthefour bandsusingmaximumlikelihood algorithm. It may be mentionedhere that the digital datawerenotcorrectedfor sun elevationangleandatmosphericconditions. Although 14 levelsof variation in sedimentpatternswerebroughtout throughthedigital analysis,it was further regroupedinto a maximumof 7 specificclassesbasedon reflectancevaluesindependentlyfor eachseasonin order to havebetterdiscrminabilityofdispersalpatternandto achievesemi-quantification.Thesegroups,viz., veryhigh, high, moderateto high,moderate,slight to moderate,slight, noneto slight depict the sedimentconcentrationin decreasingorderbetweenthehighestandthelowestas observedon thesatellitedata.

RESULTSAND DISCUSSION

Thedetailedanalysisof themultidatesatellitedataclearlybroughtout severalinterestingresultspertainingtocoastalzonedynamicsand its responsein various time scalesto changesin oceanicand continentalprocesses.Figuresla,b and2 showtheseasonalvariationin thesedimentdistributionand its dispersalpatternin thestudy

- DECEMBER 03, 1988 NOVEMBER 21, 1989

~ s--- ~ ~VERY HIGH- \.~.:::.~q_.‘._,— ___

~ (a) ~ J ~ Ib)

LAND ~JMOO�RATEI .~ :~:.:.::.‘~. ~ LAND— II ~J’’ F_I , MODERATE

I I ~ —

ESTUARJNE ~ ‘-,.--—-,f — SLIGHT To

- ~- —~-~:- !~i UTH -~ ~ ., ESTUARINE MODERATE

- - -.~- . -. ~ MOUTH

~ ~ SLIGHT

- - - _:_ ~ I ~ ~:~I~~lEHTroiBm 12m Be (depth)

The 12m Bm (depth)

Vig.la,b : Distributionof SuspendedSedimentduringPost-monsoonSeasons

FEBRUARY 21, 1991 The seasonalvariationsof sedimentdistributionin

.k~~/j—

the seanearan estuarinemouth arecausedby• . t HIGH winds,tides, currents,freshwaterdischarge,wave

- --: action, littoral drift andsedimentdistribution/7/.- — ~ ., .. MODERATE Indirectly, windsareperhapsmore important in that- ~-‘L ~iJ TO HIGH they generatewaves,which causeconsiderable

U-:: . modificationthemselves,and alsoproducelongshoreSLIGHT TO currents/18/. Along the west coastof India, the

~ ) A C MODERATE south-west monsoonstrikesthecoastalmost- .;..~ . ( orthogonallyandwith greatstrength/19/. In the- :::. k--:. SLIGHT coastalwatersoff Mangalore,windsduring south-

west monsoonandpost-monsoonperiods,apart-. - . ~ ~ from generatinglocal seasandproducingcurrents,

- —.-.-. . FS.U~’~.‘~ perhapshelp to raisethe level of theseawaterandt’~?~~•~ ~ thus enablingthelandwardrushingof theseawaters

— to meetthe seawardrushing flood watersof- :::-.::. \ . ~ i Nethravathi-Gurpurriver with greaterforce. This

could causean oscillation of a massof sediments- ~ ‘\~ neartheestuarinemouth in the seaand is probably

1~m ~am .~ - —. responsiblefor thegrowthof thebar formingduring1depth) theseperiods.In viewofthis, suspendedseidmentload

wasnoticedto bemaximumneartheestuarinemouthasFig.2 : Distribution of SuspendedSediment is evidencedfrom the high spectralvaluesrecorded

duringPre-monsoonSeason duringpost-monsoonseason(Figs.la,b). Duringthis

SedimentDynamics (5)77

seasonthesedimentdistributionis uneven,dueto manylocal forcesactingtogether.In situ observationonsuspendedsedimentloadsduringthemonsoonseasonwasrecordedto be in therangeof 116 to 178 mg/I neartheestuarinemouth and 60 to 84 mg/I in the seasaround 12m depth/15/. Also, during thewinter seasoninDecember(post-monsoon)suspendedsedimentload atthe estuarinemouth variedbetween38 and52 mg/I,while in theseait was about20 mg/I throughoutthetidal cycle.

During the pre-monsoonperiod,suspendedsedimentload werecomparativelyvery low both in the estuarmnemouthandin the seaessentiallydueto thereducedriver dischargeandtherebysedimentdispersalpatternisfoundto beuniform andparallel to thecoast(Fig.2). Certain interestingobservationsthatcould bemadeduringthis periodare(i) aregionof relativelyhigh concentrationof sedimentoccurringnot attheestuarinemouth but immediatelyafter it. This couldbe dueto relativelylower intimity of wavesandcurrentsin thepre-monsoonseasonresultingin an equilibrium positionaway from theestuaryand(ii) arelativelyhigh level ofsedimentationareanoticedbetweentheclear watersin theinshoreandoffshoreregions,perhapsthesourcecould bethe sandbar formedduring previousseasonswhich is erodeddueto bottom currentsof reflectedwavesandtransporteddueto north-northwestcurrentsas dredginghasbeenstoppedfor manyyearsin theMangalorePort. Thesesedimentmovementarecontrolledby north and north-westcurrents/5/ prevailingduringthis seasonasis evidentfrom Fig.2. Estimateson suspendedsedimentloadduringthis seasonhasbeenreportedas4 to 8 mg/I nearestuarmnemouth and 3 to 3.5 mg/I in theseas.

Apart from the suspendedsedimentload andbedload from thetwo rivers, littoral drift alsois a sourceofsedimentin thestudy area. Sedimentmovementalonganygivenstretchof coastis governedmostlyby thecomplexinterplayof forecesassociatedwith wavesand tides. Movementof sedimentalongthecoastis theresultof many individual littoral drift actingon discretecoastalsegments.During thesouth-westmonsoonseason,thelittoral drift will be directedtowardsnorth in thearea,while duringtheremainingperiod,thedriftwill betowardssouth/14,16/. However, dueto therelatively strongerwavesduringthe south-westmonsoon,therewill bea netlittoral drift towardssouth. This netlittoral drift movingnorthwardsalongthe coastissmallerwhencomparedto the quantity of sedimentbroughtdownby rivers /15,16/. Tidesare importantcurrentcausingforce in coastalwaters. Diurinal variationof tides in the study areacausedifferentflowcharacteristicsnear the estuarinemouth and thesecurrentsareapparentlymodified by riverine dischargeandshorelineconfiguration. Thestrong tidal currentsprobablyact as barrierto thelongshoretransport/3/. Thedifferencesin depthof thebottomof thesea,wherelow densitywateroverliesdenserwater, especiallyif asharpinterface existsbetweenthem, induceslopecurrents/20/. It is know that during monsoonandpostmonsoonseasons,thesecurrentsaremore pronouuncedandstrongerand thus largequantitiesof sedimentscouldbetransportedinto theseathroughtheestuaryas it is evidentfromFigs.la,b.

While consideringthe dispersalof sediment,theconceptof greatercompetenceof theonshorephaseoftheoscillatorywavecurrent indicatesthat underfavourableconditionsof wave, depthand material characteristics,nettransportshorewardmayoccur in theareaseawardof thesurfzone/21/. Thehighercurrentvelocities andthebottomsedimentspredominantlyconsistingof sandnear theestuarinemouth in thestudy areashowthatfiner grainmaterialwill becarriedaway by thesecurrentsinto the sea. Also at somepoint the onshoretransportwill becheckedby the effectof gravity, reflectedcurrentsandriver discharge.At this point,conditionsarefavourablefor an accumulationof material knownasonoffshorebar /15/. Thepositionatwhichthebar will form is dependentupon depth, slope, material and wavecharacteristics.In theseaoff MangalorecoastnearNethravathi-Gurpurestuarinemouth,theformationof bar is likely to occurmainlyduringthesouth-westmonsoonseasonas aresultof the strong waveconditionsduringthat season. However,this couldnot beobserveddueto non-availabilityof cloudfree satellitedataduring this period.

Coastalerosionis yetanotherfactorcontributing to thehigh concentrationsof suspendedsedimentsalongtheMangaloreCoast. Wavesof heightof about4 metersandof period8 secondsoccurringat this time seemto beresponsiblefor thelargescaleerosion/13/. Thenetresultis aseriouserosionof landcloseto thecoastaroundMangaloreaffectingthefishing villages, othersettlementsandcultivations. Studieshaveindicatedthatconsiderableerosionin thenorthernand middle portion of thesandspitwhich canoccurmainly dueto high andsteepwavesandlittoral currentsduringthesouth-westmonsoonseasonandalsoearlypost-monsoonseason/13/. On the southernsidenear the estuarinemouth erosioncan occurmainly dueto theflooding of theNethravathi-Gurpurriversas it canbe evidencedfromthefigures.

CONCLUSIONS

Thestudy clearlydemonstratesthat multispectraldatafrom IRS-lA could beeffectively utilised for dynamicmonitoring and for time sequenceanalysisof the estuarineand coastalenvironment. Non-availability of dataduringthemonsoonseasondueto persistentcloud cover,hashoweverlimited thescopeof the study. FurtherimprovementscouldbeachievedprovidedthesatellitedatafromIRS-i couldbecombinedwith microwavedatasuch as ERS-1so asto overcomethecloudcoverproblem. This couldlead to better informationand analysisof oceanographicparameterssuchaswave, wind, current, surfaceroughness,etc.,and help us to pinpoint thespecificcausativefactors controllingthesedimentdynamicsof estuarineand coastalwatersoff Mangalore.

(5)78 B.Manikiamet aL

ACKNOWLEDGEMENTS

The authors wish to expresstheir gratefulthanksto Shri. S. Adiga, Head,RRSSCfor providing computerfacilities, Prof.M.P.M. Reddy, Collegeof Fisheries, Mangalorefor providing valuablereferencesandShri. J. Krishnamurthy, Scientist,NNRMS for providing suggestions. Thanksarealsodue to Ms. S. Van~afor secretarial assistance.

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