Protection and Control of Coastal Erosion in India

Erosion prevailing along the vast coastline of India has a long history. Coastal erosion, very of ten, poses a serious problem The nature and degree of protection required for a given coast vary widely depending upon the environmental conditions prevailing in the area. A comprehensive environmental study of the problem is required for developing a suitable solution to any specific coastal problem. In genera I, there wiJl be more than one method applicable to protecting an eroding area Hence, it is very desirabIe to consider both shortterm and longterm effects very carefully before determining the most suitable remedial measure to cernbat erosion problem. In this manual, an attempt has been made to present some of the remedial measures including the guidelines for suitable designs to control coastal erosion with special reference to Indian condinons. While some of the basic information has been presented in the text 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 problern. This manual is first of its kind in India. It is intended to be precise and effective .and makes no claim to be exhaustive. Nevertheless, the value of a manual of th is nature, dealing with diverse aspects of coastal erosion and its protection, cannot be denied. The original idea for preparing this manual came from Professor Per Bruun, who has considerable experience of working in Indian conditions for the past fifteen years or so. His major contribution and guidance during the preparation of this manual is indeed greatly appreciated.

COVER PHOTO:

Anjuna Beach, Goa, India - protruding

rocky cliffs offering natural proteetion

to pocket beaches

I I General review on causes of beach erosion

'·2 Rise of sea level

1·3 Heavy storms, storm surges, wave

action and its seasonal effects

1.4 Littoral drift barriers, natural

and man-made conditions in India

2.3.3 Relationship between the visual

and the Instru mental data2.4 Current and tide surveys

3·1·2 Beach and bottom profiles

3· 1·3 VVave machanics aspects

3·2 Review of coastal protective measures

3.2·1 Natural and man-made coastal protection

Page

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s w =BREAKING WAVE SETUP

Sy : y - COMPONENT SETUP 0

S.~: ATMOSPHERIC PRESSURE SETUP I

SA = ASTRONOMICAL TIDE ,n

PrefaceErosion prevailing along the vast coastline of India has a long history Coastal erosion,

improving and up-dating the manual in the future

S Z QASIMDirector

Some examples of coastal erosion on the west coast of India

Schematic diagram showing attack of storm waves on beaches and dunes (ref 43)

Various setup components over the continental shelf (ref 43)

Probable elevation of maximum storm surge on the south-east coast of India

Wave setup in a breaking zone in relation to tides, beach profile and energy

dissipation (ref 12)

Wave setup along a beach profile in terms of significant wave height (ref 12)

Naturallittoral drift barriers and headlands

Natural Iittoral drift barriers, tombolo and recurved spit

Effect of man-made littoral drift barriers

A group of groins used as Iittoral drift barriers

Some problems of littoral drift at tidal inlets

Improved tidal inlets as littoral drift barriers

Shoreline at Mangalore showing the location of the Bengre fishing village (ref 34)

Developing erosion at a jetty improved tidal inlet (a) showing persistent swelJ

conditions (h) during storm wave condition

A simple procedure for measuring beach and offshore bathymetric surveys

Procedure for rapid and accurate beach and offshore bathymetric surveys

Size frequency plots

Overfill factor (RA) versus phi mean difference and phi sorting ratio (ref 17)

Renourishment factor versus phi mean difference and phi sorting ratio (ref 16)

Tracer experiments to determine the predominant direction of Iittoral drift

A simple wave observation procedure to evaluate Iittoral drift

/

Longshore transport rate versus longshore energy flux factor for field conditions

(ref 43)

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

wave angle (ref 43)

SwelJ profile and storm wave profile

Various types of wave breakers

Breaker height index versus deep water wave steepness (ref 43)

Relative depth àt wave breaking versus breaker steepness (ref 43)

Schematic of a rock mound wall in front of a dune on an open beach

Schematic of a rock revetment for dune proteetion on an open beach

Schematic of a rock revetment for protecting the valuable shore property

with a provision of an access.to the beach

Schematic of a vertical rock gravity wall (for wave heights Iess than 0.5 m)

Schematic of a double piled fascine or bag crib ( for wave heights less than Im )

Schematic of a single piled rock crib (for wave heights less than 1.5 m)

Schematic of a simple mattress or gabion wall Ifor wave height less than I m)

Schematic of a simple revetment of sand bags ( for wave heighis less than I m ~

Page3567991011121314151617202022222.32728

32333537383945464748494950

~,

For the above formula, the relation between Hbr, height of the wave at the time of its breaking

and Hp thc height of the same wave while passing the wave polc are needed (sec Fig 2.7) Further field

o servations on the following lines wcre made to arrive at a relationship between Hbr and Hp with specialreference to monsoon waves-

St e p 1: From the knowledge of the b ttom topography, refraction diagrams for different wave directions

wcre drawn to trace the path of the

waves-St e p 2: Using the data fom the Shore Proteetion Manual of the U· S· Army Coastal Engineering

Research Centrc (ref 43) for the obscrvcd directon, the height of the wave at the pole and

the approximate depth at wave breaking were determined- A marker buoy was pla ed at

that point (see Fig 2.7)

S te p 3 From the kn wledge of the distance throug which the wave had to travel the actual time

required for a wave to travel from the wave pole to the point of its breaking at the markerbuoy was noted as t seconds

Step 4: Two theo olites were set up as close as possible at a strategie locaton to get a clear view

of the crest and trough of the breaking waves and the distanc s between the marker buoy andthe theo o tes were computed-

S te p 5: A person was posted to observe the wave height of a passing wave at the pole- At the same

time he signals the other two persons at the two theodolites- A stopwatch was used by the

posted person to count the time t for the wave to travel from the po Ie to the buoy- At theend of t seconds, and on receiving the signal, the vertical angles to the ere st and a few seconds

later to the trough, were read at the buoy by the two theodolites- The procedure was repeated

for a number of waves passing the wave

pole-S t ep 6: From the abov data and after knowing the vertical angle between the crest and the trough

and the horizontal distance between the theodolites and the buoy, the height of the wave at the

time of its breaking was computed

-FIOm a sufficient number ofreadings, the following relationship between Hbr and Hpwasarrived at:

Hi»=1-45 Hp (valid for waves of almost equal steepness)

Using this relationship and adopting the above integrated formula, the amount of littoral drifteach day for a period of one year was calculated- Table 2·2 is a summary of the littoral drift calcula-

tions (ref 29)

Table 2·2 Summary of littoral drift eaIculation for Ramayapatnam

covering the period from 31·5·1972 to 1·5·1973( e f. 29)

has the right order of magnitude as compared to the drift at the Madras harbour which has a similarwave.elimate as at Ramayapatnam- The above example shows how a difficult task could be accomplishedusing simple methods-

Forces acting on a gravity seawall

Circular slipsurace for a seawall (rel- 8)

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

Some failure mechanisms for piled retaining walis (ref 8)

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

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

a structure (ref 8 )

Geological map of nortb-west coast of India

Geological map of sou th-west and south-east coasts of India

Geological map of north-east coast of India

Climatqlogical factors at Jamnagar, Marmugao, Visakhapatnam and Pamban

Succession of dune plants at Miramar beach, Goa

Proteetion of transplanred seedlings by 'Checker board' method

(a) Dune formation by Spinifix littoreus at Miramar, Goa

(b) Growth of S littoreus

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

(b) Female flowers of S littoreus

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

(b) Typical bilobed and fleshy leaves of J. pescaprae

Plate F.4 (a) Carpet flora of Cyperus arenarius on sand dune at Miramar, Goa

(b) Mixed vegeration of C arenarius and l pescaprae

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

(b) Periploca sphylla growing in-sand in an arid region

Plate

F-Plate F'6 (a) Coastal erosion of sandy beach at Miramar, Goa

(b) Coconut plantation on sandy beaches

Page

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1 9129130130131131

132132133

List of Tables

PageTable 1.1 Causes of erosion attributable to nature and man (ref 3)

Table 2·1 Steps for sampling and analysis

Table 2.2 Summary of Iittoral drift calculation for Ramayapatnam covering tbe

period from 31·5·1972 to 15·6·1973 (ref 29)

Table 3·1 Breaker type in relation to tbe parameter ~ or ~b

Table 3.2 Natural and man-made coastal proteetion (ref 3)

Table 3·3 Needs for coastal prote tion (ref 3)

Table 3·4 Coastal protective measures classified in accordance witb tbeir ability to

provide proteetion to large and small shore areas and their influence on

tbe adjoining sbores (ref 3)

Table 3·5 Coastal proteetion in relation to souree of materials and conditions of

beach profiles for beneficial versus adverse effects

Table 3·6 Details of tbe performance of seawalls (ref 3)

Table 3·7 Details of tbe performance of groins (ef 3)

Table 3.8 Details of tbe perormanc of offshore breakwaters (ref 3)

Table 3·9 Details of the performance of artificial nourisbmen t (ref 3)

Table 3.10 Future coastal protective measures (ref 3)

Table A.I Parameters of long-term distributions of individual wave heights

(ref 10)

Table 8.1 Values of r for various slope characteristics (ref 16)

Table B.2 Approximate rock sizes in kilograms for various wave beigbts, slopes

and wave periods T== 6 to 10 seconds (specific gravity 2.65)

Table C.I Grain siz scales and soil c1assification systems

Table E.I Important engineering properties of common rock types (ref 15)

Table F I Distribution of sandy beaches along the Indian coastline

Table p 2 Distribution of dune species along the Indian coast

2242936

40

40

4142434344

44

577379

9599112121

122

Fig 3.23 Mainte n ance of inlet t o improve nävigation and t o decrease loss of materia l o

dee pe r water b y ebb ftows during the rnonsoon,

Dlost couutries of the world are surrouuded b shores of alluvial matcrials derived from inland andoffshore soure s-

Erosion is caused by the forces of nature, sometimes enhanced by man-made structures or b

rnan's a tvity of removing the material from the shore for b ilding or other commercial purposes- Table 1.1summarises some of the causes leading to natur al and man-made erosion-

Nature

Tabl e1·1 Causes of erosion attributable to nature and man (ref 3)

Man

Rise in sea level

Protruding headlands, reefs and rocks

causing downdrift erosion

Tidal entrances aud river mouths eausing

interruption of Iittoral drift

Shoreline geometry causing rapid increase

of drift quantity

-Blocking of river outlets carrying sedi

-ments to the shore by flood stage barriers,

change of loe tion of outlets due to

floods, erosion, teetonic movements etc

-Removal of beach material by wind

drift-Removal of beach material b sud en

outbursts of flood waters

-Dams, dykes and other coastal structurescausing rise and concentration of tides-

Groins, breakwaters, jetties etc-, causing

downdrift erosion.Man-made entrances causing interruption

of littoral drift- This includes jetties forproteetion of tidal entrances-

Fills protruding In the ocean to an extentthat they change local shoreline geometry

radically Sueh fiUsare often bulkheaded

-Damming up of rivers without providingmaterial sluices which allow continuatien

of drift of matcrials Irrigation projectsdecreasing flow of water and sediments

to the shore

-Removal of material from beaches forconstruction and other purposes-

.Digging or dredging of new inlets, channels

and cntrances- Offshore dumping of materi

als-The following paragraplis give the overall explanations for erosion- Section 3·1 deseribes basicphysics and engineering aspeets of the erosion problem-

1.2 Rise of sea level

Alrnrst all the mores in India erode (refs- 25, 28, 3 , 35, 40 and 41) Figs- 1·1(a) to (h) showsome of the examples of beach erosion occurring on the west coast of India-

Óne general reason for erosion isthe rise of the sea Ievel- The sea level rise (refs- 2 and 13) may

sound insignificant but it is necessary to realise how narrow a beach is, as compared to the offsh re are ,

which has to be nourished by the material eroded from the beach iu order to compensate for thc rise of

the sea level- With an equal amount of the deposited material at the b ttom, it is easy to work o t how

au average sea level rise of just 1 mm per ye r could cause a shorelinc reces ion inthe order of about 0·5

metre per year- The actual rise of the sea level along the Indian coast is not wellestablished- However,

i is generally a cepted that while the sca level is rising, a consolidation by settling takes place at the same

time in the river delta" lke the Hooghly- The average rise of the sea level appears to be of the order of

1 to 2 mm per year, which is the average rate accepted universally

-1.3 Heavy storms, storm surges, wave action and is seasonal effects

It is wellk own that heavy storms including severe monsoons, hurric nes and eyclones cause themaximum erosion rates- The explanation for this is that high and steep wavesbreak onthe shores producinghighly turbulent waters an uprushes which often atta k the dunes or coastal platforms directly, thereb ,

causing eros ion and creatn vertical sc a rps , which in turn cause reflection of the waves, increase the

(a) Photograph showing beach erosion at Punnapra, Kerala

during the Monsoon of 1967. (b) Photographrooted at Punnapra,showing how the coconutKerala due to beach erosiontrees we re being up-(1967).

(c) Photograph sh o wing erosion problem at a beach at

Trivan-drum, Kerala during the Monsoon of 1976. (d) PhotographKerala due to the tesbowing the uprrminal eeffect cf a seawalloted ccconut trees at Vypeen,.

Fig.1.1 Sorne exarnples of c o astal erosion on the west coast of India

F r a ymmetrical distribution, equation C·2 wiU give the median size but for an asymmetrical

distribution, M will be the most reliable estima.e of the q, mean- Thus S and Mare perhaps the best esti

-mates of the standard deviation,IJ" and cf>mean, !.I for describing a unitnodal sediment grain size distribution

-A comrno metbo to calculate S and M is shown in Fig Co3·In this figure, size data have been plotted

as cumulative distribution on log probability paper in such a way that cf> and percentage coordinates of a

p int on the curve indicate the percentage of coarser than a given cf> size The sizes associated with 84 per

cent and 16 per cent finer by weight are interpreted directly to calculate S and M (ref land 2)

References

Ho son, R.D ]977,"Review of Design Elements for Beach-Fill Evaluation", U·S· Army Corps ofEngineers, Coastal Engineerin Research Centre, Fort Belvoir, Va- Teehuical Paper No- 77-6

2 James, W·R·, 1975, "Techniques in Evaluating Suitability ofBorrow Material for Beach Nourishment" ,

TM-60, U·S·Army Corps of Engineers, Coastal Engineering Research Centre, Fort Belvoir,

Virginia

s w =BREAKING WAVE SETUP

Sy : y - COMPONENT SETUP 0

S.~: ATMOSPHERIC PRESSURE SETUP I

SA = ASTRONOMICAL TIDE ,n

=

Wave setup caused by breaking waves

Y-component of wind setup

The wind setup components jnclude the effects of surface wind-shear stresses and 'QQttom-friction

as weIl as the.influence of.earth's

rotation-The Iargest component contributing tothe rise of the sea level during storms, cyclones or hurricanes

is the wind shear stresses acting over the surface of water- Computational procedures för the determination

of wind setup are given in a number of publications includlng ref· 43· However, it is important to notethat the wind pileup iSrl?roportio!1a1 to the second power of the wind velocity and inversely proportional

to the water depth- The wind setups or storm surges during the cyclones and hurricanes are, therefore,

large st in the shallow water areas -of the continenta] shelf as in the upper: part of the Bay of Bengal and

in the Gulf Co ast of

Florida-Fig 1·4 gives the probable elevations of maximum storm surges on.the south-east coast of India.These values are-computed based on the assumptions that a sustained wind of 40lm/sec is blowing in anonshore direction and the centralpressure depression is 35 mb when the storm is approaching the coast-

It is also assumed that the storm surge coincides with the high spring tide (ref- 33) The astronomicaltide, in general, is quite smalt in magnitude, but can be very significant at certain geographical locationslike the Gulfs of Cambay and Kutch on the west coast and the mouth of Hooghly river on the east coast·Storm surges in combination with astronomical high tides can play havo es in the coastal zone- Information

on the tides can be obtained from the Indian Tide Tables published by the Survey of India, Dehra The atmospheric pressure setup, SL',pexpressed in metres is given by

Dun-SL',p=0·13 (Pn - Po) (1 - rB/r)

where Pn is the pressure at the periphery of the storm, Po is the central pressure in cm of mercury,

r is the radial distance from the storm centre to the computation point on the traverse line and R is thedistance from the storm centre to the point where the region of maximum winds intersects the shoreline-Rand r should be in the same units say in kilometres, metres or nautical miles-

The wave setup Sw mayalso contribute significantly to the total elevation of the water level inthe region shoreward of the breaker zone- It is caused by the inflow of water by wave-breaking anddepends upon the characteristics of the wave.and the bottorn profile and their mutual interaction, tides,

energy dissipation, bottorn matcrials etc- This is described in detail in ref- 12 which gives the results offield tests on the German Island, Sylt on the North Sea coast where beach and bottom profiles and wavecharacteristics have considerable similarity to conditioris found in the nearshore areas of the east and westcoasts of India Accordingly, the maximum wave set up, "'l ' max may be written as

Lh

Factor ~ =Lb

where Lh is the distance from the breaking point until the wave height has decreased to O.5Hs (HB isthebreaker height) Ls is the wave length at the breaking point B is the width of the bre ker zone andJ.1B is

the wave setup at the breaking point Other terminologies are defined in Figs- 1·5 and 1·6

Waves in the ocean, however, are irregular having certain spectra as explained in Appendix A·

In wave science and engineering, one distinguishes between a ge neration pha se when the waves are

The uprush or runup elevation

depends upon the wave characteristics,

W a v e set up in a b r ea ki n g z on e in r e l atio n 1 t ides, b e ach pr ofil e

a n d e n er g d iss ip ati on (ref 12)

India ha s a long s ho r eline chara c terized by varieties of coa s tal features like r ocky headlan d s, coral reefs and reef-like st ructure s, ti dal i nle t s , estuar i es , lagoon s , bar ri er islands , b ays e t c Such c oa st a l fe a tures

of t en give rise to ad v erse cond it iori s affeet i ng the shore s t ability a s they would ac t as complete or partial lit t oral d r if t barr i er s t hereby p reve n r ing the dri f of t he material to downdrift sh o r e s which , as aresult ,

will be s ubjec t ed to eros i on Fig s - 1·7 and 1·8show a' few typical examples of such natural structuralbarriers foun on the Indian sh res and Fig- }·9 shows similar barriers caused by man-made structureswhich also include a group of groins (F i g 1.10) One of rnan's worst destructive actlvities on the beaches

is the cxcavation and removal of thc beaeh material for land or road fill or for other construction purposes

-Sueh a lack of understanding of the most important principle of conservation is of common occurrenceall over the world as also in India-

BEACH ERODES HERE DUE TO

MOPLA BAY, I<ERALA

WALTAIR POINT, ANDHRÀ PRADESH

SHORELINE

-:O:N J

LlTTORAL DRIFT MATERlAL FROM R I GHT

DRIFTS ON ROCK REEF PAST "HARD POINT" ( ROCK OUTCROP)

INSTEAD OF NOURISHING DOWN DRIFT BEACH

EX AMPLES :

CANNANORE , KERALA

CAPE COMOR I N EAST, PUDIMADAI<A, TAMI L NADU

-

Fig 1 7 Natur a ! Iittoral drift barriers and headlands,

Fig 1.11 shows how a natural inlet or an estuary may interrupt the longshore drift thereby causi~g

downdrift crosion- This type of situation is very frequently seen both on the east and ",:,estco~stsof India

-As it is known, sornc material wilt always b pass the inlet and this proeess m~y be assisted eit er ?y theinlet eurrents or by the presencé ofbars or b a combination of both- The vanons degre s- of effectiveness

ER O SION

HEAO L A~D LlTTORAL. DRIFTEXAMPLES :

KAKINADA I MACHILIPATNAM ANDHRA PRADESH

Fig 1 8 Na t u ra l Iittoral drift ba rriers - t omb o l o and r ecurved s p i t.

::' , ', :: : '

EXAMPLES=

MADRAS HA~BOUR ANO TUTICORIN I;IARBOUR, TAMIL NADU

PARAOIP HARBOUR, ORISSA

PORBUNDAR, GUJARAT

RATNAGIRI J MAHÄRASHTRA

OETACHED BREAKWATE , R WHICH COULD BE A SHIPWRECK

EROSION

EXAMPLES:

VISAKHAPATNAM I ANOHRA PRAOESH

ACCRETION INITIAL SHORELINE

Fig 1 9 Eff e ct o f man-made I itt o r al dri ft h a rr i e r s

of such a transfer system are described in ref- 4· Some inlets,particularly those 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 the monsoon season- This would Bush the littoral drift material farther offshore 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 theinlet ebb currents- They facilitate bypassing of a major part, if not all, of the material drifting alongshore-Such natural bar- bypassing systems are found in very large numbcrs on the Indian shorcs- Examples ofthis type of offshore bars are given in Fig- 1·11· However, natural bar bypassers are undesirable for navigationbeeause the shoals or bars cause obstruction to free navigation from the bay or lagoon to the sea- Duringthe recent years, our knowledge and understandrng of the associated physieal processes have advancedconsiderably and such problems ean be solved by introducing proper dredging or by constructing suitablejetties or both as illustrated in Fig 1·12· Such improvements invariably cause erosion or incrcase theexisting erosion on the downdrift side of the inlet- -Examples of such occurrcnccs are numerous all overthe world including India (refs- 3 and 4) As indicated in Fig· 1.12, we find some intcrcsting examples onboth east and west coasts of India such as the dredged entrance of Cochin Harbour (38ft - deep at MLW)and thc 57 ft- deep dredged channel with groin and sand-trap-protection of the Visakhapatnam Harbour

including sand-bypassing by pumping- Both these cause severe downdrift

erosion-An interesting example of the intermittent natural bypassing is found at Bengre, a fishing village near

Mangalore in the Karnataka State (Fig 1.13) Although located close to the tidal Netravati 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 monsoon(May to October) But a temporary slow down in this natural bypassing process may intensify thc existing.beach erosion problem-

Fig 1·14 indicates how a jetty and channel improvement can cause considerablc crosion- Such ancrosion often does not take place immediately after thc establishment of such a littoral drift barrier Itmay take a few ycars bcfore it starts accentuating thc problem- This is largcly due to changes in wavecharacteristics caused by diffraction of waves (spreading of waves) resulting in the deercase of wave stcepness

therebyeausing a tempoary transport of material from the nearshore bottom towards the beach This leads

to a temporary stabilization of the beach- Reference is made to Section 3·1.2'for the beach and bottomprofiles under the influence of storm waves and swclls- However, as soon as thc limited quantity of

HONNAVAR • COONDAPUR I ' KARNATAI<A

KRISHNAPATAM I MACHILIPATAM , ANDHRA PRADESH

CHILKA LAKE INLETS , ORISSA

Fi ç 1.11 So m e problems o f littoral dr i ft at tidalinlet~

Fig 1 12 I m p ro ved ti d a l i n let s as l in o r a l dri f t ba rr ie rs.

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 the beach and offshore bottoniprofiles is when the erosion of the beach starts of at a rapid rate- As a consequence of this, the nearshore

bottom in front of the beach ten s to develop a more gentleplatform-like slope and, in some c ses ashoreline recession of this type would lead to simultaneous seaward movement of depth contours caused

by temporary depositio 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 1.1

It is very important to consider all possible adverse effects noted above when any improvement isplanned or executed along the shoreline- In all problems related to coastat protection, it seems, somewhat

S E A

illogical or undesirable to construct a structure for protecting or stahilizing a shoreline- Beeause, such

an action, after taking into consideration all thc factors on a broader perspective, may tend to producemore harm than good (See sections 3.2·3, 3·2·4 and 3.3.3) It would also be wrong to allow harboutentrance structures or jetties 1.0 cause serious erosion on their downdrift side, often leading to serious loss

of valuable land and propert

y-This type of calamity can now always be foresecn and r~tified' beforeany damage could This requires a thorough study of the problem and proper planning beföre such projects are earried out

occur-A coastal engineer, confronted with a shore proteetion or improvement -problem, should be in a position

to evolve au economical and technically viabie design making the best use of the environment al data,

uptodate knowledge, experience and judgement

Fi g. 1 14 D evel o pinî e r osion at a jetty improve d tida l in l et ( a) s how i ng persi s te n t

swell conditi on s, ( b) during stor m wave condition.

Beach Surveys

Asirnple but approximate meth d of measuring beach profiles, as described below, can easily be

ado ted after establishing a reference level o a backsh rc sand dune b driving a peg or b rying a large

rock- This reference levelshould beconneetod'to amore permanent ne rby bench mark which in turn should

be conneered to the nearby Survey of India bench mark, if available In order to me sure the beach profile

at the same location each time, anothcr obje t such as a light post, corner of a building etc should beused in line with the rcference point and perpendicular tothe shore- Fix stato s at 3 mintervals (Fig 2.1)

The technique of levelling requires two pers ons, o e person to hold the graduated statf and the

other to observe from the reference point The o server sights on the vertical staff from the top

of the reference level and notes down the reading of the horizo - Sincc the line of vision to thehorizon is nearly horizontal, the reading on the pole which is graduated from the bottorn will givethe difference in height of the station below the reference level- This procedure is repeated for allthe stations and the heights noted- Generally additional reference points are required along theprofile since the staff used i's not of sufficiënt height to cover the entire drop in sand level across

the beach profile These can be easily established along the line of the stations as required- Sandlevel of each station can be computed with reference to the original reference level (Fig 2.1)

For more accurate profile surveys, and when the horizon isnot clearly visible, the usual surve

-ying methods using either plane tablc or spirit level can be adopted- In shallow water and thesurf zone, modification in the method is necessary- A wader then operates the level staff or stadiaboard- In deeper areas an electronic depth recorder (echosounder) can be used to take continuoussoundings of the bottorn- Altcrnatively, a leadline can be used to take spot soundings Fixing the

position of the levelling stations can be made from the shore using transits or pre-determined rangelines andmeasuring tapes or from the survey boat itself using sextant angles to three fixed objects on the shore

As an example, a specific procedure for rapid and accurate beach and nearshore bathymetricsurvey can be as follows: A baseline is established along the stable landward area of the beachwhich is to he used as a basic control for survey ranges (profile lines) normal to the baseli e

These ranges, along which the profiles are taken, run from the baseline across the beach seawardFor a long-term beach study, the baseline can he monumented by erecting masonry or concrete posts-Any existing structure 1ike curbs, lamp posts, fishing piers, buildings can also serve to establish andrelocate a baseline or the survey ranges- The position fixing along the range line can be secured by atransit intersecting a level staff or a sounding boat operated along these ranges as illustrated in Fig 2.2

Prior to 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 on the range at the requireddepth- The sounder operator then logs in pertinent information on the sounding record (paper chart)prior to each run, namely the station number of the range line, the starting time and the date- The boat

then proceeds towards the shore along the predetermined range line e,iher ,by thc monuments fixed

19

SANö~ " BEAcH~ : t:\ : ~:T: : : : ~ :: : : ': :-:;: : : : : ', : ~ : Z? : : 7 :: ' ! : ?2::~~lUHE '

Fig.2 1 A s i m p le p r o c ed u r e for m eas ur i ng be ac h a n d offsho r e b athymet r ic s u rve y s

LAND

SHORE UNE -'

~ J J~

-2.2 Sand sampling and analysis

2·2.1 Sand sampling: Design schemes for sediment sampling should be made in such a way

arialysis-2.2.2 Sample analysis :Although Indian Standard Classification System and Wentworth Scale

cf>can be written as (ref 17 and 18)

atcf>=f.L and inflexion points at f.L±0- where f.L is phi mean and 0-is phi sorting- Using the combination of

Fig.2.3 Size freque n c y plots.

O v erfil l jactor method: There are three different approaches for determining the overfill factor

(i) beach sediment is considered to be most stabie for the environment,

(U) the entire volume ofthe fill material placed on the beach is sorted by local processesto achieve

a grain size distribution similar to the beach material, and

(Ui) sorting processes change the fill materials into the beach-like sediments by winnowing out

a minimum amount of the original fill

-The above method proposed by James (ref 17) is based on the selection of the criticalor stabiegrain size distribution of the borrow sie sediments and it quantifies the amount by which that distribu-

tion is to he modified to resembie the be ch sediments- Actual calculations of R ( ato of the weight

percentage of the beach to that of the borrow site composite) involve complicated rnathematics but'

accurate 'graphical estimates can he obtained using the curves shown in Fig 2·4 The basic information

required is the phi mean and phi sorting values for beach and borrow site sediments

-R enou r ishment facto r method : It is a dynamic approach to describe how beach processes can

be expected to modify specific fill sediments (ref 16) This te hnique is used to estimate how often,

placement of a partienlar fiU wil! be required to maintain specific beach dimensions It attempts toevaluate long term performance of different fill materials with regard to suitability, maintenancc andcost- In this method, the active beach system is treated as a oompartment which rcceives sediments

through longshore transport and from gradual erosion of the inactive reservoir of the sediments which

form the backshore- The metbod estimates mass balance of the compartment using the relativeretreat-rate equation

log R = /;:, (~- j.l n) _ ~ 2 crn 2 , 1

where Rj = relative retreat rate (ratio of the borrow.to beach retreat rates)

b , n = subscripts for borrow and beach material respectively

~ = dimensionless parameter (0.5 to 1.5)

Using ~ as unity, Fig 2·5 shows R , contours plotted against th e standard reference axes thatcomparès textural parameters for the na tive and borrow composite- A renourishment factor of,!'means that borrow material is three times as stabie as the native sediment- On the other hand, Rj' of

3 indicates that the borrow material is one third as stable, and if used as beach fill, would require

Being simple in nature, these beach fi.ll mode Is should be used considering the practical environment

of rea I and unique engineering problems like:

(i) will a.fill factor apply to all the beach fill placed ?

CU) which sedimentary data should be selected to compute aspecific composite?

(Ui) would the placement of fill on different parts of a beach require different fill factors ?

(iv) 'how should a borrow site composite distribution be modified to ref'lect the effects of

different kinds of equipment and techniques?

Table 2·1 gives the general steps for the sampling, analysis of beach and borrow site sediments

-These are only rough guidelines- Depending on the specific requirement and unique characteristics, theyare to be modified to suit aspecific purpose-

Tab l e 2·1 Steps for sampling and analysis

1· Depending on the relatve importance of the project and gcometry of the beach, mark the stationsalong the beach at a regular interval-

2· Also mark the stations across the profiles below and above the plunger zone at certain elevationdifference dopending on the steepness of the beach-

3 Collect the surfac sediment sample from each station ju st before the monsoon and after the mon

-soon- In certain cases, it would be advisab1e to collect samples during the monsoon as most ofthe erosion takes place during this time-

4 Making 3 categories of samples (nearshore, offshore and onshore) a split of the sample should

be mixed thoroughly by adding equal weights of the splits

5 Using sieve analysis, a composite grain size distribution may be plotted as in Fig C.3· Calculate

S and M, as explained in Appendix C

6 Considering the amount available from each borrow sie, relative economics of the borrow material andthe suitability of the sediment for probable composite fill material an the b rrow site be selected-

7 Splits from the cores should be mixed in such a way that each layer gets represented in weighted

ratio depending on the thickness of each layer of different sediment

s-8 Core samples should be collected in such a way that it wiU deseribe the sediment characteristics

of the whole borrow si

te-9 Composite grain size analysis should be catried out by sieving and plots should be made as inFig C.3· Calculate S and M·

10 Using the ren urishmentfactor technique, theamount of renourishmeut eau be calculáted usingt.as unity

-2.3 Wave Surveys

2.3.1 General : Surfase wavesplay amajor role in coastal processes, design of coastal structures,

maintenance of navigation channel and port operato s- Athorough understanding of the wave regime ofthe area in question is, therefore, neces ary-

iS

There are various ways of co eering and comp ting the wave data and several methods are inuse for their analysis- The most common oncs are: (a) visual observations, (b) hind-casting and (c)

by using wave recording instruments- Most of the data on waves presently available are based on visual

o servations made from the ships- By hind-casting the wave heights are computed for an area usingthe past meteorological data of the regio - The advantage of this method is that the results will be based

on comparatively large wave data which would help in extrapolations Instrumental data pertaining towaves at present are very few- These are more reliable and necessary for spectrum analysis- The instruments

used for wave measurements are either surface (e g.wa- wrider buoys) or sub-surface (e·g. OSPOS) type

2.3.2 Wave me surements: Waves ca.r5e measured in the coastal zone by adopting easyas well

as less expensive methods- Locally availabëematerials and workshop facilitics could be made me of forthe design and fabrication of a ~mtáblesystem for the colloetion of wave data For examplc, sparbuoys, wave polcs etc-, c; utdeasily be fabricatcd and installed The measurement procedure involvedand the arialysis are quite simple- Proper training to local persons is all what is nceded for thecollection of wave data Wave observations may be made for a group of say, 3 waves and theaverage height of these waves (Hm) and thc highest 10 per cent waves (H10) are recordcd- Other obeervationsshould include breaking wave height, angle made by breaking wave with the shoreline, breaker types etc

For visual wave observations, the following table may be used for recording the wave data:

D ate Time Pe r iod H eight D irec t ion B reaker ty p e Re marks

2.3.3 Relationsbip betweenthe visual and tbe instrument al data: As mentioned earlier, the datapresently available are largely visual- Hence, it is essential to find a relationship between the visual wavehcight H, and the visual wave period Ty and the significant wave height H, and T, (zero crossing period)-

Nordenstrom (ref 27) has worked out a relationship between H', and H, based on North Sea wavedata as follows:

2·3The above equation gives the relationship between H, and H, at the same level of probability-Similarly, the relationship between Tz(zero-crossing period) and Ty (visual wave period ) can be expressed as

as described in Appendix A· This appendix includes ·information on the wave spectrum

also-2.4 Current and ti de surveys

2.4.1 General : Currents are one of the most complex phenomena in the ocean as they aregenerated by various driving mechanisms- The principal parts of a current system in the coastal zoneare tidal currents, wind driven currents, wave induced longshore currents and rip currents-

To understand the phenomena of sediment transport, current system can be divided into twoparts, v i z - , within the surf zone and beyoud the surf zone· The current systems in the surf zone arevery important 'for thc sediment transport- They derive energy from breaking waves- The current systems

beyoud the surf zone are mainly governed by the wind and tide

s-: 2 4 2 C u rrent measurements: Current measurements could be made using either of the two wellknown approa hes, viz - Eulerian or Lagrangian- In the former method, measurement ismade as water

passesa fixed point and in the latter, b y. following thc path of water particles- Both principles havetheir lirnitations but measurements at fixed.p ints with current meters are far more common than the

"path fo owing metbod using neut rally buoyant floats (Swallow's floats) or parachute drogues

-I '

I But again it is important to study thc requirement for which the measurement is necessary Inthe case of forces on offshore structures, both in· the short and long term statistics, the only type of

;data that could be used is from the Eulerian type of current measurement- The path following method

or thc indirect way of deducing currents by surface f'loats or other devices wil! most certainly be of greatimportance in thestudy of nearshore currents and ~urrents in thc surf zone, which influence the coastal processes

Current measurements in the surf zone are made by using dyes and floats- The dyes are packcd inbags which are either porous enough to allow water into thcm or dissolv ~in' water thcrebyexposingthe dye- -The rate at which the dye patch moves, givcs the speed of the current-

Measurements of currents using floats consist of a system in which a ball of say 15 cm in diameter

is connected to another ball of 7·5 cm in diameter with a fishing lino of say 3 m length- The largerbal].~sfillcd with sca water using au injector and the whole system is thrown b hand into thc surf zone

Smm the s.h re: The position of (he bigger ball is rneasured each time with respect to the floaiing srnalhall which is connected to it-

Móst comm'm and simpte equipment uscd beyond the surf zone for measurements are neutrallybuoyant floats or parach te drogues- These are releascd at known depths and tracked with the help oftheodolites from shore or they can even be folio wed by boats and their positions can be fixed by sextants-

The foll wing table can be used for recording the current observations in the field:

D a t e T i m e D e p t h ( m ) Sp ee d ( m/ sec ) D ir e t on Rcm a r ks

predierion ofextreme values- This is because the currents consist of periodic componénts (tidal components)added to the residualcomponents- First, the periodic components are to be subtractcd and later spectrumanalysis eau bemade for thc residual components- A detailed arialysis of thisnature is of much importancefor the design and operatien of offshore works and hence is not treated here in detail-

2.4.4 Tide surveys: Information on the tides is equally important as tides directy influence the limit

of wave ata k, port navigation, flushing in estuaries and inlets etc· Variation of water levels due to tides

can be rccorded either by erecting tide poles or by water levelrecorders which are readily available In the

case of tide poles,daily readings for the low and high waters should be t

aken-Measurements using a water level recorder should be a continuous process indicating the variatien

ofthe tide elevation over a required period of study

-For areas nearer to harbours and anchorage points, the Indian Tide Tables publislied by theSurvey

ofIndia, Dehra Du for the year under consideration may.be referred to·

The tidal currents vary from place to place depending upon the character.of the tide, the waterdepth and the configuratio of the coast- The tidal components of the current n the sea or bay areperiodic in behaviour and repeat themselves as regularly 'as the tides to which they are related- In

general it is necessary to study the chan e in water levels and associated currents at least for a period

21covering hoth neap tide and the ensuing spring tide at a locality Several such studies ma)', sometimes,

become necessary deponding upon the complexity of the'problem· being investigated at a givén coastal 'area.2.5 Linoral drift surveys

To evaluate the possibility and the expected rate of erosion caused by the establishment of littoral driftbarrier, it is necessary to know the predominant direction of the drift, and if possible, the magnitude of

data are important for the development of port structure or atidal inlet or when an estuary is to beimproved by jetties-or dredging Littoral drift surveys are needed forthe colleerion of data and calculations

should be made as explained in Sectien 3.1.1

Usually itis not too difficult to determine the predominant direction of the drift, particularly if theshore has some headlands or reefs or man-made structures which, by the sand accumulation pattern, winshow the predominant direction of drift (Fig 1.7) If no natura! or man-made obstruction is available, as

in the case of many shore segments along the east coast of India, a study of the migration of natural inlets

along the coast would give the desired information as the inlets migrata in the direction of the nant littoral drift It will, however, be difficult to determine the relative magnitude of the -drift in either:

predomi-direction Evaluation of the ratio between drift quantities in two directions may be done by the use oftracers Fig 2·6 illustrates how tracers ,can be used'for the determination of relative magnitude of

If tracers are injected in the beach and the samples are taken from the be ch and offshoreareas to cover one cycIe, the ratio of southward drift to northward drift along a beach of length Scan

he written as

J C s d I JCn dS=A/B

where C s and C n are the concentrations of the tracer material at distance S, from the injected areas

-S bscripts sand nare usedto indicate 'south and north directions respectively A and Bare southward

an northward drift in cubic metres per year respectively

Calling this ratio a , the stability criteria is T=A- B, where

called the predominant drift, one has

A / B = a r B =~ andT=A(a:l)

Hence, if A and a are known it wil! then be possible to compute Band T

T is the balance of drift, usually

BEACH

' ; : FLUORESCENT TRACERS IN EACH

I NJECTION TO

BE REPEATED 3 - 4 TIMES DURING A YEAR

The determination of t h e q uantity A may, how e ver , invo l ve considera b le diffic u lties unless a proper litto r a l drift b arrierjs availa b le - It co u l d b e a nat ur a I hea d land, a har b our or inlet jetties or a dredged channel- Such b ar r iers , howev e r , are gene ra ll not availa b le close to the site in q uestion- Therefore ,

computat i ons of the drift could b e made as mentioned b

3 1 1 h ave b een prepare d - S ee ref - 2 and 4 3 in , which littoral difrt form u lae have been compiled- S ec also Ch apter 6 of ref- 2 · A simple fie ld meth o was a d opt ed b y the P re - Investment Survey of Fishing Har b ours , G overnment of India , B angalore, for evaluaing the littoral drift con d itions at R amay a patnam

in Andhra P ra d esh (ref 29).

W ave data were o b tai n ed using a gra du ated po l e d r ivcn down i n t h e b ottom on the outer s l ope

of t h e offsh o re b a r Ob servations were m ad e visua l ly b y m e ans of b inoc ul ars from a 4 m hig h tower

H I l a =height of'the significant wave at the time of or just beforebreaking in metres

IXb =the angle of the breaker crest to the shoreline in degrees

IC X x x )( )()( IC X x)()( X)( )( IC IC IC IC)( X X x

BACK WASHt JUPRUSH

OF WAVE HEIGHT OBSERVATION OF BREAKER

Fi g , 2.7 A si mple wave observa t ion procedure 10 evaluate linoral grift

For the above formula, the relation between Hbr, height of the wave at the time of its breaking

and Hp thc height of the same wave while passing the wave polc are needed (sec Fig 2.7) Further field

o servations on the following lines wcre made to arrive at a relationship between Hbr and Hp with specialreference to monsoon waves-

St e p 1: From the knowledge of the b ttom topography, refraction diagrams for different wave directions

wcre drawn to trace the path of the

waves-St e p 2: Using the data fom the Shore Proteetion Manual of the U· S· Army Coastal Engineering

Research Centrc (ref 43) for the obscrvcd directon, the height of the wave at the pole and

the approximate depth at wave breaking were determined- A marker buoy was pla ed at

that point (see Fig 2.7)

S te p 3 From the kn wledge of the distance throug which the wave had to travel the actual time

required for a wave to travel from the wave pole to the point of its breaking at the markerbuoy was noted as t seconds

Step 4: Two theo olites were set up as close as possible at a strategie locaton to get a clear view

of the crest and trough of the breaking waves and the distanc s between the marker buoy andthe theo o tes were computed-

S te p 5: A person was posted to observe the wave height of a passing wave at the pole- At the same

time he signals the other two persons at the two theodolites- A stopwatch was used by the

posted person to count the time t for the wave to travel from the po Ie to the buoy- At theend of t seconds, and on receiving the signal, the vertical angles to the ere st and a few seconds

later to the trough, were read at the buoy by the two theodolites- The procedure was repeated

for a number of waves passing the wave

pole-S t ep 6: From the abov data and after knowing the vertical angle between the crest and the trough

and the horizontal distance between the theodolites and the buoy, the height of the wave at the

time of its breaking was computed

-FIOm a sufficient number ofreadings, the following relationship between Hbr and Hpwasarrived at:

Hi»=1-45 Hp (valid for waves of almost equal steepness)

Using this relationship and adopting the above integrated formula, the amount of littoral drifteach day for a period of one year was calculated- Table 2·2 is a summary of the littoral drift calcula-

tions (ref 29)

Table 2·2 Summary of littoral drift eaIculation for Ramayapatnam

covering the period from 31·5·1972 to 1·5·1973( e f. 29)

has the right order of magnitude as compared to the drift at the Madras harbour which has a similarwave.elimate as at Ramayapatnam- The above example shows how a difficult task could be accomplishedusing simple methods-