CMFRI
bulletin 31
APRIL 1984
Coastal Zone Management
MUDBANKS O F KERALA C O A S T
CENTRAL MARINE FISHERIES RESEARCH INSTITUTE (Indian Council of Agricultural Research)
P.B. No. 1912, Cochin 682 018, India
CMFRI
bulletin 31
APRIL 1 9 8 4
Coastal Zone Management
MUDBANKS O F KERALA C O A S T
CENTRAL MARINE FISHERIES RESEARCH INSTITUTE (Indian Council of Agricultural Research)
P.B. No. 1912, Cochin 682 018, India
Bulletins are issued periodically by Central Marine Fisheries Research Institute to interpret current knowledge in the various fields of research on marine
fisheries and allied subjects in India Abbreviation : Bull. cent. mar. Fish. Res. Inst.
Copyright reserved
Published by E. G. SILAS
Director
Central Marine Fisheries Research Institute P. B. No- 1912, Cochin 682 018
Edited by
K. N. KRISHNA KARTHA Scientist
Library & Documentation
Central Marine Fisheries Research Institute P.B. No. 1912, Cochin 682 018.
This issue, originally taken up for publication in the first quarter of 1982, and assigned number accordingly, was however delayed due to unavoidable reasons — Editor
Restricted circulation
C O N T E N T S
CHAPTER ONE Introduction CHAPTER TWO
Mudbanks of Kerala-Karnataka
—Need for integrated study CHAPTER THREE
Hypotheses on mudbanks CHAPTER FOUR
Source of mud at Alleppey mudbank:
Mud cone and the message it conveys CHAPTER FIVE
Mud of the mudbank: Its distribution and physical and chemical characteristics CHAPTER SIX
Ecology of mudbanks — Hydrography CHAPTER SEVEN
Ecology of mudbanks — Phytoplankton productivity in Alleppey mudbank
CHAPTER EIGHT
Ecology of mudbanks — Zooplankton CHAPTER NINE
Ecology of mudbanks — Benthos CHAPTER TEN
Ecology of mudbanks — The current system of Alleppey mudbank
CHAPTER ELEVEN
Fish and fisheries of the mudbanks
CHAPTER TWELVE References
E. G. Silas
A. V. S. Murthy, D. S. Rao, A. Regunathan, C. P. Gopinathan and K. J. Mathew
C. P. Gopinathan, A. Regunathan,
D. S. Rao, K. J' Mathew and A. V. S. Murty
18
D. S. Rao, A. Regunathan, K. J- Mathew,
C. P. Gopinathan and A. V- S. Murty 2 1
D. S. Rao, K. J . Mathew, C P. Gopinathan,
A. Regunathan and A. V. S. Murty 2 5
P. V. R. Nair, C. P. Gopinathan, V. K. Balachandran, K. J. Mathew,
A. Regunathan, D. S. Rao and A. V. S. Murty 2 8
K. J. Mathew, C. P. Gopinathan,
A. Regunathan, D. S. Rao and A. V. S. Murty 3 5
A. Regunathan. C. P. Gopinathan, K. J. Mathew,
D. S. Rao and A. V. S, Murty 4 0
K. J. Mathew, A. Ragunathan, C. P. Gopinathan, D. S. Rao and
A. V. S. Murty 4 6
A. Regunafhan. K. J . Mathew, D. S. Rao, C. P, Gopinathan, N. Surendranatha Kurup and
A. V. S. Murty gQ
72
P L A T E S
1 Sir Robert C. Bristow Frontispiece
2 Fury of the sea 4 3 Mudbank at Purakkad 4 4 Harvest from mudbank 4 5 Satellite imagery of the southwest
cost of India showing the Alleppey
mudbank area 6 6 Mud cones 20 7 Mud cones 21 8 Flurry at the mudbank 64
9 Fish landings 65
VI
/ \
'\ IF O R E W O R D
The southwest coast, bearing the brunt of the S. W. monsoon, is subject to various dynamic seasonal changes. More than a 100 km of the coastline is affected by serious erosion during monsoon months, while in some areas accretion takes place.
The continental shelf waters are highly productive and sustain a large pelagic fishery, consisting mainly of sardines, mackerel and anchovy. The neritic bottom conditions are also subject to major changes, particular!/
through the process of upwelling, during the monsoon and immediate post-monsoon periods. A considerable amount of river runoff also enters the sea to add to the nutrient load in the inshore waters. In addition to these, a very special phenomena occurs along
parts of the southwest coast, particularly along Kerala coast, which is, in simple terms, known as ''mudbanks" or Chakara. This phenomenon, taking place in the coastal zone, has been known from time immemorial, but, even to-date, we have no convincing evidence as to the origin and nature of dissipation of the mudbanks of Kerala, particularly those
found between Ponnani and Quilon. The phenomenon is all the more important as it also enables a thrilling fishery to develop in the coastal area where the mudbank appears during the height of the monsoon period.
The waters of tha mudbank remain practically an undisturbed sheet, while turbulent conditions prevail along its outer fringes, a situation brought about by the colloidal suspension of fine silt or clay particles in the water column. This calmness facilitates easy operation of fishing canoes, which are trans-
ported from fishing villages several kilometres north and south of the mudbank. Often several influxes of prawn or pelagic fishes, such as mackeral and sardine, from the adjacent open sea into the mudbank area occur, when bumper landings take place. When market conditions are good, this fishery is a boon for several thousands of coastal fishermen families, as otherwise operation of canoes in heavy surf-ridden beaches is practically impossible during the monsoon months.
With meagre facilities the late Sir Robert
C. Bristow had made a fairly exhaustive study
of the mudbank, and his work posed many
serious questions which remain unanswered
to this day. Besides, we are now aware that
mudbanks of different types occur, some, in
the estuarine area, as a result of heavy flood
waters and silt, which may be transitory,
while the type of mudbank seen south of
Alleppey sustains itself for weeks. Consider-
ing the importance of this coastal phenomena
to the fisheries of this region, a baseline
and monitoring study was initiated by me in
1971 to study the mudbanks mainly at
Ambalapuzha-Purakkad-Thottappallyarea south
of Alleppey, A multi-disciplinary team
consisting of fishery biologists, chemists
oceanographers, etc., was formed to study
the physical and biological characteristics
of mudbank south of Alleppey, in order to
have an objective idea about their origin,
life and nature of dissipation. This report
embodies some of the essential aspects of
work carried out during these studies. I
wish to place on record my appreciation to
the team headed by Dr. A. V. S. Murty,
Senior Scientist, who have under great strains and sacrifice carried out some very useful observations on the mudbank of Kerala, particularly the one south of Alleppey.
In no way could this report be considered an end to the problem of the study of mudbanks; on the other hand, we should consider this as a basis for a national effort for studying this coastal phenomena occurring along Kerala and Karnataka coasts.
It is hoped that such a national multi- disciplinary effort would go a long way in our better understanding of this problem.
This should also find an important place in
the national context of coastal zone manage- ment of our country.
The frontispiece to this publication shows the picture of Sir Robert C. Bristow, who
made a special study on mudbanks as well as on problems of coastal erosion and accr- etion, enabling him to propound some views and hypotheses on this. These have been, no
doubt, thought provoking and have stimulated a considerable amount of work, and we feel that it is only fitting that this volume be dedicated to the memory of the late Sir Robert C. Bristow as a token of our esteem.
E. G. SILAS DIRECTOR Central Marine Fisheries
Research Institute
Cochin-682018
INTRODUCTION
ONE
THE M U D B A N K S , the f o r m a t i o n of w h i c h is a curious phenomenon reported hitherto from o n l y the southwest coast of India, appear over restricted areas on t h e Kerala- south Karnataka coast during the inclement S. W. m o n s o o n as bodies of calm water, where the w a v e action the coastal waters at t h i s t i m e abounds w i t h is almost absent, largely due to a f i n e mud in a state o f suspen- sion. On a squally, surf-beaten coast, these areas f a c i l i t a t e easy f i s h i n g operations of country crafts, w h i c h o t h e r w i s e i d l e away during this haish w e a t h e r . For this reason it gained much popularity, particularly of late, among the coastal people.
Reports on mudbanks date back t o the early 18th century. But a serious scientific study on them was not i n i t i a t e d u n t i l nineteen-thirties, w h e n Sir Robert B r i s t o w t o o k it up in c o n n e c t i o n w i t h his port man- agement. His study, h o w e v e r , stimulated further interest, a n d , because of its c o n t r i - bution o f t e n t o successful fisheries, w i t h spectacular catches of prawns and f i s h , parti- cularly the shoaling f o r m s , the mudbank has attracted increasing a t t e n t i o n f r o m fishery scientists.
Central Marine Fisheries Research Institute, being the chief b o d y engaged in fisheries research, n a t u r a l l y g o t itself s o o n
i n v o l v e d in a major programme o f i n v e s t i - g a t i o n , assigning a team of scientists o f d i f f e r e n t disciplines t o study the p h e n o m e n o n in d e t a i l . The f o l l o w i n g report embodies the results of studies on some of the diverse characters of the mudbanks in general, but w i t h particular emphasis t o the Alleppey mudbank, w h i c h is by far the most important of a l l . Problems pertaining t o physical, chemical and b i o l o g i c a l aspects have been dealt w i t h in detail as far as possible, in
a d d i t i o n t o c r i t i c a l l y r e v i e w i n g the earlier d o c u m e n t e d i n f o r m a t i o n . In the l i g h t o f these studies it has been possible t o give an explanation of the o r i g i n , maintenance and d i s s o l u t i o n of the mudbanks. H o w e v e r , it has t o be confessed w i t h candour that these studies are by no means conclusive and, needless t o say, a l o t of multidisciplinary e f f o r t s t i l l remains t o be made, preferably as a part of a national programme of manage^
ment of coastal zones.
The authors w i s h t o express their thanks t o the authorities o f the Cochin and the A l l e p p e y p o r t s , especially t o Capt. Rajan, t h e t h e n Port Officer, A l l e p p e y , for f u r n i s h i n g valuable i n f o r m a t i o n on the Alleppey m u d - bank. Their thanks are also due t o the Depart- ment of Chemistry, A g r i c u l t u r a l University, Mannuthy, for p e r m i t t i n g them to carry out the chemical and mineral analyses at their labor- atory. Shri R. Vasanthakumar, Smt. C. M . A l l i k u n j u , Shri P. K. Swamy, Shri P. M . Aboobaker and Shri N. P. Kunhikrishnan deserve special thanks for their helps rendered in the chemical analysis o f t h e samples. Shri K. L. K. Kesavan has helped in the preparation o f many of the d r a w i n g s . Shri P. Narayanan o f Cochin Port Trust deserves their special thanks for procuring the p h o t o g r a p h of Sir R. C. B r i s t o w , w h i c h is reproduced here on the frontispiece. The Cochin Archives has made available some of the early literature relevant t o the present w o r k . During the f i e l d investigations, a g o o d deal of local cooper- ation and g o o d w i l l was f o r t h c o m i n g w h i c h greatly f a c i l i t a t e d w o r k , o f t e n under adverse c o n d i t i o n s . Special mention s h o u l d be made here of the help rendered by Shri Sivanandan of Ambalapuzha, Shri Devadattan of Purakkad, and t h e c r e w o f R. V. Cadalmin-I of the' Institute.
TWO
M U D B A N K S OF K E R A L A - K A R N A T A K A — N E E D F O R A N I N T E G R A T E D S T U D Y
E. G. SILAS
Central Marine Fisheries Research Institute, Cochin INTRODUCTION
We have documented reports of the mudbanks along the south-west coast between Mangalore and Quilon for well over three hundred years. However, sequential record of their occurrence in time and space is wanting. This phenomenon in Kerala, typi- cally as occurs south of Alleppey (^Fig. 1), is known as Chakara. or Santhakara ("quiet shores"). During the last two decades the presence or absence of the Chakara pheno- menon has attracted considerable attention of the fisheries sector, resulting in a number of useful studies on mudbanks covering specific aspects. Yet we are far from having precise answers to the problem of the mode of its origin, establishment, stability and dissip- ation, except propounding some hypotheses and suggestions based on physico-chemical and biological data.
The recent past has also seen large-scale erosion along long stretches of this coast and consequent serious impairment of the life of those living in the coastal zone. Since 1973, the mudbank has not developed to the extent anticipated, to sustain any major fishing activity along the coast. This, com- bined with extensive erosion, has created serious socio-economic problems in the coastal sector. Of a good mudbank season, the primary beneficiary is the artisanal fisher- man, who is able to operate his canoe from the calm waters of the mudbank and often land heavy catches of shoals which enter the area. Owing to heavy surf along other parts of the coast, fishing in the artisanal sector is generally at a standstill during the monsoon season. As such, the formation of mudbank
is eagerly awaited as it portends good fishing, and hundreds of canoes are transported by road to the sites of the mudbanks from villages even 50 to 60 km north and south.
These multifarious factors focus attention on an imperative need for developing an integrated multidisciplinary programme to study the mechanisms of mudbank formation, its life and dissipation and its impact on the coastal zone.
HISTORICAL RESUME
in the t w o volumes entitled "History of Mud Bank" Bristow (1938) refers to the early works on mudbanks, which are mainly narrative in nature. He also opines on the various possibilities of the mode of formation of the mudbanks.
Probably, the first mention of the mud- banks along the southwest coast in recorded history is a mention as early as 1678 in Pinkerton's "Collections of voyages and travels," appearing in the Administration Report of 1860 of Travancore, In his book,
" A New History of the East India," Capt.
Cope (1755) spoke of the Alleppey mudbank ("mud bay" as he calls), which, he says, is a place that few can parallel in the world.
Crawford (1860) may be credited with
attempting the first possible explanation of
the source of the Alleppey mudbank. Based
on personal observations of mud cones on
the beach and on roads of Alleppey, Crawford
found cause for linking them with the back-
waters and rivers. His observations of bursting
of mud and water, during the widening of the
Alleppey canal, and his attempt t o sound the
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CMFRI BULLETIN 33
' L i n u s ' of Chengannur River, strengthened his opinion that subterranean mud sustained t h e Alleppey mudbank. The observations o f Logam (1882) further help us t o note the importance of subterranean mud deposits. He f o u n d
" d e e p p o t - h o l e s in the lake east of A l l e p p e y and w i t h a rise of 4 ' t o 6', as occured in the f l o o d s of 1882, it can be easily b e l i e v e d that the enormous pressure thus caused w o u l d force relief w a y s for itself b e l o w the coastline through s o f t m u d s . "
King ( 1 8 8 1 ) , in a report on " C o n s i d e r - ations on the smooth water anchorage of mudbanks of Narakkal and Alleppey on the Travancore c o a s t , " discussed the m i g r a t i o n and f o r m a t i o n o f the mudbanks b e t w e e n Alleppey and Purakkad and between Cochin and Narakkal. A s i g i n i f i c a n t observation was that of Rhode (1886;//?.• B r i s t o w 1938), w h o spoke of f l u i d mud existing b e l o w Alleppey, and thereby p o s t u l a t i n g that the mudbank at Alleppey increases and diminishes as the level of the inland water rises and f a l l s , as w a s observable during the 1882 f l o o d s . Opining on the same lines Drury (1906) was of the v i e w that, in the absence of a natural o u t l e t for the vast accumulation of waters w h i c h are poured d o w n f r o m the various m o u n t a i n streams i n t o the basin of the backwater, nearer than 36 miles on either side, it is not improbable that there exists a subterranean channel c o m m u n i - cation w i t h the sea from t h e backwater, through w h i c h large quantity of mud is carried o f f and t h r o w n up again by t h e sea in the from of a bank. A c c o r d i n g t o Lake (1889), " i t is t o the observation of Mr. Rhode and of his predecessor, Mr C r a w f o r d , that w e o w e most of our k n o w l e d g e of the A l l e p p e y mudbank, and there is very little t o add t o w h a t they have already r e c o r d e d . " He also reported on the occurrence of mud cones in his report on the Alleppey mudbank.
For more on these and f o r other references t o the mudbfcnks, I w o u l d direct the reader t o the comprehensive treatise of Bristow (1938) on the " H i s t o r y of the M u d b a n k s " , wherein he has also added his exhaustive observations on the f o r m a t i o n , maintenance and movements of the mudbanks at Alleppey and Narakkal.
The mudbank at Narakkal plays an important role in the silting o f the Cochin Harbour c h a - nnel and was the cause for a Special report by
Du Cane et a i . (1938). This report also does not favour the v i e w that an increase in the water level in the lake w o u l d result in mud being pushed up in the adjacent coastal area*
due t o the insufficient pressure (2 Ibs/sq. inch) t h a t even a 5 ' rise in water w o u l d create. Nor
is t h e consistency of the mud of the mudbank t h e same as t h a t f o u n d in t h e lake, t h e latter
having a high percentage of carbon and a l o t of vegetable debris. However, I feel that a critical study of t h i s is necessary, w h i c h w i l l also necessitate borings and s o i l studies at d i f f e r e n t depths in the lake, intervening land area and the Purakkad inshore waters.
The post-war period saw a renewed inter- est in t h e studies on the coastal ecosystem.
This has l e d t o more specialised i n v e s t i g a t i o n s on t h e mudbank ecosystem as summarised b e l o w : Seshappa (1953) and Seshappa and Jayaraman (1956) have s t u d i e d the phosphate content of the mudbank at Calicut and noticed higher phosphate concentrations. Ramasastry and M y r l a n d (1959) stated that the forma- t i o n of the mudbank is associated w i t h u p w e l l i n g and divergence near the b o t t o m between 20 and 30 m along the coastal line, w h i c h produce vertical acceleration, w i t h resultant l i f t i n g of the b o t t o m waters; the l i f t e d b o t t o m water carries along w i t h it the fine mud of the b o t t o m . Nair et al (1966) have studied the mud deposited on the sandy beaches of the Vypeen island near Cochin after a storm, for its physical and chemical properties, in order t o understand the source and mechanism of mud d e p o s i t i o n . A comparative study of these sediments w i t h that obtained f r o m o f f s h o r e samples has also been made. They came t o the c o n c l u s i o n that the m u d d e p o s i t e d on the beach was f r o m the nearshore areas, as it was composed of dredged material transported n o r t h w a r d from
Ernakulam channel. Varadachari (1966) has discussed the part played by the estuaries and mudbanks o f Kerala coast on s h o r e - l i n e c p r i f i g u r a t i o n . Varadachari and Murty (1966) have made some observations on a temporary mud flat that appeared b e t w e e n C o c h i n harbour entrance channel and Elamkunnapuzha, d u r i n g a storm in December 1965. Damodaran and Hridayanathan (1966) suggest that l o w e r i n g of surface salinity and a f l o c c u l a t i o n effect caused by t h e same keeps t h e mud in suspension. Rao (1967) has given an account
F U R Y O F T H E S E A
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PLATE II. Heavy monsoon seas and coastal erosion at Chellanam and adjacent areas on the
Alleppey-Cochin coast.
M U D B A N K A T P U R A K K A D
PLATE I I I . A: Calm waters of the mudbank; B: Boat-seine, Thanguvala, operation; C: Awaiting
the catch; D: Ice packing of fish before loading on cycles; E: Catch of Lesser
sardines; and F: Heavy catch of Parapenaeopsis stylifera at the Purakad mudbank.
H A R V E S T F R O M M U D B A N K
PLATE IV. A. Oilsardine; B: a mixed catch; C: prawns spread out for drying; D: Shelling of
prawns, after drying; E: a mixed catch of prawns, crabs and fishes; and F: Fishvans
at Purakad waiting for transporting prawn and fish catch from mudbank.
of the fishery aspects of the mudbank at A l l e p p e y w i t h some considerations on the physico chemical features. Dora et al (1968) carried out i n v e s t i g a t i o n s on the texture of Narakkal mudbank sediments.
Varma and Kurup (1969) opined that the formation of the mudbank is t h e result of the interaction between the inshore and offshore transport of sediments in suspension, the former by waves and the latter by r i p - f l o w s . According to Kurup ( 1 9 7 2 ) , the c o n v e r g i n g l i t t o r a l currents in the inshore waters of the Kerala coast have influence on the s h o r e -
line changes along the coast and play an important role on the f o r m a t i o n o f t h e mud- bank s o u t h of A l l e p p e y . Iyer and M o n i (1972) evaluated the effects of mudbanks on the s h o r e - l i n e stability. Gopinathan and Qasim (1974) have investigated the f o r m a t i o n and characteristics o f the Alleppey mudbank.
The organic carbon of the mudbank sediments of Alleppey have been studied by Jacob and Qasim ( 1 9 7 4 ) . T w o recent fairly exhaustive works are; first, a detailed i n v e s t i g a t i o n o n the meiobenthos and macrobenthos of the mudbanks on the s o u t h - w e s t coast of India by Damodaran ( 1 9 7 3 ) and, s e c o n d , a s t u d y of the physical aspects of the mudbank i n c l u d i n g the texture of sediments, by Kurup ( 1 9 7 7 ) . M a t h e w et al (1977,) have studied the diurnal variations in the d i s t r i b u t i o n of zooplankton in relation t o currents and other e c o l o g i c a l parameters of the mudbank of A l l e p p e y . Recently, McPherson and Kurup (1981,) postulated a mathematical model t o explain the w a v e damping at the mudbank.
I n f o r m a t i o n was gathered from several sources, including statements by inhabitants in the f i s h i n g v i l l a g e . A fisherman (70 years o l d ) of Punnapra stated that he had never seen mud cones at Punnapra area in his l i f e - t i m e ; but he remembered that there was a w e l l -
f o r m e d mudbank at north Punnapra during 1950. A n o t h e r person stated t h a t , in the late f i f t i e s , the seat of the mudbank was at south
Punnapra w i t h its southern end at Nirkunnam, and t h i s was so u n t i l 1968. Then it shifted t o Valanjavazhi in 1969 and Nirkunnam and Ayyancoil became respectively t h e northern and southern ends of t h e mudbank. Local fishermen also stated that mud cones were observed on the land at Nirkunnam in 1969.
In 1971 and 1972 the northern periphery of the mudbank w a s observed at Kakkazham, w h e r e the mud cones were observed in 1972 on the beach and in the i n t e r - t i d a l zone. In 1973, there was no proper mudbank formation at Karoor-Ayyankoil, and the s i t u a t i o n has re- mained so in subsequent years, w h e n only ' i n c i p i e n t ' mudbanks or 'evanescent' mudbanks have occurred. Thus Purakkad was the l o c a t i o n of mudbank in 1974 and 1975. In 1976 it m o v e d further s o u t h t o appear b e t w e e n Pura- kkad and Thottappally. Since 1976, until the mudbank season in 1980, the mudbank a p p e - ared at T h o t t a p p a l l y , n o r t h of the s p i l l w a y .
In 1981 t w o mudbanks w e r e observed in the A l l e p p e y r e g i o n , one at Punnapra and the other at Thottappally-Pallana r e g i o n . However, all the post-1973 mudbanks south of Alleppey have been very evanescent ones of not any great consequence.
TYPES OF M U D B A N K S
Studies thus far carried out indicate that the mudbank c o u l d result from d i f f e r e n t f a c t - ors, such a s :
a) e r o s i o n , accretion and transport of sediments,
b) t r a n s p o r t a t i o n of sediments through u p w e l l i n g or currents in the coastal waters,
c) transport of sediments t o the river m o u t h ,
d) f o r m a t i o n of m u d cones due t o pressure in the lake site ; and
e) dredged sludge w h i c h is dumped into the inshore sea.
The need of the hour is a critical study of the various types of mudbanks and the mecha- nisms w h i c h are responsible for them under different situations.
Remote sensing and satellite imagery technique for studying mudbanks
It is necessary t o o b t a i n synoptic pictures of the river discharges in the inshore waters and the i n d i c a t i o n o f f o r m a t i o n or b u i l d up of sediments t o f o r m mudbank, and the related features b e t w e e n Q u i l o n and Mangalore. The manpower and facilities required for m o n i t - oring such a long stretch of coast w i l l be
tremendous and, as such, the time has come for techniques such as remote sensing and s a t e l l i t e imagery t o be taken advantage of t o record t h e events synoptically. This w i l l also facilitate m o n i t o r i n g the water shed of the rivers t o assess the run off and t h e amount of i n f l o w in the inshore waters. Because of t h e m u l t i f a r i o u s uses to w h i c h river waters are presently put it is only a short time before major imbalances may d e v e l o p in t h e coastal regime, affecting its f e r t i l i t y as w e l l as the other associated natural phenomena. Fortuna- tely, intensive m o n i t o r i n g systems can be d e v e l o p e d and t o d a y we are aware of the magnitude of the problems and h o w t o a p p r o - ach the same.
Some priorities for consideration in future inte- grated investigations on mudbanl(
1. The s t a b i l i s a t i o n of the coastal track by use of appropriate vegetation is an area w h i c h needs i n v e s t i g a t i o n s . This is necessary on account o f the considerable erosion along l o n g stretches o f the coast. The importance of rehabilitation of mangrove vegetation as w e l l as other plant c o m - munity for stabilizing the beaches and the
s u b l i t t o r a l s h o u l d be g i v e n priority.
2. The f l o c c u l a t i o n of cohesive sediments in suspension and the a g g l o m e r a t i o n of p a r t i , cles added by b i o l o g i c a l organisms and the subsequent rate of s e d i m e n t a t i o n , is area w h i c h is yet t o be u n d e r s t o o d . S i m i l - arly the i n t e r - a c t i o n between various b i o l o g i c a l and physical processes i n v o l v e d and accretion and f i n e - g r a i n sediments in the areas adjacent to the mudbanks is practically u n k n o w n .
3. To-date w e have no idea o f the role of epiphytic m i c r o f l o r a and d i a t o m s w h i c h produce mucus and thereby accretion of intertidal mud d e p o s i t i o n .
4. We k n o w l i t t l e about a c t i o n of inverte- brates on sediment d e p o s i t i o n .
5. Normally, suspended sedimentary material should be a repository of trace metals as w e l l as heavy metals. We have practically no i n f o r m a t i o n as t o the extent that m u d - banks c o n t r i b u t e t o w a r d s this.
6. The role o f m i c r o - b i o t a in organic d e g r a - dation and process of recycling is yet another area where i n f o r m a t i o n is lacking.
7. We have no i n f o r m a t i o n on sediment o x y - gen demand and the rate of oxygen t r a n s - ference in the mudbank.
8. The " c r i t i c a l l i m i t " or " c r i t i c a l d e p t h " f o r d e f i n i n g the boundary based on the s u s - pended or s i l t d i s i r i b u t i o n in t h e mudbank has not been properly d e f i n e d . This is important as the outer boundary may be o s c i l l a t i n g , depending on the load of suspended matter. Our present p r o j e c t i o n s in t h i s are arbitrary.
9. The question may be asked as t o whether w e have any idea of the mudbank s e d i - ments as c o n c e n t r a t i n g mechanisms of organic and inorganic materials. The answer is n o ! S i m i l a r l y , w e lack in i n f o r m - ation on entrainment, d e p o s i t i o n and transport of fine grain sediments in the mudbank.
10. Is there a w a y of estimating annual/sea- sonal total budget of substances of the mudbank, w h i c h separate f r o m mud t o the water? Particularly suspended materials and trace metals? The answer at present is no.
1 1 . A b s o l u t e l y no i n f o r m a t i o n is available from the mudbank studies on the b i o - g e o ~ chemical cycles taking place there.
12. We have no i n f o r m a t i o n on the role of microbial metabolism in t h e m u d sedi- ments and the role it may play in m o b i l i - s a t i o n of phosphorus.
13. The optimum/maximum of trace metals in this natural ecosystem of mudbank is s t i l l u n k n o w n . There is hardly any i n f o r m - ation to assess trace metal uptake in sediments and suspended metals—to un- derstand the modus operandi—whether it is through p h y s i c o - c h e m i c a l absorption or t h r o u g h physical accumulation of metal enrichment of particulate matter. Or t h r o u g h b i o l o g i c a l uptake.
14. There is a lack of information on b i o l o g i - cal uptake; the role of bacterial p o p u l a t i o n in this process and the m i n e r a l i s a t i o n of MUDBANKS
algal matter by bacteria are practically u n k n o w n .
15. The earlier w o r k s clearly indicate that nematodes c o n s t i t u t e n u m e r i c a l l y t h e most important component of the meiofauna o f t h e mudbanks. However, w e have no i n f o r m a t i o n on the nematodes-bacteria i n t e r a c t i o n . The burying and f e e d i n g a c t i - v i t y of nematodes may help in i m p r o v i n g exchange o f m e t a b o l i t e s and other essen- t i a l nutrients f o r bacteria to maintain the latter at the p o i n t o f maximum g r o w t h . Nematodes also assist in the process o f b i o - t u r b i n a t i o n . A c c o r d i n g t o Piatt and W a r w i c h (1980), nematodes are primary consumers of f o o d f o r higher organisms and t h e y play a v i t a l subsidiary role in organic d e c o m p o s i t i o n and in m o d i f y i n g the physical stability o f sediments. More w o r k on the nematodes in the mudbank area is necessary.
16. Often large quantities of benthic animals, such as t u b e - w e l l i n g polychaetes and bivalves, are f o u n d accumulated in the intertidal area w h e n the mudbank exists.
The causative factors for their displace- ment from natural beds in the mudbank area needs study.
17. Examination of core samples f r o m the mudbank and adjacent areas is necessary to understand as t o h o w l o n g this p h e n o - menon in the A l l e p p e y region has been in existence.
In the f o l l o w i n g reports stress has been made t o study some aspects of the physico- chemical and b i o l o g i c a l aspects of the m u d - bank. It is h o p e d that t h i s w i l l stimulate more
intensive studies o f an integrated nature in future by c o l l a b o r a t i o n and c o o r d i n a t i o n of
w o r k w i t h other interested agencies.
Coastal zone management—need for a national policy
The mudbank is a phenomenon of the coastal zone. The management problems c o n n e c t e d w i t h it during a year of successful mudbank f o r m a t i o n , particularly b e t w e e n Quilon and Cochin, are up t o n o w tackled on an ad~hoc or temporary basic. There is an imperative need for a national coastal m a n a - gement strategy to be d e v e l o p e d t o l o o k c o n s t r u c t i v e l y at these and other phenomena and shape public policy. A large share of t h i s responsibility w i l l rest w i t h the Department of Environment, Government of India, w h i c h s h o u l d develop a strategy, that w o u l d also improve the q u a l i t y o f l i f e of the people i n v o l - ved in various a c t i v i t i e s along the coastal zone. This w o u l d i n v o l v e also a j u d i c i o u s development o f aquaculture practices in the sea and in the adjacent inundated brackish- water areas; the interest o f the artisanal fisher- men and fisheries; the development of harbour and other infrastructure; the proper m a n a g e - ment o f the mangrove eco-systems; and the p r o t e c t i o n and s a f e t y of the estuarine areas f r o m industrial p o l l u t i o n ; besides m o n i t o r i n g o f human interference on the rivers and water- sheds, w h i c h eventually upsets run o f f i n t o the sea and thereby affects d e l e t e r i o u s l y the coastal eco-systems, problems o f erosion and accretion, and special phenomena such as mudbanks and their importance in i n f l u e n c i n g the coastal zone management.
It is hoped that p r i o r i t y w i l l be g i v e n t o e v o l v i n g a coastal zone management p o l i c y for the c o u n t r y taking also into account an integrated approach of demographic and environmental problems o f the coastal zone.
No d o u b t , the Department of Environment, Government of I n d i a , has to play a nodal f u n c t i o n in close liaison w i t h the concerned maritime States and U n i o n Territories. W e hope that this Report w i l l create an awareness in this d i r e c t i o n and stimulate p o s i t i v e a c t i o n .
CMFRI BULLETIN 31
THREE
H Y P O T H E S E S O N M U D B A N K S
A. V. S. MURTY, D. S. RAO, A. REGUNATHAN, C. P. GOPINATHAN and K. J . MATHEW
ABSTRACT 1. Subterranean Passage Hypothesis
The report gives a critical appraisal of the various hypotheses on the formation and the characteristic calmness of the mudbanks, in the ligt of the auther's findings, with special reference to the Alleppey mudbank. A detailed account on various types of mudbanks along the southwest coast of India is also presented.
INTRODUCTION
The mudbanks, t h o u g h c o n f i n e d to the near-shore waters, is a phenomenon s t i l l not f u l l y explained. The f o r m a t i o n of mudbanks at the vicinity of river m o u t h s , such as Korapuzha, Bharathapuzha, Chetwai and A z h i k o d e , and Cochin bar m o u t h s , w h e r e clay and vegetable debris brought d o w n by t h e rivers are d e p o - s i t e d on the d o w n w a r d side o f t h e littoral currents, is easy t o understand. But, t h e m u d - banks at places where there are no river d i s - charges, as at Alleppey, are rather d i f f i c u l t t o explain. There are many hypotheses put f o r w a r d to explain specific aspects, such as the source o f the mud and its role in calming d o w n the w a v e s . These hypotheses are b r i e f l y discussed here in the l i g h t of the present observations.
SOURCE OF M U D
Sir Rohert C. B r i s t o w , former A d m i n i s - t r a t o r - c u m - C h i e f Engineer of C o c h i n Harbour, w h o was also t h e chief architect of the W i l i i n g d o n Island, c o m p i l e d " T h e H i s t o r y of M u d b a n k s " f r o m the then available records, w h i c h was published in 1938. It details the v i e w s h e l d by the earlier authors on the f o r - mation and other aspects, mainly of the Alleppey and Narakkal mudbanks, and a h y p o - thesis o f B r i s t o w himself. The f o l l o w i n g is a discussion on the various t h e o r i e s , advanced before and after B r i s t o w , w e i g h i n g the merits a n d demerits of each one o f t h e m .
C r a w f o r d { I 8 6 0 ; i n : B r i s t o w 1938) was the first t o advance a hypothesis — the Subter- ranean Passage Hypothesis — t o explain t h e source of the mud of the Alleppey mudbank.
He observed t h e f o r m a t i o n of mud cones on the beaches and in the roads of Alleppey in 1855. This led him t o suggest the existence of a subterranean passage, or stream, or a succession o f t h e m , that becomes more active during heavy rains, particularly in the c o m - mencement of the m o n s o o n , carrying off the accumulating water and w i t h it vast quantities of s o f t mud from some of t h e inland rivers and backwater t o the sea.
Later Capt. Drury ( I n : B r i s t o w 1938), o b - serving the d e p o s i t i o n of so large a quantity of mud in the open sea, about 2 or 3 miles f r o m the shore and many miles from any bar mouth or outlet f r o m the backwater, suggested that it is not improbable that there exists a subterranean channel t h r o u g h w h i c h large quantity of mud is carried o f f i n t o the sea, where it is t h r o w n up in the f o r m of a bank.
He stated that the mud thus formed gradually f l o a t e d s o u t h w a r d w i t h the littoral currents and fresh banks are formed whenever the hydraulic pressure of the inland backwater increases s u f f i c i e n t l y t o overcome the s u b - terranean resistance o f the stratum o f f l u i d mud w h i c h is formed at certain places.
A c c o r d i n g t o King ( 1 8 8 1 ) , the mudbank m a y b e e n t i r e l y d u e ' t o the discharge of mud f r o m under the lands of Alleppey, Purakkad and Narakkal, being effected by the p e r c o l a t i o n or undeiground passage of lagoon water into the sea
Philip Lake(1889; i n : B r i s t o w 1938) d i f f e r e d f r o m t h e v i e w s held by the previous odservers
on the source of mud for the A l l e p p e y m u d - bank. He o p i n e d that the A l l e p p e y mudbank is formed not from t h e backwater m u d , but f r o m an o l d e r river deposit f o u n d only at particular points along the coast. He further s t a t e d t h a t , w i t h regard to the existence of subterranean channels, it might w e l l be d o u b t e d whether any c o u l d exist in such unstable deposit as found there.
J o h n Rhode ( 1 8 8 6 ; i n : B r i s t o w 1938) for- mer Master o f A l l e p p e y Port, suggested that a f l u i d mud strata exists b e l o w Alleppey.
The consensus of o p i n i o n s s t a t e d above leads t o the concfusion that there is an u n d e r - ground discharge of water, at any rate, into the sea f r o m the l a g o o n and river system behind the Aileppey-Purakkad coast during f l o o d time, the inland water being at a higher level. This passage of underground water must, more particularly during heavy rains, pour out w i t h it large quantity of the mud.
2. Hypothesis of water bearing stratum B r i s t o w (1938) ruled out the p r o s s i b i l i t y of the existence of an underground river at Alleppey. His argument is that it is impossible for the backwater t o rise more than a f o o t w i t h o u t f l o o d i n g the lower parts of the neck of land separating the backwater f r o m the sea, at many p o i n t s between Cochin and Alleppey.
Besides, a head of 5 ft., the maximum possible, w o u l d give a pressure of only about 21 Ibs/sq.
inch, w h i c h is not enough t o overcome the frictional resistance set up by solids in suspension. A c c o r d i n g t o h i m , w h a t is more likely is that a water-bearing stratum exists at a g o o d d e p t h , w h i c h brings d o w n water from the hills and crops out under the sea at varying distances from the s h o r e , thereby l i f t i n g the b o t t o m mud above it and anything s u f f i c i e n t l y buoyant that lies buried in the mud.
3. River deposition iiypotfiesis
According t o Ducane et al ( 1 9 3 8 ) , the chemical analysis of the backwater mud and t h e mudbank mud reveals d i f f e r e n t char- acteristics. The mud of the mudbank is green- ish, very o i l y , b u t m i x a b l e vvith water, whereas the mud of the backwater is black and is full of vegetable debris and is immiscible w i t h water.
This difference led Ducane-s team to conclude t h a t the mud o f the mudbank might be from an older source. They h e l d t h e v i e w that the l a t e r i t e / a l l u v i a l sediments f r o m the land are run d o w n by the rivers and are deposited on the seabed close t o the shore in a regular process of river discharge and the sediment deposit thus accumulated near the coast is churned up by monsoon waves, and thus the mudbank is f o r m e d .
This explanation is, however, c o n v i n c i n g w i t h regard t o the mudbanks f o r m i n g near river mouths and bar mouths. But the mudbank near Alleppey cannot be explained by t h i s hy- pothesis, because there is no river or back- water emptying in t h e nearby area.
4. Ttie Upwelling fiypotfiesis
Ramasastry and M y r l a n d (1959) associated the f o r m a t i o n o f mudbanks w i t h the u p w e l l i n g along t h e west coast of India during t h e s o u t h - w e s t m o n s o o n , the u p w a r d movement of water l i f t i n g the b o t t o m mud.
The presence of u p w e l l i n g according t o them is only at about 20-30 m bathymetric
lines of the coastal waters. It is w o r t h w h i l e t o mention here that the presence of u p w e l l i n g at such depths, however, does not help t o ex- plain the f o r m a t i o n ot mudbank f r o m shore t o 10 m depth, unless there is some other mechanism in the r e g i o n of the bathymetric difference of the locations of the tvyo p r o - cesses. S e c o n d l y , unless u p w e l l i n g extends d o w n t o the b o t t o m , w h i c h is unlikely, the mechanism w o u l d not be able to l i f t the b o t t o m m u d . Thirdly, w h y t h e mud banks are l i m i t e d t o o n l y certain regions w h e n u p w e l l i n g is there all along the s o u t h w e s t coast (Rama- mirtham and Rao 1973) is not explainable.
FORMATION OF M U D SUSPENSION A N D THE CALMNESS ASSOCIATED W I T H THE
M U D B A N K .
/ . The Defioccuiation iiypotfiesis
Flocculaiion is the process in w h i c h fine particles are b r o u g h t t o g e t h e r and clustered t o become heavier masses so that they w o u l d be pulled d o w n by gravity. Keen and Russel (Ducane et al 1938) f o u n d in their experiment t h a t the mud o f the mudbank completely
CMFRI BULLETIN 31 9
settled (flocculated) w h e n s a l i n i t y was greater than 20%o and it remained suspended ( d o f l o - cculated) at salinity l o w ^ r than 2.5%o. This hypothesis was a d o p t e d by Kurup (1969) and Padmanabhan and Eswaran Pillai (1971) t o e x - plain the calmness of mudbank.
As l o w as 17.04%o salinity w a s reported by Damodaran and Hridayanathan (1966) during August 1966 f r o m the surface waters of Cochin mudbank. Nevertheless, the b o t t o m waters always recorded h i g h values and never
w e n t b e l o w 33%o (Iyer and M o n i , 1972).
Periodical observations at A l l e p p e y mud- bank, w h i c h extended f r o m Valanjavazhi in the north to Purakkad in the south during the mudbanks o f 1971 and 1972. i n d i c a t e d s a l i n i t y values as s h o w n in Table 1 . It w o u l d be seen f r o m the table that the mudbank, b o t h at surface and b o t t o m , maintained w e l l above the upper l i m i t for d e f l o c c u i a t i o n .
Table 1 Salinity values (%o) at surface and bottom at the mudbank during 1971-72.
(S = Surface, B«=-Bottom).
Valanja- Valanja- Ambala- Purakkad vazhi vazai puzha
east west
1971 S
B 1972 S B
32.22 33 70 29.28 27.76
32.26 32.12 30.34 29.05
30.89 31.81 29.82 2950
30.74 30.89 3 0 0 2 31.47
Even a d m i t t i n g that surface w a t e r is d i l u t e d by freshwater influx to the o p t i m u m level at some places, b o t t o m water at no place recorded the required l o w salinity for d e f l o c c u i a t i o n , s h o w i n g that the water i m - mediately in contact w i t h t h e mud itself is not in favour of d e f l o c c u i a t i o n -2, The Hypothesis of oil in watar as an agent to cause calmness
King (1881) suggested, based on the analysis made by F. R. M a l l e t , that some b r o w n i s h - y e l l o w o i l y matter was present in the mud c o l l e c t e d f r o m t h e A l l e p p e y bank, w h i c h he t h o u g h t , w h e n released into the water, to be the main agent responsible for b r i n g i n g about calmness over the A l l e p p e y mudbank. Lake (1880), however, discredited
this hypothesis. Later the analysis of Keen and Russel s h o w e d that there was no such o i l y matter in the mud at Alleppey.
3. Hypothesis of elastic nature of mud and its role in producing calmness
Another suggestion was that the m u d is of springy or elastic nature and hence is able, by alternate contractions and expansions, as the wave passed over it, t o absorb the wave energy, so bringing them t o rest. Keen and
Russel (1983; in: Ducane et al) discarded this v i e w . According to them the primary charac- teristic of any mud is that it is plastic not elastic, i e., it w i U alter its shape or c o n f i g u r - ation under external forces, but w i l l not resume its original shape w h e n the deforming force is w i t h d r a w n .
4. Thixotropic hypothesis
From the k n o w n principles of h y d r o - dynamics and from the results of experiments Keen and Russel ( i n : Ducane et al 1938) c o n - cluded that the calming effect is due to the kinematic v i s c o s i t y and t h i x o t r o p i c properties of the muddy suspensions produced in the m o n s o o n . They are of the o p i n i o n t h a t , w h e n the heavy waves and s w e l l s of the m o n s o o n reach the shoal b o t t o m at the seaward fringe of the mudbank, the alternation of stresses associated w i t h ridge and t r o u g h of the waves brings mud into suspension. The suspended mud increases t h e kinematic viscosity of the m e d i u m . This factor w i l l t e n d t o dampen the m o t i o n of the waves on the surface and in subsurface depths. As the stress thus f a l l s , the properties of the mud suspension resemble those o f a jelly w h i c h w i l l absorb the wave energy completely. Thus, according t o Keen and Russel, the effect of t h i x o t r o p i c suspension on w a v e m o t i o n is a cumulative one. In m u d - suspended water, at high stresses, e.g., v i o l e n t w a v e m o t i o n , the kinematic v i s c o s i t y of the a g i t a t e d mud suspension produces a higher rate of damping than in mud-free water and the stresses are reduced. Then the t h i x o t r o p i c effect comes i n t o play, and the remaining stress is rapidly d i s s i p a t e d by the j e l l y - l i k e behaviour of t h e suspension.
5. Rip current hypothesis
Varma and Kurup (1 969) sought t o explain the l o c a l i s e d f o r m a t i o n of the mudbank by
attributing it to the rip currents. They said t h e rip f l o w , carrying finer o f f s h o r e sediments, prevents the onshore transport of sedin-ients by waves. Hence l o c a l i s a t i o n o f suspended sediments takes place at the rip h e a d .
A l t h o u g h the rip currents are not f u l l y u n d e r s t o o d (Sverdrup et al 1 9 4 2 ) , w e may believe that these currents are probably a s s o - ciated w i t h the surface transport o f water against the beach by t h e w a v e s (Shepard et al 1941) The rip f l o w may thus be a concentrated backlash of the waves at the beach and hence its area of action is narrow. On the other hand, the postulated mechanism requires rip f l o w from behind the mudbank ( i n b e t w e e n the mudbank and the beach). But backlash o f waves from t h i s h i n d zone is unlikely as the area is calm. The backlash of waves m i g h t be possible o n l y if the mudbank is far off f r o m the coast, a l l o w i n g w a v e action t o take place in t h e hind zone, w h i c h is usually not the case.
Therefore, t h e rip f l o w c a n n o t be a c o m - ponent o f the w o r k i n g mechanism o f the m u d - bank f o r m a t i o n .
RESULTS OF THE PRESENT INVESTIGATIONS The f o l l o w i n g is an account on the v a r i - ous more probable physico-chemical factors responsible f o r the f o r m a t i o n , maintenance and dissipation of t h e mudbanks, as revealed by the investigations carried out by the authors from 1971 o n w a r d s .
Before dealing w i t h the actual mechanism of the mudbank f o r m a t i o n , w e may have t o consider the geographical features of the areas surrounding the A l l e p p e y mudbank, including the Vembanad lake and also the rivers empty- ing into it. Vembanad lake is a vast water b o d y lying almost parallel t o the coast f r o m A l l e p p e y in the s o u t h t o Cochin in the n o r t h . Its opening t o the sea is at Cochin. The lake is separated f r o m the sea by a narrow strip o f land of only about 1 0 - 1 3 k m w i d t h . Five rivers, namely, the Muvattupuzha, the Meenachil, the Pamba, t h e M a n i m a l a and the A c h a n k o i l , d i s - charge their waters into the lake. These rivers
originate f r o m the Western Ghats in t h e east and f l o w towards the west.
There are evidences to believe that, in the past, the area, presently covered by Vembanad lake and the land strip in b e t w e e n the lake and
the sea, was under the sea. During t h a t p e r i o d , the rivers, n o w f l o w i n g into t h e lake, m i g h t have been directly discharging their water into the sea. Later o n , o w i n g t o some natural causes, such as cyclones, seismic sea w a v e s and earth quakes, huge masses o f sand and sediments might have g o t d e p o s i t e d in between the pres- ent lake and the sea t o make the Vembanad lake. Boring experiments c o n d u c t e d at various places ( B r o w n ; i n : B r i s t o w 1938) on the w e s t coast g i v e supporting evidence t o t h i s . In one of the borings at C o c h i n ( B r o w n 1928). the bed rock was f o u n d at 395 ft, w h i l e in another the hard b o t t o m was f e l t at 6 5 0 f t .
The a d m i r a l i t y charts and the recent echo surveys (Silas 1969) indicate that there is rocky substratum at about 75 m depth off the Kerala coast. Thus it seems that the entire vast area b e t w e e n the f o o t of t h e hills and at about 75 m depth o f f the coast was almost a deep basin, g o t subsequently f i l l e d up w i t h mud and sand, over w h i c h a sandy crust was f o r m e d at some places. The presence of marine shells b e l o w 40 m at place like Kadu- thuruthy near Vaikom, where the l o w - l y i n g areas are all under paddy c u l t i v a t i o n , gives a p o s i t i v e evidence t o t h i s (there is a v i e w that t h e name Kaduthuruthy is d e r i v e d f r o m Kadalthuruthu; " S e a - i s l a n d " ) . The f o r e g o i n g account suggests that, b e l o w the lake and the narrow strip of land, at least between Thottappally and Narakkal, there exists a thick
layer of u n c o n s o l i d a t e d mud, w h i c h extends i n t o t h e sea.
/ . Source of mud for Alleppey mudbank The Subterranean passage hypothesis ( C r a w f o r d 1860) and the Waterbearing stratum hypothesis ( B r i s t o w 1938) o w e their leverage respectively t o the hydrostatic pressure o f the backwater and t o the hydraulic pressure in t h e f o o t of the h i l l s . The b o r i n g experiments have revealed the presence of a clayey substratum o f varying thickness. A l - t h o u g h no mention is f o u n d t o have been made o f the presence o f a waterbearing stratum in the reports on borings of C o c h i n a n d Alleppey, the same has been reported t o exist at 1 8 1 ' and at 312' in the W e l l i n g d o n Island b o r i n g , Davey's b o r i n g s (at Alleppey) have s h o w n the presence of mud, of varying c o m p o s i f i o n , d o w n t o 3 1 6 ' , w h i l e C r a w f o r d ' s
b o r i n g s at Alleppey revealed the presence of
CMFRI BULLETIN 11
sandstone t i l l a depth of 5 0 ' and then loose mud to a depth of 8 0 ' , in w h i c h " t h e shaft sunk of its o w n f r o m 60 t o 8 0 " . Waterbearing stratum has been observed t o be associated w i t h sandy substratum, but surfacing o f the stratum has not been i n d i c a t e d in any of the b o r i n g records. In t h e absence of t h i s , it cannot be believed that water c o u l d permeate the o v e r l y i n g mud layers f r o m great
depths ( t o greater heights) t o crop up in the sea and on the shore. Further, it is d o u b t f u l whether such a massive hydraulic pressure c o u l d be d e v e l o p e d at the f o o t of the hills as t o feed the waterbearing stratum and t o push the o v e r l y i n g layers of clayey mud. C r a w f o r d ' s
experience of v i o l e n t e j e c t i o n of water and vegetable debris f r o m 1 2 ' b e l o w , at a place 200 yards f r o m t h e beach at Alleppey during the construction of the Alleppey canal, obser- v a t i o n s o f ' L i n u s ' in the Chenganur river by h i m , the presence of deep pot-holes in the water as reported by Logam (1882, in: B r i s t o w 1938) and other available i n f o r m a t i o n , all equally suggest that the pressure-head d e v e l o p e d at the bed of rivers as w e l l as at t h e backwater generates a subterranean passage of m u d , w h i c h crops out at varying distances through weaker p o i n t s b o t h on land and in the sea.
The present authors have observed, first time since C r a w f o r d , and D a v e y a n d Lake, mud cropping up in the f o r m of cones for about 9 t o 12 days during the m o n s o o n of 1972, on the beach and at the i n t e r - t i d a l zone at Kakkazham, near Ambalapuzha. Narrow (a f e w centimeters w i d e ) cracks, 10 t o 15 m long along the s h o r e l i n e , were observed on the sea side of the b e a c h - m u d cones, indicating sub- sidence, w h i l e the mud cones at the intertidal zone f e e d i n g loose mud, as w e l l as lumps of it, to the water f l o w i n g past t h e m u d cones in its t o and f r o m o t i o n across the shoreline (See paper on mud cones) These observations t o o , support the subterranean passage h y p o - thesis for the Alleppey mudbank.
//. Calmness associated with the mudbanl<
The most striking character o f the m u d - bank is its calmness. The reasons for the pre- vailing calmness over a restricted r e g i o n , w h e n all other places are highly w a v e - b e a t e n , are t o be considered. Several v i e w s have been put f o r w a r d to explain t h i s p h e n o m e n o n . It is generally accepted that t h e calmness is brought
about by the mud in suspension. But, the v i e w that a purely physical process, say, the c h u r n - ing a c t i o n of the m o n s o o n waves causes the mud into suspension is not satisfactory. The f o l l o w i n g hypothesis, e v o l v e d by the present authors, is o f f e r e d t o explain the w h o l e pro- cesses leading to the calmness associated w i t h the mudbank.
A. Wave Propagation
a) Movement of particles of the medium: The waves at the surface of the sea are caused either by w i n d force or t i d a l force. B e l o w the w a v e crest o f a progressive w a v e , the
horizontal m o t i o n of the particles is in the opposite d i r e c t i o n of that under the t r o u g h . T h e particle attains maximum h o r i z o n t a l v e l o c i t y (speed) w h e n it is just b e l o w the centre of the crest or the t r o u g h . During t h e first half of the w a v e l e n g t h , from m i d - t r o u g h to mid-crest, the particles experience vertically d o n w a r d v e l o - cities. The vertical v e l o c i t i e s reverse in the second half o f the wave l e n g t h . The vertical speed is maximum at a p o i n t w h e r e t h e w a v e passes from t r o u g h t o crest, or vice versa, and it is zero h a l f - w a y b e t w e e n the crest or the t r o u g h (Sverdrup et al 1 9 4 2 ) . Thus, as the w a v e form is propagated in the d i r e c t i o n of w a v e m o t i o n , the i n d i v i d u a l water particles i n v o l v e d in propagation of w a v e f o r m are subjected to harmonic m o t i o n s from their mean (undisturbed) p o s i t i o n .
Jeffreys ( i n : Sverdrup et al 1942) p o i n t e d out t h a t , w i t h i n surface waves (deep water waves) the individual water particles near t h e surface move in circular orbits, the radius of w h i c h is equal to the amplitude of the w a v e (a circular m o t i o n can be resolved into a simple harmonic m o t i o n ) . But the radii of these orbits, and therefore the v e l o c i t i e s , decrease rapidly w i t h d e p t h . According to t h e results of the classical hydrodynamics, the orbital paths of water particles in surface waves are e l l i p t i c a l , covering w i t h i n the same t i m e - i n t e r v a l , during w h i c h the wave travels over a distance of a w a v e l e n g t h ( D i e i t r i c h et a l . 1980). The e l l i p t i c orbit changes into a circular orbit, if the water depth exceeds half the w a v e l e n g t h , w h e n the a m p l i t u d e is very small compared t o the w a v e l e n g t h ( N e w m a n 1978). Theoretically, the diameter of orbits at a depth of o n e - h a l f the w a v e l e n g t h is only o n e - t w e n t y - t h i r d of the corresponding diameter
at fhe surface. Regardless of the actual d e p t h , the character o f t h e w a v e therefore remains unaltered, if the d e p t h to t h e b o t t o m is greater than t h a t short distance.
b) Summary of motion at surface: In a sur- face w a v e , let us assume, the w a v e m o t i o n is in the 'x' d i r e c t i o n and let the distance ' x ' and the t i m e ' t ' be reckoned f r o m a p o i n t w h e n the wave passes its equilibrium p o s i t i o n f r o m t r o u g h to crest. Then the f u n c t i o n a l repre- sentations in terms of x and t of the d e f l e c - t i o n ( y ) o f the sea surface f r o m its p o s i t i o n of rest, the h o r i z o n t a l ( VH ) and vertical ( V v ) velocities of the particles of the f l u i d medium at the surface may be w r i t t e n (Dietrich et al 1980 and Starling 1947) as
y - a S i n 2 . ("f " 7 )
V „ = . V H o S i n 2 > t ( ^ ^ - - Y )
Vv «= Vvo Sin 2»t
w h e r e x is w a v e l e n g t h , T is the p e r i o d of w a v e , a is the maximum value of y (amplitude of the wave), V H O and Vvo ai'e maximum values of VH and Vv respectively. The magnitudes of V H O and Vvo are equal for deep w a t e r waves but they d i f f e r for s h a l l o w water waves.
The behaviour o f t h e parameters of the particle m o t i o n at the surface as t h e w a v e completes a cycle o f w a v e l e n g t h is i n d i c a t e d schematically in f i g . 1 by d i v i d i n g the circle into four quadrants c o r r e s p o n d i n g t o the four quarters of the w a v e l e n g t h . The length of an arrow in each circle represents the magni- tude of the parameter f o r w h i c h the circle stands. A n t i c l o c k w i s e d i r e c t i o n is treated as p o s i t i v e . The zero p o i n t in each circle r e p r e - sents the starting p o i n t of the parameter. The rise of the w a v e is indicated by y w h i c h reach- es Its maximum at x =• - r
4 t o t h e zero level at x -. — .
height of the w a v e is negative ( b e l o w the mean level) and it reaches the negative m a x i - mum (trough maximum) at x = J x f r o m w h e r e
, thereafter it falls A f t e r w a r d ; the
.* - " -
^ _ ^
^ I If u
' f
S \ > "'
^ \ ^
\
Wavt propagation
\
VH X
4 • o-X-*"
/ /
/
J^
Fig. 1 Schematic representation of motion of a particle at the surface over a wavelegth
CMFRI BULLETIN 13
its magnitude decreases until it becomes zero at X = X.
The horizontal v e l o c i t y is f o r w a r d during the crest period and it reverses during t h e t r o u g h p e r i o d . As it varies in phase w i t h wave height, t h e magnitude of the horizontal v e l o c i t y reaches its maximum at x = -7-and at X = | x .
W h i l e the h o r i z o n t a l v e l o c i t y of the p a r t i - cle varies in phase w i t h the w a v e h e i g h t (par- t i c l e h e i g h t ) , the vertical v e l o c i t y varies out of phase w i t h it. The vertical v e l o c i t y leads the w a v e h e i g h t by o n e - f o u r t h of w a v e l e n g t h , t h e r e f o r e it is zero at the peak crest or t r o u g h .
O w i n g t o the decrease of v e i o c i t y w i t h depth, t h e f o r w a r d v e l o c i t y of the particle, w h e n it is above the mean d e p t h , exceeds the b a c k w a r d v e l o c i t y of it w h e n it is b e l o w the mean d e p t h . Therefore over a cycle of one w a v e - l e n g t h the particle does not return t o its original p o s i t i o n but it experiences a net for- w a r d m o t i o n , resulting in net f l o w in the d i r e c t i o n of progress o f the w a v e . (Sverdrup et al 1942).
In the case of s h a l l o w water, the fact that the vertical m o t i o n cannot exist at the b o t t o m m o d i f i e s the character of the w a v e s . At the b o t t o m the m o t i o n can be o n l y back and f o r t h , and, if t h e depth is small compared to t h e w a v e l e n g t h , t h e m o t i o n w i l l remain nearly horizontal at all depths. A c t u a l l y , the orbits of the i n d i v i d u a l water particles w i l l be f l a t ellipses that become more and more narrow when approaching the b o t t o m and at the bot- t o m they degenerate into straight lines. Thus*
all the particles i n v o l v e d in w a v e propoga- t i o n move in different but systematic w a y s . B. Viscosity
As the particles i n v o l v e d in w a v e p r o p a - g a t i o n experience relative m o t i o n , either h o r i - zontal or vertical or b o t h , here comes the effect of the internal f r i c t i o n ( v i s c o s i t y ) of the medium in w h i c h the waves are p r o p a - g a t e d .
a) Newtonian viscosity : If relative m o t i o n occurs, v i s c o s i t y or internal f r i c t i o n is e x p e r i - enced by the f l u i d . It was assumed by Sir Isac N e w t o n t h a t , for a f l u i d m o v i n g in parallel
layers, the shearing stress at any p o i n t — w h e r e the v e l o c i t y gradient is perpendicular t o the
du
d i r e c t i o n of m o t i o n , -j is d i r e c t l y p r o - p o r t i o n a l to the value of the gradient, so that the f r i c t i o n a l stress, f, per unit area is given by:
f = r ) - 7 -du ' dz
where f), a characteristic constant for the f l u i d , is called the c o e f f i c i e n t of v e l o c i t y . N e w t o n ' s assumption was f o u n d true as long a s t h e m o t i o n is laminar (non-turbulent), ( N e w - man and Searle 1951). The values o f v i s c o - sity (in 103 times c.g.s. units) of pure w a t e r and of seawater of 35%o salinity at d i f f e r e n t temperatures are g i v e n in the f o l l o w i n g table (from Sverdrup et al 1942.)
Temperature °C
0 10 15 20 25 30
Pure water 17.9 15.2 13.1 1 1 4 10.1 8.9 8.0 Seawater 18.9 16.1 13.9 12.2 10.9 9.6 8.7 Viscosity decreases w i t h increasing t e m - perature. Viscosity of seawater is c o r r e s p o n - d i n g l y higher than that of pure water at all temperatures. The effect o f pressure on the v i s c o s i t y in the case o f seawater is f o u n d to be insignificant.
b) Viscosity of the medium of mudban/(: Let us study h o w the waves are damped once t h e y enter into the region of mudbank. The mud particles present in a vertical c o l u m n of the mudbank is treated t o exist in three d i f f e - rent phases.
/. Phase I (Thixotropic phase) : As early as 1923,A. Szegvari and E. Schaiek (in Glasstone 195) f o u n d that w h e n concentrated pasty mass of ferric oxide is m i x e d w i t h suitable quanti- ties of e l e c t r o l y t e in aqueous s o l u t i o n , on shaking, f o r m e d c o l l o i d a l s o l u t i o n . This phenomenon has been called ' t h i x o t r o p y ' by Petrifi ( 1 9 2 7 ) . Subsequently, this was also observed i n other c o l l o i d a l systems such as alumina, s i l i c i c acid, vanadium pentoxide, zir- conium d i o x i d e , stannic o x i d e and even w i t h suspension o f fine clays. The analysis of the m u d c o l l e c t e d f r o m t h e mud cones s h o w e d that it contained ferric oxide in finest clayey f o r m .
