Manual on protection and control of coastal erosion in india

Erosion prevailing along the vast coastline of India ha, a long history. Coastal erosion. very ofeen. poses a serious problern. The nature and degree of protection required for a given COlSt vary widely depending upon the environmental conditions prevailin~ in ehe area. A comprehensive environmental study of the problem is required for developing a suitable solution to any specific coastal problem. In general, there will be more than one method applicable to protecting an eroding area. Hence. it is very desirabie to consider both shortterm and longterm efflcts very carefully before determining the most suitable remedial measure to combat erosion problem. In thts manual, an attempt has been made to present some of ehe remedial measures in::luding tha guidelines for suitable designs to control coasul erosion with special reference to Indian conditions. While some of the basic information has been presented in the eexe under various sections, more detailed information has been included separately under six appendices in the manual. Although the techniques presented in the manual are generally applicable to most of the coastal erosion problems. competent engineering judgement, based on experience, is necessary for determining their application tO any specific probiem. This manual is first of its kind in India. le is intended to be precise and effective and makes no claim to be exhaustive. Nevertheless, the value of a manual of thrs nature. dealing wrth diverse aspects of coastai erosion and its prottction. cannot be den led. The originai idea for preparing this manual carne from Professor Per Bruun, who has considerable experience of working in Indlan conditions for the past fifteen years or so. His major contribution and guidance durlng the preparation of this manual is indeed greatly appr eciat ed. I would like to express my gratefulness to my colleagues at the National Instieute of Oceanography for giving valuable support te Prof. Bruun in the preparation of this manual. Colleagues who made significant contributions to th is manual are: Or. B. U. Nayak. Mr. N. M. Anand, Dr. A. K. Jaln, Or. A. G. Untawale, Mr. B. G. Wagle and Mr. K. H. Vora. Very useful suggestions and reviews were offered by Mr. N· P. Bhakta, DIrector. Preinvescment Survey of Fishing Harbours, Bangalore and Dr. V. V. R. Varadachari. Mr. H. N. Siddiquie and Dr J. S. Sastry. The valuable assistance rendered by·Mr. K. G. Chitari of the Orawing Section and Mr. S. P. Sharma of the Planning and Data Division in connection with the printing of the manual Is gracefully acknowledged. I would like to express my gratitude te the U. S. Army Corps of Engineers, Coastal Engineering Research Centre, Virginia and Mis. Litton Educational Publishing Inc., New York for chetr kind permission to reproduce some of the material and figures from their publicatio;s. Comments and suggestions from readers on ehis publication would be most welcome for improving and updating the manual in the future.

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This manual is wriue n as a guide for those who are concerned wi.h plannin and desigru ug 0 (

practical an econornical rnerhods of proreering (he ereding shores on ihe In ian coast The sandy s hores

o t ' India s p evi a lly the barrier b eac he s of different c o a stal siates wir h a high pop u la u n d e n su y are be n g

eroded mainly by waves parti cul ar ly when these gel c ornb i ned wu h st o rm t de s- C o a s tal erosio In [ n\ k l

ha s res ul ed In the l o ss o f v aluable bea c hes and adja en t c oa st a l lan used I'o r habu ato n a gri cu l ur e a nd

re rean n loss of marshes and wetland essenual tor marine life and fishery a tiviues- Erosio also leads

to darnage and des.rucrio of coastal highways industries bridges and oiher coastal installat io n s I

increases the risk of lfe as seas encroach inro highly popuiared or urban coas.al ave as Considerablc

expertd ure has been and is being incurred almost every year in co nstructing ernporary rernedial me sures

Iar gely to handle ern irgency situarions of severely ereding shore segments

-F runa.cly India has an abundance of natura I rocks along most of i ts c oa st l ne in the f'orrn ofgra nire. basal laterite l mcstone and s andstones- These rocky ma t er i als have hr o ug h he centures

pro en hei worrh as a building marerial tor coas.al structures ro wihsrand hosulo torc s of the ~ea

Proper ly d esigned structures have largely been successtut but nothing could remain succesrul in the long: ruazanst th e co.u in ed e:o ing a cti on of the s ea wilhout proper maintenanc In c r a i n pa rts o f the In an

coa-r the situau n is 5 severe rhat hardly any extra land rernains to be lost turther as a result of shore

erosion- There is a grearer need today than ever bef'ore for proper planning and develo ing optmum

solutio n sro rhe pro 'erns of coastal erosion in India so ihat [he most su able and econornic ! me sures could

be a do pi e d to cern b at erosion of a given shore segment

Keeping the above aspects in view a-id iaking into consideration the special condin ns preval ng in

In ia this rnanual ha; been prepared to act a as guide- It is thereforc hoped rhat ihe guidelines given

In rhe manual wil be of part i cula r interest to pub l ic agen c ies and consul.ing engine rs who may want

to avoid the piualls of approving or designing inadequate and inetfective measurcs

-This manual covers the basic design aspects which should be considered in any ana'ysis [hal leads

to the se l e ti n and recornrmndarion of a spe ci fi c type of ccastal protectio: Fur hermo ·e i giv ess pec i ric

ad vic 0: 1 ihe seletion and design of coasta l pro.eerion measures for condinons prevailing in India·'

The rnanual describes several ways to reduc shorefront darnage such as:

( i ) structural e-« prevention of coastal erosion by the use of seawalls, revetrnents dunes or

d kes and groins:

( ii: co servaron - preserving and enhancing the natura! protective features lke [he dunes b sa-id

tencin and vegeration to intercept the naiural sand supply:

( ii i; re storanon 1)1' beaches and dunes by direct placement of sand tem inta d or frorn ihe ocan

bottorn or by erecting sand bypassing plarus ro restere normal lnoral drift along [he shores

In sorne c ses feeding of rhe existing groin fields suffering from starvaro rnay be advisable

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1.1 General review on causes of be a cb erosi o

Beach an shore erosion is an evil which is found all over the world Perhups there are only a fewcouut ries which do nor sutfer f rorn coastal erosion because their shores are predorninantly rocky- By faro

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2.) Wave surveys2.).1 General2.).2 Wave measurements2.).3 Relationship between the visual

and the instru mental data

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22~225252.4 Current and tde surveys

2.4.1 General2.4·2 Current measurements2.4.3 Long term analysis of current d;tta2'4.4 Tide surveys

2.5 Littoral drift surveys

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3· Coastal Protection) I Basic aspects) I I Material balance) I ·2 Beach and bettorn profiles) I.) Wave machanics as pectsj·2 Review of coastal protective measures).2· I Natural an man-made coasul protection)2.2 Pre-req uisites for coasul protection

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)2·3 Types of coasul protection

3·2· -4 Choice of protecti ve measures

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3 3 Design details

3·3· I Suwalls and revetments

3 3 2 Dunes and dykes - overflow protectio

3·3· Groins

3 3 4 Offshore break waters

3 3 5 Nourish ment of buches

3 3·6 Bypassing of muerial at tidal inlets

-4 Coastal Protection Management

Appendix A Wave Data Analysis

Appendix B Design of Sloping Rock Structures,

Uprush Elevatio and Rock SizeAppendix C Sampling Procedures and Sediment Analysis

Appendix 0 Stabiit of Earth Slopes and Retaining Structures

Appendix E Rocks for Coastal Protection

Appendix F Dune Vegetation along the Indian Coast

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( i) beach s ed i ment is c o si dered ro be mos t st ab l e tor t he en vi ronmen t.

(ii) t he ent i re v olume o f t he fill mater i al p l a ed on t he beach i s s or t ed by l ocal pro c s se s t o achievc

a grain size distributton sirniar to the beach material and

(iii) s err i ng processes change th e fill mater i a l s i n t o t he beach- li ke s ed i rnen t s by win owrng out

J m i n i mum amount o f the original fi

-T h e abo ve m e th od proposed by J atnes ( re f (7) i s b a s ed on the s e l ect i on o f t he c r icalo r s.abtc

gra i n si ze d i s r b t on o f the borrow si te s ed i rnents and i t quan tifi es t he amo unt b y which th a t

distribu-ti on i s r o be mod i fied t o r esemb l e t he bea c h s ed i men t s - A c t ual c l cu l a ti ons of R ( a ti of th e w e i gh t percentage o f the beach to that of t he borrow s i te c ornpos it e ) i nvol v e c ornpl ic a te d rnath e ma t cs but

accurate graphical estirnares c an be o tained using t he c ur v es shown i n f i g 2.4· Th b a si c irtformation

requ i red is th e ph i mean and ph i sorti ng v a l ues f or bea c h and bor r ow si te s cd i men

ts-Reno u r i shment f ac t or method : It i s a dynarn i c approach ro descr i be h ow bea ch pr oc e ss e s c an

be e x pected t o modify s pecific fill s ed i ments ( re f (6 ) Th i s techn i que i s used ro e sri ma t e ho w of te n pla c ement of a partreular fill will be required to maintain specific beach dirnensions ft atternpts ro

e v aluate l ong term per f ormance of d i fferent fi ll ma t cr i als wi th rcgard t o s u i tab ility ma i n t enancc and

c o s t In t h i s metbod the ac tiv e bea c h sy stern i s tr eated as a oompanment w h i ch rcc eiv es s ed i rn e nt s

through longshore transport and from gradual erosion of t he inact i ve reser v oir of the s cd i rnenrs w h i ch form the backshore- The method es ti mates mass balance of the cernpartment u s i ng the re l at ive

FIg 1.S Renourishment factor versus phi mean difference and phi sorting ratio (ref 16).

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Varreus setup cornponents over the continenral shelf (ref 43) 6

Wave setup in a breaking zone in relation to ti des beach profile and energy

Developing erosion at a jetty irnproved tidal inlet (a) showing persistent swell

Longshore transport rate versus longshore energy flux factor for field conditions

Longshore transport rate as a function of deep water wave height and deep water

Schematic of a rock revetment for proteering the valuable shore property

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IHITlAL SANO LEVEL;

F i g.3./8 Laberatory tests on dune-butlding with a smale and a double fence system ( ref 26).

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Fotces acting on a gravity seawall

Circular slip surface for a seawall (ref- 8)

Non-eireular slip surface for a seawall (ref 8)

S rne failure rnechanisms for piled retaining walls (ref 8)

Effect of slope angle and friction angle on stabi ty factor (ref 8)

Stability factors for failure plane passing through and below the toe of

Geological map of north-west coast of IndiaGeological map of south-west and south-east coasts of IndiaGeological map of north-east coast of India

(a) Dune formation by Spintfix littoreus at Mirarnar, Goa

(b) Growth of S linoreus

Plate F·2 (a) Development of shoot and rootlets at nodal region in Si liuoreus

( b) Ferna Ie flowers of S littoreus

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Plate F· 3 ( a) Growth of J pescaprae on the sandy dune

(b) Typical bilobed and ûeshy leaves of [ pescaprae

Pl ate F.4 (a) Catpet flora of Cyperus arenarlus on sand dune at Mirarnar, Goa

(b) Mixed vegetation of C.are n arius aod I pescaprae

Plate F· 5 (a) Growth of Periploca sp- on sand dunes of Saurasbtra

(b) Periploca sph y lla growing in sand in an arid regionPlate F'6 ( a) Coastal erosion of sandy beach at Miramar, Goa

'b) Coconut plantation 00sandy beaches

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FIK 8.16 Slo p e de p endenee of z ero - danrage s l abili l y n umber (ref 3)

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This manual is wriue n as a guide for those who are concerned wi.h plannin and desigru ug 0 (

practical an econornical rnerhods of proreering (he ereding shores on ihe In ian coast The sandy s hores

o t ' India s p evi a lly the barrier b eac he s of different c o a stal siates wir h a high pop u la u n d e n su y are be n g

eroded mainly by waves parti cul ar ly when these gel c ornb i ned wu h st o rm t de s- C o a s tal erosio In [ n\ k l

ha s res ul ed In the l o ss o f v aluable bea c hes and adja en t c oa st a l lan used I'o r habu ato n a gri cu l ur e a nd

re rean n loss of marshes and wetland essenual tor marine life and fishery a tiviues- Erosio also leads

to darnage and des.rucrio of coastal highways industries bridges and oiher coastal installat io n s I

increases the risk of lfe as seas encroach inro highly popuiared or urban coas.al ave as Considerablc

expertd ure has been and is being incurred almost every year in co nstructing ernporary rernedial me sures

Iar gely to handle ern irgency situarions of severely ereding shore segments

-F runa.cly India has an abundance of natura I rocks along most of i ts c oa st l ne in the f'orrn ofgra nire. basal laterite l mcstone and s andstones- These rocky ma t er i als have hr o ug h he centures

pro en hei worrh as a building marerial tor coas.al structures ro wihsrand hosulo torc s of the ~ea

Proper ly d esigned structures have largely been successtut but nothing could remain succesrul in the long: ruazanst th e co.u in ed e:o ing a cti on of the s ea wilhout proper maintenanc In c r a i n pa rts o f the In an

coa-r the situau n is 5 severe rhat hardly any extra land rernains to be lost turther as a result of shore

erosion- There is a grearer need today than ever bef'ore for proper planning and develo ing optmum

solutio n sro rhe pro 'erns of coastal erosion in India so ihat [he most su able and econornic ! me sures could

be a do pi e d to cern b at erosion of a given shore segment

Keeping the above aspects in view a-id iaking into consideration the special condin ns preval ng in

In ia this rnanual ha; been prepared to act a as guide- It is thereforc hoped rhat ihe guidelines given

In rhe manual wil be of part i cula r interest to pub l ic agen c ies and consul.ing engine rs who may want

to avoid the piualls of approving or designing inadequate and inetfective measurcs

-This manual covers the basic design aspects which should be considered in any ana'ysis [hal leads

to the se l e ti n and recornrmndarion of a spe ci fi c type of ccastal protectio: Fur hermo ·e i giv ess pec i ric

ad vic 0: 1 ihe seletion and design of coasta l pro.eerion measures for condinons prevailing in India·'

The rnanual describes several ways to reduc shorefront darnage such as:

( i ) structural e-« prevention of coastal erosion by the use of seawalls, revetrnents dunes or

d kes and groins:

( ii: co servaron - preserving and enhancing the natura! protective features lke [he dunes b sa-id

tencin and vegeration to intercept the naiural sand supply:

( ii i; re storanon 1)1' beaches and dunes by direct placement of sand tem inta d or frorn ihe ocan

bottorn or by erecting sand bypassing plarus ro restere normal lnoral drift along [he shores

In sorne c ses feeding of rhe existing groin fields suffering from starvaro rnay be advisable

I

I

1.1 General review on causes of be a cb erosi o

Beach an shore erosion is an evil which is found all over the world Perhups there are only a fewcouut ries which do nor sutfer f rorn coastal erosion because their shores are predorninantly rocky- By faro

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offshore soure e s

rna n ' s a':l\iy of r emovrn g the material from the shore tor buildingor other commercial purpo s e s T a b l e I -,

on-T able 1·1 Causes of erosion att but able to nature and man (ref, 3 ),

Darns dykes and ether coastal str uct ures

Groi ns breakwarers jetties etc causi ng

entrances-that they change local shoreline geome.r y

radically Such fills are ofie n bulkhead,d·

Damming up of rivers with o t providingmaterial sluices whi c h allow co n n a tio n

of drift of materiais· Irrigati u pr o jec i s

decreaving flow of water and scdmerus

to the shore

-Removal of material frorn beaches tor

construction and ether purposes

-Digging or dredging of new i n t et s. channels

and cntrances- Offshore dumping of ma' ena

ls-Pro-ruding headlands reet's and rocks

Tidal entrunc s and rver mouths causing

i rv e rru pt io n of linoral dr i f

-Shore.ne geometry causing rapid increase

of drift quanut y :

Blocking of river outtets carrying sed

change 0f location of outlets due to

Rood5. erosion teetonic movernents etc·

Rernoval of beach material by wind drift

outbursts of ftood w aters

-Tne Îo~lo.v;ng para~raphs give the overall explanations for erosion- Secnon 3·1 desc ri bes bavic

physics a-id engineering aspects of the erosion problem

-1.2 Rise of sea le\ el

Alm h l all ihe shores in India erode (refs- 25 28 34 35 40 and 41) Figs 1·1 ( a ) to (/1) show

sorne o:' he e xa-n ples of beach erosion occurring on the west coast of India

-One general reasou for erosion is the rise of the sea level- The sea level risc (refs 2 a-id 13)may

so n insignificant but it is neeessary [0 reatise how narrow a beach is as corne red o ihe offsh re area

\ hich ha, ro be nourished b he material eroded from he bcach in order [ 0 cornpensa.c for ihe rise of

[he sea level With an equa] amount of the deposired marcrial at ihc bouorn it is casy , 0 work O UL h0\\

J.'1 average sea level rse of just I mm per ycar could c use a shorelinc recession inihe order ot abo ut 05

rnt - re per year The acrual rse of the sea level atong 'he Indian coast is nor well es.ablished Howevcr

I I S genaally aceo.ed rha: wh e the sca level is rising a consolidarion by seulng iakes place at ihe sarne

tme in the rr ver del.as like the Hooghly- The average rse of the sea level appears to be of the order of

I to 2 mm per y.:ao which is the average rare accepred universally

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1.3 Heavy storms, storm surges, wave actien aod its seasoDal effects

maxrrn uu ero sion rates Tne explanaoion tor his is ha:high andsteep waves break o ihe shores producing

highly urbuleru wa e,-s and uprushes which orten auack the dunes or coast al pla-forrn s die rly thercby

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( a ) P ho t o graph sh owin g b ach er os i on a t Punnapra Kerala

durm g I e v t o nso o o f 196 7 (b) Phot ograph shcwing how thecoco rut trees

( c) P ho t c gra p f , hl"'~g er o vio n p r o blern at 1 b e ach al Tr i v an

-J r u m K e r a l a h rm g rh e v l o s o o o f 1 97 6 (d) PKheralaoto raphdue sho[Q r heingr errrur+erualprc ore ffeectd ~( r' a -caw al:<.:.) ulireel.- at '>,<:':11.

I Fig . / .1 Scme exarnples of c asral aasion on

t he "-<!SI cc a s t o f l n dra

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~ DES I GN WATER L EVEL I NCLUD I NG

-: S U RFACE - WAVE SETUP

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as higher tdes bring in higher waves causing run p to greater elevatons The worst crosion rhererore.

takes pla e when a combination of high tdes and high and steep waves occurs which leads [ 0 c r osio n

profiles as explained in Secten 3·1·2 with reference to Fig 3.3

The total rise In [he water level along the coast is the surn of all the components which lead to

changes in [he water level resulting frorn a meteorological storm plus those which are nor related to the

storm but occur simultaneously Fig 1·3(ref 43) gives the various setup cornponents contributing to the

rise in the sea water level over the continental shelf over and above he ini a] water level These are:

Wave set up caused by breaking wavesX-component of wind setup

3S well as the infiuen c e of earrhs rotat i on

-T h e l ar ge st ce rnpon c ru contributing 1 t he rse of t he sea level during storrns cyclones or hurri c ane s

is the wind s hea r s t rc s ses ac ti ng over the surfac of wa.er Cornputat i onal procedures for the determ i nation

of wind setup are giv cn in a nurnber of publications including ref 43· However i is important to note

that the wi nd p i e u is pr oporuonal to the second power of the wind velocity and in ersely proportional

ro t h e water depih The wind setups or storm surges during the cyclones and hurricane s are t herefo re

largest in rhe shal ow wa.er areas of the continent al shelf as in the upper part of the Bay of Bengal and

in ihe Gulf C o a st of Flor.da

Fig 1·4 gi v e s ihe probable elevations of maximum storm surges on the south-east coast of lu i a

These valuc s a r e cornputed based on the assumptions t hat a sustained wind of 40 m sec is blow ing I n a n

onshore diretion an rhe central pressure depression is 35 mb when the storm is appro aching ihe c oast

Tl is also assumed that ihe storm surge coincides wiih the high spring tde (ref- 33) The asto n rruc l

tde in general. is qu u e sma l in magnitude but can bc very significant at certain geograph i cal l o c uo ns

like t he Gulfs of Carnba y and Kutch on the west coas t and the mcuth of Hooghly r iv er on the e st (aast

Storm surges in combination with astronornical high tides can play havocs in the coasral zone· Info rrnatio n

o r he tdes c an be o b.ained from the Ind i an Tide Tables publisbed by the Survey of India Dehra Dun

The atrnospheric pressure set up S.:.p expressed in metres is given by

S.:.p=0·13 (Pn -Po) (I-rl/r)

w here p ; is t he pressure a: the periphery of the storm, Po is t he central pressure in cm of mercury.

r is the radial d i srance from the storm eerure to the compu t ation point on the traverse line and R I S [he

distanc from the storm centre to the point where the region of maximum winds irttetsects [he sh relnt:

Rand r should be in t he same units say in kilometres. metres or naut i cal miles

-The wave setup Sc, mayalso contribute significantly ro the total elevaiion of the water level l il

the regio sho reward of the breaker zone- It is caused by ihe inflow 0 : water by wave-break ing an

depe n d- UpO:1 the chara - ens ti cs of the wave a-id the bo t torn profile and he i rnu t ua l in-cr a to n u des

energy dssipa-o b ttorn ma.erials ere- This is described in detai in ref 12 which givcs the result s of

field tests on the Oerma-i Island. Sylt on the Nor t h Sea coast where bca c h and bo uo rn pr o fi l e s and wa ve

c ha - acte ristics have c on s id e rable s i mlar it to condinons found in the ncarshore arca s of the c ast a n w es t

c o a st s of India Ac o r d i ng l y t he maximum wave set up ' lID'" may be wrtten as

." "'u 0·3 Hos

in wh ich H:)s 1 lh recorded offsho r e significant wave he i ght lf HBS the significant breaking wav e

h eigh t in [he sur z o ne is used as a reference the maximum setup can be expressed as

11IDu=0·5 Hu \ 5 long as no field data of a similar nature are available for the shores in India one rnay use ihe

ab ve expression in rela-on to the diagrarns of ref· 12· Fig 1·5 shows schematically the wave se.up in

t he breaking zone in relaio n to [he t de bea c h profile and energy d i ss i pation Fig 1 · 6 sh ows ihe w ave

setu a'on ihe profile in relation to significant wave height

Lh

Factor ~ = Lb

whcre Lh is ihe disranc f o t the brcaking point u t the wave height has decreasec to O.5Hs (Hs is ihe

breaker hcig i ) Ls is the wave lengih ar the breaking point Bis ihe width of the brr aker zone an :.J.B is

the wave set up at he breaking point Oiher termtnologies are defined in Figs 1·5 and 16

Waves In [he ocean, however art: i egular having cerrain specta as ex lained in Appen ix A

In wave science and engineering one distinguishcs belween a generation phase when [he wa ve s are

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The uprush or -'lnup elevation

depends upon the wave characteristics,

bot torn and beach geornetry friction

and permeability characteristics-

Natu-ral sandy beaches may be considered

hydraulically srnooth and

irnpermeable-Although they are not exactly straight,

their geometry is usually sirnple and may in cross-section be approximated b a straight lne or b 0straight lines-one for lower part and the other for upper part of the beach Somelimes thc bcac h

may have a gentie slope in the

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senerared by the winds shearing [he

sea surface Next follows apeak phase

when the wind veloeities are rhe highest

and the wave heights and periods

reach their maximum values- When

winds start slacking, the wave heights

grad ally decrease whereas the ave rage

periods continue to increase because

the short period waves lose their energy

most rapidly and attenuate- This is

called [he at ten u tio n p h ase - When the

waves reach the shore they mayor

may not break but in any case they

runup o [he beach- Wave breaking

and uprush are dealt-with in Sections

J·I·J JJ·I and in Appendix B·

Fig I.S Wave setup in a breaklDg ZODeIa relation to tides, beach profile

and eDeriY d i ssipation ( re f 1 2).

The uprush on beaches and coastal structures is discussed in detail in Appendix B· For sruoorhslopes (beaches) Figs- B· ro B·8 give diagramrnatic representations from which it can be secn ihatmaximum runup or uprush occurs for the slopes of I in I to Iin 2· For rough slopes e · g · rock mo unds orrevetrnenrs the uprush decreases depending upon the character of the roughness as explained 10 Tablc B.I

t'o r regular waves (Fig B·13)

DR I FT

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101.4 Littoral drift barriers natura I and manmade conditions in India

India has a lo g shoreline characterized by varietles of coastal features like rocky headlands coral

-BEACH ERODES HERE DUE TO

PROMONTORY FUNCTIONING AS

~ LITTORAL DRIFT BARRIER

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EXAMPLES:

MOPLA BAY , I<ERALA

WALTAIR POINT , ANOHRA PRADESH

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""- I NITIAL SHORELINE

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-Ll TTORAL DRIFT MATERlAL FROM RIGHT

DR I FTS ON ROCI< REEF PAST "HARD PO I NT" ( ROCI< OUTCROP)

I NSTEAD OF NOURISHING DOWN DRIFT BEACH

F ig.1 7 N a tu r al littor a l dr ift bar n er s aad bead la ad s.

-As it is known somc material will always bypass rhe inlet and this proccss m~y be assistcd ~Ither b: the

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HEAO LAND EXAMPLES:

KAKINAOA MACHILIPATNAM t ANDHRA PRADESH.

-, : " • • ' :' :': : " "

LITTORAL DRIFT

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EXAMPLES:

MADRAS HARBOUR AHD TUTICORIH I:4ARBOUR , TAMIL NADU

PARADIP HARBOUR, ORISSA

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EXAMPLES :

V I SAKHAPATNAM , AHDHRA PRADESH

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Fig 1./0 A group of greins used as Iittoral drift barriers.

of such a transfer system are described in ref 4· Some inlets, particularly rhose with very strong ebb currents,are poor bypassers and therefore, they cause severe downdrift erosion- This condition is very widespread

in India as compared to the other littoral countries due to the fact that ebb currents become very strongduring thc monsoon season- This would flush the littoral drift material fanher otfshore where it settlesand may get lost forever from the shore-

Other inlets have large bars which are formed by the combined effects of littoral currents and thcinlet ebb currents- They facili.ate bypassing of a major part, if not all of the material drifting alongshoreSuch natural bar- bypassing systerns are found in very large numbcrs on thc Indian shorcs- Exarnplcs ofthis type of offshore bars are given in Fig- 1·11· However, natural bar bypassers are undesirable for navigationbecause the shoals or bars cause obstruction to free navigation from the bay or lagcon to the sea- Duringthe recent years our knowledge and understanding of the associated physical proccsscs have advanccdconsiderably and such problems can be solved by introducing proper drcdging or by constructing suirablcjetties or both as illustrated in Fig 1·12 Such improvements invariably cause erosion or incrcasc thcexisting erosion on the downdrift side of the inlet- Exarnples of such occurrcnccs are nurncrous all overthe world including India (refs- 3 and 4) As indicated in Fig 1·12, we find some intcrcsting cxamples onboth east and west coasts of India such as the dredged entrance of Cochin Harbour (38 It-dccp at M LW)and thc 57 ft- deep dredged channel with groin and sand-trap-protcction of thc Visakhapatnam Harbourincluding sand-bypassing by pumping- Both these cause scvere downdrift erosion-

An interesting exarnple of the intermittent natural bypassing is found a~ Bengre a fishing village ncarMangalere in the Karnataka State (Fig 1·13)· Although located close to the tidal Nctravari and Gurupurrivers the shore has been relatively stabie for a long time (ref- 34) This undoubtcdly is as a result ofnatural bypassing of material from the river along an outer sand bar, particularly during thc SW rnonsoon(May to October)- But a temporary slow down in this natural bypassing proccss rnay intcnsify thc existingbeach erosion problom-

Fig 1·14 indicates how a jetty and channel improvemcnt can causc considerablc crosion Such ancrosion often does not take place immediately after thc establishment of such a linorat drift barricr- Itmay take a few ycars before it starts accentuating thc problcm- This is largcly duc to changes in wavecharactcristics causcd by diffraction of waves (spreading of waves) resulting in thc deercase of wave stcepnessthereby causing a tempoary transport of material frorn the nearshore bottorn towards thc bcach This leads

to a ternporary siabilization of thc bcach- Rcfcrcnce is made to Section 3·1·2 for the bcach and bottornprofiles under the influence of storm waves and swclls- However, as soon as thc lirnitcd quantity of

-J • ~.r : " , : ' INLETS WITH LARGE TIDAl PRISMS CAUSE EROSION BECAUSE LITTORAl DRIFT MATERlAL IS

JETTED FAR OUT IN THE OCEAN OR IN THE BAY WHERE IT IS OEPOSITED IN SHOAlS

INLETS WITH SMALLER T I OAl PRISMS CAUSE LESS OR NO EROSION DOWN O''''T AS MATER l AL

DRIFTS ACROSS THE CHANNEL ON AN OCEAN BAR

EXAMPLES:

BAYPORE KERALA

HONNAVAR~ COONDAPUR KARNATAKA

KRISHNAPATAM MACHILIPATAM, ANDHRA PRADESH

CHILI<A LAKE INLETS, ORISSA

F i , 1.11 Some problems of Iittoral drift at tidallnlets.

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Fig. 1.12 Improved tidal lulets as linoral drift harriers.

material thus available gets exhausted erosion continues to occur due to the interruption of the drift causingstarvation of the downdrift side- The indication of such an occurrence on thc bcach and offshore bottornprofiles is when the erosion of the beach starts of at a rapid rate- As a consequence of this the nearshorebottom in front of the beach tends to develop a more gentie platform-likc slope and in sorne cases ashoreline recession of this type would lead to simultaneous scaward move ment of depih contours caused

by temporary deposition of eroded beach material in the offshore arcas The latter phenomenon may getfurther aggravated due to the formation of rip-currents along the jetty as shown in Fig 1.14

It is very important to consider all possible adverse effects noted above whcn any improvernent isplanned or executed along the shoreline- In all problems related to coastal protection it seerns somewhat

Fig J./) Shoreline at Mangalore showing the location of the Bengre fishing v i llage ( ref 34).

illogical or undesirable to construct a strueture for proteering or stabilizing a shoreline Because such

an acton aft e r taking into consideration al thc factors on a broader perspective, may tend to produce

more harm than good (See sections 3·2·3 3·2·4 and 3·3·3)· It would also be wrong to allow harbour

entranc struetures or jet es to eause serious erosion on their downdrift side, of ten leading to serious loss

of valuable land and property

-This type of calarniy can now always be foresecri and rectified before any damage could occur

-This requires a thorough stud of the problem a-id proper planning before such projeets are catried out

-A coastal engineer confronted wih a shorc proteetion or improvernent problern, should bein a posi on

to evolve an economical and technically viabic desig making the best usc of the environrnental data

uptodate knowledge, experience and judgement

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F i K I.U De v eloping eros i on a t a j euy i rnproved t i dal i n ' et ( a ) show i ng pers ist ent

s well cond i ens rb ) dur i ng storm wa v e condition.

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2.1 Bafb~'merrie surveys

A sirnple but approxima.e rnethod of measuring beach profiles as described below , can easily beadoptcd atrcr cstablishing a rercrencc level on a backshore sand dune by driving a peg or burying a large

rock This referencc level should be conneered ro a more permanent nearby bcnch mark which in turn should

be conneered to the nearby Survey of lndia bcnch mark, if available In order to measurc the beach profile

al ihc sarne location cach time anothcr object such as a light post corner of a building ere- should beused in linc with the reference point and perpendicular to the shore- Fix stations at 3 m intervals (Fig 2·1)

The technique of levelling requires two persons one person to hold the graduated staff and the

ot' the reterence level and notes down the reading of thc horizon- Sirree the line of vision to thehorizon IS nearly horizont al the reading on the pole which is graduared from the bouom will givethe difference in height of the station below the reference level- This procedure is repeared tor allthe stations and the heights noted- Generally, additional reference points are required along theprofile sirtee rhe sraff used is not of sufficient height to cover the entire drop in sand level acrossthe beach profile These can be easily established along the line of the stations as required- Sand

level of each station can be cornputed with reference to the original reference level (Fig 2·1j

For more accurate profile surveys and when the horizon is not clearly visible the usual surve

-ying rneihods using either plane table or spirit level can be adopted- In shallow water and the

surf zone modificauon in the method is necessary- A wader then operates the level staff or stadia

board In deeper areas an electronic depth recorder (echosounder) can be used to take conrinuoussoundings of the bouorn- Alternatively, a leadline can be used to take spot soundings- Fixing theposition of the levelling stations can be made from the shore using translts or pre-deterrnined rangelines andmeasuring tapes or trom the survey boat itself using sextant angles to three fixcd objects on the shore

As an example, a specific procedure for rapid and accurate beach and nearshore bathyrnetric

survey can be as follows: A baseline is established along the stabie landwerd area of the 'beachwhich is to be used as a basic control for survey ranges (profile lines) noimal to the baselineThese ranges along which the profiles are taken run from the baseline across the beach scawardFor a long-term beach srudy, the baseline can be monumented by erecting masonry or concrete posts-

-\ny exisung structure like curbs lamp posts, fishing piers buildings can also serve to establish andrelocaie a baseline or the survey ranges- The position fixing along the range line can be secured by atransit intersecring a level staff or a sounding boat operated_ along these ranges as illustrated in Fig 2-:!

Prior ro each sounding run the survey boat powered by an out board motor with a minimumcrew of two, a sounder operator and a boat operator, proceeds to a point o the range at the requireddepth- The sounder operator then logs in pertinent inforrnation on the sounding record (paper chart)

prior to each run namely the station number of the range linc the starring tme and thc date- The boatthen proceeds rowards the shore along the predetermined range line cithcr by ihc monurnents fixed

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F i g. 2.1 A s i mp t e pro c dure for measur i n b each and offsh o re bathy m etri c sur v e y s•

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on the coast in line with the range or by the range transi operator signalling wih right and lett

ttags to the boat operator - Radio comrnunicauon systern or walkie-talkies can be advantagcously

used for this purpose The depth recorder makes a conrinuous record of the depths (some ~80

soundin s per minute) The boat loc ton on the range line is determined every 1 to 15secou s by

continuous tracking and recording at the cut-in transi locarion- For approxirnate position fixing at

regular intervals a sextant can also be used in the absence of a cut-in transit

Best resul.s are o tained with the boat werking at high tde and the beach levelling done at

low tide- Correcnons for waves and tdes should be applied to the echosounding record 5 that

the depth readings with respect to a standard datum such as mean lower low water (MLlW) or

rncan se level (MSL) or chart datum are obtained for plotring

purposes-A beach sled similar to that fabricated at the Kerala Engineering Research Station Peechi can

be used for measuring the beach profiles This sled is dropped beyond the surf zone by a boat and

then pulled a ross the surf zone by one or two persons on the beach using a rope and taking

water level readings on its graduated vertical s

taff-Alternatively a wave sled sirnilar to that fabricated at the National Institute of Oceanography,

Goa can also be used conveniently- This is dcsigned to move offshore across the surf zone by the

waves and afrer the desired disrance is reached it can be reecvered from the beach by an aua hed

rope The level readings are taken from the vertical graduated staff and the distance is measured

using the rope which is rnarked for measuring distauces It is important to apply tdal corrcctions

to the readings befere plotring the beach

that sediments of rhe who Ie beach which vary: ( a) across the beach profile through the ~ariedenergy zones ( b) along the beach wit in any one energy zone ( c) between the seasons within the ihree

dimensional georne.ry of thc beach, and (d) with depth at each of the sampling spots on the acuve

profile are represented in the schcrne (ref Table 2·1)· Appendix C gives the procedures for sampling

and sediment analysis

-2.2.2 Sample an.lysis : Although Indian Standard Classification System and Wentworth Sc le

Systern of grain size analysis are generally uscd in coasial engineering praenee i has been found ihat

logarithrnic transformation '" (phi) of the Wentworth Scale is most useful lnmathematicat terminology

c pcan be written as (ref 17 and 18)

where Dis size of the partiele in mm and negative sign is used to give '" a positi ve value for finer sedirnents

To cernpare different beach sediments, a most cornmon approach is to plot the cumulative weight

percentage of each sample coarscr than a given series of size classes ( Figs - 2·3a and b)· But i

has been observed in these curves ihat in finer sizes, thc curve becomes straight and Sleep and tails out

rowards ihe coarser sizes If a number of plots of several beach samples are plotted rogether they look

sirnilar even though these are texturally distiriet from cach other- Thus a plot of weight pereenrage

for each size in c f,values is found to be more effectivo as each size class tends ro be fairly syrnmei cat

for the most frequcntly occurring sizes (Fig 2.3c) In this distribunen maximum frequency occurs

at'/J=;.t. and inf'lexion points at 1.1.±cr wherc 1.1.is phi rnean and cr is phi sorting- Using the combination ofthese J-L and cr each curve is defined independent of the other-

2.2.3 Beach fill modeis: Using cornposite rnean and sorting values of {he bcach and borrow

material the filling of the beach can be estimated using various beach fill models Basically thcre are t wo

types of mathemarical mode Is for beach fill problerns- The first metbod cnables the calculat io ofoverfill factor which is an csumatc of ihc volume of aspecific fill material needcd to crea-e a unit

volume of the native beach material Generally overfill factor excccds one indicating ihat from theborrow material unsuitable parts trom the RU will be removed by winnowing action of the water

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(refs- 10 17 and 2 ) The mathernatical model underlying these three methods are similar but fill

ratios are n r the same- Thc fill factor rnethod proposed by James (ref t7 ) provides modific tions 10

the ether two rnethods and is thus more useful- The basic assumptions in the metbod are:

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Quadrant 2

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( i) beach s ed i ment is c o si dered ro be mos t st ab l e tor t he en vi ronmen t.

(ii) t he ent i re v olume o f t he fill mater i al p l a ed on t he beach i s s or t ed by l ocal pro c s se s t o achievc

a grain size distributton sirniar to the beach material and

(iii) s err i ng processes change th e fill mater i a l s i n t o t he beach- li ke s ed i rnen t s by win owrng out

J m i n i mum amount o f the original fi

-T h e abo ve m e th od proposed by J atnes ( re f (7) i s b a s ed on the s e l ect i on o f t he c r icalo r s.abtc

gra i n si ze d i s r b t on o f the borrow si te s ed i rnents and i t quan tifi es t he amo unt b y which th a t

distribu-ti on i s r o be mod i fied t o r esemb l e t he bea c h s ed i men t s - A c t ual c l cu l a ti ons of R ( a ti of th e w e i gh t percentage o f the beach to that of t he borrow s i te c ornpos it e ) i nvol v e c ornpl ic a te d rnath e ma t cs but

accurate graphical estirnares c an be o tained using t he c ur v es shown i n f i g 2.4· Th b a si c irtformation

requ i red is th e ph i mean and ph i sorti ng v a l ues f or bea c h and bor r ow si te s cd i men

ts-Reno u r i shment f ac t or method : It i s a dynarn i c approach ro descr i be h ow bea ch pr oc e ss e s c an

be e x pected t o modify s pecific fill s ed i ments ( re f (6 ) Th i s techn i que i s used ro e sri ma t e ho w of te n pla c ement of a partreular fill will be required to maintain specific beach dirnensions ft atternpts ro

e v aluate l ong term per f ormance of d i fferent fi ll ma t cr i als wi th rcgard t o s u i tab ility ma i n t enancc and

c o s t In t h i s metbod the ac tiv e bea c h sy stern i s tr eated as a oompanment w h i ch rcc eiv es s ed i rn e nt s

through longshore transport and from gradual erosion of t he inact i ve reser v oir of the s cd i rnenrs w h i ch form the backshore- The method es ti mates mass balance of the cernpartment u s i ng the re l at ive

FIg 1.S Renourishment factor versus phi mean difference and phi sorting ratio (ref 16).

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Using ::::.as u ity Fig 2·5 shows R, contours plcued against the standard referenc axes ihat

compares exrural pararnc.crs for thc native and borrow comoosi:e A renourishmeru fa tor of 5

means ihat b row material is rhrc times as siablc as the native sediment On the ether hand RJ of

3 indicales ha: ihc b rrow m a r eri a l is one third as siable and if used as beach R il wo uld require

ren Llrshmcnt hrcc tmes as of ten as the native sedimenrs

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Being simpte in nature these beach ftll mode Is should be used co sidering the pracucal enviro ment

of re aI and unique engineering problems like:

(i) will a fill factor apply ro all the beach rill placed ')

(ii) which sedimentary data should be selec.ed to comp te a spc ific cornpo sire ?

(Îi i ) would the placement of fill on different parts of a beach require different fill factors ~

(ir) how should a borrow site composi.e distribution be mo ifted to rcr'lect (he effe ts of

different kinds of equipment and te hniques?

Table 2·1 gives the general steps for the sampling ana'ysis of bc ch and borro w site sed'menl~·

These are only rough guidelines Depending on the speciftc requirernent and unique c ha r a cr er i s t i cs they

are to be modifted ro suit a speciftc purpose

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Table 2·1 Steps for sampling and analysis·

I Depending on the relative importanc of the proje t and gcomr.ty of the bcach mark the s.auo ns

along the bcach al a regular interval

-2 Also mark the stations a ross the profiles below and above the plunger zone al ce r.ain eleva.ion

ditferenc depending on the steepness of the beac

h-3 Colleet the surfac sediment sarnple from each station just befere ihc mo soo and at.er the mo

n-soon- In c rrain c ses it would be advisable 10 colleer samples d rng ihe mo so n as most of

the erosion takes pla e during this tirne

4 Making 3 caiegones of samples (nearshore offshore and onshore) a split of the surnple should

be mixed thoroughly by adding equal weights of the splits

5 Using sieve ana'ysis a cornposue grain size distribution may be plo ed as in Fig C.3· Calculaie

S and M as ex lained in Appendix C

6 Co sidering the arnount available from e ch borrow si,e re!a ivc econ mies of the borrow materal an

the suitability of the sediment for prot-able composi.e fill ma crial and "he borrow si.e be selec.ed

7 Splits trom (he cores should be mixed in such a way iha: cash layer gers rcprcsencd in wcig ied

ratio depending on the thickness of each layer of different sedments

-8 Core samples should be colleered in such a way hat ir will deserbe the sediment charac.er isucs

2.3.1 General : Surfase waves play a major role in coas.al processes desig of coastal structurcs

maintenance of navigato n channe l and port operario ns- A thoroug understanding of ihe wave regime of

the are in quesuon is ihererore ne es

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