.~}~iTU DelftDelft University of Technology

Density currents and siltationwith particular reference toCochin -

P.V. ChandramohanFebruary 1989

Faculty of Civil EngineeringHydraulic and Geotechnical Engineering DivisionHydraulic Engineering Group

INTERNATIONAL INSTITUTE FOR HYDRAULICAND . ENVIRONMENTAL ENGINEERING

DELFT I THE NETHERLANDS

MASTER OF SCIENCE THESIS

DENSITY CURRENTS AND SIL TA TION-WITH PARTICULAR REFERENCE TO COCHIN

P. V.CHANDRAMOHAN

. Gul-

Dr.lr.G.ABRAHAMAdv••.,

Prot .Ir •H. VELSINK

F.bruary. 1889

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DICHTHEIDSSTROMING EN SEDIMENT ATIE, . -

-MÈT"ALS' SPEcïFÎ'ÉKE TÓEPAS~SING COCHIN.... . ' . .

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TIE TtE81. ON

DENSITY CURRENTS AND SILTATION-WITH PARTICULAR REFERENCE TO COCHIN

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EXECUTIYE:' ENGM=FR. COCHIN; PoRT TRUlT

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CONTENTS Page

ContentsSynopsis

(i)(iv)

Freshet dischargeSalinityVindVavesMud banksBed materialGeneral mechanism of siltationEvolution to the present shapePresent problem

1.1l.l

1.11.21.21.71.81.81.101.101.131.131.141.141.151.18

BACKGROUND OF TBB PROBLEK

1.1. History1.2. Saga of Cochin1.3. Tidal storage area1.4. Tides and tidal prisml.S. Currents1.6. Rainfall

1.0.

1.7.1.8.1.9.1.10.1.11.1.12.1.13.1.14.1.15.

C.v. & P.R.S. - 1980C.V. & P.R.S. - 1985NESA - 1988Data on dredging

2. I

2. I

2. I

2.22.32.52.62.72.7

AVAILABLE DATA AND TBBIR DETAILS

2.1. C.V. & P.R.S. - 19402.2. C.V. & P.R.S. - 19532.3. C.V. & P.R.S. - 1967 - 19682.4. N.l.O. - 1975 - 1976

2.0.

2.5.2.6.2.7.2.8.

3.0. HYDRODYNAHICS OF DENSITY CURRENT

3.1. Relevanee of theory3. I

3. I

(ii)

3.2.3.3.3.4.

Critical flow concept (open channel flow)Critical flow concept (two layer flow)Application of 'critical flow concept' to twolayer flowsDensity induced return currentsDensity induced exchange flow

3.5.3.6.

4.0. A KETHOD TO QUANTIFY DENSITY CURRENTS4.1. Rigter's graphs4.2. Procedure4.3. Practical problems4.4.4.5.4.6.4.7.

Density current equationLimitations of the methodPhysical characteristics of density currentThe complete qualitative density current graph

5.0. SCHEKATIZATION OF TIDES AND DISCHARGES5.1. Tidal flow5.2. Fresh water discharge5.3. Cross section of gut

6.0. CORRELATION OF COCHIN DATA VITO THEORY6.1. Discussion on available data6.2. Stratification parameters for Cochin6.3. Computational quantification of density current6.4. Analy~ls of data from 19806.5. Salient features at Cochin6.6. Future deepening of the channel

7.0. SEDIKENTATION7.l.7.2.7.3.7.4.7.5.

Introduction to problemTransport processes of cohesive sedimentsSedimentation during salinity intrusionComputations on sedimentationInferences from the exercise

Page3.63.73.9,

3.133.18

4.1

4.1

4.3

4.5

4'.64.64.13

5.1

5. 1

5.85.9

6. 1

6. 1

6.36.5s , 176.246.30

7.1

7.1

7.4

7.107.14

7.21

(iii)

8.4. Recommendations

Page8. 1

8.1

8. 1

8.28.6

8.0. CONCLUSIONS AND RECOKHENDATIONS8.1. Purpose of study8.2. Schematization8.3. Conclusions

List of figuresList of tablesNotationsReferenceAcknowledgements

(i)(v)

(vi)(viii)(xiii)

SYNOPSIS

In an estuary, the confluence of fresh water of uplandrivers with salt water from the sea gives rise to a complexregime of flow pattern due to the difference in densities ofthe two liquids of about 2.5%. Vhen the estuary isstratified, the heavier salt water which dives underneaththe lighter fresh water extends as a long wedge far into theupstream. The amount of saline water brought in by thedensity current into an estuary can be much larger than thetidal filling, when the freshet discharges are highcompared to the tidal prism. Density currents play a majorrole in the hydrodynamics of a harbour basin located at themouth of an estuary

The Port of Cochin on the west coast of India is locatedinside a natural lagoon with a large monsoon dischargepassing through its channels. The saline water which entersthe deep navigation channels brings in a large amount ofsilt from the sea resulting in heavy siltation.

This thesis deals with the density currents at Cochin andpresents the results of a desk study made with theobjectives

1. to analyse the hydrodynamics of density currents ingeneral with particular reference to the local situation.

2. to derive the siltation pattern from it and make a firstorder estimate and

3.to suggest solutions and make recommendations for furhterstudy.

(v)

In the study, more emphasis has been placed on thehydrodynamics of the density currents rather than thesedimentation part. The research done on the densitycurrents and salt wedge is more general in character andcould be applied universally; including to situations likethose at Cochin. A method to quantify density currents isbeing put forward. The results of the computations made,agree reasonably weIl with those obtained from actualobservations. This method, it is feIt, would prove to be ofvaluable assistance to future studies on density currentswithin its limitations. The thesis analyses the physics ofdensity currents rather than the mathematics.

Since the density current phenomenon is stronger inside theharbour basin than in the sea, as density difference isstronger there, the thesis concentrates more on theprocesses inside the Cochin gut.

The study on siltation has been based on the followingassumptions:

1.Most of the siltation inside the harbour occurs during thewet monsoon when silt concentration at sea is high due toprevailing wind and wave conditions.

2.The material for siltation is brought about from the searather than by rivers from inland.

Information about the required sediment parameters for aprimary assessment of siltation was not available readily.So quantification could only be based on assumed values. Thematter was made more complicated as the silt material iscohesive in nature. The deposition and erosion of cohesivesediments are complex due to the flocculatingcharacteristics. However a rough estimate of thesedimentation has been attempted.

(vi)

The work for the thesis included study of literature ondensity currents and the transport of cohesive sedidments,subjects which were new to the author. The findings of theliterature survey have been included only in so far asrelevant for the purpose of the desk-study,

Chapters 1 and 2 give only background information and definethe problem. The actual thesis work begins with chapter 3.The chapter wise contents are given below.

The Port of Cochin, over the ages, known as the 'Queen ofthe Arabian sea' is located at the mouth of an estuary wherea large tidal prism meets a larger freshet discharge. Theinteraction between the heavier salt water and lighter freshwater paves the way for highly stratified flow patterns. Thethesis begins by giving an orientation of Cochin in theabove context.

The second chapter glances back at the hydraulic datacollected from the area. A primary evaluation of the data ismade and interpretation is attempted.

Chapter 3 brushes up the theoretical part of the densitycurrent. The mechanism of arrested saline wedge, densityinduced return flow and density exchange flow are describedas far as they are relevant to the Cochin situation and anattempt has been made to answer the questions posed by thedata.

A method to quantify density currents is put forward inchapter 4. Rigter's graphs are given and the method isexplained. A new density current equation is introduced.This is followed by enumeration of some interesting physicsof density current and then an all inclusive graph ofdensity currents along with salt outflow, height of salt andoscillation of saline wedge is given.

(vii)

For quantification of the densityschematized tidal discharge curves

currents at Cochin,are required. The

schematization of tide and freshet discharges are attemptedin the fifth chapter.

Chapter 6 is entirely devoted to apply the above theory toCochin situation. The density currents at Cochin have beenquantified, not alone for the present draught but also forthe past and future draughts (two cases). Salient featuresof the density current and the arrested saline wedge atCochin were gone into in depth.

An attempt to quantify siltation have been made inchapter 7. The transport processes of cohesive sediments arementioned and the mechanism of deposition and erosion in thepresence of a saline wedge is explained. Computation ofsiltation with a calibrated and verified model was attemptedand inferences given.

The last chapter draws out the conclusions and recommends tostreamline the flow in the channels by additionalreclamation for increasing erosion and to instal a siltscreen at the gut to minimise transport of silt. It must bementioned, however, that determining the consequences ofthese measures and the structural difficulties involved wasbeyond the scope of this desk study.

CHAPTER 1

BACKGROUND OF THE PROBLEM

1.1. HlSTORY

Hailed as the 'Queen of the Arabian Sea' from ancient times,the Port of Cochin is situated in one of the best harbouredwaters in the world. The port is located on the Vest coastof India geographically coordinated at lattitude 9°58' Northand 76°16' East. The port facilities are located on anartificial island reclaimed by dredged spoil. The island isvery near the opening of the backwaters to the sea. Sincea knowledge of the hydrography of the area is essential forunderstanding the hydrodynamic factors at play, a broadoutline is given below.

The backwaters are a special feature of Kerala, the southernstate of India. They are huge narrow lagoons which receivethe freshwater discharges of many rivers and are open to thesea at a few places. In fact, the major opening of theCochin backwaters into sea was about 30 km nor th of thepresent position. In olden days there was a prosperous portthere Muziris But as time passed, this opening gotsilted up and in the 14th century flood waters of thenorhtern river forced its way through the present opening -the Cochin Gu t.

1.2. SAGA OF COCHIN

Sincere attempts to make Cochin a deepwater port werestarted only in the wake of the 20th century. The bar at theentrance to the harbour was the main obstacle for ships tocome inside. This bar along with an approach channel was cut

1.2

open in 1928. Inside the basin, two channels were also cut.The dredged spoil was utilised to reclaim an island in themiddle - Villingdon Island. Initially port infrastructurewas put up on the Vestern side of the island facing theMattanchery channel and later on, on the other side facingthe Ernakulam channel. Vith all the facilities, Cochin wasdeclared a major port in 1936. Initially Cochin catered tovessels up to 9.14 m draught. As time passed, Ernakulamchannel was deepened to accommondate vessels up to 10.7 mdraught.

The tiny island which was raised from the bed of thebackwaters 60 years back, has now become the nerve centreof the whole state with bustling commercial activity. Theisland is connected to the mainland by two bridges and athird one is being completed Fig. 1.1., 1.2. and 1.3.

1.3. TIDAL STORAGE AREA

Cochin backwaters extend 30 km to the north and 80 km to thesouth of Cochin. Though the length of the lagoon is 110 km,the width ranges from 1 km to a maximum of 10 km only. Inmany places the flow of water is obstructed by a number ofislands apart from the irregular shape of the lagoonitself. The total water area of the lagoon is 300 sq. km.Fig. 1.1.

1.4. TIDES AND TIDAL PRISH

The tides at Cochin are semidiurnal with marked diurnalinequality. The maximum spring tidal range is 1 m while theneap tides do not exeed 0.5 m. The tides enter the basinthrough Cochin gut, 430 m wide and propagate southwards to80 km and northwards to 30 km. Tides lose most of theirenergy as they travel farther from the gut. The tidal range,for a 0.9 m tide at the gut reduces to 0.20 m at thesouthern extremity of the lagoon. Consequently, the

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1.7

contribution of these areas at the southern extremity to thetidal prism is also limited. The tidal prism has beenassessed as 90 x 106 m3 for a 1.0 m tide. Out of this, 75%goes to the south while 25% goes to the north. This willexplain the fact that the southern water areas wereconsidered vital for maintaining adequate velocities in thenavigation channels. In fact, this was the reason for sitingthe port facilities south of Cochin Gut. A typical tidalcurve at Cochin is given in Fig. 1.4.

1.5. CURRENTS

The tidal flow towards south bifurcates and flows aroundYillingdon island giving rise to maximum current velocitiesboth in Ernakulam and Mattanchery channels but theirmagnitude is different at different locations inside theharbour. In the shallow natural channels on the northernside, velocities are lower as can be expected. Yhile, thevelocities follow a defenite pattern during dry season, themonsoon bringing in flood waters from upland areas makes itdifficult to predict the nature, period and occurance of themaximum veloeities. As can be expected, peak velocitiesoccur at the gut. Maximum velocity during the dry season atthe gut had been found to be 1.6 mis while that duringmonsoon was recorded as 2.0 mis during ebbing. Max. floodvelocities were recorded at the bottom due to densitycurrents. Max. velocities observed in the channels are givenbelow.

Ernakulamchannel

Mattancherychannel

Vypeenchannel

Premonsoon 0.96 mis 0.85 mis

Monsoon 1.42 mis 1.12 mis 1.01 mis

Post monsoon 1. 32 mis 1.18 mis 1.01 mis

1.8

Tidal currents outside in the sea are predominantly fromnor th to south during the spring tides, crossing theapproach channel with a velocity of about 0.5 mis, throughout the tidal cycle as indicated by the available data ofhydraulic observations. The current direction however tendsto reverse during the neap tides, the maximum velocitydecreasing to about 0.2 mis. The flow pattern in the seaimmediately to the west of the gut is charaterized byfunnelling action during the flood and jetting action duringthe ebb.

1.6. RAINFALL

The climate is characterized by the dry and wet seasons. Thewet season starts in late Hay and ends in November. Duringthis period, two monsoons pass by one after the other. Themajor monsoon is the south west monsoon which lasts tillSeptember. This period is usually characterized by the heavydownpour and strong westerly winds. The details of rainfallat Cochin during the past 10 years are given in Table 1.1.The average rainfall per year is 3260 mm while a majorportion of this falls down during south west monsoon.

1.7. FRESHET DISCHARGE

The Cochin backwaters receive freshwater through six riversdischarging into it at various points. Four rivers - Pampa,Achankoil, Manimala and Meenachil enter the lake at thesouthern extremity while Muvattupuzha river joins a littledownstream. The discharge into the northern part isdominated by the river Periyar. The average monsoondischarge of the southern rivers is estimated to be 1700m3/s with a peak value of 3400 m3/s. The correspondingnorthern discharges are 450 m3/s and 900 m3/s respectively.

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1.8. SALINITY

1.November to MayDuring November and December, the surface salinities are lowinside. A sudden increase is noted after December. DuringJanuary to March, the surface salinity is found to be veryhigh. Maximum salinity is observed during March and April.By late May, salinity begins to decrease. But bottomsalinity during this season is found to be quite high.Except for the period January to April, the salinitygradient is found to be steep.

2.June to OctoberAs regards the distribution of surface salinity, the patternpresents features that are fairly obvious and which can beexpected during this season especially with such influenceof fresh water. The surface layer consists of fresh water.Tongues of low salinity extending seawards at the surfaceare observed in July to early August and September. Althoughtidal influence tends to bring much volume of sea water intothe estuary and brings about a considerable mixing, thefresh water influx superpasses the saline water and surfacesalinity conditions, continue to be low whereas, the influxfrom the sea remains as a distinct layer in the backwaters.Yith the high salinities at the bottom and almost freshwaterat the surface, there is a very sharp gradient of salinityresulting from the stratificationlayer of dense bot tom water

during the season. The

change of phenomena and inassociated with the

spite ofthe backwaters is not a

turbulent conditionsin

monsoon, there is considerablewith that atresistance to the mixing of the bottom water

surface.

1.9.

Vind at Cochin is highly influenced by the land and seabreezes. The analysis of the data from 1930 to 1960 shows

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1. 13

that the wind direction changes from north east duringmorning hours to west during evening, for the period Octoberto May. During peak of south west monsoon especially fromJuly to September, predominant wind direction remains southwest both during morning and evening hours. Due to strongmonsoon winds, the effect of land winds is not dominantduring south west monsoon. The estimated annual ave rage windrose is shown in Fig. 1.5. and direction of max. wind speedshas been illustrated in Fig. 1.6.

1.10. VAVES

Yave action is strong during the monsoon months of June toSeptember which is primarily responsible for disturbing thebed material in the sea. The predominant direction of wavesfrom June to September is from the west, with south-west andwest-south-west, being contributory factors during themonths June and July. During September and October, thenorth west component acquires prominence. The analysis ofwaves in the open sea outside the harbour is given as rosediagrams in Fig. 1.7. and Fig. 1.8. In the breaker zone,littoral wave currents are set up which result in transportof material between the shoreline and -3.6 m contour.However this transport of material is notconsequence in the siltation of the approachRef .1.18.

of muchchannel.

Inside the harbour,through out the year

generally, calm conditions prevailas it is weIl protected from the

outside waves by the Fort Cochin and Vypeen coasts.

1.11. MUD BANKS

The mud banks along the coast of the Kerala form one of thespecial features of the coast line. Yhile very little isknown about the origin and causes of formation of the mudbanks, its most impressive effect is its wave attenuation

1.14

capacity. Occasionally, these banks migrate and on one suchoccasion in 1937, when the mudbank crossed the approachchannel, very heavy siltation was reported.

1.12. BED HATERIAL

The bed material on the sea bed upto -3.6 m contour consistsof very fine sand. The sea bed in the deeper contoursconsists of silty clay having a median diameter of about 2microns in the dispersed state and 100 microns in theflocculated state. In the inner channels, the bed materialis mainly fine silt and clay.

1.13. GENBRAL HECHANISH OF SILTATION

It has been observed that 70% of the siltation in thechannels occur during the 4 months of the south westmonsoon. During this period, there is high fresh waterdischarge through the channels. The rivers which dischargeinto the lagoon carry lot of sediments with them. It wouldbe only natural to jump to the conclusion that the siltationis caused by river borne sediments. But this is not so. Itis true that the rivers bring in lot of sediments but thesesettle down in the wider part of the lagoon itself, wherethe velocities are low resulting in shoaling up there. Onthe other hand, south west monsoon is also accompanied bystrong winds and consequently wave action in the sea. Theresulting high shear stresses churn up lot of sediments andkeep them in suspension. It is this silt which createsshoaling in the channels. In the approach channel, west ofturning point (T.P.)(Fig.2.1) which is about 2 km west ofthe gut, the siltation is caused by the tidal crosscurrents. In the earlier days, this was aggravated by thedumping of dredged spoil on the north side of the channelas the predominant cross currents is from north to south.

1. 15

In the inner channels and in the portion of the approachchannel east of the T.P., the siltation occurs due to theaction of the saline wedge. This would be dealt with more indetail in the coming chapters but a brief outline is drawnout below. There is a large tidal prism entering Cochinbackwaters and spreading out to north and south. The size ofthe prism varies with the amplitude of the tide. As statedearlier, for a spring tidal range of 1 m, the total tidal

106 m3.prism (northern + southern) is equal to 90 x Thefresh water discharge through the gut is also sizable. Theaverage monsoon discharge of all the rivers is 2150 m3/s.Out of this, Thottapally spillway is discharging 280 m3/sinto the sea at the south. So the net discharge through thegut is 1870 m3/s. This works out to 83.59 x 106m3 per tidalcycle. But this is only an average figure. For an averagetide, the freshwater discharge is more than the tidalvolume. So conditions are favourable for the formation of asaline wedge. A high degree of stratification is alsonoticed. The effect is that the saline water comes into thebasin as a distinct wedge. The saline water also brings in alot of cohesive sediments which are available at the oceanbed. These sediments would be in a flocculated condition insaline water and would set tIe down in the channels. It isseen from observations that not alone the inner channels butthe approach channel east of the T.P. also comes in thedensity current zone.

1.14. EVOLUTION TQ THE PRESENT SHAPE

Figures 1.9. to 1.12.backwaters during the lastthe shape of the originalopen the Cochin gut. It was

present a panorama of the Cochinseventy years. Fig. 1.9. showsVendurently island before cutting

desired that when the tidalcurrents start flowing, the island should have a streamlinedshape. So a nosing was provided in the front to streamlinethe flood tide. This was completed in the twenties. At that

I. 17

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l. 18

time, Mattanchery was the main centre of activity and therewas no siltation problem in that channel.

But in 1953, a tail was added on the southern side whichlater on acted as a guide to divert sizable ebb flow intothe Ernakulam channel (fig. 1.11.). Thus started thesiltation problem of Mattanchery channel. Thereafter wharveswere put up on the Ernakulam side and Ernakulam channel wasimproved. The centre of gravety of the port shifted to thatside. The siltation figures were also low. Out of a totalannual siltation of 2.7 x 106 m3, 2 x 106 m3 was in theapproach channel, 0.5 x 106 m3 was in the Mattancherychannel while only 0.2 x 106 m3 was in the Ernakulamchannel. To improve the flow conditions in the Mattancherychannel, a reclamation on the southern side has been takenup. This is supposed to streamline the flow into thatchannel.

1.15. PRESENT PROBLEM

There was a drastic change in the channel hydraulics ofCochin in 1983. Till then, from the date it was declared amajor port, Cochin was catering to vessels of 9.14 mdraught. But in 1983 as part of the facilities to bring indeeper draughted vessels, the approach and Ernakulamchannels were deepened and widened. Since the continentalshelf at Cochin has only a mild slope, the length of theapproach channel was also increased. In the Ernakulamchannel, the widening and deepening reduced the ebbvelocities considerably resulting in reduction in the bottomshear stresses to erode the deposited particles.Consequently siltation figures shot up considerably. Theannual maintainance dredging in the Ernakulan channel went

6 3 6 3up from 0.2 x 10 m to 2.2 x 10 m. The approach channelalso experienced an increase from 2 x 106 m3to 2.6 x 106 m3.The port is now poised for an expansion which involves asecond phase deepening of channels by another 1.5 m. The

1. 19

study about the hydro-dynamics of the channel and factorsaffecting siltation assumes greater importance in thiscontext.

CHAPTER 2

AVAILABLE DATA AND TBEIR DETAILS

2.1. c.v. & P.R.S. - 1940(Ref.1.10.)

Even though the port of Cochin has a long history dating backto the twenties and though the port facilities are located ina basin whose hydrodynamics is complex and depends on a widerange of factors, there are no records of frequent hydraulicobservations. The earliest observations were seen to havebeen taken in 1940. The velocities were measured in a crosssection across the Cochin gut to assess the tidal prism. Thecumulative tidal prism of two floods was then estimated tobe 4,400 x 106 cft (125 x 106 m3). This value is valid eventoday.

2.2. C.V. & P.R.S. - 1953(Ref.1.5.)

The necessity for further observation was felt in 1953 whenthe state government wanted to go ahead with the Kuttanaddevelopment scheme. Kuttanad which is known as the"Netherlands of Kerala" is the low lying deltaic reg;i.on ofthe southern rivers and is located close to the tidal storagearea of Cochin. To prevent salt water intrusion into thearea, in order to save paddy crops, the government wanted toput up a salt water regulator which would have cut off partof the tidal prism. Since it was feared that this wouldaffect the channel hydraulics of Cochin, a physical model ofthe whole area involved, was constructed at Central Yater andPower Research Station, Pune. The model was proved, based ondata obtained from prototype observations. Tidal levels andvelocities were observed all along the lagoon. Tidal influxvolume at various con trol stations all along the lagoon

2.2

southwards was computed. The freshwater discharges werecomputed based on the rainfall and run off. These values wereused in the model at Pune. Peak monsoon discharges agree withthose observed in 1980.

2.3. C.V. & P.R.S. : 1967 - 1968(Ref.1.18)

These observations were taken during the peak monsoon seasonof July in 1968. Attention was focussed on the flowcharacteristics in the approach channel. These are the onlyvaluable data on approach channel available even now. Thedata collected from five stations beyond the Turning pointare available for interpretation and analysis. Continousvelocity, salinity and silt profile are available. Please seefig. 2.1. for locations of stations.

10•

~CALE2 :t~·4~OOO·

. .IND.EX PLANfOR L·OCATIONS Of STATIONS fOR HyDRAULIC C8SERVAli N

FI G:2.1. OBSERVATIONS - 1968

Eventhough the stations are outside the gut, a sharp salinitygradient is visible. The presence of the surface fresh layeris stronger in stations nearer the gut (3 and 4) while astrong tendency for mixing is seen towards far outside. Thevelocity distribution is found to be typical of a stratifiedflow in stations 3 and 4. The silt content also followed thesame pattern in the same locations. Observations were alsotaken at the gut and at certain locations inside. Thevertical velocity and salinity profile establishes

2.3

stratisfied flow and shows complete flushing out of saltwedge into the sea during monsoon. See Fig.3.5.

2.4. N.l.O.: 1975 - 1976

Extensive hydraulic observations over the entire harbour areafor a whole year round was taken by the National Institute ofOceangraphy during May,1975 April,1976. The data werecollected from a total of 30 stations. The period ofobservations were so adjusted that the details of premonsoon,monsoon and postmonsoon were obtained distinctly. Again thesedata were collected for different tidal conditions spring

\

N

l..1.I-sr

••

,. .. .. -~c •.....J ~

FIG:2.2. OBSERVATIONS - 1975-76

o lOOO 1000 1000~k?-,S;;Z:::5 .. dd;;:==~==dl,...Iru

scAlE r : "000

2.4

and neap. The details of temperature, salinity, density,currents and suspended load across seven cross sections andeight separate locations in various shipping and naturalchannels were collected. These locations of the observationpositions are given in fig. 2.2. The maximum and minimumvalues of temperature, sediment cocentration and salinity inthe Ernakulam, Hattanchery, Bolghatty East, Bolghatty Vestand Vypeen channels have been recorded. The maximum currentvelocities during the pre, post and monsoon periods in theabove channels are also given. Extensive details at the crosssection of the gut are recorded. The maximum observedvelocity at the gut is recorded as 1.74 mis in postmonsoon.This appears to be .on the lower side compared to the 2.0 mis

observed by C.V. & P.R.S.

The saline wedge is seen to intrude into the basin startingfrom the bottom at first and extends to the surface duringthe later part of the tidal cycle. One important featurewhich was consistently observed is the phenomena of flushingout of the salt into the sea during higher tidal and freshetdischarges. The definite change from highly stratifiedconditions during monsoon to vertically homogeneousconditions during premonsoon was very much evident.

The average tidal prism during thecalculated to be 31.5 x 106 m3 which is

dry season has beenvery

lower side compared to the value of aboutindicated by C.V. & P.R.S. for alm tide. However the

much on the90 x 106 m3

tidalrange for this particular volume has not been specified.

Sediment concentrations have been found to be much higherduring post and premonsoon periods than during monsoon. Thiswas explained to be due to the dredging operations in theouter channel during this period.

2.5

2.5. ...;_C_.ll_o_..;...&_P_._R_.-,-S_.__ 19:....;8~O(Ref.1.11)

Extensive data on vertical velocity-salinity and silt profilewere collected from the gut, Ernakulam and Mattancherychannels during the peak - monsoon season of July 1980. Fromtable 1.1., it can be seen that this was a period when therewas excessive rainfall. The freshet discharges were high.continuous observations at 1 hour time and 2 m depthintervals were taken to collect data on salinity and velocityfor a continuous seven days from 20th to 27th of July. Thedata shows sharply stratified conditions at the gut andinside. Step by step advance of the saline wedge can bevirtually followed in the profiles. The data also gives thetime lag of advance/retreat of the saline wedge between gutand specific points in the channels.

L.I.-wr

f.IG:2.3 .. OBSERVA TIONS .-1980·

Analysis of the data in respect of salt inflow, net flow,density current and salt out flow has been presented later in

2.6

chapter 6. The data from these observations have been heavilyrelied on for correlation with the theory.

The observation locations are shown in fig. 2.3.

2.6. e.v. & P.R.S. - 1985(Ref.1.6.)

These were the first observations taken af ter the wideningand deepening of the channel to -11.9 m. The location of theobservations are given in fig. 2.4. There were three pairs oflocations in all, all inside the basin. The purpose was tostudy the extent of mixing. Out of two stations, one was inand the other was on the flank of Mattanchery channel. Twolocations were in the widened portion of the Ernakulam

,.....- .... CKAIIIIUDI."'ilO~ ""111 IlllIDiaIIlét.__._·~.. 0 OUI'hl.i . .'U)'777J CII..... "" DIMhilO., "'8U'IIIIIOi.I.'~ .. 11OU;'hlllli . • "

. •··•..QC.JI~, 11' '1'''0 D'" """'''111'... •..IIC.IIII,.' 8f .. 1111'" DAIA .

F;:G~...2 •.4 e- _

OBSERVATIONS-1985

channel. Two more stations were fixed near the Naval jetty.Velocity and salinity profiles during the monsoon season wasobserved. In the Mattanchery channel, stratified conditions

2.7

were visible while on the flank weIl mixed conditions wereseen.

One important feature was that in the widened portion of theErnakulam channel, there was variation in the advance of thesaline wedge. The wedge advanced quicker at the centre thanat the widened portion.

2.7. NESA - 1988

This is the most recent survey carried out at Cochin. But thetiming of the survey was not proper for hydraulicobservations. The observations were carried out during earlyMay. Normally, this would have been a premonsoon phase. Butuntimely rains just before the observations coloured theresults. The freshwater discharge affected velocity andsalini tyaffected

distributions. The turbidity values were alsoby the dredging operations. But valuable

bathymetric details were recorded. However, this survey wasof not much use for this desk study.

2.8. DATA ON DREDGING

The data on maintainance dredging quantities are given intable 2.1. and 2.2. below.

Year Quantity tobe removed106 m3

Quantityremoved106 m3

1984-19851985-19861986-19871987-1988

4.6082.3222.7953.654

3.1871.8512.8042.519

Table 2.1. Approach channel dredging.

2.8

Year Quantity Quantityto be removed removed106 m3 106 m3

1984-1985 2.128 1.6591985-1986 2.883 2.9801986-1987 2.064 1.9691987-1988 2.643 2.693

Table 2.2. Ernakulam channel dredging.

CHAPTER 3

HYDRODYNAKICS OF DENSITY CURRENT

3.1. RELEVANCE OF TBBORY

It is proposed to bring out some basics of the theory ofdensity currents in this chapter. Before going into that, itwas feIt pertinant that the relevance of that theory to theCochin conditions should be established.

3.1.1. The flow through the Cochin gut held the facination of all,ever af ter it was dredged deep for entry of ocean vessels.The tidal storage area accomodates a large prism and induceshigh velocities at its only opening to the sea, the gut.This is apart from the fact that there are six rivers, whosecatchment areas receive more than 3000 mm ofrainfall, discharge into the basin.

annual

Fig. 3.1. gives the schematised sinuzoidal discharge curveinduced by the tide, the steady freshet discharge and theresultant flow. Let us call this the net flow.

3.1.2. The above flow is a simple case where the complication dueto the difference in densities of fresh and river water isnot considered.360/00. The massare 997 kg/m3 anddensities is

The sea water at Cochin has a salinity ofdensities of fresh and sea water at 28°C1023 kg/m3 respectively. The difference in

3kg/m This net difference provides aa driving force for the

water front at the

26pressure difference which acts assalt water to advance into the freshbottom. This phenomenon is schematized in fig. 3.2. At slacktide, when the net flow is zero as at points d and f in fig.3.1., the salt goes inside with a velocity c and the

IC1l;>-."0• .-1C1l ........:;>0-

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3.2

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.....cri

3.3

freshwater comes out on top at the same velocity. This isthe maximum discharge of density current. The net flow inthis case is zero.

DuringdensityThe net

ebbing, the ebb velocity is superimposed over thecurrent and the latter is reduced to that extent.

ebb of a -ta. is the net flow, say at point c.ou lnHere, the lower value of ain is the density current whichwill be less than the a. in the previous case.ln

The situation reverses when there is a net flood flow, sayat point e.Then a t is the density current.ou

3.1.3. The above schematised situations are identical to the oneoccuring at the gut, according tQ field data. Please seefig. 3.3. The vertical velocity and salinity profiles showthat there is an inflow at the bot tom and an outflow at thetop as seen in the schematization itself. Ve get the netflow by subtracting one from the other giving either floodor ebb as resultant. This is ploted in fig. 3.4. over aperiod of about 30 hours. The lowest value, as mentionedearlier is the density current and it is plotted at thebottom. The height of salt at gut is also given above that.One interesting feature noticed was that there is an outflowof salt itself over the inflowing saline water. This is dueto the shear at the interface. Please see velocity verticalsfrom 0500 hrs to 0900 hrs and from 1200 hrs to 1800 hrs infig. 3.3. , 6.28 and 6.29. This phenomenon of continousoutflow of salt water is also accompanied by a morepronounced salt outflow over the entire dep th during highebb discharges of both larger and smaller tides. Please seevelocity verticals at 1100 hrs, 2100 hrs and 2200 hrs infig.6.28 and 6.29.

3.14. In fig.3.3., at 0.000 hrs. there is no salt water seen. Thismeans that the salt wedge has been completely driven out.Fig. 3.5. gives the longitudinal profile of the saline wedge

S131\31 WOU_ - ... -I ... 00. . .... _CO

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3.5

.0-----1.-.-.-r-----.._-22.7.1980

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.8

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,TaTAL OUTflOW

12

14

16

18

tDiHT Qf SAlT wflKif AT lilTI...J

:112 t--------_;:.--....c....--- ----_.--.-..__. _

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fIG:3.4.DfNSITY CURRfNT - 1980

3.6

once when it is completely inside and once when it is drivenout during low tide. It has been established that the wedgegoes out once every day, thereby confirming the saltbalance. The salt water which is coming in is driven outcompletely and there is no accumulation of salt over a longperiode Here we have two natural questions:

(i) Vhy is flushing once a day ?

(ii) Vhat is the quantity of salt water coming inside ?

These questions would be answered in subsequent sections. Toanswer question (i), the theory of an arrested salt wedge isconsidered putting emphasis on the flow rate needed to makethe length of arrested salt wedge, zero. To answer question(ii), gut is schematized .

3.1.5. The gut is schematized as a channel of finite widthseparating a fresh reservoir (backwater) and salt reservoir(sea). This schematization justified the application of thetheory of density induced return currents over a sillseparating a fresh reservoir from a salt reservoir. For thetheory to hold good, the fresh reservoir has to stay fresh.That means there should not be any salt accumulation. Ifthere is salt accumulation, the density current would bereduced considerably. The assumption of no salt accumulationis justified just af ter the salt water starts to flow in asa density current.

3.2. CRITICAL FLOll CONCEPT (OPEN CHANNEL FLOll)

Ve can treat the density current similar to a homogeneousflow and apply the densimetric concept to it. This mayperhaps be the simplest way to study stratified flow.

For open channel flow, the velocity of propagation of a longwave at the water surface, is given by

3.7

c = u ± Igh ...... 3.1

where, c velocity of propogation of long surface waveu velocity of water over the surface of which

the long wave is propagatingg acceleration due to gravityh water depth

Defining the Froude number as,

2uFr = gn •..... 3.2

where, Fr Froude number of open channel flow, inaccordance with Eq. 3.1., depending on the magnitude of theFroude number, the following distinction can be made:

For Fr <Then, Iu Ipropagatingdirection.

1, the flow is referred to as being subcritical.< I/gh I, meaning that there is one wavein the direction of u and one in the opposite

For Fr > 1, the flow is called supercritical. Then,lul<l/ghland both waves propagate in the direction of u.

For Fr = 1, ie, for critical flow, lul = I/gh I. This meansthat one wave is propagating in the direction of u and thatthe other wave is stationary (c=o)

3.3. CRITICAL FLOV CONCEPT (NO LAYER FLOV)

For two layer flow, the velocity of propagation of the longinternal wave, at the interface, is given by (Schijf andSchonfeld,1953)

3.8

2 1/2u1a1+u2a2 (). a1a2 (u1-u2) a1a2c. ± [~--- 2 ] ...... 3.31 a1+a2 p a1+a2 (a1+a2)

where, c. :veloci ty of propagation of long internal1

wavea1,a2 :Thickness of the upper layer above and

lower layer below the interface betweenwhich the long internal wave is propogating

u1, u2:Velocity of upper and lower layers{).p :Difference in densities between both

layers

Vhen the absolute value of the first term at the right handside of Eq. 3.3. is smaller than that of the second term,one wave velocity is positive and the other negative. Thismeans that there are two waves propagating in oppositedirections. Both wave velocities have the same sign as thefirst term at the right hand side, when its absolute valueis larger than the absolute value of the second term at theright hand side. Then both waves propagate in the directionindicated by the sign of the first term at the right handside.

Therefore, defining internal Froude numbers as,

..... 3.4

where, Fr1, Fr2: densimetric Froude number of upper andlower layers,

in accordance with the magnitude of (Fr1+ Fr2), thefollowing distinction can be made:

For (Fr1+ Fr2) < 1, the two layer flow is subcritical. Thenthe absolute value of the first term on the right hand side

3.9

of Eq. 3.3. is smaller than that of the second term. Hencethere are two waves propagating over the interface inopposite directions.

For (Fr1+ Fr2) > 1, the two layer flow is supercritical.Then the absolute value of the first term at the right handside of Eq. 3.3. is larger than that of the second term andboth interfacial waves propagate in the direction indicatedby the sign of the first term.

For (Fr1+ Fr2) = 1, the absolute value of the first term atthe right hand side of Eq. 3.3. is equal to that of thesecond term. Then one wave is propagating over the interfacein the direction indicated by the sign of the first term andthe other wave is stationary (c.= 0).

1

3.4. APPLICATION OF 'CRITICAL FLOV CONCEPT' Tb NO LATER FLOVS:

3.4.1. ARRESTED SALINE WEDGE

Yhen a river discharges fresh water into a saline sea forwhich the tidal range is zero, an arrested salt wedgeresults. As the name indicates, the resultant velocity ofsalt water inside a saline wedge is zero

FIG.3.6. ARRESTED SALINE WEDGE

Assuming the freshwater flow to be in the positivedirection, for an arrested salt wedge,

3.10

· ..... 3.5

and

· ..... 3.6

where, qfr fresh water flow rate per unit width.

Substituting Eq 3.5. and 3.6. into Eqs 3.3. and 3.4. givesthat for an arrested salt wedge,

2qfr--3~lp

· •.... 3.7

and that the sign of the first term at the right hand sideof Eq. 3.3. is positive. This means that when thisstratified flow is supercritical, both internal wavespropagate in the direction of the river flow, ie, out of theestuary. Only when the flow is subcritical, there is oneinterfacial wave which can propagate in the directionopposite to that of the river flow and hence can penetrateinto the river.

On the basis of the above information, the followingexperiment can be performed. Assume that a river issues intoa fresh water sea, ie, a sea filled with fresh water inwhich the range of tide is zero. Assume further that, fromone moment to the next , all water in the sea is made salt.Then salt water penetrates into the river as long as theflow at the mouth of the river is subcritical andconsequently an internal wave is capable to intrude into theriver.

The larger the quantity of salt which has intruded into theriver, the smaller is the thickness of the upper layer, alat the mouth of the river. Once at a given moment, the value

3. 11

of al may become sufficiently small to make (Fr1+ Fr2) equalto one at the mouth of the river. (See eq. 3.7.). Reducingal further, requires more salt to penetrate into the river.This is impossible, however as reducing al further makes theflow super critical, which for the arrested salt wedge meansthat internal waves can penetrate only out of the river.

This means that salt stops to penetrate into the river, whenat its mouth,

2qfr--3 = 1~p

· .... 3.8

Knowing the value of al at the mouth of the estuary, thelength of the arrested salt wedge can be computed using theback water curve equation for stratified flow, which relatesthe slope of the interface with the velocity and thicknessof upper and lower layers. A discussion of two layerbackwater curves is given by Rigter (1970) ref. 1.15. Schijfand Schonfeld (1953) ref. 1.16. found the length of thearrested salt wedge to be given by

L.= a~ [_!__2 - 2 + 3F02/3 - 5~F0

4/31 '+Ki 5Fo

· .... 3.9

in which, FO2

qfr--3~

p

· .... 3.10

Yhere, L.1

Length of the arrested salt wedge(see fig.3.6.)

FO densimetric Froude number based on river flowa depth of riverk. interfacial shear stress coefficient1

The river flow may be so large that there are no internalwaves which can penetrate into the river even when a2 0

3.12

and therefore a. In accordance with Eq 3.7., thiscondition arises when

·.....3.11

Hence salt cannot penetrate into the river and so noarrested salt wedge can be formed when the fresh water flowrate is sufficiently large to satisfy eq. 3.11. whichrepresents the condition of flushing. The significanee ofeq. 3.11. from Cochin is discussed in 6.5.5. Comparisionwith data is also made.

In both layers of an arrested salt wedge, the pressure ishydrostatic. For the upper layer,

· .... 3.12.

and for the lower layer,

·....3.13.

where, p hydrostatic pressurex,y: horizontal and vertical co-ordinates

(fig.3.6.)P1 . density of upper layer.

For the upper layer, the horizontal pressure gradient isgiven by:

· .... 3.14

and for the lower, layer by

·....3.15

3. 13

In eqs. 3.14. and 3.15., a (al + a2)/ax represents the slopeof the water surface and aa2/3x, the slope of the interface.

In an arrested salt wedge, the fresh water flow isacceleratedthe sea.

as al decreases with decreasing distanceThe pres sure gradient required foris due to the surface slope (Eq. 3.14.).

fromthisTheacceleration

lower layer velocity is zero. This means that neglecting theinterfacial shear stress, the pressure gradient acting onthe lower layer is zero. In accordance with Eq. 3.15., thisimplies that for the lower layer, the pressure gradient dueto the surface slope is balanced by the pressure gradientdue to the slope of the interface. As 6p« p, thisrequires the slope of the interface to be much larger thanthat of the water surface.

3.5. DENSITY INDUCED RETURN CURRENTS

For an arrested salt wedge, the slope of the interface is solarge that the lower layer velocity is zero. If the slope ofthe interface could be made larger, this would cause a flowof the lower layer in the direction opposite to that of theupper layer flow. Consider a long weir, connecting afreshwater reservoir with another with salt water, over

Fresh wat:r freserVOlr

FIG.3.7. ARRESTED SALT WEDGE ON LONG WIER

which there is a net flow qfr from the fresh water reservoirto the salt water reservoir. The length of the weir may be

3.14

so large that above the weir, an arrested salt wedge can beformed. fig.3.7. Then the flow above the weir corresponds tothe flow described in the preceding section.

A different situation arises when the length of the weir issmaller than that of the arrested salt wedge. Then the slope

CJ'rI r·t T UI-- .......

LOf

»:

FIG.3.8. DENSITY INDUCED RETURN CURRENTOVER SHORT WEIR

of the interface may become so large that there is a lowerlayer flow of salt water from the salt water reservoir tothe freshwater reservoir, the direction of which is oppositeto that of net flow over the weir. fig. 3.8.

The larger the slope of the interface, the larger is theabove density induced return current, q. In thisrconnection, an experiment can be performed by bringing theinterface separating the fresh water above from the saltwater underneath, continuously at a lower position in thefresh water reservoir and at a higher position in the saltwater reservoir, keeping the netflow over the weir constant.This will lead to an increase of q as long as the resultingrtwo layer flow remains subcriticalover the entire length ofthe weir.

This stops to be true when the flow at x = 0 and x = Lbecomes critical. The flow being critical at both these

3.15

locations, at x = 0 the sign of the first term at the righthand side of Eq. 3.3. is that of u2,(1 u2a1 1 > 1 u1a2 I),while at x = L it is that of u1, (I u1a2 1 > 1 u2a1 I). Thismeans that internal waves cannot any longer penetrate intothe area above the wier as both at x = 0 and x = L, one ofthe internal waves is stationary while the other propagatesin a direction away from the weir. Consequently, loweringthe interface further in the freshwater reservoir orbringing it up at a still higher level in the salt reservoirhas no effect on the flow over the weir.

The above considerations imply that for a given value ofqf ' the return flow q has its maximum value when both atr r

x = 0 and at x = L, the flow is critical. These criticalflow conditions cannot be selected arbitrarily as the valueof a2 at x = 0 is linked to that at x = L by the back watercurve equation for stratified flow. This link can bemathematically expressed as

L= a2,O + Io

•..••3.16

Yhere a2,O,a2,L: value of a2 at x = 0 and x = L

The problem of how to select the critical conditions atx = 0 and x = L so that Eq. 3.16. is satisfied has beensolved by Rigter (1970)(ref. 1.15) who gives design graphsfor the maximum value of q as a function of qf ' the lengthr rof the weir and the interfacial shear coefficient whichrelates the shear at the interface with the difference invelocities between the layers over it. Rigter furtherderives formally that qr has its maximum value when the flowis critical at both x = 0 and x = L, confirming the abovephysical reasoning. For this purpose, Eq. 3.16. is writtenas

3.16

L · .... 3.17

The integral of Eq. 3.17 is a function of its limits a2,Oanda2,L. Further it is a function of qr' as for constant qfr'the slope of the interface determines q. Therefore, therintegral may be represented as

· .... 3.18

where A: functional relationship of a2,0,a2,L,and qr.As A= Land L is constant, differentiating A with respect toa2,oand a2,Lgives

aA aA aqr 0--+ aYe aa2~Ö=aa2,0

aA aA aqr 0--+ aq . aa2,L =aa2,L r

·.••.3.19

•.••.3.20

The condition at x = 0 and x = L is selected so that qr hasits maximum value ie,

o ....•3.21

or in accordance with Eqs. 3.19 - 3.21, when

aA 0aa2,0 =

aAaa2,L = 0 ·...•3.22

Substituting Eq. 3.18 into Eq. 3.22, qr is found to have itsmaximum value when

o and at x L · .... 3.23

3.18

As 3a2/3x is propotional to (Fr1+ Fr2 _1)-1 (Rigter1970),this means that maximum values of qr are found when the flowis critical at both x = 0 and x = L, (Fr1 + Fr2 = 1)

3.6. DENSrTY INDUCED EXCHANGE FLOli

For qfr = 0 (no net flow over the weir), there occurs adensity induced exchange flow over the weir which isconsidered in section 3.5. For such exchange flows,

..... 3.24.

where, q : density induced exchange flow rateex

The magitudegiven by Rigteroccurs when Lfinds

of qex(1970)

is equal to the maximum value of qr'for qf = o. The maximum value of qr ex

= o. Then, from Rigter's design graphs one,

1 (~ )1/2qex = 7+ a p ga ...•.3.25

Experiments summarized by Yih (1965) give a value of about0.22 instead of the factor 1/4 of Eq. 3.25.

The curves given by Rigter have wide application on variousconditions of density currents. In the following chapter, amethod to quantify density currents through the gut, fromthe net flow, using Rigters graphs is explained in detail.

CHAPTER 4

A KETHOD TO QUANTIFY DENSITY CURRENT

4.1. RIGTER' S GRAPHS

Rigter (1970) gives a graph 1inking the interna1 Froudenumbers of the two 1ayers and the interfacia1 shear. Froudenumbers are defined as

..•..4.1

where,e:F1 and F2:for upper and 10wer 1ayers respective1y. See

fig. 4.1. In the graph, F1 is p10tted against F2 and 1inesof equa1 k.L have been drawn. k. is the interfacia1 shear

1 1

coefficient and L is the ratio of the 1ength of the si11 tothe water depth. See fig. 4.2 .

... -U, (- q,)

•

+x

Fig .4.J. Oefinition sketch

4.• 2

::0,.ij"

Ö::::J

. C"',.~,.,.::::J

. Fig.4.4Relation betweenkl, ,F1,Fi

E.9O~igin~l Data - solidApP~oxiMation ~.dotted

.189••••• '.' ••••••••• '•••••••••••••••••• I ••••••••••••••••.•••• '.' ••• ,. '.' ••••• '.

. : : : : '. . : : : : :: . : : : : . . :....": : : ~ ; , , ; :....... ". : : : : : . : :.. '~ ~ t ~ : : ; ! ~ .

. . : . : : . : : .. -1- -1- r·: '1' ~ { j + .

.~ ,:, i i i ~ : : .~ : : : : : :': : r j "1' i' 0 '1' 0 • t 0 0 0 ' ••

o . • 0 • io0 - ••• t 0 0 •• ~ + 0, 0 • ~ • 0 •• 0 ol, 0 ••• 0 ;' 0 0 0 ooi 0 0 o .. 0 'j' 'l' 0 •• j• ; 0 ••• : • ~. 0 •• 0 : ~ •••••• o~ ••••• 0 .~ •• '.' 0 •• i

, r ::::q::i':}f:;::::I

. 149

.969

.929

.969

E 9..... _._-- ..

'.- .FIG:4.1 I~· - F.J vs .~

4.3

4.2. PROCEDURE

The starting point is the tidal discharge curvefresh water discharge through the gut. It maymind that the schematised discharge curve

includingbe kept in(Fig.3.1. )

indicates the 'net flow' through the cross section withoutseparating the density currént. This in effect is q1 - q2'From Eq. 4.1. we have,

.... 4.2.

or

..... 4.3

a

the net flow per unit width directlyread out from the discharge curvesdepth of flow6p/ p

€ depends on the salinity of the sea. So all the parameterson the left hand side of Eq. 4.3. are known to calculateF2- F1• From fig. 4.2., values of F2 and F1 can be read outalong any curve of interfacial shear in such a way that thedifference between the values of F1 and F2 on each axis, isequal to the (F2 - F1) as calculated by Eq.4.3. Once F1 andF2 are known, Eq. 4.1. can be used to compute q2 and q1' Thelowest of the two values is the density current. See fig.3.2. Vhat is obtained is the value of density current atthat particular point of time. Several points along thedischarge curve are to be put through the above procedurefor getting the density current graph for the entire tidalcycle. The computed graphs are given in figs. 6.1. to 6.20.

4.3. PRACTICAL PROBLEMS

4.3.1. As mentioned earlier, the first thing which is required isthe schematized tidal discharge curve with fresh water

4.4

discharge superimposed on it. Some sort of tidalcomputations will have to be carried out to obtain this. Thetidal schematization attempted for this particular casewill be described in detail in the next chapter.

4.3.2. Another important point is to calibrate the theoreticalvalues with the actual observations. Vertical distributionof velocities and salini ties at the estuary mouth arerequired to draw the graph of density discharge. Asexplained earlier and as evident from fig. 3.2. , the lowestof the two, either ebb or flood may be plotted as densitycurrent (q2)' At the same instance from the net flow,(ql - Q2)' (F2 - F1) can be read out from the graph forvarious values of kiL. F2a/€ga or F1a/€ga whichevervalue represents density current as given by Rigters curvesmay be computed. For each value of k.L, there is a set of

1

values of Fl and F2. The kiL value which is more fitting tothe density current plotted from the prototype data of thevélocity distribution graph may be adopted for furthercalculation. For example a value of k.L = 0.05 has been

1

found fitting in the case of Cochin. The fitness may be seenfor a number of values, at one hour intervals. The graph maybe drawn for the entire tidal cycle.

k. is the interfacial friction coefficient which has a value1

of 0.001.

L is dimensionless length I/a where I is the length ofthe sill

4.3.3. In the case of a place like Cochin where the tides show amarked daily inequality, the schematization has to be donefor two consecutive tides and the graphs should be drawn forone larger and one smaller tide. This means that for aparticular combination of tides and for a certain freshwater discharge the calculations will have to be repeated atleast 25 times if values are to be obtained every hour.

4.5

Doing these calculations further, for other combinations oftides and freshet discharges could be labourious.

4.4. DENSITY CURRENT EQUATION

To overcome the above difficulty and to avoid the longprocedure, a short cut is also put forward. Since, in thecase of Cochin, kiL 0.05 curve was found fitting,attention was focussed on the same. For all the values of(F2 - F1), the lowest values of Flor F2 were taken from thegraph and a curve was fitted to the points.Please see fig.4.3. and fig. 4.4.

• ..... 4.4.

Yhere F2 is the lowest of Fl and F2·

Since, q2 = F2 a/€ga, •••••• 4.5.Substituting for (F2 - F1) from Eq. 4.3. and for F2 from Eq.4.4. into Eq. 4.5., we have,

2q2 = 0.174 a/e,ga - 0.483 q + 0.274 q

aJe,ga ••••• 4.6

h

nq

density currenth+ ncosrot= dep th of flowmean water dep th andtidal amplitudetidal + freshwater discharge per unit widthor net flow (q2 - q1)

where,q2a

Eq.4.6. can be termed as the densitybecause, now the procedure is simply

current equationto substitute the

variables directly in the equation and get the density flow.On examination, we can find that the only independentvariabIe is the time 't' during the tidal period and therest are all dependent variables making it a far simpIerprocedure.

4.6

4.5. LIKITATIONS OF TUE KETBOD

As explained in section 3.1.5., for applying Rigter'stheory, the gut is schematised as a channel of finite widthseperating a fresh water reservoir (backwaters) from a saltwater reservoir (sea), neglecting the effect of saltaccumulation. This assumption is justified when the saltwater starts to flow in as a density current, just af ter theperiod of flushing. However, as the field measurements whichare described in chapter 6 show eventually, the flowthrough the gut is influenced by the accumulation of saltwater inside the gut as an extended saline wedge. Then thedensity current equation overestimates the density currents.Please see Fig.6.22 to 6.26.

4.6. PHYSICAL CHARACTERISTICS OF DENSITY CURRENT

Vhile evaluating the density currents for the Cochin estuaryand while analysing field data, certain general features ofdensity currents through tidal inlets like the gut came tolight. It would be easier to explain these features with thehelp of a theoretical density current graph made for aparticular case. Such a graph is produced in fig.4.4. Thegraph contains the schematised tidal discharge with a freshwater flow of 6 m 2/s. The dep th of flow is 12 m. The rangeof the first tide is 0.75 m while that of the second tide is0.30 m. Neglecting friction, a 90° phase lag between thehigh water and the maximum discharge has been assumed, anassumption elaborated upon in section 5.1.2.

4.6.1. Following the procedure mentioned earlier, the densitycurrent graph has been plotted. As indicated in Eq. 4.6. thedriving force for density current is I€ga. € and gareconstant for a particular estuary. So the driving forcesolely depends on la. Since the dep th of flow of densitycurrent is proportional to the total depth of flow of theestuary, the total density discharge is proportional to a/a

4.7

u.7,-----------------~-----------------r-----------------r----------------~bt Ti,j.:: 0.75111, 2nd Tide: u.30&\,

fl'~::tl.:t, dÜc.t~l'\J~: G. u bl::./:.

U.ó+------------------+------------------+-----~-----------+------------------~O.5+------------------+------------------+------------------+------------------~0.4 'r-,0.3 -7\~0.2 , \O. 'I

I

0 I

.[ -0. 'Iöi>-.!! -o.~ii...ï=

-0.3 I

-0.4

I\ ,I

I

/

Ol.........iioi

F

r<:5+-~------~---~~--~----~--~-------------+------------~: ~( G ~~,~q_!~ ..~ .., .'.I "" •••• , v-u- .\ -t t :«, ' ~,.~, ~,.. ,.I , .! - ,•••..: r: '. t· ':.' .. t

i) ,,(0 •• ,. \ ' .. : ' 10.,.,': .... ," '. ," .-J)..-Er

-;

~ /I.c: ~. 1_.Ol ,,'(1 l'";~ 0 , ............'Oie:..Cl

I

cl

-5+----------------~------------~---~~-----------------4-----------------~o

~ltllOl

FIG:4,4. DENSITY CURRENT GRAPH

4.8

Eq. 4.6. The opposing factor is the freshet and tidal (net)discharge.

4.6.2. MAXIMUM VALUE OF OENSITY CURRENT

It can be easily deducted from the above reasoning that themaximum value of density current occurs when the opposingfactor is minimum ie, when the net discharge is zero. In atidal cycle, this may happen two times ie, at instances whenthe tidal flood becomes equal to the freshet flow. As can beseen from Fig.4.4., the density current is maximum at E andH when the discharge q is zero. But both maximum values arenot equal. Now, it is time to take a look at the top graphof tidal levels which determines the value of 'a'. The netdischarge q being zero, the density current at H will belarger then that at E because the tidal elevation at H ishigher. So H gives the maximum value of density current.There is also a third maximum at J, during the secondsmaller tide when the discharge is the minimum but not equalto zero. But this will be lower than the other two values.

4.6.3. MINIMUM VALUE OF OENSITY CURRENT

As we have seen, when the net discharge q is zero the valueof density current is maximum, and when q goes beyondcertain value, the density current becomes zero as q is theopposing force. Here Band 0 are the points of zero densitycurrent. This happens during the lowering phase of the tideas against the previous case; so at point 0, the densitycurrent is zero even at a lower discharge than at B becauseof a lower water depth and hence a lower driving force.Please note the upward slope of line KL. From Eq. 4.6.,q2 = 0 for values of q/a/€ga equal to or above 0.5.(Negative values of q2 are not considered). For a particulardischarge q and water depth 'a' this condition will remainthe same. If the estuary is deepened 'a' increases and the

4.9

points Band D will come closer reducing the period of zerodensity current.

4.6.4. CONDITION FOR COMPLETE FLUSHING

Complete flushing is said to have occured when the entireTheoretically

by Eq. 3•10 • issaline wedge is driven outthis happens when the

of the estuary.Froude number given

much before that in practice. Theone. But flushing occursmechanism is that after the density current has gone to zerothere would be an outflow of salt. From B to C, 'q'increases and ,a' decreases so that conditions arefavourable for the salt to be driven out. It has been foundfrom data that at Cochin, flushing takes place whenFO= O.7.see 6.5.2. It is not necessary that either flushingor zero density current should occur during a tidal cycle.Both or neither can occur depending on the balance betweennet flow q and water depth 'a'.

4.6.5. VARIATION OF DENSITY CURRENT YITH FRESHET DISCHARGE ANDTIDAL FILLING

In the above schematisation, the physics of density currentsbecomes more interesting than ever. The above property ofdensity current is closely interlinked with its variationwith tidal filling current. Please see the portion of thecurve EGH. The tidal filling starts at E and goes on to amaximum value at F. Correspondingly density current has itsminimum value at G. It may be noted that the same salt watercomes in as density current and tidal filling current in theform of a wedge. But as is evident from fig.3.2. , thetendency of tidal filling current is to reduce densitycurrent. Please see fig. 6,1 to 6.5. As the tidal currentincreases to a maximum, the density current decreases to aminimum.

4.10

The direct tendency of increase in fresh water discharge, isto reduce density current. But another property is to reducethe tidal filling. If we start from a higher freshetdischarge and go on reducing it, the density current wouldgo on increasing; but the tidal filling would increasefaster than that and so its effect would be to reduce thedensity current. So there would be an optimum freshwaterdischarge when the balance between the two gives a maximumdensity current. On both sides of this point, the densitycurrent would be on the decrease. But at the same time, thetotal quantity of salt water which is density current andtidal filling, would be always on the increase. Of course,

...Ol...";........'0 ~VI ~... ,0 ~Ol

3ä;;..

Fresh water di~chargeFIG:4.S. VARIATION Of fRESH WITH SALT

when freshet discharge becomes zero, density current alsosuddenly drops to zero. Please see fig. 4.5. This is on theassumption that stratification is still present.

4. 11

4.6.6. QUANTUM OF SALINE YATER

Sometimes it is required to find out the total quantity ofsaline water entering inside.This is either for calculatingthe amount of salt coming inside or to quantify the siltbrought in by the sea water. The inflow is given by the areaof the density current graph for a tidal cycle. This can beobtained by integrating the density current equation 4.6.,within the time limits of the tidal cycle. Tidal fillingshould be added to this to get the total inflow of saltwater.

4.6.7. LACUNAE NOTICED

During the preparation of density current graphs for Cochinwith varying tidal and freshet parameters, it was noticedthat there was a large quantity of salt water coming insidedue to density current. But there was no negative phase inthe curve whereby the salt which had come inside would goout. In the absence of a regular outflow tothe inflow, there would be accumulationbasin. This was not true since the dataflushing out of the salt wedge.

compensate forof salt in the

showed complete

Another interesting fact which came to the attention wasabout the quantity of salt water which had gone inside thebasin according to 1980 data in the form of a wedge. Thisquantity was found to be more than 4 times the volume ofthe saline wedge itself, even for a peak fresh waterdischarge. For the average freshet discharge conditions,this figure was 8. This disproportionality had to beexplained.

4.6.8. EXPLANATION TO LACUNAE

Part of the explanation to both this lacunae is given by theexperimental results put forward by Keulegan (1966).

4.12

, .. e c: e e,E 0 u. 0 0,..:i 2 ... 0'.) .. e ... .:;J u- .. >- ';; -~ iii .. ...

::J iii - U .;: .. iii.. :5 c c: u.. c)I 0 - ~ ..t Q... ): ëi 'ij ... ,.. 0 0 Q QQ. Q IJ) Q >

Ah. (Oc.an En',anc.)

fig. 4.G .. fully stratiffed estuary.

2.5

o

I I

Waler Surface ..r--o-+0J>

0

0JI.

0

Fresh Water 0

--'l

00""

. . o· _ .0. lf-. oorPInterface_:) o_o

00o ...

00

,.QcpeP Solt ...,edge

( u/ü

2.0

1.5

1.0

y/hz·0.5

05

1003 o 0.3 0.6. 09 1,2

ffg. 4.'L· Veloc1ty profl1e in str'atff1ed_ .f1_0w.. af ter Keulegan .(1966), .

Keulegan established that there is a large scale circulationwithin the saline wedge giving rise to a salt underflow.Please see fig. 4.6. and 4.7. The difference in pressure atthe same elevation between the downstream and upstream side

4.13

of the saline wedge and the interfacial shear areresponsible for the salt underflow below the zero velocityline and the salt overflow above this line. The analysis ofvertical velocity distribution at the Cochin gut for sevencontinuous days also established the above fact. In fig.,4.6., when Q > Q ie, the salt inflow is greater than salts soutflow, the saline wedge is advancing into the estuary.

the outflow, the wedge isYhen the inflow is equal to,and when Q < Q,s sarrested that means the wedge is

retreating.

This fact also gives rise to the concept that velocitystratification is different from the density stratification,though the former is a consequence of the latter. Due to thefactors mentioned above and also as established by prototypeobservations, velocity stratification occurs at a lowerlevel than the density stratification. Accurate data aboutthe salt outflow at the interface is still not available asthe prototype data give velocities only at 2 m depthintervals. So it is suggested that in future, while takingvelocity and salinity observations, as soon as densitystratification is noticed at a particular water depth (inthe form of higher salinity), velocity observations shouldbe carried out at closer intervals till the velocitystratification is pinpointed. Then we will get a clearpicture of salt out flow which is highly essential forestablishing salt balance in an estuary.The explanation to the above phenomena would be completeonly after further studies.

4.7. TBR COMPLETE QUALITATlVE DENSITY CURRENT GRAPH.

Putting together the data obtained from theory and analysisof prototype observations, a conceptual graph on thehydrodynamics of density currents is presented in fig. 4.8.The graph includes curves of net flow, density current, saltoutflow, height of saline wedge and schematic oscillations

4.14

T _..,.

,.,

C-,J

i>-!:::. IVlO ,Z......LoJu-CI~

0

~.....0............ a.<U- iVIS0 I

~..... I....J-e Hf1GHT 0VIU- .I0::t: (......0..

l -, I:LoJ (Q , - l.

I

w ~ ÀfICJ0 lw CJ:. z..... :ï:_, VI4( JVI

_,u... OS 110H OF WEDGE fRO Tu, !

(;)

j!:

ffi_,

FIG:4.8. COMPLETE Q.UALJTATIVE DENSITY CURRfNT GRAPH

4. 15

of the wedge front during the entire twin tidal period. Thegraph has been drawn to indicate the phenomena taking placeduring a twin tidal cycle with a marked daily inequalityand a high fresh water discharge.

At F2, the salt inflow and consequently the outflow at theinterface starts. The inflow increases to Band then - addedby the tidal filling - to C. The length of the saline wedgewould be on the increase at this stage, the outflow beingvery small. At point C, the salt inflow starts reducing butis still much more than the outflow. In other words, 0' > 0s s(para 4.5.8.). Af ter point D, the ebb discharge of the 2ndtide increases and start driving out the density current. Sosalt outflow increases and at Al' 0' = 0 . This is the firsts spoint of arresting. From Al to A2, there is a larger saltoutflow (0' < 0) and so the salt wedge starts retreatings suntill at A2, (0' = 0 ) when it is arrested for a seconds stime at a shorter length. From A2 to A3 the salt inflowoutbalances the outflow (0' > 0 ) and the wedge goes in.s sThere is a brief moment of arresting at A3 for the thirdtime and then the larger tidal + freshet (=net) dischargeincreases the salt outflow sharply. Af ter the point of zerodensity current 0, there would be salt outflow for theentire water depth until at Fl when complete flushingoccurs.

It can be seen from the foregoing that for a twin tide, thesaline wedge gets arrested at three points at variouslengths. These points and the various lengths of the salinewedge can be determined only when we are in a position toschematise quantitatively, the salt outflow which has twocomponents - salt overflow and outflow over the whole depth.This should be the topic for further research, inconjunction with the effects of accumulation.

CHAPTER 5

SCHEKATISATION OF TIDES AND DISCHARGES

5.1. TIDAL FLOV

The tidal flow through the gut is due to the tidally inducedvariations of the water level at sea. It is controlled bythe tidal response of the area behind the gut. This area ischaracterized by a net work of channels of varying width anddepth, flanked by shallow areas. A first estimate of thetidal response can be obtained by schematising the area as anetwork. More accurate results can be obtained by performingtwo dimensional numerical tidal computations. These types ofcomputations, however, were outside the scope of this deskstudy.

Because of the above reasons, the tidal component of thepresent study has been limited to distinguishing that thetidal response of the area behind the gut depends on thelength of its separate channels. For this purpose, the tidalflow through a channel of rectangular cross section withconstant width and dep th has been considered.

5.1.1. CHANNEL OF CONSTANT DEPTH AND VIDTH

A computational method for the tidal flow through a channelof constant dep th and width has been developed by Ippen.From the literature, the following physical feature emerges.The considered channel has a length L. At one end (x = 0),it is in open connection with a sea basin. At its other end,it is closed. In the sea basin, the water level varies, withtime, in accordance with

5.2

11. 2n11 Sln T t · .... 5.1

t

T

vertical position of water level with respectto time mean sea leveltimeduration of tidal cycleSymbol denoting amplitude of parameters

where,11

involved.

Vhen in the basin, water level rises or falls, in thechannel, water level tends to do the same. This is effectedby tidal waves entering the channel. Vhen in the basin,water level rises, tidal waves entering the channel arepositive. Vhen in the basin, water level falls, negativetidal waves enter the channel.

Over a period of time, t, the height Zo of an individualtidal wave which enters the channel is given by

· .... 5.2

Consequently,

· ..... 5.3.

and

· ..... 5.4.

Vhere,zo (tO): height of tidal wave entering the channelat t = to

Zo (to + ~ T), Zo (to + T): the same at ~ Tand T later.

The velocity of propagation of tidal waves and the velocityand flow rate which the waves induce are given by:

c ± Igh · ..... 5.5.

u

and

q Bzc

where,czghuBq

5.3

ziïc ...... 5.6

••.••• 5. 7 •

velocity of propogationheight of tidal wave at any instantacceleration due to gravitydepth of channelvelocity of flow induced by tidal wavewidth of channelflow rate induced by wave (q = Buh)

For deriving Eq. 5.5., the assumption is made that n « h.Further, the velocity of flow induced by tidal action isassumed small compared with Igh.

At the closed end of the channel, the velocity of flow mustbe zero under all conditions. Consequently, at the closedend of the channel an incoming tidal wave gets reflected.This reflection is positive meaning that after reflection,an incoming positive tidal wave remains positive whentravelling back to the basin. An incoming negative waveremains negative af ter reflection.

The positive reflection may be explained by assuming thatthe incoming tidal wave passes through the closed end of thechannel. In doing so, at x = L, it must be met by a tidalwave of equal height propogating in the opposite direction.Then at x =L the condition u = 0 is satisfied as, because ofEq. 5.6., the net velocity induced by both waves is zero.

The water level variations in the large sea basin do notdepend on the flow in the channel. This leads to a negativereflection, meaning that a wave which returns trom x = L to

5.4

x = 0 as a positive wave, reenters the channel as a negativewave. A wave which returns to x = 0 as a negative wave reenters the channel as a positive wave.

The tidal wave which returns from x = L to x = 0 loses its

dx .. tghde Jb"

[..to+t2L :positive wave- - - ... - .. :negative wave.... .....CD -foL.... damping factor a=..... e@' ® damping -2Jl-L'" factor ...,,- e.,,-~: e-~L"" damping factor •

t;to+[4L etc.

[-[0+[6L.... ..... .... ....6);

""""'",.""[;[0+[8L

FIG:5.J. FATE OF POSITIVE WAVE ENTERINGCHANNEL AT t • a

height when it propagates into the sea basin as the width ofthe sea basin is much larger than the width of the channel(eq. 5.7.). It does not affect the water level at x = 0;when at x = 0, it is met by a wave propagating in the

5.5

opposite direction, with equal absolute height and oppositesign. This explains why at x = 0 the reflection is negative.

The flow induced by the tidal waves is counteracted by bedshear. Therefore the wave height decreases with the distancecovered by the wave in accordance with

·..... 5.8.

lJdistance covered by tidal wavedamping coefficient (Ippen 1966 Ref.1.9.)

Vhere,x

The above phenomena are represented schematically in fig.5.1., which shows a positive wave entering the channel att = to. It arrives at x = L with a damping factor e-lJL • Atx = L, the reflection is positive. The reflected wave whichremains positive, arrives at x = 0 with a damping factor-2lJLe • At x = 0, the reflection is negative etc.

The negative reflection at x = 0 occurs at time t = tO+t2L,where t2L represents the time needed for the tidal wave tocover a distance 2L. This time and corrensponding times t4L,t6L etc. are given by

2L19h etc ·..••• 5.9

The length of the channel may be expressed in terms of thelength of the tidal wave L given byv

L T/ghw

· .... 5.10.

Vhen L= ~Lw' t2L = ~ T. The individual wave which enters thechannel from the sea basin at to + ~ T has the same absoluteheight as the wave which entered at t = to' with oppositesign (Eq. 5.3.). Hence for this length of the channel, thenegative wave entering at t = to + t2L and the negative waveinduced by the negative reflection of the wave which entered

5.6

at t = to reinforce each other. In a similar way, eachindividual wave which enters into the channel from the seabasin is reinforeed by its predecessors in such a way thatits effective height tzo is given by,

where tzo :combined height of all waves which at given timet propogate from x = 0 into a channel ~ L long.w

Vhen L = ~ Lw ,t2L = T. The individual wave which enters thechannel from the sea basin at t = to + T has the same heightand sign as the positive wave which entered at t = to (Eq.5.4.) Hence for this length of the channel the positive waveentering at t= to + t2L and the negative wave induced by thenegative wave which entered at t = to counteract eachother. In a similar way, each individual wave which entersthe channel from the sea basin is couteracted by itspredecessors. lts effective height tzo is given by

The above considerations imply that there is a maximumamplification of the tidal motion when the channel lengthcorresponds to a quarter of the tidal wave leng th L.wNeglecting shear, (~ 0), for this channel length, theeffective wave height becomes infinite (Eq. 5.11.). In thiscondition of resonance, the bed shear stress must beaccounted for as it keeps the effective wave heights finite.

There is a minimum amplification of the tidal motions whenthe channel length corresponds to half of the tidal wavelength.

Neglecting shear (~ = 0), at any station along the estuary,maximum tidal veloeities occur when the water level is atits highest or lowest level. Maximum veloeities are found to

5.7

occur at a later point of time when the effect of bed shearis taken into account. The above processes are described inmathematical terms by Ippen(Ref.1.9). The phenomena ofresonance is discussed in detail by Pugh.

The above schematisation could not be applied to Cochinbecause of the irregular shape of the Cochin backwaterslike varying length, width, depth and other obstructions.It did not give any reasonable values for velocities. Itbecame evident that these values could be obtained to anyreasonable accuracy only by a mathematical modelling. Sincethis was not within the scope of this desk study, it wasconsidered necessary to depend on other simpIer methods.

5.1.2. SCHEMATISATION FROM TIDAL PRISM

It was decided to depend on the tidal prism to arrive at thetidal discharges. The tidal prism computed by C.V.&P.R.S inthe case of Cochin is around 90x106 m3 for a spring tide of1 m. Since the area of cross section of the gut is known,the tidal discharges could be schematised equating the tidalprism to the product of the area and the depth averaged,time integrated velocities. This schematisation was done forfour tides of various ranges and the values have been usedin the computation of density currents in Chapter 6.

5.1.3 PHASE LAG

As mentioned in 5.1.2., the velocities through the gut wereschematized using the simple method of tidal prismassuming a phase lag of plus or minus 90° between the highwater level and the maximum discharges. This has beenreproduced in the discharge curves, see fig. 6.1. to 6.20.The analysis of the extended data which was made availableat a later stage of the study af ter computations wereperformed showed,however, that maximum ebb discharges occursimultanously with the low water. see fig. 6.22. to 6.26.

5.8

For a rectangular channel of constant width and depth, thiscan happen only when the length of the channel is equal tothe resonance length of L/4. The direct length of the Cochinbackwaters is much too small for that. The phenomenon oftide at Cochin is thus identified as the effect of storageareas. For mathematical modelling of a tide, the situationcan be schematized as an estuary containing embayments. Thewater in the embayments does not participate in thelongitudinal tidal transport. However the water fills theempties with the change of water surface elevation. So theembayment acts as a storage and in Cochin, since the surfacearea of such embayments is a significant percentage of thetotal surface area, the schematization should represent thestorage action. But as mentioned earlier this could beachieved only through elaborate computations which wasoutside the scope of this study. So the 90° phase differenceis retained, fig. 6.1. to 6.20.

This was done because the density currents depend primarilyon the net discharge through the gut and since the timehistory of the discharges used as an input into thecomputations remains to be of the correct order ofmagnitude. In addition, depth variations due to water levelfluctuations are smalle Eventually computations are to beperformed using as input, tidal discharges obtained frommore elaborate tidal computations.

5.2. FRESH VATER DISCHARGE

It is estimated that during the south west monsoon (SYM)period of 4 months, June, July, August and September, thefreshwater discharge from all the five rivers from the southof Cochin increases rapidly. The peak monsoon discharge isrecorded as 3400 m3/s. Out of this, Thottappally spillwaydischarges a peak of 560 m3/s at an average of 280 m3/s intothe sea. The river Periyar has peak and average dischargesof 900 m3/s and 450 m3/s respectively from the northern side

5.9

of the gut. Except for a shallow silted up opening 30 kmnorth of Cochin, all the above water has to find its way outthrough Cochin gut. This quantity comes to 3740 m3/s and1870 m3/s for the peak and average discharges. Varyingvalues of freshwater discharges were superimposed on thetidal velocities for density current computations.

5.3. CROSS SEC'rION OF GUT

As can be seen from fig. 6.31., the gut has an irregularcross section. The southern side is deeper. For computationpurposes the following section was adopted.

+0 ')---• tNt ----

430m -12.0

FIG.5.2.SCHEMATISED SECTION OF GUT

The schematized discharge curves for various freshetdischarges and tidal amplitudes are given in figures 6.1. to6.20.

CBAPTER 6

CORRELATION OF COCHIN DATA VITB TBEORY

6.1 DISCUSSION ON AVAILABLE DATA

6.1.1 C.V. & P.R.S. :1967-1968

As discussed earl ier in 2.3, valuable data on flow outsideand inside the gut were collected during 1967-68. But thedata as such were not available for analysis for the purposeof this study. Vertical velocity profiles at a certaininstant of time for certain locations in the approach channelhave been given. Tarapore et al (1977) Ref.1.18. The data attwo stations near the gut show marked stratification. Basedon a detailed analysis of the above data, Gole and Vaidraman(1969) Ref.1.8 give the longitudinal salinity profile at thesurface and at bottom. Fig.3.5. It shows sharp longitudinalgradient at the surface suggesting stratification. Theprofile at low water clearly shows that the saline wedge ispushed out and the basin is clear of salt. It may beremembered that the observations were taken during July whenhigh freshet discharges were expected.

6.1. 2 NIO : 1975-76

As discussed earlier in 2.4, these were the first completeobservations covering all the seasons, tides and locationsinside the Cochin basin. The report mainly contains meanvalues of analysed data for three seasons: pre, post andmonsoon. Hean salinity profiles at various locations havebeen arrived at. The mean values of monsoon season clearlyshow stratified conditions. This is evident not alone at thegut and navigation channels but also in the shallow naturalchannels on the northern side. The 'percentage flowdownstream graph' also indicates that at the surface, thereis continuous ebbing during monsoon with a sharp change atthe bottom. The phenomenon undergoes a change during the postand pre monsoon seasons. Vertical salinity gradient at thegut during a day in July (monsoon) have been shown as an

6.2.

example in the report.Ref.1.13. The values have been recordedat close depth and time intervals. It clearly shows theadvance and retreat of the saline wedge until it iscompletely flushed out. The salinity profile shows sharpstratification also. Vertical distribution of currentvelocities during one day in July (monsoon) and February (premonsoon) have been given by V.S. Rama Raju et al; 1979 (Ref.1.14).The data in July show a clear velocity stratificationwhile the one during February presents a normal verticalgradient.

6.1.3 e.v. & P.R.S. :1980

As earlier mentioned, for the purpose of this study, thisdata had been made use of for analysis. The data werecollected from a location in the gut and two locations eachin Ernakulam and Mattanchery channels. The period ofobservations was between 20th and 27th, July, 1980. FromTable 1.1, it can be seen that during this month, there was arainfall of 749.40 mm compared to an average for the month of614 mmo The previous month of June also registered a highrainfall of 888.90 (average 844 mm). This has certainlyresulted in a more than average freshwater discharge. At alllocations, vertical profiles of velocity and salinity at one.hour time intervals and 2 m. depth intervals were taken. So acontinuous data for seven days is available. The data athourly intervals at the gut were available for this study.Data collected from the channels were not available in fullfor analysis.The velocity and salinity verticals showed aconsistent pattern for all the seven days. On all the sevendays, the salt wedge was flushed out during high dischargesof the larger tide. Af ter the brief moment of flushing, thewedge starts to come inside and starts oscilating with theincrease in discharge of the second tide. This is followed bythe quick retreat of the wedge and subsequent flushing.

6.3.

6.1.4 C.V. & P.R.S. :1985

These observations were important because they were the firstobservations taken after the channmel was deepened andwidened. There was no observation at the gut. Two locationsside by side in Mattanchory and Ernakulam Channels and twonear the naval jetty were subjected to observations. The datacollected were similar to those of 1980. In Mattancherychannel, stratified conditions were observed while outside inthe flank of the channel fully mixed conditions wereprevailing. In the Ernakulam channel, at the widened portion,a lateral variation in the advance of the saline wedge wasobserved. Stratified conditions were alsostations near the naval jetty

observed at

6.1.5 NESA :1988

Eventhough these observations were taken with mostsophisticated equipments and by expert personnel, ·the resultscould not be used for the purpose of this desk study. Thesebeing the latest observations would have been much useful ifcarried out at the right time. The observations were madeduring May early which was supposed to be pre monsoon. Butthere had been some untimely rain during April and thechannels were subjected to some amount of freshwaterdischarge. This had affected the data collected, badly. Adiscrepancy noticed was the excessively high velocitiesthroughout the depth without a sizable freshwater discharge.

6.2 STRATIPICATION PARAMETERS POR COCHIN

From the analysis of the above data two valid conclusionscould be arrived at.

1. There is a sharp stratification. in the estuary duringthe monsoon season.

6.4.

2. The freshet discharges were so high compared to thetidal filling that at a certain stage of the tide, thesaline wedge is pushed completely out of the estuary

To check these facts with the theory, the stratificationparameters were computed.

6.2.1 SIMMON'S RATIO (Simmons, 1955)

..... 6.1

where OfT

river flow rateduration of tidal cycle

Pt volume of sea water entering the estuary on theflood tide

For 3a peak monsoon discharge of 3,740 m Is and for a maximumrange of 1 m tide, Simmon's ratio was found to be 2.48, whilefor an ave rage monsoon discharge of 1,870 m3/s and a moderatetide of 0.75 m, the Simmon's ratio works out to 1.24 which ismore than the required 1 for stratification.

6.2.2 ESTUARY NUMBER (Harleman & Abraham, 1966)

F 2o .....6.2ex

where,U 2

F~ : Froude number = ~hUo :Maximum profile averaged

at mouthh :time mean value of depth of estuary at mouth

value of velocity

As in the earlier case, the estuary number E worked out to be-4 -37.90 x 10 for peak discharge and 1.9 x 10 for ave rage

6.5.

discharge which is less than 5 xstratification.

-310 ,the upper limit for

6.2.3 INTERNAL ESTUARY NUHBER (Thatcher & Harleman, 1972)

=F~1

Ot = E/1p/ p ..... 6.4

EO was computed to be 0.031 and 0.074 for peak and averagedischarges respectively which were less than the maximumlimit of 0.2 for stratification to be present.

6.2.4 RICHARDSON NUHBER (Fisher,1972)

~ Qe g f

b U3t

.....6.4

where, :rms tidal velosity averaged over profile.:width of the gut

The Richardson number worked out to be 2.91 for peakdischarge for alm tide and 2.25 for average discharge for a0.75 m tide both of which is more than the lower limit of 0.8for highly stratified conditions.

The above parameter values are compatible with theobservation made from the field data that Cochin is a clearcase of a stratified estuary during monsoon. Ye will nowproceed to make a quantitative and qualitative study aboutthe resulting density currents.

6.3 COKPUTATIONAL QUANTIFICATION OF DENSITY CURRENT THROUGH GUT

This section gives a theoretical estimate of the densitycurrents through the gut using the density current equation(Eq.4.6.), schematisïng the net flow through the gut asindicated in section 5.1.2. Those values would be used herefor computing density currents. Because of the marked diurnal

6.6.

inequality, twin tides werefollowing tidal combinations

considered together. Thewith five varying freshwater

discharges were used for the computation

lst Tide range 2nd Tide range

(1)

(2)

(3)

(4)

0,75 m0.75 m0.50 m0.50 m

0,30 m0.15 m0.30 m0.15 m

Freshet Discharges

(1) 8.16 m3/s/m width(2) 6.00 m3/s/m width(3) 4.08 m3/s/m width(4) 2.33 m3/s/m width(5) 1.00 m3/s/m width

Following the procedure laid down in Chapter 4, densitycurrent graphs were prepared f~r each combination of tidesand for varying fresh water discharges. The sets of graphsare given in Fig. 6.1. to 6.20. The variation of densitycurrent with the decreasing freshet discharges can be clearlyseen from the graphs for all the four combinations of tides.The tidal filling (salt water) is also seen increasing withdecreasing freshet discharge. The results of the computationsare given in Table 6.1. For the 0.75 m tide, it was found

2that for a freshet discharge of 6.00 mis, the densitycurrent was maximum. The density current was found to bereducing for higher and lower values of qfr. But the totalsalt water coming inside is density current plus tidalfilling, which was always on the increase (Fig. 6.21). Fortides of lower range, the optimum qfr was much lower than6.00 m2ls. .

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From the point of view of siltation, the total silt charged(saline) water coming inside the harbour basin is veryimportant. At Cochin, the silt charge in the sea water isappreciable only during the SVM months of June, July, Augustand September. Using the average conditions, the total volumeof salt water coming inside during this period has beencomputed to be 11.18 x109 m3. This is done by integrating thedensity current and tidal current over the tidal cycle. Thedensity current was found to be 3.54 times the tidal fi1ling.This is a measure of the importance of density current forsiltation. The values of density current given in table 6.1.represent an upper limit for reasons explained in section6.4.

6.4 ANALYSIS OF DATA FROK 1980

This section gives the magnitude of the net flow and thedensity currents through the gut as obtained from the 1980

TABLE 6.1 DENSITY CURRENT AND TIDAL FILLING

PRESENT SITUATION:Draught: -11.9 m

Densitl current Tidal fillins: Ratio:densitf

lst 2nd Total lst 2nd Total Total currenqfr HitI~;2m /s Tide Tide Tide Tide Saline

water

1.lst Tide 0.75 m, 2nd Tide 0.30 m

8.16 60,704 42,523 103,227 15,257 0 15,257 118,484 6.776.00 62,737 71,621 134,358 40,851 0 40,851 175,209 3.294.08 60,972 102,376 163,348 70,028 332 70,360 233,708 2.322.33 59,141 111,827 176,968 101,432 17,289 118,721 295;689 1.491.00 58,026 122,757 180,783 128,152 39,270 167,428 348,211 1.08

2. lst Tide 0.75 m, 2nd Tide 0.15 m8.16 60,704 39,097 99,801 15,257 ° 15,257 115.058 6.546.00 62,737 69,427 132,164 40,851 ° 40,851 173,015 3.244.08 60,972 100,560 161.532 70,028 ° 70,028 231,560 2.312.33 59,124 132,357 191,480 101,432 0 101,432 292,912 1.891.00 58,026 147.923 205,949 128,152 11,061 139,213 345,162 1.48

3. lst Tide 0.50 mz 2nd Tide 0.30 m8.16 53,643 40,872 94,516 0 0 ° 94,516 CD

6.00 73,656 69,379 143.035 5,254 0 5,254 148,239 27.224.08 82,418 102,376 163,348 70,028 332 70,360 233,708 2.322.33 87,556 115,340 202,896 53,334 17,266 70,650 273,546 2.871.00 88,675 120,235 208,910 78,549 39,276 117,825 326,735 1.77

6J9

TABLE 6.1 (cont.)

PRESENT SITUATION.

Density current Tidal filling

1st 2nd Total 1st 2nd Total Totaldensitvcurren1:il~tI~;qfr

2m /s Tide Tide Tide Tide Salinellater

1st Tide 0,50 m, 2nd Tide 0.15 m

8.16 53,643 37,031 90,674 ° ° ° ° GO

6.00 73,656 67,120 140,776 5,254 ° 5,254 146,030 26.794.08 82,418 98,100 180,517 26,104 ° 26,104 206,621 6,922.33 8.7,556 129,909 217,364 .53,334 ° 53,334 210,698 4,081.00 88,675 145,339 234,014 78,549 11,061 89,610 323,624 2.61

field data (section 2.5).estimate of the densityabove net flows as anequation.(Eq.4.6.).

It further gives a theoreticalcurrents through the gut using theinput into the density current

The velocity verticals at one hour intervals were analysedfor density currents. The curves of Tidal levels, NetDischarge, Density current and Height of salt at gut has beenplotted on the same sheet. Sincé the data is continuous forseven days, the plots also could be made continuously. Thesefield data are presented in Fig. 6.22 to 6.26. The verticaldistribution of velocity for every hour during 22-7-1980 isalso given in Fig. 6.27 to 6.29 for a better appreciátion ofthe phenomena. For calibration with the prototype data, thetheoretical curve for a k.L = 0.05 value has been plotted on

1

top of the density current graph from data. The correlationis marked at the beginning of the density current af ter

•

6.20

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6.23

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FIG:6.21. VElOCJTY VERTICALS-1910,

1600 HRS~ "-,,..,..

101 _..1

170. hr.

WoIt"""

.....22 .7 .1'"

2200 HRS 23•• hra

\0,

....

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,.00 hr.WoI,.,TI

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1900 hrs~n1~----~--~~-.

•

23 .7 .1910

.,00 hr.

FIG:6.2'. VrLDCITY VERTICALS-1980

SoIloIt,IPPn1 - ...

2100 hrs

•...

OlOO hra

•

...• ti

r.~.

••

._. --_ .. __ . -----_._---------- ------------------- _ .._------_._------------

11 41

6·24

flushing. As to be expected for reasons given in section 4.5,af ter the initial portion, there is a deviation of thetheoretical from the actuals. This is where the accumulationand the friction is coming into play. Inside the gut, thereare two main channels in Cochin. The wedge goes into bothchannels. So the interfacial friction is higher resulting ina slowing down of the density current. The value of k.is~given by 10-3. This means that the length of the gut is 600m. The effective width at site also would be around the same.Because of the abov~ findings, the figures given in thesubseqent table 6.2. indicate upper limits for the densitycurrents.

6.5 SALIENT FEATURES AT COCHIN

6.5.1 As briefly mentioned in 6.1.3, the field data belonged to themore than average monsoon conditions because of the excessiverainfall during that periode The freshet discharge at thattime is estimated to be around 8.00 m2/s. The general picturegiven by Fig. 6.23 to 6.26 is the same for all seven days.Because of the predominance of freshwater discharge, thetidal flooding was considerably less and there were instanceswhen there was continuous ebb over three tides. But thepattern is that, af ter flushing, the density current startsand goes on to the two maximum values at zero net flow. Thedensity current is then brought to zero by the ebbing phaseof the smaller tide also, without flushing really occuring.The density flow starts again after the peak ebb of thesecond tide and reaches a third maximum before diminishing tozero prior to flushing.

6.5.2 One interesting feature was the reversal of the current overthe whole depth during the peak discharge of the smallertide. Example: 1300 hrs on 22-7-80 in Fig. 6.28. Eventhough,there is no flushing at this discharge, the wedge goesbackward to a point near the gut and goes in again when thenet flow is reduced. This physical bodily shift of the saline

6.25

wedge is in line with the concept given in 4.5.9. This isalso in line with the theoretical estimate of Fig. 6.1 whichis prepared for the peak discharge conditions. Thisphenomenon results in a high salt outflow over the wholedepth during this period which is essential for keeping upthe salt balance in the estuary.

6.5.3 As earlier stated, the salt outflow consists of twocomponents - salt overflow and outflow over the whole depth.The velocity and salinity observations have been taken at 2 mintervals. The readings have been joined by straight lines.So a sharp interface, which is no doubt present, is notsharply depicted in the vertical profile. But it is notdifficult to see the salt overflow above the velocitystratification. See velocity verticals from 0500 hrs to 0900hrs and from 1200 hrs to 1800 hrs in fig.3.3.,6.28.,and6.29 .. The concentration of salt seems lower but flows outwith a higher velocity .The salt outflow (whole depth) ismainly concentrated over the region where the density currentis zero. See velocity profiles from 1900 hrs to 2200 hrs infig.6.29. The largest salt outflow occures during the periodbetween the point of zero density current and flushing.During this period, the entire wedge flows out. This isevident from the velocity/salinity verticals.

6.5.4 A phenomena to be observed is the abnormal salt outflow af terflushing has occurred and while the density current is juststarting. Fig. 6.22. It may be remembered that just beforethis period, all the salt had been driven out of the estuary.That means there is no salt left in the estuary to go out.This is a phenomenon particular to Cochin. There is a highvelocity at the gut but in the channels, the veloeities arelow. Horeover the channels have irregular shape and areobstructed by islands. So when salt is driven out at the gut,it is not fully driven out in the basin with the result, somesalt is trapped inside. This is flushed out later while beingmixed with the predominantly fresh water in the estuary.

6.26

Please see velocity verticals at 0200 hrs in Fig.6.27. It isnot clear at which location the salt is trapped in. It couldbe in the shallow areas in Mattanchery or Vypern channels.

6.5.5 Various data on discharges at instances of flushing have beenanalysed and it has been found that it would be correct tosay that flushing occurs at Cochin at a Froude Number of 0.7against a theoretical value of 1. One explanation is that asper theory given in Chapter 3, when the estuary meets thesea, ° there is a sudden change to infinite width of the seafrom the finite width of the estuary. But physically, this isnot so. The channel continues into the sea with high contourson the sides. The sea bottom slopes only mildly. So part ofthe wedge is still formed outside. This has been proved rightby data collected in 1967-68 as discussed in 6.1.1.

The Froude number of 0.7 corresponds to a discharge of 13.5m2/s for a water dep th of 12.20 m at the gut and 10.00 m inthe channel. This was the case before 1983 when the channelswere deepened. For the same Froude number, for the deepenedconditions, a discharge of 15.60 m2/s would be required toflush out the salt wedge. Ve do not have any observation~taken at the gut af ter deepening of the channel (to -11.9 m).In all probability, flushing may be a rare phenomena now. Itis all the more reason to ascertain this by observations.

6.5.6 The special hydrography of the gut and the bifurcating innerchannels induce a peculiar pattern at Cochin. Because of thehigh velocity regime at the gut, there is deep scouring andnatural deeper draughts are available there. But the channelsare to be maintained by dredging even at a shallower draught.Though the dep th is more at the gut, the area of crosssection is less. The result is that the driving force for thedensity current is proportional to the depth of the innerchannel while the discharge depends on the area or °depth atthe gut. So if the gut is deeper more density current wouldcome in.Fig. 6.30.

6.27

If the channel is deeper, the velocity of flow would behigher. But widening the channel cannot increase densitycurrent as the control is at the gut. If the depth of the gutcould be controlled, there could be substantial reduction inthe density current.

--=-I

A~'I

"7»"""'" '~ I /'11),~n",ru" ,J~ . Irl

PLAH

1.0NGITUDlHAL SfCTIOH

FIG:6.30. GUT ANC Ö-iANNELS

6.5.7 The saline wedge at Cochin is not a simple case of a wedgeentering an estuary with an almost rectangular cross section.There are two deep navigation channels joining at the gut.There are three natural shallow channels on the northernside. Out of this, Vypern channel joins right at the gut. Twoother channels branch off from the Ernakulam channel towardsnorth. At present, the deepest is the Ernakulam channel at-11.9 m. The wedge goes into it first. As the wedge advances,the height of salt at the gut also rises. When the interfaceis above -9.8 m the wedge finds its way into the Mattanchorychannel also. If the height of the wedge climbs above -2.5 m,it would start entering the northern Vypeen channel on theflanks of the Ernakulam channel. Subsequently it may find itsway into the eastern channels as weIl. In èffect, the advancewould be in the form of a multipronged wedge.

628

6.5.8 In the Ernakulam and Hattanchery channels itself, the entirewater area is not occupied by the deep channels. On theflanks of the channels, there are shallow portions. Thefreshet discharges here are weak. It is mostly guided throughthe deep channels. The discharge over the flanks are notsufficient to create or maintain stratified conditions. SoweIl mixed conditions could be expected there. This has beenestablished by the 1985 data.

~.5.9 By taking simultaneous observations at two locations in thesame cross section in Ernakulam channel, it was found thatthere is a lateral variation in the advance of the salinewedge through that. channel. This was partly due to thewidening at this location which has resulted in lateralvariation in freshwater flow and tidal influx. (Dixit 1985,Ref.1.6.) The southern part of the gut is scoured deeper thanthe northern part Fig. 6.31. This could be because 75% of thetidal prism and the upland discharge is coming from thesouth. Because of the variation in depth along the width, thesaline wedge enters the southern part earlier. This will alsoinduce a lateral variation in its advance in the early stagesand is expected to be corrected later. Ye do not have anydata to prove this yet.

FIG:6.31. CROSS SECTION OF GU .

6.5.10

6.29

Before deepening of the Ernakulam channel, the width of thechannel was half that of the Mattanchory channel. Since bothwere of the same depth, the density current was distributedbetween the two channels in the ratio of width. So two thirdof the density current used to go into Mattanchery channeland the balance into the Ernakulam channel. But in 1983,Ernakulum channel was widened equal to Mattanchery channeland deepened to -11.9 m. So now the ratio of distribution ofdensity current roughly became proportional to a/a, where ais the water depth. This ?orks out to 0.57 : 0.43 in favourof Ernakulam channel. It can be seen that the situation wasreversed. This acquires more significance since densitycurrent carries silt into the channels .

.. lil • '.. .... . ... .'.- . ' ~~::~~~~~~~~--~----• '. ••••••• 111: • '. • • •• .' • lil •• '., ., • • • •

, : SA~iNE. WEll<i,' : .' .. '.' " ':.' '. I. " t,' •. ...' . '. '.... '.' . .'. : . . ',: '. .... ' ..'. .,...., '.. . .. . .' . . .,. '. . . . -, ,.q :. .

F\t;. &.3~ LOti~\TUDltiAL.. .SEC,.lON

't •

-5r-----------r---------~----------_4----------~

6.5.11

:::0TiIKë' (T r::.ü)

FIG:6.32. DENSITY CURRfNT WHEN DRAUGHr"IS REDUCED

One feature which was not taken into account for thequantification ofsudden reduction

density current, described in 6.3. is thein the density current when the wedge

6.30

reaches the end of the channel. Please see Fig. 6.32. and6.33. The driving force at that stage is reduced to thereduced draught of the channel. So there is a sudden decreasein the density flow, which is shown schematically inFig. 6.32. The point at which the sudden decrease occursdepends on the velocity of advance of the saline wedge. Thisis a field to persue further research.

6.6 FUTURE DEEPENING OF THE CHANNEL

The Port of Cochin propos es to deepen the Ernakulam channelfurther for accommodating deeper draughted vessels asrequired by trade. The first stage involves a deepening ofapproach channel to -14.3 mand inner channel to -13.40 m.During the second phase, there would be a further deepeningof approach channel to -16.10 and inner channel to -15.20 m.It would be interesting to note the developments in thedensity current pattern. Computations revealed the followingfacts.

TABLE 6.2 DKNSITY CURRENT FOR DEEPENED SITUATIONS

SITUATION Total salt water Ratio of DensityDuring monsoon Current to Tidal

filling

LPast Case 9.34 x 10 m3 2.79(Draught - 9.8 m)

2.Present Case 11.18 x 109 m3 3.54(Draught -11.9 m)3.Future Case (1) 13.80 x 109 m3 4.60(Draught -13.4 m)4.Future Case (2) 17.58 x 109 m3 6.13(Draught -15.2 m)

6.31

It may be noted that from 1 to 2, the increase was 19.7 %while from 2 to 3 it would be 23.4 % and then there would be27.4 % increase. For a better appreciation, the theoreticaldensity current graphs for the various situations for fivedifferent freshwater discharges are given in Fig. 6.34 to6.38. It has to be kept in mind that there will not be anyappreciable increase in the tidal filling due to deepeningbecause of the comparatively large tidal prism.

6.32<J1::'.~: r, .'. f:: ;:::~.:; ~f": ',' .::0{.:,:':';" ...l:·: " I I y: ..: "l ::'::'.~~,::.. , {;.:: }t , •• , ' , •, . .'.... ,,' ~:····;·,:r:.:.. : ..." "" ........

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6.34

\

CHAPTER 7

SEDlHENTATION

7.1 INTRODUCTION TO PROBLEM

7.1.1 The biggest single problem facing the port of Cochin issiltation in its channels. The port is spending a huge amounton maintenance dredging to keep up the draught requirements.As discussed in the earlier chapters, there are threenavigation channels at Cochin - the approach channel outsidethe basin and the Ernakulam and Mattanchery channels insideit. Yhen Cochin was declared a major port in 1936, thedraught in the inner channels was - 9.8 mand the approachchannel, - 11.3 m. The siltation was keeping the same patternuntil 1983. The annual siltation in various channels is givenbelow.

Table 7.1. Dredging data

Channel Quantity

1. Approach channel 6 32.0x10 m2. Ernakulam channel O.2x106 3m3. Mattanchery channel 6 3O.5x10 m

Total 6 32.7x10 m

But in 1983, the shipping channels were deepened and widenedto accommodate bigger ships.

7.2.

1. The approach channel was deepened from -11.3 m to- 12.8 m. The continental shelf at Cochin being onlymildly sloping, this deepening resulted in.a substantiallengthening as weIl. The channel was also widened from137 m to 200 m. The quantity of siltation jumped up from2.0 x106to 2.6 x 106 m3/year.

2. The new deep draughted facilities were to be put up inthe Ernakulam channel. So this channel, upto theproposed fertilizer berth was deepened to -11.9 m. Theconfiguration generally underwent sizable wideningjnear the berths the widening being as high as 500 magainst the earlier 250 m. The siltation made aspectacular increase from 0.2 x 106 to 2.2x106 m3/year.

3. There were no changes made in the Mattanchery channel.This has resulted in a change in the relative positionbetween the two channels. Before 1983, there was moresiltation in the Mattanchery channel. But now this hasreversed though there had been a slight increase.

7.1.2 OBJECTlVE

The objective of this study is not to go in detail into themechanism of siltation. It is also not intended to quantifysiltation accurately as various parameters of the siltrequired for the model are simply not available. The mainintent of this thesis is to go in depth into the densitycurrent phenomena. During that process, the total densitycurrent and the tidal filling current entering the harbourwas quantified. Since the saline water entering the harbourduring the southwest monsoon would be heavily silt chargeddue to strong wave action outsidej it would be worthwhile toexamine what happens to the silt during the period the salinewedge stays inside.·

7.3.

7.1.3 SCOPE

As mentioned earlier, para 1.13, the mechanism of siltationis different in approach and inner channels. Since thisthesis is concentrating on density current, attention isfocussed on silt carried by the density current and tidalfilling current. It is needless to say that this phenomenonis mostly confined to the inner basin only though datasuggest that the interface extends to a small distanceoutside the gut. Again, more importance is given to Ernakulamchannel as the major siltation presently is there.

7.1.4 HYPOTHESIS

A large tid~l prism is meeting a larger fresh water dischargeat the Cochin gut. This has given rise to highly stratifiedconditions inside the basin. See 6.9. The slow movement of adistinct saline wedge and a circulation of salt waterwithin, has been established. Large fresh water dischargesoccur during the four months of the southwest monsoon.Incidentally this period is accompanied by strong westerlywinds resulting in high wave action outside in the sea. Theshear stress induced by these waves erode a lot of bedmaterial and keep them in suspension. The saline water whichenters the basin in the form of a bot tom wedge brings thissilt into the navigation channels. Ultimately, before thewedge goes out at the end of the twin tidal cycle, a certainportion of the material is deposited èreating shoaling in thechannels. This process happens every time the wedge entersthe channel during the four months of the southwest monsoon.

Therefore the relevant processes involved can be summarisedas:

ErosÏ'on by waves;Transportation by tidal and density currents;Deposition inside the wedge;

7.4.

Erosion outside the wedge.

These processes will be discussed in the coming two sections.

7.2 TRANSPORT PROCESSES OF COBESlVE SEDlHENTS

7.2.1 FLOCCULATION

Flocculation can be defined as the forming of large particleaggregates from smaller particles. Flocculation is broughtabout by collision of particles due to:

1) Brownian motion;2) Velocity gradients within the suspending fluid and3) Differential settling of the suspended particles.

Turbulence increases·· the probability of collision aidingflocculation but also acts as a check on the size of flocs bybreaking them up.

Salt is a natural flocculating agent. Yhen two particlesmeet, they will repulse each other because of the clouds ofpositive kations surrounding the negatively charged suspendedsolids. But there is a smal~ attractive tendency due to Vander Vaals forces. The repulsive forces depend on the amountof positive ions in water. An increase of the concentrationof positive ions results in a compression of the cloud ofpositive ions around the sediment particle. This will resultin a decrease in the repulsive forces thus aiding thecoagulation of colliding particles. A salinity of 20/00 isgiven as a point of flocculation. Apart from saltflocculation there is bioflocculation which takes place dueto the action of bio-organisms. Polymers are found to beadsorbed around particles when they come in contact withággregates.

7.5.

From the foregoing, it can be seen that the flocculationprocess is determined by:

the properties of the sediment (minerological, organiccontent etc.);the properties of the fluid (salinity, pH, temperature);hydraulic conditions (turbulence);Sediment concentration.

The aggregated particles could be much larger than the ionsin the dispersed state and would be possessing a highsettling velocity.

Fig. 7.1 gives a particle size distribution graph of the bedmaterial of Ernakulam channel, showing both the dispersed andflocculated states. The d50 is 3.5 microns and 25 micronsrespectively for dispersed and flocculated conditions.

According to Stokes' law,

ys = ...... 7.1

So, fall velocity increases as the square of the diameter. Inthe above case, Y increases 50 times. But since the flocsconsists of loosely packed particles, its density would below. This makes it difficult to predict the fall velo~ity ofa flocculated mass. So it is common practice to take actualmeasurements. Fig. 7.2 shows astrong dependence offlocculation on concentration. The following relationship isoften used.

n= me ...... 7.2.

where, usually, 1 < n < 2

~NI

lI...lZ':Ja. ..I

~C

. ;)~

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7. 7

Krone(1962) derived a theoretical value of 4/3 for n.

Fig. 7.2 shows TJ varying between 10-5 -3 formis and 10 missconcentrations between 100 and 1000 mg/l. The lower valuecorresponds to the mean diameter of the dispersed material(3.5 ~m) using Stokes' law.

3 3(pd = 2,600 kg/m , Pw = 1,025 kg/m , v =

I

QJ

Clmu/II-

FJG:7.2. COMPARJSON OF W,.For the flocculated state, (d = 25 ~m - 100 ~m), the fallvelocity may be estimated using some typical values for thefloc density as derived from literature. Krone (1978) givesvalues ranging from 1.050 to 1.250 kg/m3. Using Stokes' law,

-5 -3the fall velocities become 10 to 10 mis.

7.8

7.2.2 DEPOSITION

Once fall velocity is decided by flocculation, the nextimportant sediment parameter is the critical shear stress fordeposition (~ D). Krone (1962), through a series of flumecrexperiments arrived at the conclusion that when the bot tomshear stress of the flow reduces to a certain value,deposition starts to take place. This value is termed as thecritical shear stress for deposition. If the bottom shearstress is more than this vaIue, no deposition would occur,although Krone did not confirm this. Krone had expressed thisas a fraction of the concentration near the bed.

• . . . •. 7.3

where,DVsCbP

Deposition rate (kg/m2/s)Fall velocity (mis)Concentration near the bed (kg/m3)probability that a particle sticks to the bed,'expressed as:

P~b

= 1 -'tcrD

••.•.• 7 •4

Bottom shear stressCritical shear stress for deposition

For a natural mud, Krone obtained a value of 0.06 Pa. (in arecirculating fIume).

Experiments performed by Partheniades and Hehta(1978),however indicate that for a value higher than the criticalbed shear stress, a fraction of the initial sediment candeposito This "degree of deposition" is a function of the bedshear stress. Because the application of the theory topractical problems is rather cumbersome, the expression byKrone is most widely used.

7.9

Both theories however agree to the fact that completedeposition occurs below some critical bed shear stress. Mehtaand Partheniades found values of 0.12 and 0.15 Pa forkaolinite in a circular flume. Reanalysed data from othersshow values ranging as follows.

Table 7.2. Typical values of ~ o(table derived from Ref.2.8)cr

Referenee Sediment ~crD

" "

natural mudkaolinite(distilled water)kaolinite(salt water)kaolinite-natural mudmixture in salt water

0.060.180.15

Krone(1962)Mehta/PartheniadesMehta/Partheniades

" "" " natural mud(salt)

0.120.10

0.040.07

Partheniades(1968)Partheniades(1965)Rosillon andVolkenborn(1964) 0.08

It follows that the critical shear stress depends upon thesediment and fluid properties.

7.2.3 EROSION

Identical to the critical shear stress in the case ofdeposition is the concept of ~ E' Above this shear stress,crthe particles start eroding. Although, there are a largenumber of expressions available from literature (Mehta,Parchure, Ariathurai), the most practical and usually usederosion function is given by (Kandiah Ariathurai etal,Ref. 2.15).

7JO

E'tb

M(-'t- - 1)crE

•••••• 7 • 5

'IIhere,EM

2erosion rate kg/m Is2erosion parameter (kg/m Is)

't E: critical shear stress for eros ioncr

M depends amongst others, upon the sediment type and the porefluid and can be determined in the laboratory.In general, this expression (7.5) holds for compact, ratherconsolidated layers. Some values of Mand 't E are given in

5 c~3 2the tabIe. Typical values of Mare 10- to 10 kg/m Is.

Table 7.3. Typical values of Mand 't Ecr

Refenence Sediment 2M (kg/m Is) 't E (Pa)cr

Ariathurai, ArulanandanSargunam et alCormaultThorn/Parsons

Yolo loamGironde mudGrangemouth,Brisbane,Belavan mud

-4 -35x10 - 5x10-3 -21.4x10 - 1.6x10

2x10-4-4 -41.3x10 -3.4x10

2 - 31 - 80.1 - 0.90.05-0.34

It can be seen that M as weIl as 't E can vary in orders ofcrmagnitude.

7.3 SEDIMENTATION DURING SALINITY INTRUSION

Siltation is the net result of deposition and erosion.Normally any siltation process will have a deposition phasewhen particles will be deposited and an erosion phase wherethe bed would be eroded. It is not necessary that both these

two phases should be present in any particular case. If thereis only deposition then it may come to a stage that the newdepths would induce an equilibrium condition when there wouldbe no deposition at all. Similarly erosion can go on till adeeper equilibrium is reached.

Let us now see how this process is taking place when a salinewedge is present. Please see Fig. 7.3.

-';I"

-_"'''__EROSIONFlr..7.3. OErOSITION FROM SALINE WEOt::E

7.3.1 DEPOSITION FROH VEDGE

The density current has a smaller velocity than the tidalcurrent and freshet discharge. The advance of the wedge wouldbe at a lower velocity still. See chapter 4. The densitycurrent would be coming in as salt underflow and normallygoes out as salt overflow. As given in Fig. 4.8., during thepeak discharges of the smaller tide and that of the largertide prior to flushing, there is salt outflow throughout theentire depth. The velocity of this flow is much smaller.·compared to the pure tidal velocities. Horeover this is anoscillating movement with the wedge moving to and frospending considerable time at zero velocities. The importantfact is that this flow carries silt from the sea. Due to lowvelocities inside the saline wedge, the material getsdeposited in the channels all along.

7. 12

7.3.2 EROSION OUTSIDE VEDGE

Vhen the saline wedge retreats, the wedge front is alwaysfollowed by a high fresh water discharge. See Fig. 7.4. Thisis the freshwater which was detained in the estuary by theflood tide (abed = a'b'e'd') and coming out during the ebbtide. As can be seen from the figure, this discharge wouldpossess high velocities and could be capable of erodingmaterial deposited by the wedge. But this eroding processcould start operating only after the wedge has withdrawn fromthat reach.

J;'IC.7.4. f.IillSHET f1EETINr. SALINE WEDGE

7.3.3 SALINITY INTRUSION CONTROLLED OR TIDE CONTROLLED

It is a usual phenomenon that the tidal amplitude andconsequently tidal shear reduces with increasing distancefrom the ocean. So there would be a point beyond upstream of

7·1 j

which the tidal shear would be less than the critical shearstress of the bed and that point is called 'point of noerosion'. Inside the salt wedge there is a point where thetime averaged near bottom velocities cross the point thezero velocity line meets the bed and turn back in seawarddirection. This point is called th~ 'null point'. In the caseof estuaries where the sediment is coming from the sea, thismarks the upstream limit of shoaling. Depending upon thelocation of these two points, an estuary can be classifiedinto two of the following.Ref.l.12.

(I) If the null point is downstream of 'point of noerosion', all the deposited material would be subjectedto erosion of some magnitude. So the estuary isclassified as tide controlled.

(11) If the null point is upstream of point of no erosion,the net sedimentation would be around the null point. Sothe estuary is salinity intrusion controlled.

7.3.4 EFFECT OF VEDGE ON SILTATION

(I) Due to the density current, the volume of salt waterbringing in silt into the basins is increased. Thisquantity increases with the depth of the channel. At thepresent draught of - 11.9m, the ratio of density currentto tidal filling is 3.54, which gives a factual pictureof the magnitude of the role of density current.

(11) The wedge is retained over the channels for a very longtime. This increases deposition due to increasedretention time. Af ter the wedge has receded, there isonly a small period during which the tidal shear isallowed to erode material before the wedge comes inagain. This increases net sedimentation.

7.T 4

7.4 COMPUTATIONS ON SEDlKENTATION

If all the sediment parameters for deposition and erosion areavailable and the flow parameters are also schematised, theproblem boils down to only applying the equations. Butunfortunately none of the parameters have been evaluated. Socomputations were made for an array of values and a processof elimination based on the site data available, was _adopted.For easiness of comparison, the computed deposition anderos ion was expressed as a fraction of the total amount ofsediment brought inside during the southwest monsoon.

OTOLB~ = --S.ln

ETELBCl]: .. S.ln

S. = Yflood·csealn

· .•... 7.6

• • • . •• 7.7

· • . . .. 7.8

~-Cl]: = ex ·..... 7.9where, TO: time for deposition (18 to 22 hoursO

TE: time for eros ion (7 to 3 hours)L channel lenghth under the wedge(three reaches

totalling 4400 m was taken for calculation inthis case)

B channel width (varying for various reaches300,400 and300 m)

The bed shear stress is computed by taking the total saltinflow as taking place over half the water depth as anaverage case.

Vflood1 hB2:

The bed shear stress follows from:2u

"tb = pg C2

u = · ...•. 7.10

· ..... 7.11

where, C Chezy's coefficient (taken as 100 g mis forthe smooth channel)

Please see Fig. 7.5 and 7.6. On one side, P vs Lb isfor various values of LcrO' On the other side, «0 isagainst TO' the retention time for deposition, for constant

-5 -4 -3fall velocities of V = 10 , 10 and 10 . If we have thesvalue of Lb' than we can proceed from the left side to therelevant LcrO and go on to the required «0 for a desired TO'The graph gives an overall idea about how deposition would

plottedplotted

vary with the variation of different parameters.

The erosion graph is given in fig. 7.7. On the left handside, Lb is plotted against log E for constant values ofLcrE' each again for three constant values of H. On the righthand side, log E vs log TE' where TE is the time for erosion,is plotted. The graph is drawn for sediment concentration of200 mg/litre. The same procedure of entering on the left sideof the graph for the relevant values of Lb' LcrE and H, andcoming to the value of ~ for the appropriate value of TE canbe followed here as weIl.

From field measurements, the amount of dredging can bereduced to the amount of sediment brought in by the totalinflowing salt water. Assuming a bulk density for the dredgedmaterial, the bed concentration follows from

Pb - Pw) ...... 7.12cb = (p - Pw Pgg

where, cb bed concentrationPb bulk densityPw mass density of waterPg mass density of grains

Af ter this, the sediment mass dredged during the monsoonperiod is calculated as :

.'.7.16

)

.....

t

0.05 0 5 10 15 20 25+---T (hu)-'"rrb(Pa) •

FIG. 7.5. P vs Ts' W = 10-4s

\.. ,

0.15 0.10 0.05 0 5 10 15 20 251---- rrb

(Pa) T. (hr5) ...FIG. 7.6. P vs T

s ' W "10-3s

7.17

IC

NIC

where, net siltationVolume dredged

7.4.1 CALIBRATION

7.18

...... 7.13

In the present case, the volume of salt water passing overthe Ernakulam channel during the southwest monsoon wasestimated to be 6.40 x 109 m3. It is assumed that thisquantity comes in with a silt charge of 200 mg/l. Seefig.7.8. The average annual dredging in the Ernakulam channel

6 3 .as per data is 2.2 x 10 m. For an assumed bulk density of1,200 kg/m3, the amount of sediments works out to 6.27 x 108kg.

I•... I~ _ <111.,SA,:,,,,rv '''' "pr,

o ., 0-.1.,l.r '""' ..,..' '" ",.T.FIG.7.8.SILT CHARGE AT GUT

Ot

86.27 x 1096.40x10 xO.2

0.49, say, 0.5

7.19

(It is interesting that for a eoneentration of 100 mg/l, therequired « value would be one, meaning that all the silt thatenters, deposits inside)

So the required value of « for various eombinations of «0 and~ is 0.5. There woul~ be several eombinations of thesevalues sinee there are several varying sediment parameters.

The «0 and ~ values are given in Table 7.1 side by side.

The region of the values of «0 whieh ean lead to «0 - ~values of 0.5 are shown by arrows. For a partieular set ofparameters for deposition whieh gives a «0 value, there is nolimitation on the seleetion of erosion parameters exeept for~erE value. Generally,

~erE ~ ~erD . . . . . .. 7.14

So for two sets of deposition parameters, welarge range· of eros ion parameters. ie,

-5 -3 20.30 Pa and H = 10 to 10 kg/m Is.

still have a0.20 to~ E-cr

TABLE 7.4.«0 & ~: CALIBRARION

DEPOSITION EROSION

~erV~\ 0.05 Pa 0.10 Pa 0.15 Pamis

10-5 0.00021 0.00385 0.007610-4 0.0021 0.0385 0.07610-3 0.021 0.385 0.76

0.10 Pa 0.20 Pa 0.30 Pa H2kg/m Is

0.00885 0.0014 0.00034 10-50.0885 0.014 0.0034 10-40.885 0.14 0.034 10-3

7. 20

7.4.2 VERIFICATION

The same procedure was done for the past case when thedraught of the channel was -9.8 mand the width of Ernakulamchannel was much less. The computations were carried out for

-4 -3~crO= 0.10 Pa and 0.15 Pa, Ys 10 mis and 10 mis,-4 -30.20 Pa and 0.30 Pa and H = 10 and 10 . As per the~ E=cr

dredgingto 0.09.now be

quantities at that time, the required « worked outThe disposition of « is shown in table 7.2. It canseen that the region of possible values have beendown considerably especially with the erosionnarrowed

parameters. The region is shown by arrows. In fact, only twosets of reasonable values can be derived from this. which,looking at the accuracy of the computations, are almostsimilar.

TABLE 7.5. «0 & ~: VERIFICATION

OEPOSITION:cn EROSION:~

~crY\ 0.10 Pas 0.15 Pa 0.20 Pa 0.30 Pa H

10-4 0:043610-3 0.436

0.0770.77

0.1461.00

0.05490.549

(I)(II)

-3 -3~ 0=0.142 Pa, ~ E=0.284 Pa, Y =10 mis, H=10 ;cr cr s -4 -3~ 0=0.15 Pa, ~ E=0.30 Pa, Y =8.325x10 mis, H=10 .cr cr s

7.4.3 PREOICTION

An attempt could now be made to predict the value of possiblesiltation if the Ernakulam channel is deepened to -13.4 m.The above two sets of values were made use of. The values of

.« obtained were 0.256 and 0.252. The siltation worked out to

7.21

be 1.50 x 106 m3. Incidentally it may be mentioned that thisis less than the present quantity of 2.2 x 106 m3 in thesecalculations. This is because, in the case of the deepenedchannel, there is a net increase' in the velocity of thedensity current. This has increased the shear stressesslightly in the sensitive region of deposition making way forsubstantial reduction in sedimentation. But the velocitiesinside a salt wedge have to be quantified using a twodimensional (vertical) or a three dimensional model forobtaining realistic figures.

7.5 INFERENCES PROM TBE EXERCISE

(I) Increase in depth would increase the quantity of siltcharged salt water being brought inside. But this alsoincreases the velocity of circulation in the wedge.Deposition is affected largely by the velocities; notonly by the total quantity of silt brought inside. Sodeepening need not increase sedimentation. But deepeningcan reduce erosion by decrease in tidal velocities.Deepening could increase retention time increasingdeposition. Therefore a more accurate modelling of thehydraulic conditions is needed in order to predict theconsequences of a further deepening of the innerchannels.

(11) Videning of the inner channel does not increase densitycurrent as Gut is the con trol. Videning will reduce bothvelocity of density current and tidal velocities. Thiswill increase deposition and reduce erosion inducing adouble fold increase in sedimentation.

(III)Slight increase in the roughness of the channel mightincrease the bottom shear and reducing siltation.

(IV) The quantity of sedimentation is highly sensitive to thesilt parameters and flow parameters and so accurate

7.22-

following parameters by laboratory

Therefore, inevaluating the

and field

evaluation of the samefuture, attention should

is required.be paid in

measurements:~crE' ~crD' Ys' H, csea at gut, the bulk density of thedredged material and a roughness value for thebed (Chezy).

CHAPTER 8

CONCLUSIONS AND RECOKHENDATIONS

8.1 PURPOSE OF STUDY

The Port of Cochin, located at the mouth of a large tidalbasin, exhibits fascinating flow patterns due to theconfluence of a large tidal prism with a larger freshwaterdischarge. The fact that the entire exchange has to takeplace through a narrow opening called Cochin Gut makes itmore interesting than ever. The port experiences a largeamount of siltation in its channels presumably from the siltbrought in by density and tidal filling currents. Thesiltation in the inner channel on the Ernakulam sideincreased sharply, after that was deepened and widened in1983. Yith a further deepening of the channel in two stagesin the offing, it became apparent that a research into theflow pattern bringing in sediment into the basin wasinevitable.

8.2 SCHEMATISATION

This study has concentrated on the physics of densitycurrents at Cochin. Cochin backwaters was considered as afresh water lake connected to the saline sea by the gut andthe theory developed by Rigter (1970) was applied withcertain modifications. This was developed into a method toquantify density currents. Extensive application of thismethod was made to Cochin for quantification of the siltcharged saline water entering the basin. Density currentgraphs made for various tidal and freshet dischargecombinations brought into focus interesting general physicalfeatures of density currents. Certain other features could be

8.2.

explained with the help of the analysis of prototype datacollected at Cochin. The open questlons which remained areincorporated in the recommendations to follow.

8.3 CONCLUSIONS

Host of the conclusions from the analysis are given inChapter 6 and 7. However an important few are listed below.

8.3.1 DENSITY CURRENT

(1) It was found that the deeper the channel, the more wouldbe the density current. The density current isproportional to a/a. Using the density currentequation (Eq.4.6.) and schematising the net flow throughthe gut as indicated in section 5.1.2., thequantification of density current shows that densitycurrent fraction of the total salt water movement wouldincrease with increase in depth. In the present case,the total density current during the whole south westmonsoon is 3.54 times the tidal filling current. Theratio was 2.79 before deepening and would go up to 4.60and 6.13 aftertotal quantityneglecting the

the future two cases of deepening. Theof salt water entering the basin,effect of accumulation as mentioned in

section 6.4., during the southwest monsoon over a periodof 120 days is estimated to be 11.18x109 m3.

(11) The gut was found to be the control and constraint forthe density current. But the gut has a deeper draughtand large cross sectional area which admits a largequantum of density current partly due to the increasedcombined area of the two channels together inside. Thegut is deeper on the southern side and so salt wedgeadvances on that side first.

8.3.

(III)From data analysis, it was shown that the salt wedge isflushed completely ou~side at a Froude number 0.7 ie,at a net discharge of 13.5 m2/s until 1983. For a deeperchannel, the discharge required is 15.60 m2/s makingthis phenomenon rarer. But if there is no flushing somesalt is left behind during each tide. This accumulatesinside the basin and reduces further density currents.

(iv) It has been found that the freshwater discharge forwhich the density current is maximum, at Cochin is 6m3/s per metre width of the gut. On both sides of thisdischarge the quantity of salt brought in by the densitycurrent would go on decreasing. This has only academiesignificanee as the total salt coming inside because oftidal filling and density currents combined would alwaysdecrease with increasing. freshet discharge.

(v) It was found that some salt is getting trapped in somecorner of the basin and is flushed out at a later stageafter the main wedge has gone out. The location of thetrap could not be discovered (See 6.5.4.)

(vi) Before 1983, Mattanchery channel was wider because ofthe stream moorings there and therefore drew most of thedensity current .So the ratio of salt water enteringthese two channels was 0.67 : 0.33 approximately in theratio of their widths. But af ter deepening and wideningof Ernakulam channel, the situation changed to equalwidth and more depth, changing the pattern of densitycurrents and siltation.

(vii)A final conceptual graph on density current could beprepared based on the above study. (Fig.4.8.) Thispoints out the necessity for further research on thesubject apart from exposing some interesting features.

8.4.

8.3.2 SEDIMENTATION

Only a limited study was made on the actual process ofsedimentation. The various sediment parameters were notevaluated. So a method of narrowing down the range of assumedvalues was employed. The following conclusions were derived.

(i) Deepening the channel would increase the volume of thedensity current and so increase the quantity of sedimentcarried in from the sea by the salt water. It wouldalso increase salt water veloeities which has a reducingtendency on deposition. But the latter fact is beingstated here with a certain amount of caution. Theimportant thing is to get correct values of theveloeities inside the wedge by a two dimensional modelin the vertical.plane. The accuracy of this could affectdeposition computations. Deposition is also proportionalto the area of the channel exposed to silt chargedwater, not only the total quanti ty of water itself. But

deepening can increase retention time, increasingdeposition and reduce time of erosion. Please seeChapter 7.

(ii) Videning the channels does not increase volume ofdensity current as the gut remains the constraint butwould reduce the veloeities inside the wedge,encouraging deposition.

(iii)Roughness of the channel can increase shearreduce sedimentation.

and would

(iv) The values of ~ ,V, Mand Care to be evaluatedcr saccurately as total sedimentation quantity is highlysensitive to these parameters.

..c .l

"/,".. . .,.r:....

.\. \.\

.,..

.,.,

"

" ;cS !: 8..5..,

. I I ...., JC

-=,'",.......;;:

,l/i

'1,'/'.'1.:1 .

.;.'/

·.l,1/

r ..I

. iI .

..

.__

--_ -. -

...reO'a.;.

V'

:IC. , a.:" . Lol •.. ", u.!a.,_

• >-0 >;:z:

rJ'

8. 6

8.4 RECOMHENDATIONS

The recommendations made here are of two types.

(i) To reduce density current and reduce sedimentation whichhowever need further study and cannot be based only onthis desk study.

(ii) Certain studies to be included in the imminentfeasibility study for the Container Transhipmen~Terminal at Cochin.

8.4.1 (i) One recommendation is to streamline the flow in thechannels by training them so that ebb velocities areincreased. Please see Fig. 8.1. The increased dischargewould flush out the wedge from the channel quicker. Ofcourse, the situation at the gut would remain unchanged.But higher velocities in the channel would reduceretention time of the silt wedge, and increase erosion

FI~:8.2. SILT SCREEN

8.7

time, thereby reducingerosion.

deposition and increasing

(ii) Another innovation is the silt screen which was triedout at Botlek harbour in Rotterdam. Please see Fig. 8.2.

The screen would act as a barrier to the bottom portionof the density current and would also prevent mud flowinto the basin. As mentioned earlier, the constraint isthe area of gut. The screen would, in fact, reduce thedepth of the gut. To certain extent this would overcomethe difficulty described in 6.5.6.

(iii)The value of Chezy coefficient could be decreased byincreasing the roughness of the channel. This wouldincrease the shear stress sharply. It should be studiedhow this could be done in practice. Increased shearstress can reduce siltation considerably.

(iv) Further studies should be made on density current andsedimentation on the desk as weIl as in prototype andmodel especially for a deepened situation when therewould be no flushing.

It must be mentioned that determining the technicalfeasibility and economie efficiency of (i), (ii) and (iii)was outside the scope of this desk study.

8.4.2 For the future study on the Transhipment Terminal:

(i) Even among all the data described in chapter 2, we donot have a single reliable silt observation. So theimportant need of the hour is to have reliable data onsilt charge during pre, post and monsoon periods. It isoften documented that southwest monsoon is accompaniedby strong wind and wave action. This has to be checkedup. The hypothesis of origin of silt from sea has been

8. 8

taken for granted. This is to be verified to someextent.

(ii) The phenomenon of flushing was weIl established before1983. This phenomenon after the deepening and wideningin 1983 has to be studied and verified.

(iii)It would be interesting to pin point density andvelocity interfaces. So, as soon as the densityinterface is noticed, more frequent observations alongdepth are to be made until velocity interface isobserved. This would be useful in the study of saltoverflow in highly stratified estuaries.

(iv) The 1980 observations available are of peak dischargeconditions. The data of average conditions would be muchwelcome for quantification over a periode

(v) Careful evaluatian of silt parameters like ~crO' ~crE'V , Hand C is required before any definite quantitativesstatement on the effect of deepening can be given.

(vi) As velocities inside the wedge are important in thequantification of siltation, a 2-0 model in verticalplane should be employed for the study. This is moreimportant for a deeper draught (to accommodate thirdgeneration vessels) when the salt accumulation due tononflushing would be more important.

This study was undertaken for educational purposes. Becauseof the limitations which are inherent to a desk study, it isprimarily exploratory.

LIST OF FIGURES

1.1. General plan for Cochin backwaters showing thevarious rivers

1.2.1.3.1.4.1.5.1.6.1.7.1.8.1.9.1.10.1.11.1.12.

2.1.2.2.2.3.2.4.

3.1.3.2.3.3.3.4.3.5.3.6.3.7.3.8.

4.1.4.2.4.3.4.4.

General plan for the port of CochinPort infrastructureTidal curve at CochinVind rose diagramDirection of maximum wind speedVave rose-heightVave rose-periodBefore 1920Af ter 1930Af ter 1953Af ter 1982

Observations 1968Observations 1975 - 1976General plan for the port of CochinObservations : 1985

Net flow GraphDensity current mechanismVelocity verticals - 1980Density current - 1980Profile of saline wedgeArrested saline wedgeArrested salt wedge on long wierDensity induced return current over short wier

Definition sketchRelation between kiL, F1' F2(F2 - F1) Vs F2Density current graph

4.5.4.6.4.7.4.8.

5.1.5.2.

6.1.

6.2.

6.3.

6.4.

6.5.

6.6.

6.7.

6.8.

6.9.

6.10.

6.11.

6.12.

6.13.

6.14.

(ii)

Variation of freshet with saltFully stratified estuaryVelocity profile in stratified flowComplete qualitative density current graph

Fate of positive wave entering channel at t=OSchematized section of gut

Density current graph, lst tide: 0.75m,2nd tide: 0.30 m, qfr = 8.16 m2/sDensity current graph, lst tide: 0.75m,

22nd tide: 0.30 m, qfr = 6.00 m IsDensity current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 4.08 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 2.33 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 1.00 m2/sDensity current graph, lst tide: 0,75m,2nd tide: 0.15 m, qfr = 8,16 m2/sDensity current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 6.00 m2/sDensity current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 4.08 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 2.33 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 1.00 m2/sDensity current graph, lst tide: 0.50m,2nd tide: 0.30 m, qfr = 8.16 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 6.00 m2/sDensity current graph, lst tide: 0,75m,2nd tide:0.30 m, qfr = 4.08 m2/sDensity current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 2.33 m2/s

6.15.

6.16

6.17.

6.18.

6.19.

6.20

6.21.6.22.6.23.6.24.6.25.6.26.6.27.6.28.6.29.6.30.6.31.6.32.6.33.6.34.

6.35.

6.36.

6.37.

6.38.

(iii)

Density current graph, lst tide: 0,75m,22nd tide: 0.30 m, qfr = 1.00 m Is

Density current graph, lst tide: 0,75m,2nd tide: 0.15 m qfr = 8.16 m2/sDensity current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 6.00 m

2/s

Density current graph, lst tide: 0,75m,2nd tide: 0.30 m, qfr = 4.08 m2/sDensity current graph, lst tide: O,75m,

22nd tide: 0.30 m, qfr = 2.33 m IsDensity current graph, lst tide: 0,75m,

22nd tide: 0.30 m, qfr = 1.00 m Isqfr vs tidal fillingDensity current - 1980, 21-7-80Density current - 1980, 22-7-80Density current - 1980, 23-7-88Density current - 1980, 25-7-80Density current - 1980, 26-7-80Velocity verticals 1980Velocity verticals 1980Velocity verticals 1980Gut and channelsCross section of gutDensity current when draaught is reduced.Longitudinal sectionIncrease in density current with deepening:qf = 8.16r -Increase in density current with deepening:qfr= 6.00Increase in density current with deepening:qfr= 4.08Increase in density current with deepening:qfr = 2.33Increase in density current with deepening:qfr = 1.00

7.1. Size distribution of bed material

Scheme tosiltation

8.2. Silt screen

7.2.7.3.7.4.7.5.7.6.7.7.7.8.

8.1.

Comparison of VsDeposition from saline wedgeFreshet meeting saline wedge

-4= 10 mis-3= 10 mis

p vsp vsLogESilt

<iy}

T , Vs sT , Vs svs log TE' c =200seacharge at gut

mgll

streamline veloci ties to reduce

LIST OF TABLES

1.1. Rainfall data

2.1. Approach channel dredging2.2. Ernakulam channel dredging

6.1. Density current and tidal filling6.2. Density current for deepened situations

7.l.7.2.7.3.7.4.7.5

Dredging dataTypical values of ~crDTypical values of Mand«oand ~: Calibration«oand ~: Verification

~crE

NOTATlONS

a

c

g'h

kiL

1L.1

LwHPPPtq

:h + ~ :Yater depth at any instance:Depth of flow of upper 1ayer:Depth of flow of lower 1ayer:Yidth of channe1:Yidth of estuary:Ve1ocity of propogation of tide:Ve1ocity of propogation of interna1 wave:Initia1 concentration:Bed concentration:Chezy's coefficient:Deposition rate:Erosion rate, Estuary number:Interna1 estuary number:Froude number:Froude numer of upper 1ayer:Froude number of lower 1ayer:Froude numer of fresh water flow:Acce1eration due to gravity:€g : Densimetric g:Mean water depth:Interfacia1 shear stress coefficient:l/a : Dimension1ess 1ength of si11, 1ength of channe1:Length of si11:Length of arrested saline wedge:Length of tida1 wave:Rate of erosion:Probabi1ity that a partic1e sticks to the bed:Pressure at any point:Tida1 prism:ql - q2 : Net flow, flow rate induced by tida1 wave

Ql,q2:Discharge rate of upper and lower 1ayersQ :Density induced exchange flow rateex

(v i.i )

qr :Density induced return flow rateqfr :Freshet discharge rateOs :Salt overflow0' :Salt underflows0fr :Fresh water dischargeRiE :Richardson numberS :SedimentationS. :Total sediment carried insidel.n

t :period till that instantT :Tidal periodu :Velocity of flowu1' u2:Velocity of flow of upper and lower layersUt :rms tidal velocity averaged over profileV :Fall velocitysx :Horizontal coordinate, distance covered by tidal wavey :Vertical coordinateZo :Height of individual tidal wave

"tb

ratio of bot tom shear to:Simmon's ratio, ~interfacial shear

:Ratio of deposition to total sediment carried inside:Ratio of erosion to total sediment carried inside

PI + P22

:Mass density of lighter fluid:Mass density of heavier fluid

(p2 - PI)/lp

P

:Bottom shear stress"t :Critical shear stress for depositioncrD

:Critical shear stress for erosion:Interfacial friction

2n=T:Damping factor:Symbol denoting amplitude of parameter involved

A :Function

"tcrE"t.l.

{Al

n :Vatersurface elevation~. :Interfacial shear of arrested saline wedgel.

REFERENCES

1. DENSITY CURRENTS

1.1 Abraham G., "Reference notes on density currentstransport processes". Lecture notes, IHE,1982-83.

and

1.2 Abraham G., Jong P.D. and Kruiningen F.E.V., "Large scalemixing processes in a partly mixed estuary". Delft Hydraulicscommunication no. 371, 1986.

1.3 Abraham G., Eysink V.D., "Magnitude of interfacial shear inexchange flow". Journalof Hydraulic Research, vol. 9, 1971,no . 2.

1.4 Barr, O.I.D., "Densimetric exchange flow in rectangularchannels". La Houille Blanche, November 1963 and June 1967.

1.5 C.V. & P.R.S., "Specific note no. 441" of 16.9.1957.

1.6 Dixit, J.G., "Lateral change in the mixing characteristics asa result of widening of the Ernakulam channel". InternationalSymposium on new Technology on model testing in HydraulicResearch 24-26, September 1987, India.

1.7 Eysink, V.D., "Sedimentation in harbour basins small densitydifferences may cause serious effects".

1.8 Gole C.V., Vaidyaraman P.P., "Deepening the approach channelto the Port of Cochin". 13th Conference of InternationalAssociation for Hydraulic research, Kyoto, vol. 3, 1969.

1.9 Ippen A.T., "Estuary and coastline Hydro-dynamics." McGraw-Hill Inc., 1966.

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1.10 Joglekar D.V., Gole C.V. and Kuiekar S.N., "Studies insiltation of Cochin Port". XIX International NavigationCongress, Section 11, Communication 3., London, 1957.

1.11 Naik A.S., Kanhare V.N., Vaidyaraman P.P., "Effect ofsalinity on siltation in the Cochin Port". InternationalConference on Coastal and Port Engineering in Developingcountries, Colombo, March 20-26, 1983.

1.12 Partheniades E., "Salinity Intrusion in Estuaries and itseffect on shoaling". River Mechanics vol. 11, Hsieh wen Shen,1966.

1.13 Rama Raju V.S., Udaya Varma, Abraham Pylee, "Hydrographicobservations in the inner harbour of Cochin Port for Harbourdevelopment works". National Institute of Oceanography, 1976.

1.14 Rama Raju V.S., Udaya Varma, Abraham Pylee, "Hydrographiccharacteristics & Tidal Prism at the Cochin Harbour mouth".Indian journalof Marine sciences, vol. 8, June 1979 p.p.78-84.

1.15 Rigter B.P., "Density inducedchannels". Publication no. 83,1970.

return currents in outletYaterlookundig laboratorium,

1.16 Schijf J.B. and Schonfeld J.C., "Theoretical considerationson the motion of salt and fresh water". 1953.

1.17 Simmons H.B., "Some effects of upland discharge on estuarineHydraulics". Hydraulics division of ASCE, 1955.

1.18 Tarapore Z.S., Kanhare V.N. and Naik A.S., "A study ofsiltation at Cochin Port". Seminar or Coastal Engineering atNational Institute of Oceanography, GOA, 23/24, March 1977.

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1.19 Udaya Varma P., Abraham Pylee, Rama Raju V.S., "Tidalinfluence on the seasonal variation in current and salinityaround Yillington island". Mahasagar, Bulletin of NationalInstitute of Oceanography 14(4), 1981, 225-237.

2. SEDIMENTATION

2.1 Allersma E., "Mud in estuaries and along coasts". Publicationno. 270, Yaterloopkundig laboratorium.

2.2 Burt T., "Field settling velocities of estuary muds ",

Proceedings of a workshop on cohesive sediment dynamics withspecial reference to Physical Processes in Estuaries, Tampa,Florida, November 12-14, 1984.

2.3 Gole e.v., Tarapore Z.S. and Brahme S.B., "Prediction ofsiltation in harbour basins and channels". Proceedings of14th Conference of IAHR, Paris, 1971, vol. 4, paper 05, p.p.33-40.

2.4 Kranck, "Settling behaviour of cohesive sediments".Proceedings of a workshop on cohesive sediment dynamics withspecial reference to Physical Processes in Estuaries, Tampa,Florida, November 12-14, 1984.

2.5 Krone R.B., "Flume studies of the transport of sediment inestuarial shoaling processes Final report". HydraulicEngineering laboratory and Sanitary Engineering ResearchLaboratory, University of California, Berkely, 1962.

2.6 Krone R.B., "The significance of aggregate properties totransport processes". Proceedings of a workshop on cohesivesediment dynamics with special reference to PhysicalProcesses in Estuaries, Tampa, Florida, November 12-14, 1984.

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2.7 Leussen, V.V., "Aggregation of particles, settling velocityof mud flocs". International Symposium on Physical Processesin Estuaries, The Netherlands, September 9-12, 1986.

2.8 Mehta A.J. and Partheniades E., "An investigation of thedepositional properties of flocculated fine sediments".Journalof Hydraulic Research, International Association forHydraulic Research, vol. 13, no. 14, 1975, p.p. 361-381.

2.9 Mehta A.J., "Characterization of cohesive sediment propertiesand transport processes in estuaries". Proceedings of aVorkshop on Cohesive Sediment Dynamics with Special Referenceto Physical Processes in Estuaries Tampa, Florida, November12-14, 1984.

2.10 Mehta A.J., "Cohesive Sediment in Estuarine Environment". AGUChapman Conference, Bahia Blanca, Argentina, June 1988.

2.11 Parker V.R. "On the observation of Cohesive sedimentbehaviour for Engineering purposes". Proceedings of aVorkshop on Cohesive Sediment Dynamics with Special Referenceto Physical Processes in Estuaries Tampa, Florida, November12-14, 1984.

2.12 Partheniades, E. "A fundamental frame work for Cohesivesediment Dynamics". Proceedings of a Vorkhop on CohesiveSediment Dynamics with Special Reference to PhysicalProcesses in Estuaries Tampa, Florida, November 12-14, 1984.

2.13 Partheniades, E. "Erosion and deposition of Cohesivesediments". River Mechanics Vol 11, Hsieh wen Shen, 1971.

2.14 Partheniades, E. Cohesive sediment Transport Mechanics andEstuarine Sedimentation - Lecture notes.

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2.15 Ranjan Ariathurai and Kandiah Aru1anandan, "Erosion rates ofCohesive soi1s". Journalof the Hydrau1ic Division, ASCE,February, 1978 Vol 104, pp 279-283.

2.16 5i11s" G.C., "The transition from sediment suspension tosettling bed."

2.17 Tetsuya Kusuda, Teruyuki Umita and Youichi Awaya, "Erosiona1process of fine Cohesive sediments" Memoirs of the facu1ty ofEngineering Kyushu University, Vol 12, no. 4, December 1982.

2.18 Trimbak H. Parchure and Ashish H. Hehta, "Eros ion of softCohesive Sediment Deposits". Journalof Hydrau1ic EngineeringASCE, Vol 111, no. 10, October 1985.

2.19 Villiams, D.J.A., "Rheo10gy of cohesive suspensions".Proceedings of a Yorkshop on Cohesive Sediment Dynamics withSpecial Reference to Physica1 Processes in Estuaries Tampa,Florida, November 12-14, 1984.

ACKNOVLEDGEMENTS

At the onset, the author places on record his gratitude to Prof.Ir. H. Velsink of Delft Technical University/Port AdvisoryServices who was the driving force behind the entire programme.He is grateful in no small measure to his guide Dr.Ir. GerAbraham (Delft Hydraulics) who introduced the magie of densitycurrents to him and to Ir. Kees Kuijper (Delft Hydraulics) whohelped with fundamentals of sedimentation. At this moment, theauthor recalls his memorabie days at the Estuaries and SeasDivision in zout-zoet-hal. He cannot but remember with gratefulsatisfaction all his E-Z colleagues under the leadership of Ir.Ad van Os who made Delft Hydraulics a home away from home forhim.

Many thanks are due to Ir. J.H.C. Viersma of IHE who madeperfect arrangements at the Institute. The author is aware thathe owes a lot to the Directorate General of InternationalCooperation, Ministry of foreign affairs, Government ofNetherlands who made all this financially possible. The authorremembers with gratitude the organisations NEDECO and PortAdvisary Services B.V., Netherlands for making his dream of thisresearch a reality. This moment is utilised to thank DelftHydraulics for making available the excellent facilities of thegreat institution.

Hr A. Ananthakrishnan of Ministry of Surface Transport,Government of India had been a souree of inspiration to theauthor through out his professional career; especially duringthe study at IHE. The author expresses his gratitude to him. Theinterest shown by Dr P.P. Vaidyaraman and Hr J.G. Dixit ofCentral Vater and Power Research Station, Pune to send valuabledata, is gratefully acknowledged. The author also acknowledgesthe official support provided by Hr H.K. Hanoharan and Hr H.H.John, Cochin Port Trust.

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Above all, the author thanks from the bottom of his heartDr Kaniben and little Swapna, his wife and daughter who helpedand spurred him on to the final destination.

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