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Utrecht
REPORT ON BEACH EROSION IN BALI
by I r . D. Kooman M i n i s t r y of Transport and P u b l i c Works Delta Department The Netherlands
31 August 1983
C O N T E N T S
Page
1. Introduction
1
2. Terms of reference
2
3 . General approach to coastal engineering
3
3 . 1 . Introduction
3
3 . 2 . Research and design approach-
5 7
3 . 3 . Work done to date 3.4. Work to be done
9
3 . 5 . Organisational aspects
10
3.6
11
A e s t h e t i c a l aspects
4 . General remarks on beach protection 5. S p e c i f i c problems
in B a l i
12 14
5 . 1 . Kuta Beach
14
5 . 2 . Nusa'Dua
15
5 . 3 . Sanur Beach
16
5 . 4 . Lebih
16
5 . 5 . Batumadeg
17
5 . 6 . Pulaki
18
5 . 7 . Gumbrih
18
6. The problem of the coral digging
20
7. Summary
22
8. Recommendations
23
9. Acknowledgement
24
10. L i s t of References
25
2. TERMS OF REFERENCE Objective The main o b j e c t i v e of the v i s i t of a Dutch coastal engineering expert was to i n v e s t i g a t e the e f f e c t of coral digging on beach erosion problems in B a l i . Coral diggers make a l i v i n g out of
this
work, and a complete stop of the digging would create a s o c i a l problem. Hence, the problem must be considered from both an engineering and a s o c i a l point of view.
Scope of work 1. Study a l l previous surveys and the p r e l i m i n a r y design for the B a l i Beach P r o t e c t i o n Scheme. 2. Make a s i t e
visit.
3. Provide comments, suggestions or recommendations f o r a l l
the
works that have been done to date including construction works r e l a t e d to the protection scheme. 4 . Suggest f u r t h e r measures for f u t u r e p r o t e c t i o n works against e r o s i o n . Since B a l i i s one of Indonesia's centres for t o u r i s m , a l l works proposed should meet a e s t h e t i c a l requirements and, whenever p o s s i b l e , they should blend with the natural environment. Consideration should also be given to the source of any sediment required for beach p r o t e c t i o n , since very l i t t l e i s a v a i l a b l e l o c a l l y . Transportation of sediment from the nearshore should be avoided.
- 3 -
3.
GENERAL APPROACH TO COASTAL ENGINEERING
3.1.
Introduction The problem of defending a coast against the attack of the sea occurs both in developed and developing countries with sea boundaries. The attack of the sea i s dynamic and the sea takes land at one time from one place and gives i t back somewhere e l s e . This dynamic process depends on the waves, the t i d e s and on ocean c u r r e n t s , a l l of which vary in time and p l a c e . An always varying topography, the ever varying t i d a l movements ( s p r i n g t i d e - neaptide c y c l e , 29-day c y c l e , half y e a r l y and y e a r l y v a r i a t i o n , e t c . ) , the seasonal v a r i a t i o n s in meteorologic
conditions
e t c . , a l l i n f l u e n c e the amount of energy that i s transported to a c o a s t . For t h i s reason, a s t a t i c e q u i l i b r i u m of a c o a s t , whether i t comprised of sand, mud or r o c k , i s never p o s s i b l e . If state e x i s t s at a l l , i t
is
an e q u i l i b r i u m
is dynamic, e s p e c i a l l y in the case of sandy
c o a s t s . When people use a p a r t i c u l a r beach, erosion i s f e l t as a real l o s s , whereas accretion is g e n e r a l l y not a cause for concern. In the protection of human i n t e r e s t s against the s e a , structures are made with the o b j e c t i v e of l i m i t i n g , or even stopping, the coastal changes. Of course considerable amounts of money can be involved in these s t r u c t u r e s . Not « n l y in the c o n s t r u c t i o n i t s e l f , but also in the i n v e s t i g a t i o n s required to obtain a proper design and in the design work i t s e l f . A d i f f i c u l t question i s always how much money can be j u s t i f i e d for such defence works. This depends on the i n t e r e s t s to be protected. If
these
are l a r g e , then, from an economic point of view, the investments can be l a r g e . In the Netherlands, for example, enormous amounts of money are spent on coastal defence, s i n c e 60% of the land i s below mean sea l e v e l . In B a l i , large investments are made in i n f r a s t r u c t u r e !
works such
as roads, a i r p o r t , bridges and hotel b u i l d i n g s , a l l r e l a t e d to the t o u r i s t i c development of the i s l a n d . It
i s a n t i c i p a t e d t h a t , up to the year 1990, US$ 300 m i l l i o n w i l l be
invested in such s t r u c t u r e s . Most of the hotels are situated
adjacent
to or near beaches, and i t follows that loss of the beaches, due to
]
- 4 -
e r o s i o n , w i l l have a detrimental
e f f e c t on t o u r i s t development. The
beaches are very s m a l l ; there are very l i m i t e d sources of sediment, and the surrounding coral r e e f s are being dug, g i v i n g the sea more opport u n i t y to reach the beaches. Often hotel b u i l d i n g s and other
structures
are b u i l t very near to the beach, and any l o s s of beach i s a threat to i t s t o u r i s t i c use and sometimes to the b u i l d i n g s themselves. For t h i s reason, the l o s s of every meter of beach i s considered to be a serious t h r e a t and a s t a t i c state of e q u i l i b r i u m of the beach in which there i s no erosion
i s h i g h l y d e s i r a b l e . To achieve t h i s many a r t i f i -
c i a l s t r u c t u r e s w i l l have to be b u i l t and in the long r u n , the whole c o a s t , required for t o u r i s t i c development, must be protected by s t r u c tures l i k e g r o i n s , l a t e r a l dams and s e a w a l l s . A r e l a t i v e l y large amount of c a p i t a l f o r investments in beach p r o t e c t i o n works should be a v a i l a b l e because of the huge economic i n t e r e s t s to be p r o t e c t e d . This money should, in f a c t , be incorporated in t o t a l developing c o s t s ; roads 'and also beach p r o t e c t i o n works are part of the works which should be c a r r i e d out in r e l a t i o n to the development of South B a l i . Up to now, the costs of the defence works have always been considered as too much.
It
i s stressed that t h i s i s not the correct way to view beach p r o t e c t i o n . There i s an urgent need f o r a " m a s t e r p l a n " , based on a systematic approach which can be used as a framework and "touchstone" f o r a l l the s t r u c t u r e s needed. It
i s e s s e n t i a l for such a masterplan, that one
government department i s made responsible f o r coastal defence problems. The i n v e s t i g a t i o n s , the design and the c o n s t r u c t i o n of a l l coastal prot e c t i o n works have to be c o n t r o l l e d by one responsible o r g a n i z a t i o n . The d a i l y control
and maintenance should be delegated to the l o c a l O f f i c e of
P u b l i c Works, under the s u p e r v i s i o n of a central government department. The coastal engineering r e s e a r c h , any mathematical and physical modell i n g and consultancy should be concentrated in one central
institute.
DPMA at Bandung i s to be considered the most appropriate for t h i s r o l e . In the long term, the c r e a t i o n of regional research centers should c a r e f u l l y be c o n s i d e r e d . A l l the money to be spent on research a c t i v i t i e s must be considered as an investment for the f u t u r e .
- 5 -
Without sound research no design of coastal s t r u c t u r e s can be o p t i m i z e d . The optimal construction i s that for which the c a p i t a l and maintenance costs are the minimum; the inadequately designed coastal structure i s a c e r t a i n way to waste money. 3.2.
Research and design approach Figure 2 gives an i n s i g h t into the work r e l a t e d to the design of beach p r o t e c t i o n works. The diagram shows a general way to develop optimal designs for h y d r a u l i c s t r u c t u r e s and the s i t u a t i o n which has to be reached i n the long r u n . Comparison of t h i s diagram with the work that has been c a r r i e d out to date gives an i n s i g h t into what has to be done in the f u ture. Some major points in the whole approach are considered below in
relation
with the problems on B a l i . The d i s i g n of caostal s t r u c t u r e s i s not only a technical
problem.
Many i n t e r e s t s are involved and the influence of these i n t e r e s t s on the problem should be q u a n t i f i e d
as much as possible
to give a proper basis
for the design c r i t e r i a . As stated b e f o r e , the problems on South B a l i , e s p e c i a l l y at Nusa Dua, ' r e q u i r e , i d e a l l y , a s t a t i c e q u i l i b r i u m state of the beach. This c o n c l u sion i s based on discussions with the various groups involved in Nusa Dua, e s p e c i a l l y the BTDC ( B a l i Tourism Development C o r p o r a t i o n ) . They recognize the problem (Item I i n the diagram) and have economic i n t e r e s t s (Item 4 ) . The P r o v i n c i a l Department of P u b l i c Works, the Environment Bureau, P r o v i n c i a l Planning Board, Bureau of Development are also i n v o l v e d . They a l l , working together,
should define the problem
(Item 2 ) , in cooperation with the t e c h n i c a l c o n s u l t a n t , DPMA. From an o r g a n i z a t i o n a l point of view there should be one r e s p o n s i b l e group which acts as the p r i n c i p a l for the c o n s u l t a n t s . Up to now the s i t u a t i o n B a l i i s somewhat confusing. DPMA at Bandung has no o f f i c i a l
in
principal
to
report to and there i s no p r i n c i p a l who can support the DPMA proposals on the research which has to be done and the r e l a t e d f i n a n c e s . This a l l leads to the undesirable s i t u a t i o n that DPMA, (and not only in B a l i ! ) ,
-6-
ill
Problem i d e n t i f i c a t i o n (4)Basic f a c t o r s -Economic i n t e r e s t s -Environmental " -Social -Hydraulic aspects
s (2)
Problem d e f i n i t i o n
**
..
Conceptual model of p h y s i c a l processes
Theory available
Physical scale model (s)
no ?
Mathematical, empirical or p h y s i c a l scale model (s)
Dimensional analysis
Governing parameters
Boundary conditions
Data c o l l e c t i o n - hydraulic - bottom topography - sediment properties
C a l i b r a t i o n of the model (s)
4 Verification on present s i t u a t i o n ,^ OK?
no
yes Possible solui:ions i n the model
tested
Aesthetical criteria
Data c o l l e c t i o n - economic - environmental - ecological
P r e d i c t i o n of hydraulic changes
Design data
Economic evaluation of design alternatives
Design
criteria
design Design Approach
Figure 2
- 7 -
quite often acts as i t s own p r i n c i p a l and reports to several
institu-
t i o n s with no a t t e n t i o n to non-technical aspects. Even the problem r e c o g n i t i o n i s often done by DPMA without the involvement of any governmental group. Rumours form, too o f t e n , the basis of the DPMA a c t i o n s ,
'
which has to i n t e r e s t a l o c a l p u b l i c works o f f i c e . (See, for example, erosion problems at Jepara on the north coast of Central J a v a ) . DPMA cannot be blamed for t h i s since i t
,
i s due to a s i t u a t i o n in which r e s - /
p o n s i b i 1 i t i e s are not c l e a r l y defined and delegated. Most of the other items of work, indicated in Figure 2 , belong to the task of the i n v e s t i g a t o r and designer, in c l o s e cooperation with the p r i n c i p a l . The conceptual model i s e s p e c i a l l y important. It
i s a des-
c r i p t i o n , in words, of the problem and the physical processes i n v o l v e d , based on t h e o r e t i c a l knowledge, experience and " i n s i g h t into nature".
It
i s used in two ways. F i r s t as a basis for d i s c u s s i o n with the p r i n c i p a l in order to ensure that the problem i s w e l l - d e f i n e d and understood and also to explore possible ways of s o l u t i o n : mathematical model, physical model or empirical model. A proposal for the i n v e s t i g a t i o n s required and the costs involved can also be given by the c o n s u l t a n t . During the course of the d e s i g n , the consultant reports to the p r i n c i p a l on both t e c h n i c a l and f i n a n c i a l progress.
3.3.
Work done to date A f t e r two weeks of intensive discussions with many people i t became c l e a r to the author, that only DPMA i s attempting to solve the problems on B a l i q u a n t i t a t i v e l y .
However, the f i n a n c i a l resources of the coastal
section of DPMA are very l i m i t e d ; only a small budget of US $ 20,000 i s a v a i l a b l e f o r a l l the fieldwork to be done in the whole of Indonesia. As a consequence, only the seabed p r o f i l e s at a s t r i c t l y
l i m i t e d number of
l o c a t i o n s have been e s t a b l i s h e d . At one l o c a t i o n wave measurements have been c a r r i e d out over a short p e r i o d . Tidal measurements such as water l e v e l s and current v e l o c i t i e s , have never been c a r r i e d out; at Nusa Dua there i s some i n d i c a t i v e data a v a i l a b l e about inshore current v e l o c i t i e s , but these are too l i m i t e d to draw c o n c l u s i o n s . There are no
J
- 8 -
measurements of l i t t o r a l d r i f t and sediment t r a n s p o r t . Wave measurements during both the west and the east monsoon w i l l be e s s e n t i a l to form the basis for the understanding what i s happening on the beaches of B a l i . Only one set of photographs, which covers some of the beaches and was taken in 1970 by the F A 0 o r g a n i z a t i o n , i s a v a i l a b l e . The a v a i l a b l e data are i n s u f f i c i e n t to e s t a b l i s h a set of boundary conditions for a , whatever i t s form may be, model. In f a c t , the data i s hardly
sufficient
to form a base for a conceptual model. As a q u a n t i t i v e basis for design work, they are i n s u f f i c i e n t and only rough q u a l i t a t i v e conclusions can be drawn at t h i s stage. The question a r i s e s , n a t u r a l l y , of whether t h i s ideal approach should be followed every time. Figure 1 i s intended as a g u i d e l i n e ; The d e t a i l to be followed depends on the importance of the p a r t i c u l a r problem. Let us c o n s i d e r , as an example, the Nusa Dua s i t u a tion. The f o l l o w i n g assumptions are made: r jj 1. Investmensts: US $ 300 m i l l i o n 2. Yearly income: US $ 200 m i l l i o n If we assume t h a t , due to the beach e r o s i o n , the a t r a c t i o n of B a l i to t o u r i s t s diminishes by 5%, i . e . US $10 m i l l i o n , then t h i s amount can be justified
f o r use for structures to keep the beach in a good c o n d i t i o n .
Of course, t h i s i s an extreme example and perhaps not very r e a l i s t i c , but i t demonstrates, that the economic i n t e r e s t s involved are large and that beach protection works in such cases not should be viewed as simply annoying and not to be taken s e r i o u s l y . During the d i s c u s s i o n s at B a l i i t became c l e a r that the erosion problems a r e , in f a c t , viewed s e r i o u s l y but that they are not considered to be infrastructural
works r e l a t e d to the development of t h i s t o u r i s t i c a r e a .
However, i t must be stressed that the beach protection works are inherent to the t o u r i s t i c beach development at B a l i in j u s t the same way as roads, waterworks, sewerage and e l e c t r i c a l s e r v i c e s .
j
- 9 -
Apart from the lack of economic aspects in the planning a c t i v i t i e s , the problem of a not well-equipped consultant e x i s t s . With the very small budget a v a i l a b l e , DPMA i s doing what i t c a n , but both the number of experienced people in the coastal engineering section and the a v a i l a b l e budget are too s m a l l . An increase in the budget i s absolutely necessary to c o l l e c t s u f f i c i e n t d a t a , so that the physical processes can be understood and at l e a s t empirical r e l a t i o n s h i p between governing parameters can be estimated, on which to predict changes in physical processes in the f u t u r e .
In addition these data w i l l be e s s e n t i a l for the design of
the s o l u t i o n s i n v o l v e d . 3.4.
Work to be done
3.4.1 Data c o l l e c t i o n As stated above, more prototype data have to be c o l l e c t e d , i n c l u d i n g : 1. As a time s e r i e s to estimate boundary conditions and to c a l i b r a t e models. 2 . As s t a t i s t i c a l
information in r e l a t i o n to the determination of design
c r i t e r i a f o r r e l i a b l e and optimal
structures.
The measurements should i n c l u d e : - Bottom topography Necessary f o r wave r e f r a c t i o n and d i f f r a c t i o n models, t i d a l models and l i t t o r a l d r i f t models, and also to get i n s i g h t into the dynamic changes of the c o a s t . Coastal changes have to be measured over a period of several y e a r s , both in dry and wet seasons. - Waves Height, periods and d i r e c t i o n s , in both dry and wet seasons. - Tidal e l e v a t i o n s and current
velocity
At a number of places during a whole t i d e c y c l e during both spring and neap t i d e s . Both magnitude and d i r e c t i o n must be measured. - Sand transport Sand concentrations have to be measured, in combination with the current v e l o c i t y measurements, in order to predict coastal changes.
To get a l l t h i s information involves time and money. R e l i a b l e equipment
I
- 10 -
has to be bought and people have to be t r a i n e d . The knowledge i t s e l f
is
present in DPMA, the problem i s more one of f i n a n c e . 3 . 4 . 2 Model s t u d i e s The models necessary for i n v e s t i g a t i o n s into coastal processes have been already developed elsewhere. They have to be implemented on the DPMA computer and people have to be t r a i n e d in the understanding of what the models are doing. This means t r a i n i n g
and education by experts; both the
Dutch Government M i n i s t r y of Transport and P u b l i c Works ( R i j k s waterstaat)
and the D e l f t Hydraulics Laboratory can give support in t h i s
aspect. . A l l the a c t i o n s , mentioned above, are in the medium and long term. During the implementation of the a c t i o n s , things have to be done in the short term on B a l i . These short-term actions are described in more detail 3.5.
in Chapter 5.
Organisational aspects Coastal defence i s a matter of national concern and belongs to the r e s p o n s i b i l i t y of the Government. To be sure that a l l i n t e r e s t s are taken into account one department should be r e s p o n s i b l e for a l l the measures to be taken to keep Indonesia safe from the s e a . This department must be responsible for solving the coastal problems and should have the author i t y to make decisions on budget and p r i o r i t y . This department must also be r e s p o n s i b l e for the coordination of a l l i n t e r e s t s involved and must act as p r i n c i p a l to consultants and research i n s t i t u t e s ,
and also to
c o n t r a c t o r s . Of course there should be delegation of auhority to p r o v i n c i a l and l o c a l public o f f i c e s , , but supervision must always be by the central government department. This department should have a d i v i s i o n containing not only technical knowledge and s k i l l , but also environmental
and economic knowledge to ensure the coordination of a l l
interests
affected by the problems. An i n s t i t u t e l i k e DPMA should be incorporated within t h i s d i v i s i o n to ensure the d i r e c t l i n k between r e s e a r c h , design and coastal defence management.
- 11 -
3.6.
A e s t h e t i c a l aspects Because of the t o u r i s t i c
use of the beaches of B a l i , beach protection
structures should not s p o i l the appearance of the beaches, and they should not form an obstacle to people walking along the beach. The f i r s t condition i s d i f f i c u l t to meet for a l l beach protection
structures.
Adverse e f f e c t s can, however, at l e a s t be minimised. The use of l o c a l m a t e r i a l s , such as coral stone, r i v e r stone and wood, such as the trunks of coconut t r e e s , could be considered. Gentle slopes on both s i d e s , covered with a mixture of asphalt and coral sand to blend with the colour of the beach w i l l make the structures more acceptable. Beach r e s t o r a t i o n , by f i l l i n g up eroded l o c a t i o n s with sand, i s the best way to f u l f i l
a l l the requirements. Further recommendations are made in
Chapter 5. Another aspect of beach appearance i s the presence of b u i l d i n g s near the shore. From a coastal engineering point of view, the distance of a b u i l ding from the shore should be not less than approximately 100 m, in view of the dynamic behaviour of the natural beach berm. However, B a l i i s so attractive,
because of the combination of t r o p i c a l
beach and c u l t u r a l
v e g e t a t i o n , white
background. Hotels have been b u i l t so near to
beach that they d i s t u r b the t r o p i c a l
the
b e a c h l i n e , the berm of which
c o n s i s t s of a white beach with coconut t r e e s , banana trees and other t r o p i c v e g e t a t i o n . From t h i s point of view, the distance .between large b u i l d i n g s and the beach berm should be much more than the 50 m. This i s now the standard at Nusa Dua. The author would l i k e to make a plea for a minimum distance of 200 m, which should be used for planting vegetation.
tropical
- 12 -
4.
GENERAL REMARKS ON BEACH PROTECTION IN BALI Because of the demand for a s t a t i c beach e q u i l i b r i u m , the c o a s t l i n e should be d i v i d e d into sections in which a l l the sand, present in a sect i o n , i s r e t a i n e d . The boundaries of such sections should be high and long impermeable g r o i n s , perpendicular to the c o a s t . The c o a s t l i n e between these high groins i s more or l e s s f i x e d , using simple, small permeable groins of various lengths. If
insufficient
sand i s a v a i l a b l e in
one s e c t i o n , sand from elsewhere must be used. A f t e r some time i t may be necessary to move from one place to another w i t h i n a s e c t i o n ; t h i s
is
normal beach maintenance. The main aim i s to prevent the loss of sand from the s e c t i o n s . The general p r i n c i p l e i s shown in Figure 3. The long impermeable
s t r u c t u r e s , extending beyond the Low Water Line
(LWL), should be b u i l t of large stones, founded on a f i l t e r t i o n , to ensure the s t a b i l i t y . struct
construc-
The shape at the beach should not ob-
p e d e s t r i a n s . An idea i s sketched in Figure 4 . The groin has
gentle slopes covered by an impermeable l a y e r , for example, asphalt. The surface of t h i s layer c o n s i s t s of an adhesive mixed with coral sand and shaped to f o l l o w the beach i t s e l f . The groin on the beach should look l i k e a bump, so that i t
i s not immediately obvious that i t
i s an a r t i f i -
c i a l s t r u c t u r e . From t h e - h y d r a u l i c point of view, t h i s i s the best shape* of a g r o i n . The permeable groins should be made from l o c a l m a t e r i a l s . The trunk of the coconut tree i s s t r a i g h t
and therefore convenient to use as p o l e -
work. Whether or not t h i s tree can be used, depends on i t s seawater r e s i s t a n c e . Otherwise other wooden poles should be used, see Figure 5 . At the dry beach, the poles must not be more than approximately 0.5 m above the beach p r o f i l e . These allow some of the sediment transport pass, but not a l l . A f t e r some time i t
to
is p o s s i b l e to remove sand from
the accretion zones to the eroded zones. The idea i s to create one s y s tem in which the sand i s transported w i t h i n the s e c t i o n , but not removed outside the system f u r t h e r along the c o a s t . This i s because of the demand for a s t a t i c beach instead of the normal dynamic beach l i n e . The permeable groins can comprise two rows of poles and i f necessary, depending on f u t u r e beach developments, a stone f i l l i n g can be used between the pole rows.
- 14 -
In a l l ways i t makes sense to use l o c a l m a t e r i a l s as much as p o s s i b l e and to design f l e x i b l e s t r u c t u r e s . Knowledge about the phenomena along the coast of B a l i i s very l i m i t e d , and i t
i s very p o s s i b l e that s t r u c -
tures are not well l o c a t e d . The removal and re-use of the materials may be p o s s i b l e .
5.
SPECIFIC PROBLEMS
5 . 1 . Kuta Beach North of the a i r p o r t runway there i s erosion and some structures have already been made. In part of the Pertamina Cottages a beach wall has been c o n s t r u c t e d , the toe being protected by t e t r a p o d s . Several hundred meters northwards a low groin has been constructed which i s in a bad c o n d i t i o n and forms an obstacle to pedestrians and i s not long enough to be e f f e c t i v e , s e e Figure 6. To protect the beach, t h i s groin should be made longer and higher, with gentle s l o p e s , blending to the beach i t s e l f , see Figure 4 . On both sides of t h i s groin smaller groins should be made, c o n s i s t i n g of polework.
Figure 6 - Kuta Beach
- 15 -
A long permeable groin should be constructed at the northside of the Pertamina cottages seawall to prevent erosion p r o g r e s s i n g . Nusa Dua The demand for a s t a t i c beach means also the c r e a t i o n of systems in which a l l the sediment i s r e t a i n e d . The beach has to be divided into sections and on the boundary of each section a l o n g , high impermeable groin should be made. Between these g r o i n s , smaller permeable groins have to be b u i l t in order to give the beach l i n e r e q u i r e d . Sand i n p u t , to f i l l the systems, completes the beach r e s t o r a t i o n and conservation works. Of c o u r s e , downdrift of the system, new erosion problems w i l l occur. When these become s e r i o u s , the same system of protection
should
be extended northwards. The e x i s t i n g groins are not s t a b l e . Because there i s no bottom protect i o n and a sound foundation the groins w i l l c o l l a p s e after a c e r t a i n time, see Figure 7.
Fugure 7 - Nusa Dua
- 16 -
Sanur Beach Beach protection has to be c a r r i e d out in the area-of Werdhapura using the system descriped in Chapter 4. Long impermeable groins with the smaller permeable constructions and also s a n d f i l l i n g w i l l be necessary. In the long term, the whole beach should be divided into sections as described above, the timing of the work and the extent depending on the l o c a l s i t u a t i o n . From a coastal engineering point of view these are a l l s m a l l - s c a l e works, but they are necessary, since any loss of beach cannot be t o l e r a t e d . Structures have already been made in the area of the B a l i Beach H o t e l . These function rather well and only the shape and the foundation of the functions can be c r i t i c i z e d . A l l the beach protect i o n works should be founded on a f i l t e r construction as mentioned in Chapter 4. The shape must be convenient to p e d e s t r i a n s .
Lebih At Lebih the beach, used for r e l i g i o u s purposes, i s being eroded. * Because the l i t t o r a l d r i f t i s from east to west, a long groin with a c r e s t height HWL - 0.5 m, should be constructed u p d r i f t of the present o u t l e t of the r i v e r , to cause accretionon the east s i d e . Because of the erosion expected on the west side afterwards, the groin should be b u i l t with roots extending well into the e x i s t i n g dune. At the same time a second dam should be b u i l t in the r i v e r behind the dune to prevent erosion of the bank of the r i v e r basin and to prevent the r i v e r forming a new o u t l e t , _>
east of the g r o i n , see Figure 8 and 9 . Dam
to
be
midt
Figure 9 - Lebih 5 . 5 . Batumadeg The road from Manggis to Karangasem, 1.5 km east from Sengkidu, touches the c o a s t . The road has been protected by a masonry revetment, see Figure 10. Coral digging is c a r r i e d out here i n t e n s i v e l y and because of t h i s , the seabed has been deepened by up to 1.5 m. As a consequence, waves of 0.8 a 1.0 m are able to reach the beach. The toe of the beach, wall w i l l be eroded as there i s no bottom p r o t e c t i o n . The l i t t o r a l i s c l e a r l y from east to west and therefore b u i l t westwards of the beach w a l l , s t a r t i n g
drift
a groin system should be at an e x i s t i n g s p i t , and
founded on the coral bank. The toe of the bach wall should be protected using the l o c a l stones, graded from small to l a r g e , in a f i l t e r c o n s t r u c t i o n . Coral digging must be stopped
immediately.
J
- 18 -
5 . 6 . Pulaki At P u l a k i , on the north coast of B a l i , the road reaches the c o a s t l i n e at the toe of the mountains. To protect the r o a d , a seawall has been b u i l t . An extension of t h i s seawall i s planned r e l a t e d to road widening, see Figure 11. No bottom p r o t e c t i o n has been provided at the toe of the beach wall and during storms serious erosion w i l l
undermine the
s t a b i l i t y of t h i s w a l l . A bottom p r o t e c t i o n , as described f o r Batumadeg, should be c o n s t r u c t e d . To design the bottom p r o t e c t i o n , simultaneous wind and wave measurements during bad weather conditions must be made at high t i d e . These measurements, together with wave generation computations w i l l
give an i n s i g h t into the p r o b a b i l i t y of the wave
heights and periods needed for the design of the toplayer of the protection.
Figure 11 - Pulaki 5 . 7 . Gumbrih West of the o u t l e t of the River B a l i an serious erosion i s taking place where high waves attack the coast d i r e c t l y around high water. The s t a b i l i t y of the coastal road here i s threatened.To protect the coast here, see Figure 12, w i l l
be very expensive. Structures must be able to
- 19 -
destroy the incoming wave energy and, n a t u r a l l y , remain s t a b l e . From a v i s u a l impression of the wave height i t
is obvious that the breakers on
such s t r u c t u r e s w i l l create very large dynamic f o r c e s . To the author
it
seems that i t may be much cheaper to reconstruct the road more landwards and to rehouse the people l i v i n g t h e r e , rather than to b u i l d huge wave b r e a k e r s . If
s t r u c t u r e s are b u i l t , some of the bigger l o c a l r i v e r
stones
could be used, but heavy equipment would be necessary to handle them. Wave measurements, as proposed
Figure 12 - Gumbrih
for P u l a k i , should be c a r r i e d out.
- 20 -
6.
THE PROBLEM OF THE CORAL DIGGING For a long time, coral digging has been a custom and a way of earning a l i v i n g for a number of people in B a l i . The c o r a l stone i s used for
the
production of lime for b u i l d i n g purposes. However, the coral reef acts as a very good destroyer of the wave energy, coming from the ocean. Not only are the waves broken at the offshore edge of the r e e f ; the reef itself
acts as an energy destroyer because of the high r e s i s t a n c e of the
shallow rough bottom. Removal of the c o r a l , i . e . c r e a t i n g deeper water inshore, w i l l
always have a negative influence on the wave energy
reaching the beach. If the coral digging was a s m a l l - s c a l e o p e r a t i o n , then time would be a v a i l a b l e to implement and control r e g u l a t i o n s aimed at phasing t h i s way of earning a l i v i n g , but from a report development of the lime industry in B a l i i t
(4) on the
i s very c l e a r that i t
i s not
a s m a l l - s c a l e a c t i v i t y . About 1200 people are a c t i v e in the f i e l d and y e a r l y remove approximately 150,000 m3 of c o r a l . At some p l a c e s , the s i tuation
i s not yet so alarming, since the width of the coral reef i s be-
tween 500 and 1000 m. But there are only a few such l o c a t i o n s . At most p l a c e s , the width i s about 100 to 200 m. The removal of the coral here w i l l
have a serious detrimental
the beach. In the future the beach protecting structures w i l l
e f f e c t on have to be
much more extensive and expensive than in the present s i t u a t i o n . By not stopping the coral digging now, Indonesia w i l l
have to spend a lot
money in the f u t u r e . The complete cessation of coral digging require an e f f e c t i v e control
system which w i l l
of
will
cost money and w i l l
create s o c i a l problems. Coral digging was p r o h i b i t e d a long time ago, but there has been no control
and the law has not been maintained. From
the point of view of Government t h i s should not be t o l e r a t e d ; to issue a law but then not to execute i t ,
throws doubt on government
credibility.
The p r o v i c i a l government t r i e s to eliminate coral digging by changing from coral stone to limestone for lime production. The opening of limestone quarries and the construction of p r o f e s s i o n a l lime k i l n s
is
part of t h i s p l a n . This change is not only needed because of the coral digging but also to improve the q u a l i t y of the lime. Lime, produced in the t r a d i t i o n a l
way has a poor q u a l i t y and a l o t of firewood i s used in
i t s production. The Government of B a l i t r i e s to t r a n s f e r the coral diggers to the limestone q u a r r i e s . But t h i s i s not enough. Only 100 people in the Nusa Dua area have been helped in t h i s way. Other sources of employment, proposed and
- 21 -
p a r t l y implemented by the B a l i Government a r e : c i t y c l e a n i n g , seaweed planting and employment in the b u i l d i n g i n d u s t r y . The B a l i T o u r i s t Development Corporation has imported from East Java 1500 people to work i n the development scheme in Nusa Dua. These people had to be educated for t h i s . w o r k .
This would also have applied to the 1200 coral diggers. A
drawback of employing coral diggers in the b u i l d i n g industry i s that the whole f a m i l y i s not involved and that the f a m i l y income would decrease. This i s a very common phenomenon over the whole world in t h i s time of economic r e c e s s i o n . It
seems that the Government of B a l i i s on the r i g h t track to solve the
problem except for the non-existence of a legal enforcement system. The c e n t r a l problem i s more one of f i n a n c e , because the implementation of a l l the measures w i l l
cost money and there i s i n s u f f i c e i n t
available.
It
would be wise to place enough money at the disposal of the B a l i Government, so that i t can carry out i t s p l a n s , otherwise the cost w i l l be paid many times over in the f u t u r e in the c o n s t r u c t i o n of large and environmentally undesirable s t r u c t u r e s .
- 22 -
SUMMARY In g e n e r a l , c o a s t a l defence in Indonesia i s not yet a matter of national i n t e r e s t . The approach i s not systematic and does not cover a l l
the
i n t e r e s t s i n v o l v e d . One governmental department should be responsible for coastal defence and should be given a u t h o r i t y to budget and e s t a b l i s h p r i o r i t i e s . A coastal defence d i v i s i o n should be created. The Hydraulic Research I n s t i t u t e , DPMA, should operate as adviser to t h i s d i v i s i o n . In the long run, a systematic approach to a l l coastal problems should be f o l l o w e d . In the present s i t u a t i o n t h i s i s impossible. The amount of money, a v a i l a b l e to the i n v e s t i g a t o r s , i s t o t a l l y insufficient.
The budget a v a i l a b l e and the personnel have to be
increased c o s i d e r a b l y to reach the s i t u a t i o n that physical processes can be understood, based on prototype measurements and the modelling of these processes in order to predict the consequences of engineering measures and to make optimal designs. This would be investment for
the
future. Beach p r o t e c t i o n works should be shaped as i f they were part of the beach, at l e a s t on the beach i t s e l f . Seven s p e c i f i c coastal problem areas (Kuta Beach, Sanur, Nusa Dua, L e b i h , Batumadeg, Pulaki and Gumbrih) have been t r e a t e d , in a q u a l i t a t i v e way o n l y , because of the lack of data and model r e s u l t s . The coral digging problem, based on the ideas and the p a r t l y
implemented
measures of the B a l i government, i s more of a f i n a n c i a l problem than anything e l s e . More money should be made a v a i l a b l e to solve t h i s problem, otherwise i t w i l l be paid for many times over in the future by large expensive, beach protection works. B a l i i s of national
interest.
The r e l a t e d problems should be the r e s p o n s i b i l i t y of the central ment.
govern-
- 23 -
RECOMMENDATIONS A. Organisation 1. Consider coastal defence as a matter of national
interest.
Consider a coastal problem always in r e l a t i o n to the economic, environmental, e c o l o g i c a l and s o c i a l i n t e r e s t s
involved.
2. Make one goverment department r e s p o n s i b l e for the coastal defence of Indonesia. 3. Create a coastal d i v i s i o n in t h i s department with a u t h o r i t y to budget, e s t a b l i s h p r i o r i t y and coordinate a l l the i n t e r e s t s v o l v e d . This devision should be the p r i n c i p a l for matters
in-
related
to consultants and c o n t r a c t o r s . 4. Use DPMA at Bandung as the central adviser to t h i s d i v i s i o n on coastal problems. 5. Extend the budget of the coastal section of DPMA c o n s i d e r a b l y . Enlarge the number of personnel. Educate the people with the help of Rijkswaterstaat and D e l f t Hydraulics Laboratory. 6. Give a u t h o r i t y for proposing of measures, management and maintenance of e x i s t i n g s t r u c t u r e s to p r o v i n c i a l
and l o c a l public
offi-
c e s . Let them act as delegated p r i n c i p a l s . 7. Consider the p o s s i b i l i t y of asking the Dutch Government to give a s s i s t a n c e in the formation of the o r g a n i s a t i o n and the r e l a t e d measures. B. Technical Aspects 1. Devide the coast of B a l i into sections from which no sediment can be removed by the sea. Treat these sections as closed beach systems. 2. Design structures in such a way that the construction material can be removed and used elsewhere. 3. Use l o c a l material
( r i v e r stones, c o r a l , wood) as much as pos-
sible. 4. Design proper foundations, constructed as f i l t e r s for beach prot e c t i o n works.
\
- 24 -
5. Carry out prototype measurements w i t h i n the scope of a research s t r a t e g y . Always put the question " i f
not
what e l s e " ,
6. Shape s t r u c t u r e s on a beach to resemble bumps on the beach. Try to give them the shape and colour of the beach Coral
itself.
digging
1. Give more money to the Province of B a l i to implement t h e i r
solu-
t i o n s of t r a n s f e r r i n g the coral diggers to other j o b s . 2. Maintain the law, control
and punish the o f f e n d e r s .
3. Do f i r s t Item I and, i f t h i s i s not s u f f i c i e n t , do Item 2 . 4. Remove the coral diggers from the coral reef in Batumadeg i n f r o n t of the seawal1.
Acknowledgement The author wishes to thank the D i r e c t o r and s t a f f of DPMA, Bandung and Mr. Lukman of the Department of P u b l i c Works, Jakarta for t h e i r
assis-
tance during his v i s i t to Indonesia. E s p e c i a l l y he would l i k e to thank I r .
Syamsudin for accompanying him i n
B a l i and Bandung. He thanks Mr. Kardana f o r a l l his help and Mrs. A r i f for typing the r e p o r t . The d i s c u s s i o n s with people of several governmental groups in B a l i and of the BTDC were most h e l p f u l . Thanks to them a l l .
- 25 -
L i s t of References
(1) .
Diephuis, J.G.H.R. Report on beach preservation on the Island of B a l i ; 1972.
(2) .
T s u c h i y a , Y. F i n a l report of coastal engineering consultant with special reference to beach changes i n Sanur Beach, B a l i ; 1978
(3) .
S i l v e s t e r , R. Beach s t a b i l i z a t i o n South of Sanur Beach, B a l i ; 1979'.
(4) .
D a b e l s t e i n , N. Development of the lime industry
in B a l i
Report from Seminar and Workshop; 1980.
rijkswaterstaat deltadienst hoofdafdeling waterbouwkundige werken afdeling waterbouwkundige werken west
i r . C.J.H. Oudshoorn i r . J . Slagter Rijkswaterstaat-Hodfddirectie Koningskade 4 2596 AA ' s Gravenhage
uw kenmerk:
burgh-haamstede,
uw brief van:
ons kenmerk:
onderwerp:
Dienstreis B a l i
14 September 1983
verzonden: bijlagen:
H i e r b i j doe i k u toekomen :
1. Rapport inzake de kusterosie van B a l i . 2. Aanvullende t o e l i c h t i n g en i n t r o d u c t i e t o t het gesprek tussen i r ' s Slagter, Oudshoorn en Kooman over het vervolg van de missie.
1
Ik heb s l e c h t s een exemplaar van het rapport met k l e u r e n f o t o s . Dit heb i k aan i r . J . Slagter
gestuurd.
Met v r i e n d e l i j k e
^ . i r . D"^-Kooman
postbus 5002 4328 ZV burgh-haamstede verzoeke bij uw antwoord kenmerk en
havenplateau burghsluis
datum dezes te vermelden en slechts
telefoon (01115)50 00
sen zaak In een brief te behandelen.
telex 55456
qro&i,
rijkswaterstaat deltadienst
notitie a a n :
i r . J . Slagter, i r . C.J.H. Oudshoorn
v a n :
i r . D. Kooman
datum:
12-9-1983
onderwerp:
Dienstreis naar Indonesie
Duur : zaterdag 13 augustus t o t zondag 4 September 1983.
15- 8-1983
: Bezoek departement Openbare werken Jakarta. Gesproken met i r . Mardjono, hoofd planningbureau. Term of reference ontvangen. Gesproken met d i r e c t e u r Directorate r i v e r s and t i d a l i r r i g a t i o n .
16- 8-1983
Bezoek Bandung : Hydraulisch i n s t i t u u t (DPMA).
t/m
Gesproken met i r . S a d e l i directeur
18- 8-1983
:
i r . S o e l a s t r i p r o j e c t manager i r . Syamsudin hoofd coastal engineering section Probleemanalyse erosie strand B a l i . I n s t i t u u t voor bouwmaterialenonderzoek UPMB, d i r e c t e u r i r . Karman, i.v.m. probleem k o r a a l r i f e x p l o i t a t i e .
19- 8-1983
: Bezoek Jakarta. Gesproken met S.G. Departement Openbare werken i r .
Radinal.
20- 8-1983
Bezoek B a l i . B e z i c h t i g i n g probleemgebieden (7 stuks, v e r -
t/m
spreid over geheel B a l i ) .
27-8-1983
: - discussies met departement Openbare werken van de prov i n c i e B a l i . Hoofd : i r . Tambing. - discussie met B a l i Tourism Development Corporation. President Directeur : i r . Gandjar.
26- 8-1983
: Eindbespreking o . l . v . i r . Tambing met grote groep vertegenwoordigers van 13 overheidsinstanties u i t B a l i , Bandung en Jakarta. A l l e i n s t a n t i e s z i j n belanghebbend i n kusterosieprobleem B a l i .
27- 8-1983
: Reis naar Jogyakarta.
28- 8-1983
: Bezoek Jepara. Noordkust Centraal Java i.v.m. kusterosieprobleem
29- 8-1983
landbouwgronden bedreigd.
: Reis naar Bandung
rijkswaterstaat behoort bij: bladnr.:
30-8-1983
notitie
nr.
2
Rapport s c h r i j v e n .
t/m 1-9-1983
:
1- 9-1983
: Eindbespreking
6 . 1 . v . i r . S a d e l i i n Bandung met ver-
tegenwoordigers van Hydraulisch i n s t i t u u t DPMA, research i n s t i t u u t voor bouwmaterialen UPMB en P.W. Jakarta. 2- 9-1983
: Reis naar Jakarta.
3- 9-1983
: Eindbespreking S.G.
op departement P.W. Jakarta o . l . v .
i r . Radinal.
Aanwezig : Ply. D.G. Cipta Karya (soort v o l k s h u i s v e s t i n g ) . D.G.
Waterresources Development
Directeur Directorate r i v e r s and t i d a l
irrigation.
Met medewerkers. Rapport u i t g e r e i k t . 4- 9-1983
: Aankomst Nederland.
De f i l m "Dutch Delta" 3 maal vertoond
(26-8-1983, 1-9-1983,
3-9-1983).
De VIP-brochure en stropdas u i t g e r e i k t aan : i r . Radinal
; D.G. Waterresources Development en i r . Mardjona.
Overig foldermateriaal Oosterscheldepro.ject u i t g e r e i k t aan deze 3 heren en aan vele anderen.
rijkswaterstaat behoort bij: n o t i t i e bladnr.:
nr.
3
In de eeste week onduidelijkheid over het doel van de missie. Voorbereiding was matig. Terms of reference pas na aankomst ontvangen.
Korte probleemomschrijving In r e l a t i e met de t o e r i s t i s c h e ontwikkeling van B a l i i s erosie van het strand een probleem. Het k o r a a l r i f dat voor de kust l i g t en i n breedte v a r i e e r t van 100 t o t 1000
m wordt afgegraven door koraalpikkers.
D i t r i f fungeert a l s g o l f e n e r g i e v e r n i e t i g e r . Door de verdieping van r i f b e r e i k t meer golfenergie de kust met
het
erosie a l s gevolg.
Het rigoreus stoppen van deze a c t i v i t e i t creeert een s o c i a a l probleem, aangezien ca. 1200
gezinnen hieraan hun inkomen ontlenen.
Bestudeer d i t probleem vanuit technisch en s o c i a a l oogpunt. Naast de stranderosie wordt de kustweg op diverse plaatsen bedreigt door e r o s i e . Bestudeer ook d i t .
Bevindingen Het bijgevoegde rapport bevat mijn bevindingen. Naast het ingaan op de vragen gesteld i n de terms of reference ben i k v r i j u i t v o e r i g ingegaan op de organisatorische problematiek die samenhangt met
kustverdediging
a l s nationale taak. Verder heeft de algemene probleembenadering nogal wat aandacht gekregen. D i t bleek precies aan te s l u i t e n op de verwachtingen die de Secretaris-Generaal van het M i n i s t e r i e had, de Minister, zoals de S.G.
en ook op die van
mij vertelde (De Minister van P.W.
zou de
meeting bijwonen maar werd een uur voor de aanvang weggeroepen door President
Soeharto).
Ik heb het gevoel dat van een succesvolle missie gesproken mag
Enkele opmerkelijke
worden.
punten :
1. Er i s geen enkele i n s t a n t i e verantwoordelijk
voor
kustverdediging.
De behandeling van actuele problemen i s o n d u i d e l i j k , ad hoc en f r a g mentarisch. Maatregelen worden of v o l s t r e k t onoordeelkundig genomen zonder i n z i c h t i n de n a t u u r l i j k e v e r s c h i j n s e l e n of, indien voorgesteld door DPMA te Bandung, s l e c h t s g e d e e l t e l i j k en dan nog a l t i j d k l e i n e r van afmetingen dan aanbevolen. Het gevolg i s een k l e i n aantal verspreid liggende k l e i n e constructies die nauwelijks
e f f e c t hebben, integendeel z e l f s ; n i e t op elkaar aan-
s l u i t e n en obstakels vormen voor t o e r i s t e n die op het strand wandelen.
rijkswaterstaat behoort bij: n o t i t i e bladnr.:
nr.
A
De b e l a n g r i j k s t e klacht i s geldgebrek.
I k z e l f denk dat d i t
o n j u i s t i s . Er worden zeer grote bedragen i n B a l i geinvesteerd (tot 1989 ca. 1 m i l j a r d gulden) maar de p r i o r i t e i t s s t e l l i n g
ge-
beurt met veronachtzaming van de strandverdediging. Een zeer k l e i n e ombuiging i n p r i o r i t e i t e n zou het f i n a n c i e l e probleem oplossen. Daarvoor i s nodig het besef dat kustverdedigingswerken ook
.nfrastructurele werken z i j n en dat ze onderhouden moeten
worden net zoals men
dat b i j wegen wel doet. Ik geloof dat deze bood-
schap goed overgekomen en begrepen i s .
Het koraalpikkersprobleem
i s meer een f i n a n c i e e l / b e s t u u r l i j k dan
een ander probleem. Het p r o v i n c i a a l bestuur heeft tenminste van vervangende werkgelegenheid. De implementatie stagneert door geldgebrek.
4 bronnen
van deze plannen
Het probleem i s echter acuut geworden
door de enorm toegenomen bouwactiviteiten op B a l i . Het koraal wordt gebruikt voor de productie van bouwkalk en voor de bouw van tuinmuren b i j hotels en restaurants. Deze a c t i v i t e i t e n het r e s u l t a a t van de o n t w i k k e l i n g s a c t i v i t e i t e n van de Centrale
zijn Overheid.
De ermee samenhangende problemen z i j n echter een zorg van de l o c a l e overheid, zonder dat deze fondsen k r i j g t 6m ze op te lossen. Dit i s een b e s t u u r l i j k e i n c o n s i s t e n t i e . Het koraalpikken i s a l meer dan 20 jaar b i j de wet verboden. Controle en sancties z i j n echter achterwege
gebleven.
De c o a s t a l engineering s e c t i o n van DPMA te Bandung fungeert a l s consultant voor geheel Indonesie
(60.000 km
kustlijn).
Het budget voor alle^ veldwerk bedraagt ca. f. 60.000,00 gulden per j a a r . Dit i s absoluut onvoldoende om z e l f s het minimaal nodige voor a l l e e n -
B a l i te doen . Ook de personeelsbezetting i s k w a l i t a t i e f onvoldoende. De i r r i g a t i e s e c t o r i s , b e g r i j p e l i j k veel beter bemand en beschikt over a a n z i e n l i j k meer geld en meetmiddelen. Enige verschuiving van zou mogelijk moeten z i j n , aangezien voedselbehoefte v o o r z i e t .
gelden
Indonesie nu voor 95% in. z i j n eigen
rijkswaterstaat behoort bij: bladnr.:
notitie
nr.
5
Vervolg De S.G.,
i r . Radinal heeft de vraag gesteld wat de RWS
doen b i j het opzetten van de noodzakelijke o r g a n i s a t i e ,
zou kunnen reglementen
en technische maatregelen. Ik heb geantwoord dat de RWS wel enige a s s i s t e n t i e zou kunnen leveren, maar dat een v o o r s t e l door Indonesie gedaan zou moeten worden. Hij heeft t i j d e n s de meeting opdracht gegeven om voor 15 September 1983 een v o o r s t e l te maken dat zowel de b e s t u u r l i j k e a l s technische kant beh e l s t van kustverdediging. H i j staat een stap voor stap opbouw voor, waarbij de eerste stap uitmondt i n een allesomvattende aanpak van de kust van B a l i . Daarbij spelen aspecten een r o l a l s : 1. Eigendom van de kust (nu p a r t i c u l i e r ) , over welke breedte i n r e l a t i e met e f f e c t i e f beheer en onderhoud. 2. Wie wordt de verantwoordelijke i n s t a n t i e , wat moeten de bevoegdheden z i j n . 3. Wat wordt gedelegeerd aan wie. 4. Opstellen "masterplan" voor strandprotectie B a l i . 5. Voorontwerpen van strandverdedigingswerken. 6. Uitvoeringsaspecten, organisatie van onderhoud e t c .
H i j z a l tevens v o o r s t e l l e n mij i n december 1983 nogmaals naar Indonesia te laten gaan. A l l e experts (4 stuks) z i j n i n het droge seizoen geweest. De s i t u a t i e i n het natte seizoen kan aanmerkelijk anders z i j n , zo i s mij gebleken u i t gesprekken met de p l a a t s e l i j k e bevolking, d i e er d i r e c t de gevolgen. van ondervinden. Het i s goed mogelijk dat i n een of twee weken i n de maand december het grootste deel van de erosie p l a a t s v i n d t . D i t kan een ander l i c h t werpen op de problematiek. De b e l a n g r i j k s t e bron van informatie i s het visuele beeld. De enkele meting d i e wordt uitgevoerd i s of eenmalig of eenmaal per jaar, t e r w i j l tenminste beide seizoenen eenmaal gemeten zouden moeten worden. Voor de onderbouwing en de d e t a i l l e r i n g van maatregelen i s daarom een bezoek i n het natte seizoen noodzakelijk.
rijkswaterstaat behoort bij: bladnr.:
notitie 6
Op 26 September 1983 i s er een gesprek tussen i r . Slagter i r . Oudshoorn en ondergetekende over de follow up. Het voorgaande i s bedoeld a l s i n t r o d u c t i e van d i t gesprek
L
^uta^aWWh
f
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