Coastal erosion problems at Nusa Dua Beach, Bali

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11-COASTAL ERO ION PROBLEMS

AT NUSA

DU

lACH

BALI

RESULTS OF TBE S'l'UDY REALlSED IN TB

MXtN

Ot '1'BE MEMORANDUMOF

UND

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UTNlDntG

BE'l'WEENTB MINISTRY OF PUBLté ~ _ POJtLIC OF INDONESIA ANDTIII

MtMllftY OF TRANSPOJtr

»m

~f.,le tfQN(I

cr

ft! NETBEIUANDS.

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I N D E X

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Chapter 1. Introduction 2. Terms of Reference 3. General Conclusions

4. Existing Studies, Investigations and Knowledge in respect to the beach behaviour in Southern Bali

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5. General description of the coastal processes at the Southern part of Bali

6. Description of the processes at Nusa Dua

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7. Measures taken up till now against the erosion of the Northern beach of Nusa Dua

8. The effect of construct ion works on a sandy coast 8.1. General

8.2. The effect of perpendicular groynes 8.3. The effect of parallel groynes

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9.

The effect of protection works to the Nusa Dua coast

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9.1. The groyne system build in 1981

9.2. The groyne system as suggested by Sogreah 9.3. The plan of Tourdevco, Svasek and ADC

consuiting engineers

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9.4. Sandsuppletion

10. Choice of the most suitable solution 11. Realisation of the project

12. Acknowledgement

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Photo annex page 2 Page 3 3

4

5

6 10 17 18 18 19 25 29 29 31 32 33 35 40 40

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page 3 1. Introduction

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On 21 june 1985 a meeting took place between the Secretary-General of the Indonesian Ministry of Public Works and the Secretary-General

of the Dutch Ministry of Transport and Public Works. During this

meeting assistance in terms of Manpower from the Dutch Ministry was requested for a number of projects. As a result of this meeting the

author ir. G. van Bochove of the Rijkswaterstaat was selected to

assist the Indonesian Gouvernemental Departments in respect to two projects.

The first project concerning the Coastal Zone Development on the

Isle of Bali is discussed in this report. In the period from 22

October up till 16 November 1985 a visit was paid to Indonesia.

During this visit meetings were held with pers ons involved, existing

reports on the subject were studied, and site observations were

carried out.

The mission has resulted in an advice concerning the best solution for the beach eros ion problems at Nusa Dua beach.

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2. Terms of reference

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- Objective

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Main objective of the visit of the Dutch coastal engineering expert to Bali, was to advice on the best suitable solution for the erosion problems at Nusa Dua Beach, Bali.

As weIl short term solutions as permanent solutions had to be taken into consideration.

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- Scope of work

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1. Study all previous reports on coastal erosion problems and

coastal behaviour in Bali 2. Carry out site observations

3. Establish the reasons for the occuring erosion and the main

acting processes in the coastal zone

4. Give a judgement on the different suggested alternative solutions for the existing erosion problems

5. Carry out studies and develope ideas and advice in close

coperation with the departments involved, in order to make the visit more fruitful and the advices better understood.

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page 4 3. General Conclusions

a) The dominant influence for the beaches at southern Bali is the ocean swell coming from the South.

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b) theThe mainshellssourcestransportedof sedimentby theforswellthe beachestowards of southernthe coasts Baliand arethe material coming from the eroding cliffs of Bukit peninsula.

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c) The main reason for the erosion of the Nusa Dusa beach north of Nusa Kecil is the blocking of the north going littoral drift, by the peninsuia of Nusa Kecil. Near lot N5 and N6 the tidal and wave induced ripcurrent becomes a very important feature that can not be neglected.

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d) All structures build on the beach have decreased the attractivity of the beach, however they have not solved the eros ion problem. e) The solution for the erosion problem of Nusa Dua as accepted

during a meeting of the l5th of Ocotober 1985 will not solve the erosion problem either.

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f) WhenTourdevcodifferentSvasek solutionsand ADC givesare money"

compared, the suggestéd plan by without any doubt most "value for

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g) In case this plan can not be carried out within a couple of years a sandsuppletion could be considered as a temporary measure.

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h) Design and project preparation should be coperation between Tourdevco Svasek and order to increase the knowledge with BPMA.

carried out in close ADC and the BPMA in

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page 5

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4. Existing studies, investigations and knowledge in respect to the beach behaviour in southern Bali

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Uptill about 10 years ago very little attention was paid to the coastal behaviour in Indonesia. Then because of a number of erosion problems that occured, the interest grew, and under the department for rivers of the Hydraulogic Institute in Bandung (BPMA), a section for coastal engineering, has been erected. During many years this section was assisted by the United Nations. Up till 1976 Tsuchiya of the UN was involved with the development of knowledge with the section. He gave lectures introduced measuring technics and supplied measuring equipment (stadia wave meater). In his final report dated 1978, Tsuchiya comes to the conclusion, that the section has shown the ability to perform coastal measurements and prepare reports about these measurements. That however no solutions are presented by the sections and too little knowledge is gathered from international publications. Now, 7 years later, the author has the impression that the knowledge of the section has increased considerable, and that a lot of knowledge about low-cost solutions for coastal defence has been gathered while these structures have been tested in practise, at different locations. However the section still knows its limitations; for example:

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- The measuring equipment of the section is far from ideal

- Wave data (period, heights and angles) and measurements at coastal changes over a long period still are missing

- Computer programmes on combined refraction and diffraction and coastline changes have to be developed

Little practical experience exists in relation to the design of large scale coastal defence works.

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As far as the beach of Nusa Dua is concerned a general idea about the reasons for the occuring erosion exists. Further a number of small groynes have been constructed from 1981. The effects of these groynes are known. Coastal profiles, tides, winds waves, currents and bottom materials parameters have been measured too. However the number of these measurements are limited and do not always give an answer to the existing questions.

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Besides the Hydraulogical Institute in Bandung and UN also other specialists have been advising on the erosion problems of Bali. a.o. Diephuis (the Neth.) Silvester (Aus.) and Kooman (the Neth.);

and special on the Nusa Dua erosion problem the Sogreah Hydro. lab.

(Fr.) and Svasek and ADC Engineering Consultants (the Neth.) together with their Indonesian partner Tourdevco. The advices of these two institutions will be discussed more thoroughly in the next chapters.

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page 6

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.

5. General Description of the coastal processes at the southern part of Bali

---From the gathered information a.o. areal pictures .aps, .easuring data and observations by the author of the beaches at the southern part of Bali, the following general morphological system could be determined:

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(

'BAL.J

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PP'l1"'C.U.U."

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Fig. 1.

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page 7

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The dynamical forces acting on a beach are: wind, tidal currents and

wave attack.

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- The wind as measured at the Airport of Denpasar shows Westerly

directions in the period mid November until April and from April

up till mid November directions between west and southwest. The

wind velocity is in general about 5 m/sec. with a max. wind

velocity of 12 m/sec. during occasional storms. These windspeeds

are not able to move the rather coarse sand of the beacher

(0,5<dm<2 mm).

- The tide at Benoa shows a period of 12 hours and a tidal

difference from 2.70 m (Spring tide) till 1.10 m (neap tide)

Besides in the major tidal gully leading to the lagoon north of

cape Benoa and in the openings in the coral reefs the tidal

currents have very little influence on the beaches. The reason

for this is that most of the beaches are protected by coral reefs,

while the wave action in the near shore zone tends to push the

tidal current to deeper waters.

- It is very clear that the waves attacking the coasts of southern

Bali are the main reason for the coastal changes. Two types of

waves can be distinguished, the swell comming from the south and

waves caused by the local wind influence.

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Already Tsuchiya tried to solve the problem of the missing wave

data, by using the wind data from the Denpasar airport in order to

predict the average wave climat.

However the position of Java to the west and the islands of Nusa

Penida and Lombok to the east causes limited fetches in both

dominant local wind directions.

Further the occuring storms are limited in number strength and

duration, leading to a situation where the permanent ocean swell

becomes the most dominant influence to the beaches.

This result is in agreement with available US Navy wave statistics

showing at the location 9 -12 S/113 -116 E a predominant role of the

southern directions (SE: 21,3% of time, S 35,4% and SW 16,4% of

time) .

The wave period of the swell is rather large (observed by the author

10-15 sec) while the wave heights can be considerable (Hbr uptill

4,50 m, on 6 Nov. '85) .

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page 8

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Now what will be the effect of these swell waves to the coasts of

southern Bali?

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1. Because of the large wave period and related large wave length at

deep water. (To =12 sec, Lo=225 m) waves will "feel" the bottom

at rather large water depth ( d= + 110 m).

This will aswell result in the turning of the

the depth contours because of refraction, as

bottom material towards the shores

Longuet-Higgins).

2. When the waves reach the shores and start breaking they will

cause a destructing force to coastal rock formations and further

cause a longshore current, able to transport beach materials

along the coasts. The equations describing the transport

capacity of the wave induced longshore current are mostly written

wave crests towards

in the transport of

(according to

I

as:

I

Q

A

Ho'"

.s. V"\

o<~..(.,0scx~ -. ( 1)

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where:

Q

=

Transportcapaci ty in [m3/sec.]

Ho= The deepwater wave heigth in [m]

tl<j,,7 The an g le of wave attack at the b reak er line

A

=

a constant factor

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From this equation it can be seen that because of the refraction the

dominant direction of wave attack, coming from the south, will lead

aswell at the southwest as at the southeast coast to a sand

transport to the n ort h . (see fig. 1).

For this reason the existance of the large sanddeposits along the

southwest coast and southeast coast of Bali only can be explained if

a large source for beach materials is present near the Bukit

peninsuia at the far south of the island of Bali.

This source will most probably be the limestone cliffs of the Bukit

peninsuia itself. These cliffs are heavily attacked while no coral

reef is present to protect them. Further the see bottem material

transported towards the coast by the swell waves, existing mainly of

shell particals, will be a second important source for beach

material.

From beach sand samples along the coast it can be seen that the sand

exists of limestone sand and shellparticals.

Now two examples will be presented, confirming this general theory

about sand transportmechanism in southern Bali.

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page 9

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A) Sanur beach near Hotel Bali Beach.

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In front of the Hotel Bali Beach a pier has been constructed.

South of this pier a nice beach exists however north of the pier hardly any beach has been left.

Simular accretion- and erosionpatterns can be observed all along

the Sanur coast where perperdicular groynes have been

constructed, proving a dominant northern sand transport.

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B) Kuta beach near the extended Ngurah Rai airport.

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A couple of years ago the airship of the Bali airport has been extended to the west out into the sea; blocking all sandtransport

near the beach. As a result of this extention south of the

runway a nice accreting beach of coarse sand exists.

However north of the runway over several kilometers eros ion has occured, proving a northern sand transport aswell.

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page 10

6. Description of the processes at Nusa Dua.

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Nusa Dua is located at the eastern coast of the Bukit peninsuia. The area has been developed by the Bali Tourist Develop.ent Coop.

(BTDC) for tourism and at the moment four luxurous hotels are in operation located north and south of the two small peninsulas. The fifth hotel is under construction (see fig. 2).

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Fig. 2.

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·

...

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page 11

The beaches

consist of

white coars

sand (0,5<d.<2•• )

the beaches

show slope of 1:5 uptill 1:9.

In

front of

the beaches

there

is a

shallow flat

with a

rather

constant depth of ±1.00 m ~MSL,

and a

width of 400 • uptill 600 ••

This flat

is bounded

by a

coral reef

with a

depth equal

to MSL

uptill

0,80 ~ MSL.

Outside the

coral

reef

the depth

increases

suddenly to 7 mand

more.

The coral reef shows two holes, one south

of the two Nusa's and one in front of Lot N6

.

An other

remarkable fenomena is the

step in the beach

line at the

northern boundary of Lot N6, (see fig

.

2).

'

.

00....

+M$L 500

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page 12

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Fig.

3 shows a typical cross-section of the beach at Nusa Dua.

The waves attacking

the shores are loosing most of

there energy at

the coral reef.

The remaining wave

that has been reduced in height

considerable, will loose its energy

co.pletely in the breaking zone

near

the sandy

beach.

The breaking

of the

waves

will cause

a

longshore current and a set-up of the water level.

Further the turbulence will cause suspension of the beach material

.

The longshore current and the suspended material will be responsible

for the littoral drift as described in chapter 5.

The longshore

current will be

concentrated in the two

zones where

breaking of the waves takes place.

At

the

coral reef

the

longshore

current

will hardly

move

any

material, as hardly any material

is available.

Only near the beach

materialtransportation

will

take

place

by

the

littoral

drift

resulting from the already reduced waves.

Thus it will be clear that

the actual longshore transport is only a

small part of the longshore transport capacity that results from the

attacking waves.

For

this reason

the coral

reef is

of ut_ost

importance for

the

protection of the coasts of southern Bali.

Besides the littoral drift also the wave set-up, that can be up till

50

cm for the existing situation,

can play an important part in the

coestal processes.

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.",,:1\\1'ê

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N

RIt' (..L..I,...l..~ê,\1i _r .". 'w .\'.C t

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---

,-.

____,-

_

;.

--,.:___

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, I,

t

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;._. ;

/

,

\

i

~

.

_,.; _,.

-.

1111

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N2. :,'" -;"

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\\

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" -_

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Fig

.

4. I

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page 13

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As shown in fig. 4 the coral reef has a interruption of the seaward breakerzone.

The waves reaching the hole will enter the gully without breaking and will decrease height because of diffraction. (see fig. 5).

hole, causing an

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Fig. 5.

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..

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page 14

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The wave energy entering through the hole will be distributed along the diffracted wave crest resulting in reduced wave height that roughly can be described as:

H2

=

Hl

rai'1)

(2)

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When this diffracted wave starts breaking the breakerheight and thus the wave set up will be very limited, leading to a drop in waterlevel in longshore direction. The difference in water level will result in a current towards the hole as indicated in fig.

4.

The waterflow will be feeded by the overtopping waves.

Further it might be clear that the velocity of the current will increase, going towards the hole and result in a ripcurrent.

What will these currents mean to the beach behaviour?

An area as indicated in fig.

6

will be considered.

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Fig. 6.

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page 15

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Through the west and south boundary no sandtransport will take

place.

Through the east boundary the littoral drift Q3 caused by the

breaking of the waves near the beach will pass the boundary. As the

dominant waves come from the south this Q3 will be directed north.

The magnitude of Q3 can be expressed as:

Q3'::

A2H~...

.z.sV-tO(WI.CO.H)(,!t"1·••••••••••••••••••••••••••••••••••• (la)

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where and

0("",".1.= the an g le of wave-at tack of the second b reakin g 1ine

H~l=

the wave height at the second breaking line.

In shallow water the wave height no longel' has arelation with the

deepwaterwave height, but has a lineair relation with the water

depth.

I

with

H,-"=

. ...••.•....•••••..•...•.•.• (3)

h.

=

r

=

the maximum wave height in shallow water

the waterdepth

a co ffici nt related to the bottom slope

(0,4

<

Y

<

1,2)

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As the flat in front of the beach is very shallow (~1.00 ~

MSL)

a

deepwater wave height of 0,60 m or 5,00 m will result in about the

same magnitude of Q3. Only the difference of wave set up will have

some influence on the waterdepth (and the wave angle ()(~z.) as:

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with

Hb ..,.

r(h_

»)

(3a)

~I= the waterdepth related to the waterlevel

outside the breakingzone

~ =

the wave set-up

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Through the east boundary the rip current Q2 is leaving the area

while Ql caused by the wave oVertopping is adding material to the

area.

The magnitude of Ql and Q2 will be influenced by the value of Hbrl

as the wave set up is the basic reason for the existance of Ql and

Q2 and has a lineair relation with Hbrl.

When the attacking waves are low, for example lower than 0,6 m. they

will not break and no wave set-up will occur so Ql and Q2 will be

zero.

During falling tide a current will leave the gully while during

rising tide hardly any current will occur. The sand transport

because of this tidal current will be limited because of the low

flow velocity near the bottom (+ 0,20 m/sec.). When the attacking

waves are high, for example 4,50-m the influence of the tide can be

neglected, and a wave set-up will result of appr. 10% of the wave

height, leading to astrong rip-current with velocities up till

lm/sec. weIl able to transport a considerable amount of material

off-shore. The suppletion of sand by wave overtopping will be

limited because the sand concenctration in the top of the waves is

limited.

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page 16

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During observations in October and November 1985 it appeared that during the complete tidal circle the rip current was present, while even during the rising tide almost a tourist drowned because she was captured by the current and was draged offshore.

Further from underwater observations it appeared

sandtransport near the bottom occured as aresult directed to the gully.

that a lot of

of the current

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The different flows passing the boundaries of the area considered will lead to an erosion E, of the area that can be written as:

E = Q3 + Q2 - Ql (4)

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Because of the important influence of the waterlevel to the values of Q, equation (4) can be written as:

E = Q3 ...•...•...•..••...•... (48)

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during highwater and average wave attack, while during low water (4) will become:

E = Q2 - Ql (4b)

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or even when ave rage wave attack is considered

E = Q2 (4c)

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From surveys carried out in 1970 and 1981 a loss of 1,50 m of beach

per year can be calculated, thus E equals about 15.000 m3/year.

From equation (4) it is clear that the area concerned will always face erosion unless waves of limited height are approaching from the northeast which will only happen occasionally.

For this reason it is surprising that the north beach of Nusa Dua exists at all.

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The only way this can be explained is by assuming that in recent

history the two islands (Nusa's) where still real islands alowing the littoral drift to pass between the islands and the coast. Some prove for this theory can be found on the topographical map of 1924 where the two nusa's have clearly been drawn as loose islands.

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page 17

7. Measures taken uptill now against the erosion of the northern

beach of Nusa Dua.

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Af ter the first Hotel had been completed at the northern beach of

Nusa Dua, it became clear that erosion was occuring.

For this reason a number of groynes has been constructed in 1981

using square hollow concrete blocs that were filled with co ral stone

and concrete afterwoods. The

7

groynes have been constructed on the

boundaries of the different hotellots as indicated in fig.

7.

In 1983 Vincent from Sogreach studied the eros ion at the northern

beach and came with additional suggestions. At that time the

groynes 6C and 6B showed good results, the groynes 6A and N5/6 were

still in a good condition while the groynes N3/4, N4/5 showed heavy

eros ion at the northern side. For this reason the foundation of the

groynes has been strengthened and additional parallel groynes have

been constructed. Further an additional groyne 6AA has been

constructed.

The additional groynes constructed in 1983 have been indicated in

fig. 7 by a dotted line.

It appeared the erosion itself had decreased south of groyne N 3/4,

while accretion had appeared between groyne 6C en 6B. However

erosion had increased near groyne 6A where nor th of this groyne a

recession of the beach of lOm was registered.

At this moment the beach north of groyne N 3/4 is still eroding

while the area between 6AA and GC has become stable and tbe area

south of N 3/4 still shows a limited erosion.

As a result of all the constructions build along the coast, the

beach bas lost a lot of its beauty as can be seen from the foto's

however the erosion problems have not been solved!

In order to get a better idea about the general effects of

protectionworkes. Is the next chapter, the effect of different

types of coasted protection works will be discussed.

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N

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;' Fig. 7.

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\\

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-

. '_

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page 18

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8. The effect of constructionworks on a sandy coast.

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8.1 General

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All sandy coasts have been build by nature itself. A beach can be eroding, accreting or show an equilibrium. However this equilibrium in genera 1 is a dynamic equilibrium that can be disturbed easily by minor changes of the circumstances. Of ten these changes are caused

by men. If a beach is not in equilibrium it will try to reach an

equilibrium by changing in shape and position untill the equilibrium is reached.

This very natural process however is of ten not accepted by men and the coastal behaviour becomes aproblem. "Problems exist only in the minds of man".

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In order to solve their problems order to improve the situation. improvement has been reached and build.

men start to build structures in Of ten it appears however that no additional structures are being

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In order to find a suitable solution,

utmost importance:

two things are of

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A very thorough knowledge of the coastal processes occuring.

111 A very thorough knowledge of the effect of solutions to the processes.

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item I. has been discussed in chapter 6. In this chapter attention will be paid to item 11.

When the erosionproblem of a coast has to be solved the following measures are of ten used:

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a) Construction of groynes perpendicular to the coast. The groynes

can be high, low, long, short, permeable or impermeable, with

large interspacing, single or with short intgerspacing.

b) Construction of groynes parallel to the coast. These groynes can also be high or low, long or short, close or far from the beach, single or with smallor large interspacing.

c) Construction of T-groynes of different types. d) Replacing the beach by an unerodable structure. e) Using sand suppletion.

f) Do nothing at all and accept the losses.

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Ofcourse also combinations of these solutions are being used. Up

till now the solutions a and bare mainly used on Bali. The effect

of these types of solutions will be discussed in detail, taking

current- and wave-act ion into consideration.

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page 19

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8.2.

The effect of perpendicular groynes.

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ç

-I

,

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_t

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D:.

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Fig. 8.

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A

straight coast is considered with a constant angle of wave attack.

Furthermore the cross-section is supposed to be an equilibrium

profile. From eq. (1) can be seen that the sand transport because

of the breaking waves in every cross-section perpendicular to the

coast will be:

I

Q

=:

A

Ho

2.

.s

lJ'\.co(.

c.o

S0<

A

second equation that is very important to describe the coastal

processes is the equation of continuity.

In words the equation says that if a certain area is considered the

erosion in this area during a certain time period will equal the

difference of material leaving the area and material entering the

area. For a beach:

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E

= (

Q2 - Ql ) dt ( 5 )

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page 20

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For the beach considered Q2

=

Ql. So no eros ion nor accretion will occur. Now a groyne is constructed as indicated in fig. 8 in cross-section 111. If the groyne is high and long enough no sand-transport can pass the groyne and Q3

=

O.

This will lead to an accretion in front of the groyne equals to Ql K dt. However at the left side at the same erosion will start, that will as weIl equal Q3 K dt

=

Ql K dt. The shape of the beach will develop as indicated in fig. 8.

At a certain time the beachline will reach the end of the groyne and sand will start passing the groyne. However equilibriu. will only be reached if the shoreline at the right side of the groyne has moved forward over its total length. At this time the coastline at the left side will show the same quantity of erosion (see fig.

9)

and Ql

=

Q2

=

Q3 again.

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Thus forAt a groyne:= - Et (6)

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*

0<

-r.

Q1

-'"

i I

I

c

/

~

T

o \6

Î

I

/

1:.

!

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-

t

J

I

/

o.,

o, ·

Q'1

/

1

QI.:Q.,•

.

-Fig.

9. I

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page 21

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From the above can be concluded that construct ion of a dam or groyne at a beach with a dominant wave direct ion will not improve the beach as a whole. It just improves a part of the beach (but not in a tourist friendly way) and at the same time it will cause an equally large eros ion at another part of the beach.

If not one but a row of groynes is constructed the development of the coast will be as shown in fig. 10.

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Q~=O

-

Q~/=

0

-t

I

_-_--co

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Fig. 10.

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page 22

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Again

E

=

-

A and

E'

=

A'. The shore line at the right side of

the first groyne and the shore line at the left side of the last

groyne will react the same way as left and right side of the single

groyne. In between however rat her quickly an equilibrium will be

reached and Q will become zero. It might be clear that the

transport only can become zero if the distance m between the groynes

is not to large compared to the lenght of the groynes, and the angle

of wave attack is limited. If tg. '%

>

2 l/m the beachline

in-between the groynes will reach the end of the groyne and a reduced

transport will continue to pass along the groynes (see fig.

11).

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t

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Fig. 11.

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page 23

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For groynes interspacing 250 mand an angle of wave attack of q 30

grades this means that the lenght 1 should be at least 150 meters or taking the beach slope into account 180 meters in order to prevent transport of sediment. Also landinwards the groyne should have a lenght of q 150 meters, leading to a total lenght of 320 meters. Now the effect of a groyne will be considered if current is the main force working on the beach.

I

---I

i

~---

I

_

j

I

1 I

I

Fig. 12.

I

If only current is present the groyne will act as a crib in a river. The current pattern is shown in fig. 12. If discontinuities appear, erosion and accretion will occur. However in this case eros ion will mainly appear at the upstream side of the groyne, while accretion will appear in the eddy at the left side of the groyne. So just the other way around as under wave attack circumstances. In a row of groynes all groynes will act as individual groynes, unless the interspacing is very limited and the eddy will reach the next groyne.

I

Now a combined situation will be considered where during high water waves from the right attack the beach, and during low water the current from the right side becomes dominant. Under these circumstances the groyne will not be able to block the transport in front of the groyne as the deposited sand at the right side of the groyne, during wave attack, will pass the groyne because of the current during low water. The effectiveness in blocking the transport will depend on the strenght of the current. If the strength of the current increases, the effectiveness of the groyne will deminish.

I

(24)

I

Groyne N5/6 at low water. Note that because of the

current the leeside shows more sandstorage than the

upstream side.

I

Groyne N5/6 showing astrong north going current to the

hole of the reef

I

(25)

-I

I

page 25

8.3

The effect of parallel groynes.

Generally the parallel groyne has the sa.e effect as the

perpendicular groyne; it is blocking the longshore current.

..

_

-

---' ....

-

_--- ---_

...

--

"

-"'!.4~

~,-,~sT-!..

--

-

-Fig. 13.

However the blocking is not caused by the structure itself but by

the turning of the waves behind the groyne because of diffraction

(see fig. 13) and because of the decrease of wave height behind the

structure.

(26)

I

page 26

I

When the wave-attack is perpendicular to the coast the typical

to.bolo-shape will appear as indicated in fig.

13.

Mote that the

total accretion has to be equal to the total erosion.

I

Wh en the waves attack under a constant angle, the longshore

transport will be blocked and the coast at the right hand side will

build out while the coast at the left hand side will erode with the

same a.ount. (see fig. 14). When the coastline at the right hand

side has reached the groyne the transport no longer will be blocked

and an equilibrium has been reached again.

If now the effect of current is considered hardly any effect can be

expected from the parallel groyne (see fig. 15).

I

Fig. 15.

I

(27)

I

page 27

The

li.ited

increase

of

velocities

could

cause

some

erosion.

However it

is .ore logic that

more water will pass

seaward of the

groyne as the resistance between the groyne and beach has increased.

Now a situation with current during low water and wave attack dur

i

ng

high water will be considered.

During high

water a

bo.bolo will

start to

build up.

During low

water

because of

contractor of

the flow

lines so.e

erosion will

occur.

However because

of the increased resitance

.ore water will

pass along the seaside of the groyne.

It will

be clear that

the distance of the

groyne to the

beach is

important

to the

quantity

of

erosion during

low

tide.

If

the

distance is considerable the distribution

of water at both sides of

the groyne

will hardly change and

the erosion of the

tombolo will

increase and thus the effect of the groyne will be less.

Finally a situation

will be considered where current

and waves are

acting at the same time.

In general

such a situation

will be very unusual.

Dijker studied

the sandtransportmechanism in combined wave current situations.

He came

to the conclusion that

the shearstress near the

bottom is

increasing more than proportional if

a velocity is co.bined

w

ith an

orbital velocity making an angle with this velocity

.

For a

coast where for example

a tidal current and

wave-attack are

present it means that the

velocity distribution of the current will

be

changed and

the

effect of

the

current to

the

beach can

be

negLec

t

ed .

(see fig.

16 )

Pt~TRI8I.(T'ON OF

C.LA.(.l.E1\IT VEI.Oc..Ty PERP[NO'('ULÄ'l __

-,

To

THE

GoM'T

(...,'TH

'-NO w'T!oioUT

""A\I~~)

(28)

I

page 28

I

Thus for a syste. of perperdicular groynes a coabined situation of

current- and wave-attack is not interesting.

For the parallel groynes however the situation is different,

specially if the distance between the groyne and the coast is large

and if the length of the groyne is long.

In that case the waves at the seaside of the groyne will increase

the bottoaresistance while between the groyne and the beach the

orbital velocity is liaited, leading to a reduced resistance.

-I

_./

QC~2Z~~~~~~ZZ~~~~---~ ~

I

-I

I

----~

__

-

---I

I

Fig. 17.

I

As a result more water will pass at the inside of the groyne and the

tombolo shape has hardly a possiblity to develop, as the sand

transportèd by the wave-act ion will not settie but be transported by

the current to the leeside of the groyne, where weaves will move it

further along the shore.

As aresuit the effect of this groyne under wave and current

conditions will be very limited.

I

(29)

___

.

I

'

I

-

_

..

----

-

-page 29

9. The effect of protectionworks to the Nura Dua coast.

---With the knowledge of the acting forces to the beach as described in

chapter

6 and the knowledge

of the different protection syste.s the

different used and suggested solutions will be discussed.

9.1 The groyne system build in 1981

N

\ ~.

.>:

\ ---~.

Fig.

18.

(30)

I

page 30

I

The basic problem of the north coast of Nusa Dua is that the

littoral drift at A is blocked completely while at B a littoral

drift Qo is leaving the area. Further a wave induced current is

transporting material away from the area to the gully. The

resulting material loss is insufficiently compensated by Ql caused

by wave overtopping.

Now from the theories described in chapter 8 the following can be

expected:

I

1. The groynes N 3/2, N 3/4 and N 4/5 will mainly act as groynes

under wave-attack, so strong accretion at the right side and

eros ion at the left side can be expected.

I

2. As the length of the groynes is short compaired to the distance

and in relation to the angle of wave-attack, af ter a wile sand

will start to move in front of the groynes. However the

sandtransport passing will be less than originally.

I

3. The groynes N 5/6 and N 6A will be comparable with the groynes

under the influence of waves during high tide and under the

influence of current during low tide. Thus the effect of these

groynes will be less, while the littoral sand transport will be

larger than the littoral drift near groyne N 3/4 and N 4/5.

I

4. The groynes 6B and 6C will be effective again. As the distance

between the groynes is small compaired to the length in relation

to the angle of wave attach. The beach wil be stabie here.

5. At the leftside of groyne 6C erosion will continue and might even

increase.

I

6. In general the total erosion of the area between crosssection A

and B has hardly been deminished, as Q2 still is present and Q3

only has been reduces a little bit.

7. Because of the difference in effectiveness of the groynes, the

erosion near N 3/4 and N 4/5 is expected to decrease while the

erosion near N 5/6 and N 6A will increase.

I

From chapter 7 may appear that the real development is in agreement

with this-expected development.

I

(31)

I

page 31

9.2 The groyne syste. as suggested by Sogreah

-

---In fig. 19 the groyne syste. as suggested by Sogreah has been

drawn. On the l5th of October 1985 this plan has been accepted.

•

/ / /

-

-~

\

\,

\

\,~.

:

\ lil';

\

_.\" I \\

\\

___

. -_ ; I

-

--~., Fig. 19.

The plan exists of the construct ion of a parallel groyne with a

length of 300 m at a distance of 200 m from the beach. Further the

extension of the groynes N 4/5 t N 6AA and N 6C and a sand filling

south of groyne N 4/5 are part of the plan.

The result-of the construction of these groynes will be that:

a. At the south side of groyne N 4/5 an almost stabie situation will

occur. However the length of the groyne will not be long enough

to prevent all transport. For that reason a limited erosion will

(32)

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page 32

I

b. Between the groyner N

4/5

and N GAA the waves will try to aake a

tombolo shape using the beachmaterial in front of lot 5 and some

material at the right side of groyne GAA. (indicated in fig.

19. by a dotted line). However the current will concentrate

between the groyne and the beach and will continue to take

material away to the gully as indicated in chapter

8.3.

For this reason no equilibriun will be reached, and a heavy loss

of beach material will occur in front of lot

5,

decreasing the

value of this lot considerable.

c. Between groyne N GAA and N GC a more or less stabie situation can

be expected.

I

d. Left of groyne GC an increased eros ion will occur.

I

9.3

The plan of Tourdevco, Svasek and ADC Consuiting Engineers

---I

In this plan no real groynes are being used. The protection works

consist of sand peninsuias protected at the seaside by a shore

protection as indicated in fig. 20.

I

a

\_:~.

:-

-,

-

'

_

__,,-; .' .... .- ~.: :..

1 ···

N

I

/

I

-

_

, ",\ \ ...-_I i\

I

Fig. 20.

I

In this solution as weIl the littoral drift as the wave induces

current is blocked completely. For this reason no erosion will

occur in front of any of the lots. North of lot NG however the

(33)

~ ...

_-

-

- ~---_._-

--I

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page 33

I

9.4 Sandsuppletion

I

A different approach to the problem is not using any structures but

just using sandsuppletion in order to maintain a stabie situation.

Such a solution is attractive because it is one of the few solutions

where the eros ion problem at a certain location is not shifted to

the next location. Specially along coasts where the value of the

beach along the coast shows little difference, this type of solution

became very popular.

In principle it can take place by a continuous suppletion or by a

large suppletion that will be repeated for instance every 5 years.

When the amount of suppletion per year has to be established, the

following should be taken into account.

I

a. Notthe allsmallerthe sandparticalssuppletedcan willeasilybe beaddedtakento the beach.to deep water.SpeciallyFor

this reason losses up till 30% are not uncommen.

b. The quantity needed to stabilise an eroding beach is of ten

underestimated.

This can be explained as follows (see fig. 21) .

I

If for example sandtransport Qo, 20.

a groyne is constructed on a beach

the coastline will develop as indicated

with a in fig.

I

Fig. 21. )(

-I

I

(34)

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page 34

I

Af ter a certain period it can be noticed that the area where erosion

is taking place, has extended considerable although the total

quantity of eroded beachaaterial stays the saae every year. (Qo x

dt). At a certain location A it looks as though the erosion is

decreasing every year. While the total a.ount of the erosion in the

area AB, at t5 will be equal to Q t 5.

However in order to restore the equilibrium we have to supplete Qo.

In this example Qo

= -

4 x Q t5.

At the coast of Nusa Dua from the coastal measurements in 1970 and

1981 an average erosion of 20.000 m3/year results for the area

between Nusa Kecil and the end of lot N6. However this amount will

only be a part of the Qo value that has to be suppleted. for this

reason Qo and the necessary suppletion easily can be 100.000 m3/year

and more.

I

To supplythe rest atsuchthe an amountbeaches considerably.of beach material by truck, will disturb

I

If: 1 truckload

=

2 m3 200 x 8

=

1600 working hours/year Qsupply

=

100.000 m3/year

I

every 2 minutes a truck will arrive on the beach continuously during

working hours.

For this reason it is more logic to supply an amount of sand by

hydraulic transport every 5 years. This solution can be considered

as an alternative for the use of structures.

I

(35)

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page 35

I

10. Choice of the most suitable solution.

---I

The choice of a certain solution will not only depend on the

effectivity of the solution in relation to the erosion itself, but

also on the price, the way it effects the environment, the

additional effects , etc. For this reason the different solutions

will be considered further.

I

willAll threebe comparedsolutionsin thisas describedchapter. in the chapters 9.2, 9.3, and 9.4

I

a. Costs involved

I

Theresultingcosts involvedin an annualwithcostthe of apr."groyne solution"$ 180,000.will

As this solution is not preventing the erosion

additional sand suppletion will be necessary,

additional 5000 m3 of apr. $ 20,000. per year.

yearly cost of $ 200,000. be $1,500,000

I

completely an this means an Resulting in a

I

The costs involved with the "is land (peninsuia) solution" will be $ 4,500,000. resulting in an annual cost of $ 540,000.

However this solution will add 190,000 m2 of valuable land to

the resort area. This means that the average cost of

constructing 1 m2 of land is $ 23.68 which is considerably lower

than the present price of land in Nusa Dua. Thus the eros ion

problem can be solved without any additional costs !

I

The costs involved with the "suppletion" solution are hard to

establish as the real amount of sand needed per year is not

known. It is estimated to be between 50.000 and 100.000 m3

resulting in an anual cost of

1

250.000 to

1

500.000.- assuming a suppletion every 5 years.

I

(36)

I

Visual effect of Nusa Dua beach protected by groynes

I

-.--",-...~_; _----_ ";~jI

I

Visual effect of the Nusa Dua beach without protection works

(37)

I

page 37

I

b. The visual effect of the solutions

I

The "groyne" solution is a rather ugly solution specially the 300 m long and (during low tide) 5 m high parallel dam will deminish the quality of the location considerable. Further the large erosion north of the extended groyne N 4/5 will neither add to the attracktiveness of the beach.

The "island" solution changes the total appearence of the location. The visual effect in general will improve, as all the beaches will become stabie beaches without protection works, the islands will offer the posibility for an enriched surrounding, while further the de ad coral in front of the beach will be dredged away.

The suppletion solution will not charge the vue at all. Just during a couple of month every five years suppletion of material will spoil the attractivity of the beach.

I

c. Safety of the solutions

I

At the moment a rather dangerous situation exists for swimmers. Specially during high waves astrong current is going to the hole in the reef. As mentioned before during the authors visit a tourist had to be rescued.

If a tourist is captured by the current it is not possible to reach the beach anymore, as the reef is preventing this.

At this moment hardly any tourists use the beach in front of lot N5 or N6 as these lots are not used up till now. In the near future however th is situation will change and then accidents are bound to happen, even if a good watchsystem and a standby boat are present. Besides the accidents the present situation is limiting the use of the coast for touristical purposes.

In respect to this highly dangerous situation the "groyne" solution is slightly worsening the situation. The "island" solution will solve however the problem completely while the new created situation will even make it possible to swim through the hole in the reef in order to visit the coral fields outside the reefs.

The "suppletion" solution will have no influence at all.

I

(38)

I

page 38

d. Other effects of the solutions

Besides the effects mentioned before, other advantages:

the "island" solution has two

I

1. At the moment two areas along the north beach of Nusa Dua, have been reserved for the original population at the Nusa Dua area. These areas are located between lot N4 and lot N5 and between lot

N5 and N6. The areas are being used at the moment for storing

boats while local people are selling handicrafts and massage

treatments to tourists.

Although there is interest from the side of the tourists for this type of business other tourists hate to be bothered all day long by salesmen.

By shifting these areas to the northern island these problems

could be solved.

Such an island could grow out to alocal market and close to the touristical zone and further could serve as a boating beach for local fishermen etc.

I

2. The southern island has been designed as a small harbour. Such a

harbour could improve the touristic possibilities considerable, like glassbottom boat trips, small cruisseships, a marina etc.

Besides the harbour the island could serve as alocation for a

sea-aquarium, snake garden, restaurants, etc.

I

In order to compare the different types of solutions in fig. 22 to

all the effects concerned, a value has been attached. This value

can be -5 uptill + 5. 5 uptill 0 means a negative effect, +5

uptill 0 means a positive effect, while 0 means no charge of the

original situation.

From the total effect the "value for money" figure can be calculated.

In fig. 22 all effects are considered equally important resulting in a negative result for the "groyne" solution a highly possitive result for the "island" solution and a possitive result for the "suppletion" solution.

Even if the different effects would be appreciated differently the groyne solution never will become the most attractive solution.

I

(39)

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page 39

Parallel groyne Sandislands Sandsuppletion a. initial costs

b. total costs

expressed per year

$ 1,500,000.-$ 200,000.-$ 4,500,000.- $ 1,500,000.-$ 540,000.- $

350,000.-I

l. reached stability + 3 + 5 + 4 of Nusa Dua 2. stability of coast

-

2

-

3 + 4 north of Nusa Dua

3. visual effects - 4 + 1 - 1 4. danger for tou-

-

1 + 5 0

rists 5. interaction tou- 0 + 4 0 ristsjlocal people 6. touristic posi- 0 + 4 0 bilities

I

c. total effect (1 up till 6) d. value for money

- 4

+ 16 +

7

- 2 _{+ 3} ₊

₂

I

fig. 22.

I

From the comparison can be concluded that from a more coastal engineering point of view the sandsuppletion will be the best solution. The best solution, however, giving most value for money is the solution with the sand islands.

I

(40)

I

page 40

11.Realisation of the project

I

Up till now a general disign has been completed by Tourdevco Svasek

and ADC while recently these consulting engineers have been invited

by the Drector General of the Ministry of Tourisme to prepare a

feasability study for their design.

In this feasability study, special attention has to be paid to the

availability of sand of acceptable quality and quantities and the

desired shape of the islands.

When the feasability has been approved and accepted a final and

detailed design and contract document has to be prepared. This

design will request the use of advanced knowledge and

computerprogrammes in order to establish the equilibrium chape of

the islands. For this reason the assistance of a qualified

consultant is essential.

As the engineering consultants have already put a lot of effort in

this design, while it is in the interest of BPMA to develop their

knowledge as far as coastal engineering is concerned, it would be

advisable to have the design prepared in close coperation between

Tourdevco Svasek

I

ADC and BPMA.

A more general guidance could be supplied by the Rijkswaterstaat.

The total preparation is expected to take half a year.

I

Uptill now an amount of money of $ 1,500,000.- has been made

available for the protection of the coast of Nusa Dua. This amount

is not sufficient to carry out the most desired solution.

The most logic approach would be, to find additioned funds on short

notice.

In case this is not possible it will be difficult to carry out a

part of the plan and other temporary measures will be required,

leading to an increase of the total costs.

As a temporary measure the use of sandsuppletion could be

considered. This method has a reasonable value for money factor as

indicated in chapter 10.

I

12.Acknowledgement

The author likes to thank all the staff members of the Gouvernmental

Institutions who were so kind to help the author in collecting the

necessary data and information and assisted him in every possible