The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version)
Chapter 10 Field Survey and Sediment Analysis for the Candidate Site
10.1 Overview
Several kinds of field surveys have been carried out to obtain the necessary information for sediment analysis, planning, and design. Finally, the comprehensive analysis based on the obtained data and survey results has been conducted to grasp the mechanism of littoral drift and resulting beach change.
Table 10.1.1 shows an overview of Chapter 10.
Table 10.1.1 Overview of Chapter 10 Ref. No Item Objective Obtained Result Applied to -To check the difference of tide change between southwest and east Same tide condition can be employed design & Tide M easurement at Southwest 10.1 coast between southwest and east coast construction and East Coast -To confirm the design tide (Candidasa) in the design method condition -To understand the bathymetric -Geographical information could be design, features as part of the formulation obtained sediment Sounding (Bathymetric and 10.2 of a coastal conservation plan - The sediment process at Kuta offshore analysis & Topographic Survey) -To know the sediment process at was identified construction southwest coast method - At Candidasa, significant change of profile was not observed since 2006 -To know the process of beach - Nourished sand are maintained the erosion and mechanism of littoral stable condition at Kuta north. design & Beach Profile Survey and Sand 10.3 drift - Change of profile was observed at sediment Samp ling -To know the relation between Legian, Seminyak area, but not so analysis beach slope and grain size significant - Beach slope and grain size was identified - There is a sand potential area at - To identify the potential area of offshore of Candidasa, but obtaining of construction Diving Survey for Potential Area sand borrow for nourishment at sand with good specification is some 10.4 method & of Sand Borrow offshore of surrounding project area difficulty. cost estimate -There is a sand potential area at out of Kuta reef.
-Westward littoral drift exists for whole Comprehensive Analysis for -To know the comprehensive area of Candidasa. planning & 10.5 Sediment Transport at mechanism for littoral drift at - Possibility to increase the organic sand design Candidasa Candidasa is not expected
Comprehensive Analysis for -To know the comprehensive -Obvious littoral drift doesn’t exist from planning & 10.6 Sediment Transport at mechanism for littoral drift at north Kuta, Legian to Seminyak area design Southwest Coast Southwest coast
10-1 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version)
10.2 Tide Measurements along the Southwest and East Coast
Measurement of sea water level was carried out to check the difference in tidal change between the southwest and east coast, which will be considered in the design study.
Each self-recorded pressure sensor type tide gage was simultaneously installed in both Kuta and Candidasa, and continuous measurement of sea water level was carried out from November 22-December 22, 2011. The measured data were compared with the tide prediction for Benoa Port. Findings based on the comparison are as follows:
¾ Similar tendency of tidal change was observed between the measurements (from bathymetric and topographic surveys) in Kuta, and prediction in Benoa. On the other hand, tidal change in Candidasa was different from that in Kuta. The difference might be caused mainly by the effect on the existence of coral reef in Candidasa. ¾ Significant difference in maximum sea water levels (HWL) was not observed. This means that the same tide condition can be employed for both the southwest coast and east coast (Candidasa) in the design study
350 Candidasa 300 Kuta 250 (cm)
Benoa 200 Level
150 Water
100 Sea
50
0 11/22 11/24 11/26 11/28 11/30 12/2 12/4 12/6 Date
300
250 (cm)
200 Level 150 Water 100 CANDIDASA Sea 50 KUTA BENOA 0 12/9 12/11 12/13 12/15 12/17 12/19 12/21 12/23 Date
Figure 10.2.1 Result of Sea Water Level Measurements in Candidasa and Kuta (Source: JICA Study Team)
10-2 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version)
10.3 Sounding (Bathymetric and Topographic Survey)
(1) Outline
Bathymetric survey was carried out Candidasa and from North Kuta to part of Canggu to understand the bathymetric features as part of the formulation of a coastal conservation plan for the candidate sites (Figure 10.3.1). The range and intervals of the bathymetric survey are shown in Table 10.3.1.
Figure 10.3.1 Bathymetric Survey Areas (Source: JICA Study Team)
Table 10.3.1 Range and Intervals of Bathymetric Survey Length of Coverage Area Location Interval of Coastline Line Spacing from Shoreline (Survey Date) Sounded Point (km) (m) Candidasa 5 km 25 m at inner reef 25 m (max.) Approx. 1.2 km, (December 1, (from peninsula area or reached 2011) to the west) 100 m at outer maximum depth reef area North Kuta to 6 km 100 m from Kuta 25 m (max.) Approx. 1.5 km, Petitenget (from Kartika Art Market to or reached a (November Plaza to end; depth more than 24-25, December Petitenget 25 m from 25 m 2, 2011) Temple) Kartika Plaza to Kuta Art Market (Source: JICA Study Team)
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(2) Sounding Results
a) East Coast (Candidasa)
In Candidasa, the coral reef in the eastern edge located in front of Puri Bagus Hotel is developing continuously across the coastline from eastern edge, with a 300 m width from coast to reef edge. The width of coral reef is decreasing gradually in a westward direction, and the reef ends in front of Puri Bagus Hotel. The coastal water depth at eastern edge and western edge of Candidasa is shallow with about 10 m in the vicinity of 400 m from the coast. The coastal water depth of the central part, in front of Sub Ocean Hotel, is relatively deep with about 20 m in the vicinity of 400 m from the coast (Figure 10.3.2).
Figure 10.3.2 Bathymetric Chart for Candidasa (Source: JICA Study Team)
b) Southwest Coast (from North Kuta to Part of Canggu)
In Kuta, the coral reef developing around the coastal area of runway is decreasing drastically northward and finally disappears as it reaches the front of the Bali Garden Hotel. The coastal water depth is shallow from the southern edge to Bali Garden Hotel, and from Bali Holiday Resort Hotel northwards. Said depth is about 5 m in the vicinity of 400 m from the coast. The coastal water depth from Bali Garden Hotel to Bali Holiday Resort Hotel is relatively deep with about 5 m in the vicinity of 200 m from the coast (Figure 10.3.3).
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Figure 10.3.3 Bathymetric Chart for the South East Coast from North Kuta to Part of Canggu (Source: JICA Study Team)
(3) Comparison of Cross Section Chart
a) East Coast (Candidasa)
The results of bathymetric survey in Candidasa were used to investigate the sediment (littoral drift sand) transport and beach change. The representative survey lines to evaluate the bathymetric features are shown in Figure 10.3.4.
L10 L9 L8 L7 L6 L5 L4 L3 L2
L1
Figure 10.3.4 Location of Representative Survey Lines in Candidasa (Source: JICA Study Team)
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The cross section charts of representative survey lines, based on the results of bathymetric survey carried out in Candidasa last December 2011, are shown in Figure 10.3.5. The sea-bottoms at shallower water depth than 10 m have gentle slope of 1/20 to1/50. In contrast, as for the sea-bottoms at deeper water depth than 10 m have, L1 at eastern edge is a gentle slope of 1/30 and other lines are sharp slope of 1/10 to 1/15.
It was assumed that it may not be possible for the littoral drift sand to return to the beach once the sand has been transported offshore considering that the sea bottom slope in the coastal area of Candidasa is steep when water depth is deeper than 10 m.
20 20 2011 Year Cross Section Line 2011 Year Cross Section Line Line L1 Line L6 Line L2 Line L7 Line L3 0 Line L8 Line L4 0 Line L9 Line L5 Line L10
-20 -20 Elevation (m) Elevation Elevation (m) Elevation -40 -40
-60 -60
-80 -80
0 400 800 1200 1600 0 400 800 1200 1600 2000 Distance (m) Distance (m)
Figure 10.3.5 Cross Section Charts of Representative Survey Lines in Candidasa (Source: JICA Study Team)
b) Southwest Coast (North Kuta to part of Canggu)
The sediment (littoral drift sand) transport and beach change from North Kuta to part of Canggu were investigated, based on the results of bathymetric survey carried out in Kuta last 2006, 2008, and 2011. The representative survey lines considered for evaluating the bathymetric features are shown in Figure 10.3.6. Among the representative survey lines, C1 to C5 belonged to the nourishment sites implemented under the Phase-1 Project, while C6 to C10 belonged to sites not implemented under the Phase-1 Project.
Figure 10.3.6 Locations of Representative Survey Lines in Kuta (Source: JICA Study Team)
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The comparison results of cross section chart for each representative survey line, based on the results of bathymetric survey, are shown in Figure 10.3.7 and Figure 10.3.8. The results of coastal accretion and erosion conditions at each survey line are summarized in Table 10.3.2.
Both C4 and C5, located in the north edge of the nourishment implemented in the Phase-1 Project, have an accretion trend when the sea bottom is deeper than 8 m. This was likely due to some of the nourishment sand, which were transported offshore and accumulated on the sea bottom at C4 and C5. C6 to C10 located at the sites which have not been nourished in the Phase-1 Project do not exhibit clear trend of erosion and accretion. It appears that the nourishment sand in the Phase-1 Project has not been transported northward from C6.
Table 10.3.2 Results of Coastal Accretion and Erosion Conditions at Each Survey Line Survey Accretion and Erosion Conditions Line C1-C2 Sea bottoms (at water levels deeper than 5 m) more than 400 m from the reference point showed different trends between 2006 to 2008 and 2008 to 2011, and exhibited no clear tendency of erosion or accretion. C3 Sea bottoms (coast side) within 1000 m from the reference point showed same accretion trends between 2006 to 2008, and 2008 to 2011. Sea bottoms (offshore side) more than 1000 m from the reference point showed different trends between 2006 to 2008, and 2008 to 2011, and exhibited no clear tendency of erosion or accretion. C4 Sea-bottoms at coast side (with higher elevation than 0 m) and offshore side showed same accretion trends between 2006 to 2008, and 2008 to 2011. C5 Sea bottoms at coast side (with elevations higher than 0 m) and offshore side showed same accretion trends between 2006 to 2008, and 2008 to 2011. C6-C10 Sea bottoms showed different trends between 2006 to 2008, and 2008 to 2011, and exhibited no clear tendency of erosion or accretion. (Source: JICA Study Team)
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8 8
Cross Section Line C1 Cross Section Line C2 4 4 2006Year 2006Year 2008Year 2008Year 2011Year 2011Year
0 0 Elevation (m) Elevation Elevation (m) Elevation -4 -4
-8 -8
-12 -12
0 400 800 1200 1600 2000 0 400 800 1200 1600 2000 Distance (m) Distance (m)
8 8
4 Cross Section Line C4 Cross Section Line C3 4 2006Year 2006Year 2008Year 2008Year 2011Year 2011Year 0
0
-4 Elevation (m) Elevation (m) Elevation -4
-8
-8 -12
-16 -12
0 500 1000 1500 2000 2500 0 400 800 1200 1600 2000 Distance (m) Distance (m)
8 8
4 Cross Section Line C5 4 Cross Section Line C6 2006Year 2006Year 2008Year 2008Year 2011Year 2011Year 0 0
-4 -4 Elevation (m) Elevation Elevation (m) Elevation
-8 -8
-12 -12
-16 -16
0 400 800 1200 1600 2000 0 400 800 1200 1600 2000 Distance (m) Distance (m)
Figure 10.3.7 Comparison on the Results of Cross Section Charts for the Southwest Coast (Source: JICA Study Team)
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8 8
4 Cross Section Line C8 4 Cross Section Line C7 2006Year 2006Year 2008Year 2008Year 2011Year 2011Year 0 0
-4 -4 Elevation (m) Elevation Elevation (m) Elevation
-8 -8
-12 -12
-16 -16
0 400 800 1200 1600 2000 0 400 800 1200 1600 2000 Distance (m) Distance (m)
8 8
4 Cross Section Line C9 Cross Section Line C10 2006Year 4 2006Year 2008Year 2008Year 2011Year 2011Year 0
0
-4 Elevation (m) Elevation Elevation (m) Elevation -4 -8
-12 -8
-16 -12 0 400 800 1200 1600 2000 Distance (m) 0 400 800 1200 1600 2000 Distance (m)
Figure 10.3.8 Comparison on the Results of Cross Section Charts for the Southwest Coast (Source: JICA Study Team)
10.4 Beach Profile Survey and Sand Sampling on the Beach
(1) Outline of Survey
Beach profile survey and sand sampling on the beach were carried out at two candidate sites namely, Candidasa and from North Kuta to part of Canggu for the study on the mechanisms of littoral drift.
Location maps for the beach profile survey and sand sampling on the beach are shown in Figure 10.4.1 and Figure 10.4.2.
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Figure 10.4.1 Location of Beach Profile Survey and Sand Sampling at Candidasa (Source: JICA Study Team)
Figure 10.4.2 Location of Beach Profile Survey and Sand Sampling at the South East Coast from North Kuta to Part of Canggu (Source: JICA Study Team)
10-10 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version)
The outline of sand sampling on the beach is shown in Table 10.4.1. Basically, sand samples were taken from three different positions (berm top, sloping part, and flat part) in one cross line as shown in Figure 10.4.3. The sand taken from the berm top is possibly influenced by sorting effect of blowing wind. To avoid this effect and obtain the same condition as that after nourishment as baseline condition, it was considered to take the sand sample at berm top 30 cm below the surface as shown in Figure 10.4.3. By comparing the difference in grain size taken from berm top and sloping part on certain lines in areas of coastal retreat, the grain size with no contribution to form the beach can be roughly evaluated.
Table 10.4.1 Outline of Sand Sampling on the Beach Location and Survey Sampling Point Analytical Parameters Date of Survey Sites Candidasa 34 3 points per 1line Sieve analysis (grain sieve distribution December 2011 (berm, slope, and flat and specific gravity) parts) Analysis of component materials (coral, volcanic, shell, foraminifera) North Kuta to 26 3 points per 1line Sieve analysis (grain sieve distribution Petitenget (berm, slope, and flat and specific gravity) November 2011 parts) Analysis of component materials (coral, volcanic, shell, foraminifera) (Source: JICA Study Team)
Slope Berm
Flat
Figure 10.4.3 Position of Sand Sampling in a Cross Section Line (Source: JICA Study Team)
(2) Results of the Beach Profile Survey
a) East Coast (Candidasa)
The beach profile survey in Candidasa has been carried out from east side peninsula to Alilla Manggis Hotel, which is located 5 km west side from the peninsula. The total number of survey lines taken was 204 as shown in Figure 10.4.1.
Some profile lines can be compared using the previous survey data. Figure 10.4.4 shows the typical results of beach profile for comparison with the previous condition. Here, three profile data which were taken in August 2005, February 2006, and December 2011(in this study) were compared.
Most parts of the beach area have already been divided into sections by the existing T-shape groins and offshore breakwaters constructed from 1989 to 1998 by the Indonesian government. Sandy beach has almost disappeared as of date, and the remaining small amount of sand at each section on the beach is trapped by groins. Due to this, no significant difference in each section was not observed from 2006 to present.
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b) Southwest Coast (North Kuta to Part of Canggu)
The beach profile survey has been carried out from the north part of coral reef area in Kuta (north side of last offshore breakwater, BWN3, which was constructed during the Phase-1 Project until Petitengat Temple located in the north of Seminyak area, which is roughly 6 km alongshore. The total number of survey lines taken is 120 as shown in Figure 10.4.2.
Some profile lines can be compared using the previous survey data. Figures 10.4.5 and 10.4.6 show the typical results of beach profile for comparison with the previous condition. Here, four profile data which were taken in November 2000, December 2008 (just after the nourishment during the Phase-1 Project), June 2011, and December 2011 (in this study), were compared as follows:
¾ In the north of coral reef area from line K1 to K8, most of the nourished sand has flown out, restoring the condition to that before the nourishment as shown in Figure 10.4.5 (K6). ¾ From line K9 to K20, the beach became significantly wider due to the nourishment during the Phase-1 Project compared to the previous condition. Moreover, the beach condition did not seem to change significantly after the nourishment. Some of the profile indicates accumulation of beach as shown in Figure 10.4.5 (K18). ¾ From line K22 to K32, the beach width evidently increased than that just after the nourishment as shown in Figure 10.4.5 (K28). ¾ Line K38 shown in Figure 10.4.6 is located at the end of nourishment area in the Phase-1 Project, which has normal constant width. The beach width that forms part of nourishment was gradually reduced from said line to the north (up to line K45), which is approximately 300 m in distance as transition area. The beach condition was almost the same just after the nourishment. ¾ Line K45 shown in Figure 10.4.6 is the end point of the nourishment. A slight increase of the shoreline was observed. ¾ From line K46 to K64, the beach slightly accumulated compared to just before the nourishment (see Figure 10.3.6 showing K60) ¾ There was no significant change of beach condition in Seminyak area (from line K70 to K90) in the last ten years.
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7.00 6.00 L4 5.00 4.00 3.00 2.00
Height (m) Height 1.00 0.00 -1.00 -2.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
Distance (m) August-05 February-06 December-11
7.00
6.00 L12
5.00
4.00
3.00
Height (m)Height 2.00
1.00
0.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 -1.00 Distance (m) August-05 February-06 December-11
7.00 6.00 L23 5.00 4.00 3.00 2.00 Height (m)Height 1.00 0.00 -1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 Distance (m) August-05 February-06 December-11
7.00 6.00 L33 5.00 4.00 3.00 2.00 1.00 Height (m)Height 0.00 -1.00 -2.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 Distance (m) August-05 February-06 December-11
Figure 10.4.4 Comparison on the Beach Profile Taken at Typical Survey Lines in Candidasa (Source: JICA Study Team)
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6.00 K6 (in front of Discovery Moll) 5.00
4.00
3.00
2.00
1.00 HEIGHT (M) 0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 DISTANCE (M)
Nov. 2000 Dec. 2008 Jun. 2011 Dec. 2011
6.00 K18 (Near the Hard Rock Cafe Hotel) 5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
DISTANCE (M)
36842Nov. 2000 December-08Dec. 2008 (BF) Jun.40705 201140889 Dec. 2011
7.00 K28 (South of Bragd Istana Rama Hotel) 6.00
5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00
DISTANCE (M)
Nov. 2000 Dec. 2008 Jun. 2011 Dec. 2011
6.00 K38 ( End of nourishment area with normal width) 5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
DISTANCE (M)
Nov. 2000 Dec. 2008 Jun. 2011 Dec. 2011
Figure 10.4.5 Comparison on the Beach Profile Taken at Typical Survey Lines from North Kuta to Seminyak (Source: JICA Study Team)
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6.00 K45 (End of nourishment area) 5.00
4.00
3.00
2.00
1.00 HEIGHT (M)
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 180.00 DISTANCE (M) Nov. 2000 Dec. 2008 Jun. 2011 Dec. 2011
7.00 K60 (North side of Padma Hotel) 6.00
5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 DISTANCE (M) Nov. 2000 Dec. 2008 Dec. 2011
6.00 K76 (South side of Dubble Six) 5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 DISTANCE (M)
Nov. 2000 Dec. 2008 Dec. 2011 6.00 K86 (Gado-gado Restaurant) 5.00
4.00
3.00
2.00
HEIGHT (M) 1.00
0.00
-1.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 DISTANCE (M)
Nov. 2000 Dec. 2008 Dec. 2011 Figure 10.4.6 Comparison on the Beach Profile Taken at Typical Survey Lines from North Kuta to Seminyak (Source: JICA Study Team)
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(3) Results of the Sand Sampling on the Beach
a) East Coast (Candidasa)
i) Grain Size Distribution of Seabed Material (Figure 10.4.7)
The medium grain size and fine sand content of seabed on the beach in Candidasa are shown in Figure 10.4.7. Except for the area surrounding Sunrise Hotel (CD14-CD17) where the beach was nourished with mountain sand and has now become a gravel beach due to sand erosion, the grain size distribution has a high percentage of fine sand (grain size: 0.074-0.42 mm). Furthermore, the grain size distributions obtained from three different positions (berm top, sloping part, and flat part) exhibited the same tendency wherein the percentage of fine sand is increasing in a westward direction. Meanwhile those obtained from the flat part have a particularly distinct tendency.
ii) Slope of Seabed (Figure 10.4.7)
The slope of seabed on the beach in Candidasa is shown in Figure 10.4.7. The range of slope is from 9 to 16, and the average is about 12.
iii) Composition of Seabed Materials (Figure 10.4.8)
The composition of seabed materials on the beach in Candidasa is shown Figure 10.4.8. They are composed of a high percentage of volcanic matter from land in the eastern and central parts of Candidasa. Other parts have a high percentage of biogenic matter such as shell and foraminifera.
b) Southwest Coast (North Kuta~Part of Canggu)
i) Grain Size Distribution of Seabed Materials (Figure 10.4.9)
The medium grain size and fine sand content of seabed material on the beach in the southwest coast are shown in Figure 10.4.9. The overall grain size distribution has a high percentage of fine sand (grain size: 0.074-0.42 mm). Seabed materials obtained from flat areas appeared to have a higher percentage of sand with large grain size than those obtained from the other areas (berm and sloping part). The distribution of grain size along the coastline showed a wavelike pattern with the peak of fine sand at Location No.1 and No.13.
ii) Slope of Seabed (Figure 10.4.9)
The slope of seabed on the beach in the southwest coast is shown in Figure 10.4.9. The range of slope was from 8 to 14 and the average was about 10.
iii) Composition of Seabed Materials (Figure 10.3.10)
The composition of seabed materials on the beach in the southwest coast is shown in Figure 10.4.10. Seabed materials are composed of a high percentage of biogenic matter in the eastern part, and appeared that the percentage of volcanic matter from land is increasing towards the north.
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D50 (mm) 4.00 3.50 3.00 (mm)
2.50 D50 2.00 B
Size 1.50 S
1.00 F
Grain 0.50 0.00 12345678910111213141516171819202122232425262728293031323334 Sample Name
CONTENT OF FINE SAND 120 (%) 100
Sand 80
60 B Fine
of S 40 F 20
Content 0 12345678910111213141516171819202122232425262728293031323334 Sample Name
FORESHORE SLOPE 18 16 14 12 )
n 10
:
1
8 (
6
Slope 4 2 0 12345678910111213141516171819202122232425262728293031323334 Sample Name
Figure 10.4.7 Alongshore Distribution of Medium Grain Size (D50), Contents of Fine Sand and Average Slope in Candidasa (Source: JICA Study Team)
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100 Foraminifera Coral 80 Shell Volcanic 60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Content of Material Obtained from Berm of Beach at Candi Dasa
100 Foraminifera Coral 80 Shell Volcanic 60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Content of Material Obtained from Slope of Beach at Candi Dasa
100 Foraminifera Coral 80 Shell Volcanic 60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Content of Material Obtained from Flat of Beach at Candi Dasa
Figure 10.4.8 Seabed Material Composition in Candidasa (Source: JICA Study Team)
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D 50 (mm) of Kuta Sand Sample 1.6 1.4
) 1.2 1.0 (mm
0.8 B Size 0.6 S 0.4 F Grain 0.2 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Sample Name
CONTENT OF FINE SAND 120
100
(%) 80
Sand 60 B
Fine S 40
of F
20
Content 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Sample Name
16
14
12 ) 10 n :
1
8 (
6
Slope 4
2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Sample Name
Figure 10.4.9 Alongshore Distribution of Medium Grain Size (D50), Contents of Fine Sand, and Average Slope in the Southwest Coast from North Kuta to Part of Canggu (Source: JICA Study Team)
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100 Foraminifera Coral 80 Shell Volcanic
60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Content of Material Obtained from Berm of Beach at Kuta
100 Foraminifera Coral 80 Shell Volcanic
60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Content of Material Obtained from Slope of Beach at Kuta
100 Foraminifera Coral 80 Shell Volcanic
60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Content of Material Obtained from Flat of Beach at Kuta
Figure 10.4.10 Seabed Material Composition in the Southwest Coast from North Kuta to Part of Canggu (Source: JICA Study Team)
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10.5 Diving Survey for Potential Area of Sand Borrow
(1) Location of the Survey
In order to understand the potential areas of sand borrow, a diving survey was carried out in the offshore area of southeast and southwest coast in November and December 2011. The outline of diving survey is shown in Table 10.5.1 while the locations are shown in Figure 10.5.1 and 10.5.2.
Table 10.5.1 Outline of Diving Survey Location Survey Field Survey Items/ Sampling Point (Survey Date) Site Laboratory Analysis Candidasa 43 Outer reef: 34 points Filed survey items: elevation, (December 2011) 26 points with laboratory test thickness of sand layer, sand 11 points without laboratory color, main contents of sand test materials (coral, volcanic, shell, Inner reef: 9 points foraminifera) 9 points with laboratory test Laboratory analysis: grain sieve distribution, specific gravity North Kuta to Part 32 Offshore: 26 points Filed survey items: elevation, of Canggu 26 points with laboratory test thickness of sand layer, sand (November 2011) Nearshore: 6 points color, main contents of sand 6 points with laboratory test materials (coral, volcanic, shell, foraminifera) Laboratory analysis: grain sieve distribution, specific gravity (Source: JICA Study Team)
Figure 10.5.1 Diving Survey Area in Candidasa (Source: JICA Study Team)
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Figure 10.5.2 Diving Survey Area in the Southwest Coast from Kuta to Part of Canggu (Source: JICA Study Team) (2) Results
a) Candidasa
The results of diving survey of seabed materials in Candidasa is shown in Table 10.5.2. The seabed materials contained more volcanic materials of land origin as indicated by a grey color. In contrast, the seabed materials contained more biogenic materials such as foraminifera, coral, and shell shown as brown color. As for the color of seabed material on the outer side of the reef in Candidasa, about 60% of the total was grey. On the inside face of the reef, about 80% was biogenic material with brown color. As for the grain size, the seabed materials contained more volcanic materials with higher percentage of fine sand and smaller percentage of medium size. In contrast, the seabed materials contained more biogenic materials that have lower percentage of fine sand content and bigger percentage of medium grain size.
b) Southwest Coast from Kuta to Part of Canggu
The seabed in the southwest coast from Kuta to part of Canggu was characterized with significant variation. of yellow sand, foraminifera sand, grey sand, fine sand, and mud. Also, the seabed was occasionally rocky. The results of diving survey of seabed materials in the southwest coast is shown in Table 10.5.3. As for the color of seabed material on the offshore and nearshore areas in Kuta, about 70% of the total was grey in color. As for the grain size, , the seabed materials contained more volcanic materials with higher percentage of fine sand content and smaller percentage of medium grain size. In contrast, the seabed materials contained more biogenic materials with lower percentage of fine sand content and bigger percentage of medium grain size.
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Table 10.5.2 Results of the Diving Survey of Seabed Materials in Candidasa
Location: CANDIDASA Result of sieve analysis EL. Thickness Surface or Description No. Color Contents of Fine Sand D50 Gs (m) (cm) Below (Main contents) 0 50% 100% % (mm) (kg/cm3) C1 -12.6 50 SURFACE Light Brown Shell 55% BELOW Light Brown Shell 55% C2 -8.3 33 SURFACE Dark Grey Volcanic 50% BELOW Dark Grey Volcanic 50% C3 -7.9 40 SURFACE Light Brown Shell 60% BELOW Light Brown Shell 60% C4 -26.2 100 SURFACE Dark Grey Shell 50% 91% 0.25 2.87 BELOW Dark Grey Coral & Shell 45% 75% 0.28 2.89 C5 -15.1 100 SURFACE Grey Volcanic 45% BELOW Grey Volcanic 45% C6 -25.3 55 SURFACE Grey Volcanic 70% BELOW Light Grey Coral 50% C7 -19.3 50 SURFACE Grey Coral 50% 73% 0.30 2.85 BELOW Light Grey Coral 90% 40% 1.05 3.00 C8 -16.6 60 SURFACE Light Brown Coral 60% 48% 0.43 2.81 BELOW Light Brown Coral 90% 28% 1.10 2.81 C9 -7.2 70 SURFACE Light Brown Shell 70% BELOW Light Brown Shell 70% C10 -25.4 100 SURFACE Grey Shell 55% 70% 0.29 2.86 C11 -26.8 100 SURFACE Grey Coral 50% 56% 0.36 2.91 C12 -21.7 120 SURFACE Grey Coral 60% 45% 0.50 2.82 C13 -22.2 85 SURFACE Grey Shell 45% 82% 0.24 2.96 BELOW Grey Shell 55% 61% 0.30 2.94 C14 -13.8 48 SURFACE Grey Volcanic 90% BELOW Grey Volcanic 65% C15 -23.3 80 SURFACE Brown Coral 90% 46% 0.46 2.90 BELOW Brown Coral 90% 15% 3.00 2.50 C16 -13.9 50 SURFACE Light Brown Coral 90% 18% 1.70 2.91 BELOW Light Brown Coral 85% 8% 2.45 2.86 C17 -18.0 120 SURFACE Light Brown Coral 80% 15% 1.85 2.86 BELOW Light Brown Coral 90% 9% 3.10 2.82 C18 -26.1 90 SURFACE Brown Coral 90% BELOW Brown Coral 90% C19 -30.1 80 SURFACE Brown Coral 90% 21% 1.00 2.82 BELOW Brown Coral 90% 6% 6.50 2.83 C20 -18.4 120 SURFACE Grey Volcanic 55% 88% 0.25 2.92 BELOW Grey Shell 55% 87% 0.27 2.98 C21 -11.9 85 SURFACE Dark Grey Volcanic 95% BELOW Dark Grey Volcanic 95% C22 -27.5 > 120 SURFACE Grey Coral 85% 40% 0.70 3.05 BELOW Grey Coral 85% 36% 0.70 2.99 C23 -13.6 80 SURFACE Grey Volcanic 80% BELOW Grey Volcanic 90% C24 -17.9 95 SURFACE Grey Shell 65% 90% 0.25 2.99 BELOW Light Grey Coral 80% 62% 0.32 2.95 C25 -20.1 95 SURFACE Light Grey Shell 75% 86% 0.30 2.92 BELOW Light Brown Coral 50% 25% 2.20 2.88 C26 -13.0 48 SURFACE Grey Volcanic 80% BELOW Grey Volcanic 60% C27 -19.1 110 SURFACE Light Grey Shell 55% 79% 0.31 2.93 BELOW Light Grey Shell 50% 53% 0.40 2.89 C28 -18.9 120 SURFACE Light Brown Shell 55% 36% 0.52 2.88 BELOW Light Brown Coral 50% 29% 0.60 2.84 C29 -21.4 120 SURFACE Light Grey Volc & Shell 45% 92% 2.89 BELOW Light Grey Volcanic 50% 75% 2.91 C30 -19.4 120 SURFACE Light Grey Shell 50% 91% 0.22 2.85 BELOW Light Grey Shell 75% 61% 0.30 2.92 C31 -18.9 90 SURFACE Light Brown Coral 60% 62% 0.32 2.85 C32 -25.0 100 SURFACE Grey Volcanic 60% 87% 0.28 2.92 BELOW Grey Volcanic 50% 75% 0.31 2.80 C33 -25.0 > 120 SURFACE Dark Grey Volcanic 55% 92% 0.21 2.78 BELOW Dark Grey Volc & Shell 50% 91% 0.22 2.73 C34 -23.0 100 SURFACE Light Brown Coral 80% 22% 1.75 2.88 BELOW Light Brown Coral 80% 15% 2.75 2.94 CR1 -0.8 30 SURFACE Light Brown Foraminifera 70% 16% 1.50 2.80 CR2 -0.8 40 SURFACE Light Grey Shell 40% 67% 0.29 2.86 CR3 -0.5 30 SURFACE Light Brown Foraminifera 65% 8% 1.40 2.80 CR4 -0.8 30 SURFACE Light Brown Coral 40% 56% 0.38 2.84 CR5 -0.7 50 SURFACE Light Grey Coral 70% 36% 0.55 2.86 CR6 -0.6 50 SURFACE Light Brown Coral 60% 22% 0.90 2.86 CR7 -1.1 40 SURFACE Light Brown Coral 40% 95% 0.22 2.82 CR8 -0.5 50 SURFACE Light Brown Shell 55% 57% 0.38 2.84 CR9 -0.5 55 SURFACE Light Brown Shell 65% 81% 0.27 2.89 (Source: JICA Study Team)
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Table 10.5.3 Results of the Diving Survey of Seabed Materials from Kuta to Canggu
Location: KUTA Result of sieve analysis EL. Thickness Surface or Description No. Color Contents of Fine Sand D50 Gs (m) (cm) Below (Main contents) 0 50% 100% (%) (mm) (kg/cm3) K1 -20.8 5 Surface Light Brown Shell 70% 14% 1.20 2.81 K2 -8.1 40 Surface Brown 77% 0.35 2.79 K3 -19.3 5 Surface Light Brown Shell 60% 11% 2.10 2.85 K4 -9.7 10 Surface Light Brown Shell 50% 4% 1.85 2.76 K5 -17.1 120 Surface Light Grey Shell 80% 82% 0.30 2.92 K6 -10.9 120 Surface Brown Shell 80% 14% 0.60 2.89 K7 -19.1 5 Surface Brown Shell 60% 59% 0.38 2.81 K8 -11.3 40 Surface Light Brown Shell 60% 66% 0.37 2.81 K9 -20.7 90 Surface Grey Volcanic 90% 89% 0.19 -0.51 K10 -9.1 5 Surface Light Grey Shell 90% 96% 0.20 2.84 K11 -16.9 120 Surface Dark Grey Volcanic 80% 94% 0.21 2.89 K12 -19.9 80 Surface Dark Grey Volcanic 80% 98% 0.20 2.83 K13 -30.0 55 Surface Grey 97% 0.22 2.92 K14 -18.7 > 120 Surface Dark Grey Volcanic 90% 94% 0.16 2.83 K15 -31.2 120 Surface Light Grey Shell 70% 60% 0.35 2.83 K16 -19.0 > 120 Surface Grey Volcanic 90% 96% 0.16 2.79 K17 -29.0 10 Surface Light Grey Shell 50% 29% 1.30 2.85 K18 -19.6 90 Surface Light Grey Shell 70% 91% 0.30 2.97 K19 -28.2 90 Surface Grey Volcanic 95% 83% 0.20 2.74 K20 -20.0 > 120 Surface Grey Volcanic 70% 99% 0.14 2.79 K21 -29.3 85 Surface Grey Coral 65% 16% 14.00 2.75 K22 -18.9 > 120 Surface Grey Volcanic 80% 99% 0.16 2.66 K23 -30.0 > 120 Surface Light Grey Shell 50% 20% 0.75 2.74 K24 -19.5 85 Surface Grey Volcanic 90% 97% 0.19 2.61 K25 -24.5 60 Surface Grey Volcanic 70% 95% 0.75 2.69 K26 -19.4 > 120 Surface Grey Volcanic 90% 93% 0.18 2.71 KS1 -8.8 90 Surface Light Brown Shell 90% 73% 0.35 2.75 KS2 -7.9 10 Surface Brown Shell 70% 19% 0.62 2.86 KS3 -10.8 5 Surface Light Grey Volcanic 100% 93% 0.10 2.80 KS4 -10.4 20 Surface Light Grey Volcanic 90% 99% 0.17 2.93 KS5 -10.3 40 Surface Grey Volcanic 80% 96% 0.29 2.97 KS6 -3.9 110 Surface Grey Volcanic 80% 99% 0.22 3.02 (Source: JICA Study Team)
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10.6 Comprehensive Analysis for Sediment Transport in Candidasa
The comprehensive analysis on sediment transport based on several collected data is very important to achieve collective understanding of sediment transport data, prior to examination of the planning and design of beach conservation measures.
The following data and information were applied for the comprehensive analysis of sediment transport in two candidate areas:
¾ Site investigation data; ¾ Results of shoreline change and topography analysis based on aerial and satellite photos; ¾ Sounding data (bathymetric and topographic data); ¾ Shoreline monitoring data; ¾ Sand sampling and sieve analysis results; and ¾ Numerical analysis results (computation for waves and longshore sediment transport).
(1) Beach Condition Obtained from Site Investigation and Shape of Shoreline based on Satellite Photos
The sediment cell in Candidasa was identified from Tanjung Iti to the peninsula westside from Amankila Hotel, which was approximately 6 km longshore distance. To present the current beach condition, this area was divided into three subareas namely, “West Area”, “Central Area”, and “East Area” as shown in Figure 10.6.1
Figure 10.6.1 Candidasa Area (Source: JICA Study Team)
a) West Area
The west area has roughly 2 km alongshore distance, with an existing coral reef of 300 m width in maximum. The position of each photo taken in November 2011 at west area (during low tide) is shown in Figure 10.6.2. The representative beach conditions are discussed as follows, and shown in Photos. 10.6.1 and 10.6.2:
¾ Predominant wave direction in Candidasa was from S to SSE based on the numerical computation for wave field. Due to this, the westward littoral sand drift occured in the whole area. ¾ However, the strength of littoral sand drift depends on the angle of shoreline to the incident waves, and width of coral reef.
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¾ The angle of shoreline in the westside was almost perpendicular to the predicted incident wave direction. Consequently, a stable black sandy beach was formed (Positions 1 and 2). However, the angle of shoreline on the other area moved to induce the westward littoral drift. ¾ Significant beach and land erosion were observed from Position 3 located at the downdrift side from small peninsula, which was covered by concrete seawalls (Position 4). ¾ Significant beach erosion was also observed from Positions 5, 7, and 8, and all beaches were covered with vertical concrete seawall. ¾ Between these eroded areas, small sandy beach partially existed (Position 6). This was probably influenced by the existence of the shoal on reef flat. ¾ The angle of shoreline changed from Positions 8 to 10 with almost the same angle as that of Position 1. Moreover, the sandy beach was maintained at this limited area even though the width of the beach is narrow.
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Figure 10.6.2 Location Map Showing Where Photos were Taken in the West Area of Candidasa (November 2011) (Source: JICA Study Team)
Position 1 Position 2
Position 3 Position 4 Photo 10.6.1 Beach Condition in the West Area of Candidasa (November 2011) (Source: JICA Study Team)
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Position 5 Position 6
Position 7 Position 8
Position 9 Position 10 Photo 10.6.2 Beach Condition in the West Area of Candidasa (November 2011) (Source: JICA Study Team)
¾ From these results, it was concluded that it is difficult to maintain the sand without costal structures such as groin and headland to interrupt the westward littoral drift. ¾ However, the strength of littoral drift is different in each area depending on the angle of shoreline and width of coral reef. Thus, a detailed analysis, taking this view into account, will be required for the finalization of layout of coastal structures and predicted beach shape.
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b) Central Area
The west area has roughly 1 km alongshore distance. The width of its coral reef is narrower by more or less 100 m compared to that of the west and east area. The positions where each photo was taken in November 2012 are shown in Figure 10.6.3. The representative beach conditions shown in Photos 10.6.3 and 10.6.4 are discussed subsequently.
Figure 10.6.3 Location Map Showing Where Photos were Taken in the Central Area of Candidasa (November 2011) (Source: JICA Study Team)
Position 1 Position 2
Position 3 Position 4 Photo 10.6.3 Beach Condition in the Central Area of Candidasa (November 2011) (Source: JICA Study Team)
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Position 5 Position 6
Position 7 Position 8 Photo 10.6.4 Beach Condition in the Central Area of Candidasa (November 2011) (Source: JICA Study Team)
¾ No sandy beach due to erosion was observed in almost the whole area (Positions 1 and 2). ¾ Fifteen T-shape groins and 15 offshore breakwaters existed in Candidasa. However, the crown height (+2.2 m) of each structure is lower than HWL (+2.6 m), and their function to interrupt the westward sand drift is very low. ¾ If the geographical shape of shoreline was checked in detail, it can confirm that sandy beach with caved shoreline shape exists in the area (Position 4).
c) East Area
The west area has roughly 3 km alongshore distance, and the width of its coral reef is wider than that of the central area by 250 m maximum. There exists a peninsula (Tanjung Nti), which is 270 m long, in its east end. The position where each photo was taken in November 2012 in the east area is shown in Figure 10.6.4. The representative beach conditions discussed below are shown in Photos. 10.6.5 and 10.6.6.
¾ Most of the westward volcanic littoral sand from eastside of the peninsula are interrupted by said peninsula. However, according to the result of sand sampling along the beach, the contents of sand nearby the peninsula were not coral but volcanic. From the result, some parts of the volcanic sand might be due to inflow occurrence through the peninsula. ¾ Local government executed beach nourishment with the improvement of existing T-shape groins. However, most of the filled sand (black sand) has flown out and only pebble
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stones have remained. Beach landscaping and utilization of the beach have deteriorated due to this improvement. (Positions 1 to 3).
Figure 10.6.4 Location Map Showing Where Photos were Taken in the East Area of Candidasa (November 2011) (Source: JICA Study Team)
Position 1 Position 2
Position 3 Position 4 Photo 10.6.5 Beach Condition in the East Area of Candidasa (November 2011) (Source: JICA Study Team)
¾ Sand has almost disappeared in most areas despite the existing T-shape groins and offshore breakwaters (Positions 6, 7, and 9). However, sandy beach partially remained in a limited area, especially at locations surrounding the T-shape groins. ¾ The beach condition nearby the peninsula is better than other west side areas, with volcanic sandy beach of certain width that was maintained (Position 10). This condition
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might have been maintained by inflow of volcanic sand through the peninsula, and the shadow effect of the waves due to existence of the peninsula.
Position 5 Position 6
Position 7 Position 8
Position 9 Position 10 Photo 10.6.6 Beach Condition in the East Area of Candidasa (November 2011) (Source: JICA Study Team)
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(2) Distribution of Sand Composition on the Beach in Alongshore Direction
The distribution of sand composition on the beach in alongshore direction has been investigated in the study (Section 10.3). The obtained result on the foreshore slope is shown in Figure 10.6.5. From the result, the following were observed:
¾ Organic contents (foraminifera, coral and shell) dominated the beach sand materials. This means that the main sand source is the coral reef. ¾ However, volcanic content was also dominant in two limited areas (Line Nos. 1 to 7, and Nos.14 to 22). The former is located from the peninsula (Tanjung Nti) to the west end of Puri Bagus Hotel, while the latter is in an area where beach nourishment using volcanic black sand was carried out by the local government. The contents of volcanic sand gradually decreased to the westward. ¾ From the result, occurrence of westward littoral drift was recognized. Further, the sand inflow from east side through the peninsula was also expected to exist. 100 Foraminif Coral 80 Shell Volcanic 60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Figure 10.6.5 Seabed Material Composition on Foreshore Slope at Candidasa (Source: JICA Study Team)
(3) Comparison of Beach Condition Based on Aerial and Satellite Photos Taken between 2004 and 2011
Figures 10.6.6 to 10.6.7 show the comparison of beach condition based on aerial and satellite photos taken between 2004 and 2011. Based on these, the following were observed:
¾ It seems that no significant difference in sand existence area was observed between 2004 and 2011, even though the tidal conditions shown in the two photos were unknown. Especially, there was no change in the white coral sand area. ¾ On the other hand, the volcanic black sand in the west end of Puri Bagus Hotel (Figure 10.6.6) seems to have increased than before. ¾ From the result, the productivity of organic sand, which was produced on the reef flat, has not changed and still unexpected to increase in quantity for at least a short period. On the other hand, the volcanic sand, which was due to inflow occurrence through the peninsula, seems to have slightly increased. However, this is still insufficient to contribute in forming the sandy beach. ¾ Above findings indicated that it is difficult to recover the sandy beach in certain years without artificial filling of sand.
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2004
2011
2004
2011
2004
2011 Figure 10.6.6 Comparison on the Beach Condition between 2004 and 2011 (Source: JICA Study Team) 10-34 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version)
2004
2011
2004
2011
Figure 10.6.7 Comparison on the Beach Condition between 2004 and 2011 (Source: JICA Study Team)
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(4) Understanding the Beach Condition Based on Numerical 45 (SW) Analysis
22.5 Figure 10.6.8 shows the relation (SSW)
between the offshore wave direction Direction
and the incident wave direction in (S) 0 Wave Candidasa, obtained from the
numerical computation for offshore ‐22.5 (SSE)
waves shown in Figures 10.6.9 and Incident 10.6.10. From this, it appeared that the incident wave directions in
(SE) ‐45 Candidasa are more likely from SSE ‐45 ‐22.5 0 22.5 45 to S. (SE) (SSE) (S) (SSW) (SW) Offshore Wave Direction Figure 10.6.8 Relation between Offshore and Incident Wave Direction in Candidasa
(Source: JICA Study Team)
East Coast Area 1.2 Candidasa 1.0
0.8
H/H0 0.6
0.4 SSW 0.2 Nusa Dua S Sanur SSE Unda River 0.0 20 30 40 50 60 70 80 90 100 Line No.
East Coast Area 60 Candidasa 40
20
0 20 30 40 50 60 70 80 90 100 -20 Direction
-40 SSW Nusa Dua S Sanur -60 Unda River SSE -80 Line No.
Figure 10.6.9 Distribution of the Rate of Wave Heights and Directions along the East Coast Area (Source: JICA Study Team)
Figure 10.6.9 shows the distribution of the rate of wave heights and directions along the east coast area, which was obtained from the result of numerical computation for offshore waves. The definition of wave direction is shown in Figure 10.6.10.
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Figure 10.6.10 Definition of Wave Direction (Source: JICA Study Team)
¾ The incident wave height in Candidasa significantly decreased as compared to that in Sanur and Nusa Dua, due to the shadow effect of the existing Nusa Peneeda Island. The shadow effect in case of SSW direction was the most significant where the wave height was the lowest. ¾ The incident wave direction in Candidasa always inclines from east to west. This wave direction induces westward littoral drift of sand.
Figure 10.6.11 shows the calculated littoral sand transport along the shore of Candidasa. Further detailed study is required to evaluate the quantity of littoral sand transport; thus, the non-dimensional ratio of littoral sand transport was presented instead. The westward littoral drift in the whole area of Candidasa was determined from the numerical computation. However, the strength of the littoral drift changed due to the varying shoreline geometry.
2500.0 Shore Line 2000.0
1500.0
1000.0
500.0
0.0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 4750
Longshore Littoral Transport Rate 1.0
0.5
Q/Qmax 0.0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 4750 Distance from Peninsula (m) Figure 10.6.11 Calculated Longshore Sediment Transport Rate in Candidasa (Source: JICA Study Team)
(5) Summary of Sediment Transport Characteristics in Candidasa
The characteristics of sediment transport in Candidasa were summarized as follows: ¾ Westward littoral drift due to S to SSE incident wave direction occurred over the whole area of Candidasa. ¾ The strength of the littoral drift was influenced by the change of geometry of shoreline and by the difference in wave action mainly due to the changing width of coral reef flat.
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¾ The sand source in Candidasa was mainly from organic origin, which was produced on the coral reef flat. However, the volcanic origin sand was also included in the westside nearby the peninsula. This means that some portions of volcanic sand were due to inflow occurrence through the peninsula. ¾ Possibility to increase the organic sand was not expected.
10.7 Comprehensive Analysis for Sediment Transport in the Southwest Coast
A comprehensive analysis on sediment transport in the southwest coast (from north Kuta to Canggu) was carried out by applying the same approach as that of Candidasa. There are no remarkable coastal structures and obstacles to identify the dominant littoral sand transport. Thus, careful analysis based on several data and information is required to determine the characteristics of sediment transport in this area.
(1) Beach Condition Obtained From Site Investigation and Shape of Shoreline Based on Satellite Photos
This area was divided into two areas with different characteristics of beach evaluation. One is the area in the north boundary of coral reef, and the other is a continuous sandy beach area located in the north side of Kuta Beach. To present the current beach condition, this area was divided into three sub-areas, namely, “North Kuta Area”, “Legian-Seminyak Area”, and “Canggu Area” as shown in Figure 10.7.1
Figure 10.7.1 Southwest Coast Area
(Source: JICA Study Team)
a) North Kuta Area
The North Kuta area belonged to the Phase-1 Project area, which has roughly 2 km alongshore distance. The south part of this area is located at the edge of coral reef, and the alignment of the beach is in the southwest-northeast direction. The dominant direction for incident wave in this area is SW to SSW based on the result of numerical analysis. Due to this geometry of the beach and incident wave direction, the northward littoral drift is significant in the south part of this area. However, the alignment of the beach suddenly changes to southeast-northwest in the south side of Hard Rock Café Hotel. This location is outside the coral reef area, with the sandy beach continuing up to Canggu.
The position of each photo taken in September 2011 in the west area (during low tide) is shown in Figure 10.7.2. The representative beach conditions are shown in Photos. 10.7.1 and 10.7.2. Photos from Position 5 show two different tide conditions: low tide condition (LWL) taken in September 2011, and high tide condition (HWL) taken in March 2012.
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¾ Positions 1 and 2 show the beach condition in the south and center of Kartika Plaza Hotel. The width of coral reef has suddenly decreased, which eventually disappeared. Position 2 is the end boundary of the coral reef. As presented in the previous chapter, sand filled from the Phase-1 Project has been washed away, and the revetment has been completely exposed. Sand was mainly transported to the north side due to the oblique incident waves. ¾ Position 3 is located in the north side of Kartika Plaza Hotel, which is just outside the coral reef area. The alignment of the beach starts to change from southwest-northeast direction to south-north direction. The revetment is covered again by gradual filling of sand. ¾ Position 4 shows the concave shape of shoreline. Wider beach can be maintained in this area. According to the monitoring result shown in Figure 5.15 of Chapter 5, it was realized that the stable beach was maintained from this area up to the north side. At said location, there are no coastal artificial structures, and the continuous sandy beach exists up to Canggu.
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Figure 10.7.2 Location Map of North Kuta Showing Positions Where Photos were Taken (Source: JICA Study Team)
Position 1 Position 2
Position 3 Position 4 Photo 10.7.1 Beach Condition of North Kuta (September 2011) (Source: JICA Study Team)
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(During LWL) (During HWL) Position 5 Photo 10.7.2 Beach Condition of North Kuta (September 2011) (Source: JICA Study Team)
¾ Position 5 is located on the beach nourishment area without coastal structures. Right side photo shows the condition during LWL (approximately +1.0 m) and left side photo shows the condition during HWL (approximately +2.5 m). A wider sandy beach can be secured despite the high tide condition.
b) Legian–Seminyak Area
The positions where each photo was taken to observe LWL (September 2011) and HWL (March 2012) conditions are shown in Figure 10.7.3. The representative beach conditions, as discussed below, are shown in Photos. 10.7.3 and 10.7.4:
¾ From the site investigation alone, it is difficult to identify possible occurrence of littoral drift in this area since there are no coastal structures or obstacles to identify the sand drift. ¾ However, the alignment of the beach in this area is perpendicular to the dominant incident wave direction. Furthermore, even though some contents of yellow colored sand were observed on the beach in the south part of this area, the color gradually changes to gray or dark gray northwards. From the result, this area is expected to be a balanced area on littoral sand drift. ¾ Position 1 is the north side from the boundary of beach nourishment in the Phase-1 Project. Beach width was narrower than that in North Kuta. ¾ Positions 2 to 5 show beach condition of this area. The beach space was very limited during the high tide condition. ¾ According to the mapping analysis using the old aerial and recent satellite photos, the beach retreat of 5 to 15 m was observed during the past 30 years (from1981 to 2010).
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Figure 10.7.3 Location Map of Legian-Seminyak Showing Positions Where Photos were Taken (Source: JICA Study Team)
(During LWL) (During HWL) Position 1
(During LWL) (During HWL) Position 2 Photo 10.7.3 Beach Condition of Legian-Seminyak (Source: JICA Study Team)
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(During LWL) (During HWL) Position 3
(During LWL) (During HWL) Position 4
(During LWL) (During HWL) Position 5 Photo 10.7.4 Beach Condition of Legian-Seminyak (Source: JICA Study Team)
c) Canggu Area
Canggu area is from the Brawa River mouth to north. Basically, sandy beach still continues in this area; however, beach rock and sea cliff appeared at several locations. Several small river
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mouths were also observed in this area. It is useful to check the beach condition around these obstacles in order to know the behavior of littoral drift.
The positions where each photo were taken are shown in Figure 10.7.4, and the representative beach conditions, discussed as follows, are shown in Photos. 10.7.5 and 10.7.6.
¾ Photos for Positions 1 and 2 were taken at both sides of the Barawa River mouth. A beach scarp was observed in the south side of the river mouth, and the sand in the north side seems to have been accumulated. Based on this, the sand is expected to move southward for this moment. However, it is necessary to check the seasonal change of the beach to ensure the net sand transportation. ¾ The photo for Position 3 was taken at the next river mouth located in the north side of the Barawa River mouth. The same tendency as that for said river mouth (north side is accumulated while south side is retreat) was observed. ¾ New land development in the area up to the beach riparian area was observed from Position 4. According to the regulation on beach riparian area, such development is illegal. ¾ The photos for Positions 5 to 7 were taken in Echo Beach (Batu Mejan Temple). Position 5 shows the south side of the beach, Position 6 shows the center, and Position 7 shows the north side. A significant difference in beach change between both sides was not observed. Some parts of the revetment were damaged due to wave overtopping. ¾ The photos for Positions 8 to 10 show the beach condition on both sides of the small sea cliff located 3 km away from Echo Beach. Since the beach in the south side has been eroded, protection by filling of armor stones was provided. On the other hand, the beach condition in the north side seems stable. ¾ The satellite photos show the gradually changing alignment of the beach in Canggu area, which is more in the counterclockwise direction, as compared to that of Legian –Seminak. This beach angle will induced the southward littoral drift based on the presumed incident wave direction (SE-SSE). The result of beach change at Positions 8 to 10 might prove this phenomenon.
d) Summary
¾ The north part of North Kuta is located in the south edge of the coral reef. Due to the relation of the angle between the beach alignment and dominant incident wave direction, significant northward littoral drift occurs. The main reason for the sand outflow after the beach nourishment is due to this sand movement. ¾ However, this sand transported northward did not contribute as sand inflow to the north beach area (Legian-Seminyak) based on the site investigation and result of shoreline monitoring. The existence of coral reef gap might have been influenced by the dynamic sand transportation in this area. ¾ The stable sandy beach with certain width could be well maintained in the south part of North Kuta (from Hard Rock Café Hotel to Alam Kul Kul Hotel) due to the beach nourishment in the Phase-1 Project, despite the absence of artificial coastal structures. ¾ The littoral sand movement in Legian-Seminyak area was expected to be balanced based on the site investigation and relation of the angle between the beach alignment and dominant incident wave direction in this area. However, the beach space becomes narrow especially during tidal conditions. This condition is disturbing the activities of tourists in the beach, which is a frequently visited resort in Bali.
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¾ The sandy beach continues up to Canggu area. However, beach rocks, sea cliffs, and several small river mouths exist. Occurrence of southward littoral drift is expected in the north part of Canggu based on the site investigation.
Figure 10.7.4 Location Map of Canggu Showing Positions Where Photos were Taken (Source: JICA Study Team)
Position 1 Position 2
Position 3 Position 4 Photo 10.7.5 Beach Condition of Canggu (Source: JICA Study Team)
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Position 5 Position 6
Position 7 Position 8
Position 9 Position 10 Photo 10.7.6 Beach Condition of Canggu (Source: JICA Study Team)
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(2) Short- and Long-term Shoreline Changes
a) Short-term Shoreline Change
The short-term shoreline change was examined by applying the monitoring results after the completion of the Phase-1 Project, and the results of beach profile survey carried out in this study. Obtained understandings are as follows:
¾ According to the shoreline monitoring result, the beach retreat rate in North Kuta (from Hard Rock Café Hotel to Alam Kul Kul Hotel, with 1.3 km alongshore) was observed to be roughly 1 m/year for ten years, prior to the Phase-1 Project. ¾ After the beach nourishment in the Phase-1 Project, there was no observed significant change in shoreline although small scale seasonal changes were observed. On the other hand, most of the filled sand has been already washed away from the north of coral reef area in Kuta (offshore breakwater BWN2 to BWN3), with a volume of 50,000 to 60,000 m3. According to the bathymetric survey result, significant sand deposition was observed on the outer area in the north edge of coral reef. From the results of sand sampling and sieve analysis, the color, grain size, and contents of sand on this deposition area were similar to the nourished sand taken from Kuta Beach. Furthermore, a westward strong offshore current through the reef gap in the north edge of the coral reef was observed during the investigation carried out in the detailed design stage of the Phase-1 Project. From the result, there is high possibility that the sand on north of coral reef area was transported mainly to the outer reef area trough the reef gap. ¾ In the north end of the nourishment area near Alam Kul Kul Hotel, sand filling was conducted to reduce the beach width gradually from 10 m to 0 m, within a 300-m distance. According to the monitoring result, it did not significantly change the shape at this area. At least, evidence of outflow to the north was not observed.
b) Long-term Shoreline Change
¾ According to the mapping analysis using old aerial and satellite photos, long-term beach retreat in Legian and Seminyak area (from Alam Kul Kul Hotel to Sofitel Hotel, with 2.3 km distance) was observed at roughly 5 m south side to 15 m north side in 30 years. ¾ The most significant beach retreat was observed in the north area from river mouth of Petitenget Temple to the Brawa River mouth, which is 1.6 km alongshore. The beach shore in this area changed from 15 m to 20 m in 30 years. However, there was no significant beach retreat further north of the area (Canggu area).
(3) Sand Composition and Relation between Grain Size and Beach Slope
Sand sampling on the beach was conducted as shown in Figure 10.7.5. Alongshore distribution of sand composition obtained from the sloping part of the beach is presented in Figure 10.7.6. Alongshore distribution of grain size (D50) and beach slope are also shown in Figure 10.7.7. Corresponding findings based on the sand sampling are as follows:
¾ Coral contents appeared only from Line Nos. 1 to 10 (until the north boundary of nourishment in the Phase-1 Project), and were not observed in the north area. From this, coral sand produced from the coral reef in Kuta is not transported to the north area. ¾ The contents of volcanic sand gradually increased toward the north side. However, volcanic contents were also observed in the sand samples taken from the south area. Such volcanic sand might have been transported from the north area. Based on this, southward littoral drift is expected to occur.
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¾ According to the alongshore distribution of grain size (D50) and beach slope as shown in Figure 10.7.6, grain size (D50) in Legian–Seminyak area (Line Nos. 9 to 14, which is from north end of nourishment in the Phase-1 Project to Petitenget Temple) was approximately 0.3 mm, and the beach slope was 1:10 to 1:12. On the other hand in Canggu area (Line No.15 to the north, which is from the Petitenget River mouth to the north), D50 was roughly 0.4 mm, and the beach was steeper with 1:8 to 1:9 slope. In the North Kuta area (Line Nos. 2 to 8), D50 was 0.3 to 0.4 mm, and the beach slope was 1:8. From the result, Legian–Seminyak area seems to be the transition between the south and north areas. ¾ As a result of bathymetric survey, a shallow water area existed in the nearshore zone (-5 m to -10 m) of Legian–Seminyak area. It is suggested that the fine contents of volcanic sand might have been deposit around this area.
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Figure 10.7.5 Sand Sampling Points in the Southwest Cost (Source: JICA Study Team) Foraminifera Coral Shell Volcanic 100
80
60
40
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Figure 10.7.6 Alongshore Distribution of Sand Composition in the Southwest Coast (Source: JICA Study Team)
Figure 10.7.7 Alongshore Distribution of D50 and Beach Slope (Source: JICA Study Team)
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(4) Checking of the River Mouth and Beach Change due to Obstacles
It is useful to check the beach change due to obstacles such as sea cliff and bending of river mouth in order to know the dominant direction of littoral drift. Table 10.7.1 shows the result of checking of the river mouth and beach change due to obstacles from Seminyak area to North Tanah Lot, based on several old aerial and satellite photos. The following show the result of checking.
Table 10.7.1 Result of Checking of the River Mouth and Beach Change due to Obstacles
Area Check Point Oct-09 Oct-02 Jul-02 Small river mouth at Double Six Straight ？？ Small river mouth at south side of Sofitel Hotel Straight Straight ？ Seminyak Small river mouth at Oberoi Hotel Straight ？ to south River mouth at Petitenget to south ？？ River mouth at Brawa Beach to south to south blockade River mouth at Batu Bolong Beach to south to south blockade Small river mouth at south of Echo Beach Straight blockade Small river mouth at north of Echo Beach Straight to north ？ Small river mouth (No.9) to north ？ Canggu Small river mouth (No.10) ？ to north Small river mouth (No.11) Straight Straight Small river mouth (No.12) Straight Straight Obstacle (Rock 1) to south to south Obstacle (Pura Gede Luhur Batungaus) to south to south River mouth (No.13) to south to south Tanah Lot Small river mouth (No.14) Straight Straight Small river mouth (No.15) Straight Obstacle (Rock 2) to north to north Small river mouth (No.16) Straight Obstacle (Jetty) to north North of River mouth (No.17) ？ to north Tanah Lot River mouth (No.18) Straight River mouth (No.19) to north River mouth (No.20) to north River mouth (No.21) to north Obstacle (sea cliff) to north to north (Source: JICA Study Team)
¾ Significant direction of littoral drift was not observed in Seminyak area. ¾ The occurrence of southward littoral drift in Canggu area was recognized. ¾ From Tanah Lot to north area, the occurrence of northward littoral drift was recognized.
(5) Understanding of Results Based on Numerical Analysis
The numerical analysis was also conducted to verify the above mentioned results related to littoral sand drift obtained from field investigation and collected data.
A numerical computation for wave field and longshore transport rate along the shore based on the shoreline change model was conducted. Figure 10.7.8 shows the result of the wave field for representative two offshore wave directions, S and SSW. The image of incident wave direction for each area, which was obtained from the result of numerical computation for waves, is shown in Figure 10.7.9.
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Figure 10.7.8 Distribution of Wave Height and Direction (Source: JICA Study Team)
Figure 10.7.9 Image of Dominant Incident Wave Direction Obtained from Numerical Computation (Source: JICA Study Team)
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Figure 10.7.10 Calculated Longshore Sediment Transport Rate in the Southwest Coast (Source: JICA Study Team)
In case the offshore waves incident from S-direction, the diffraction area due to the shadow effect for the existence of Bukit Badung is covered up to the north of Tanah Lot. On the other hand, when the offshore waves incident from SSW-direction, the diffraction area becomes small up to nearly Canggu (shown in Figure 10.7.8). In the diffraction area, the wave direction turns clockwise, which induce the southward littoral drift in north Canggu.
¾ The incident wave direction in Legian – Seminyak area is almost perpendicular to the shoreline. From this, no significant littoral drift was formed. ¾ The incident wave direction in Kuta area is shown in Figure 10.7.9. Due to this oblique wave direction, northward littoral drift becomes significant.
Figure 10.7.10 shows the calculated longshore sand transport in the southwest coast for two cases of SSW and S (T=16s).
¾ In Kuta area, significant northward littoral drift was identified based on the numerical result. ¾ This northward littoral drift suddenly decreased from North Kuta to Legian and Seminyak. ¾ Slight northward littoral drift was identified in Canggu area. ¾ Obvious southward littoral drift was identified from North Canggu to Tanah Lot. ¾ These tendencies obtained from the numerical analysis was quite similar to the result obtained from several collected data and site investigation.
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(6) Summary for Sediment Transport in the Southwest Coast
The characteristics of sediment transport in the southwest coast are summarized as follows:
¾ It is evident that northward littoral drift occurs in Kuta’s reef area. Moreover, most of the transported sand in the north has probably flown out to offshore through the reef gap in the north edge of the coral reef. A part of the sand from the south area flows to the north Kuta area; however the quantity of such inflow is limited. ¾ It is obvious that littoral drift does not occur from North Kuta, Legian to Seminyak area due to change of alignment of the beach perpendicular to the dominant incident wave direction. ¾ The slight southward littoral drift was expected in the North Canggu area, especially the north part. ¾ The northward littoral drift was observed again from Tanah Lot to the north direction.
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Chapter 11 Planning and Design for Beach Conservation Measures
11.1 Overview
The overview of this chapter is presented as shown in Table 11.1.1.
Table 11.1.1 Overview for Planning and Design for Beach Conservation Measures Ref. No. Title Outline of Contents
-Validity of Phase-2 Project was summarized based on 1)Development policy, 2) Contribution of Phase-1 Project, 3)Needs for Phase-2, 4) Necessity for improvement of Kuta and 5) Necessity 11.2 Validity of the Project of Japanese technology and experience on beach conservation project - Necessity for improvement of Kuta (Phase-1 Project Area) was mentioned
Facing problems and objective of the project for each site as Phase-2 11.3 Objective of the Project Project was summarized based on the results of Chapters 6, 9 and 10.
11.4 Scope of the Project Scope of the project for each site was presented.
Comparison study was conducted to determine the suitable beach Study for Suitable Beach conservation measures. For the improvement of Kuta, the technical 11.5 Conservation Measures study for the design of L-shape groin based on the numerical analysis was presented.
Basic Layout and Design at Basic design at Candidasa was carried out and present the typical 11.6 Candidasa layout and cross section
Basic Layout and Design at Basic design at North Kuta ~ Legian ~ Seminyak was carried out and 11.7 North Kuta ~ Legian ~ Seminyak present the typical layout and cross section
Basic Layout and Design Basic design for the improvement of Kuta was carried out and present 11.8 for Improvement at Kuta the typical layout and cross section
Principle for Planning of Beach Principle for the planning of beach control at southeast coast, which is 11.9 Control at other Southeast Coast out of candidate area for Phase-2 Project, was presented. Area
11.2 Validity of the Project
(1) Validity from the Indonesian Development Policy
The validity of Phase-2 Project was examined by checking the Indonesian development policy and plan.
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a) National Medium-Development Plan (RPJMN), 2010-2014
In the “National Medium-Development Plan (RPJMN), 2010-2014”, which was Multi-Function issued by BAPPENAS in 2010, the necessity and importance for the Conservation of Tourism Conservation of conservation and utilization of the Balinese Culture & natural environment was mentioned in Society order to support sustainable economic Conservation of growth and to increase the welfare of the coastal Conservation of environment people. Further, the necessity for the Property development of infrastructure to protect Coastal Protection the land against the impact of climate change was mentioned. As the funding source for the national development, it Figure 11.2.1 Multifunction of Beach was recommended to utilize the Conservation Project “public-private partnership (PPP) (Source: JICA Study Team) scheme”, “corporate social responsibility (CSR)” and “donation” to accelerate the infrastructure development.
The beach conservation project has multifunction as shown in Figure 11.2.1. These functions are consistent with the abovementioned development policy.
b) Bali Province Medium Development Plan, 2008-2013 (Bali Province Regulation No. 9/2009)
The proposed Phase-2 Project is valid to the Bali Province Medium Development Plan 2008-2013. One of the objectives of the Bali Province at tourism sector is to expand the coverage of tourism object which at present still concentrates at south Bali. To reach the goal, the infrastructure development policy is directed to improve the tourism infrastructure, to protect sustainability of environment condition to keep the quality of Bali tourism, and to improve the quality of natural, cleanliness, safety, and preservation of tourism object. These mean that the proposed Phase-2 Project which is a beach conservation work is in accordance with the Bali Province Development Plan.
c) Spatial Plan in Bali Province 2009-2029 (Bali Province Regulation No. 16/2009)
The validity of Phase-2 Project views from Bali Province Spatial Plan 2009-2029, is in accordance with the spatial plan. Almost all of the shoreline at Bali is designated as tourism area. The recommendation of the development policy on water resources is to protect the shoreline from erosion and abrasion.
The proposed Phase-2 Project aims to conserve and protect the eroded beach from further damage. Moreover, the Phase-2 Project will restore the beach’s potential to become a world-class tourism destination.
d) Strategic Plan of the Ministry of Public Works (MoPW), 2010-2014 (MoPW Regulation No. 02/PRT/M/2010)
The policy of the MoPW strategic plan at the coastal sector is to protect the shoreline from erosion and abrasion by constructing coastal protection works, or rehabilitating and maintaining the coastal structure. For the current period, the concentration on coastal protection works mostly made to protect the outer limit of the Indonesian territory line, the small islands, and the economic growth area.
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The proposed Phase-2 Project area was indicated as high economic growth area for Bali Province. The Legian-Seminyak area at present is the top tourism destination at south Bali, and Candidasa at east Bali has high economic growth potential if the sustainable beach condition can be maintained. From these conditions, the proposed Phase-2 Project is valid to MoPW Strategic Plan 2010-2014.
(2) Validity from the Contribution of the Phase-1 Project
Bali is the world famous tourism area in the Southeast Asian area, and the beach is the main tourism resource. Based on the real condition of the Phase-1 Project that was conducted at Sanur, Nusa Dua, and Kuta beaches, the beach conservation project to recover the sandy beach from beach erosion had a great contribution to the tourism development in Indonesia. The number of foreign tourists who visited Bali increased from 1.4 million in 2004 (just completed Sanur and Nusa Dua projects) to 2.5 million in 2010 (1.8 times increase). The number of domestic tourists further increased from 2.0 million in 2004 to 4.7 million in 2010 (2.3 times increase). Furthermore, the active renovation and development of new hotels, restaurant, and shops along the coastal area were observed at the project area. These induced the multiplier effect to the development of tourism.
The contribution of the beach conservation project was not only on tourism, but also on the welfare of the residents to conserve the culture, religion, and their lives. A lot of local people came to the recovered sandy beach, and they make use of the beach area for religious ceremonies and recreation as shown in Photo 11.2.1. This change in the beach area also contributed to the significant increase of income of the communities through parking fees.
From the abovementioned points of view, the beach conservation project has great potential to contribute to the economic welfare of residents.
Photo 11.2.1 Contribution for Welfare of Residents (Source: JICA Study Team)
(3) Needs for Phase-2 Project
The Phase-2 Project sites were selected mainly taking into account two points, such as “contribution to the economy” and “condition of beach erosion and utilization as world beach resort area” as presented in Chapter 9. The following two sites were selected as the result of the evaluation with several criteria:
East Coast: Candidasa and Southwest Coast: North Kuta ~ Legian ~ Seminyak
As presented in Chapter 9, the necessity of the new project for the two candidate sites is summarized as follows:
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¾ The international tourism area at east coast area is only Candidasa, and it is one of the major beach resort areas in Bali Island; ¾ On the other hand, beach erosion has occurred since the 1970s, owing it to the tourism development even though the beach conservation project was undertaken by the Indonesian government in the 1990s; ¾ The sandy beach has almost disappeared and the coastal line at Candidasa is covered by artificial concrete type seawall. This has disturbed the desirable beach utilization as an international resort area and caused a negative impact to tourism; ¾ Some development plan of infrastructure, which are conducted at east coast area in order to deviate the concentration of tourism at the south coast, are great potential for further tourism development at Candidasa; ¾ The North Kuta ~ Legian ~ Seminyak area is currently the top tourism area in Bali Island, and it is necessary to maintain the present multifunction on beach utilization, such as sunbathing, walking, surfing and body boding, and sunset site seeing; and ¾ The retreat of the shoreline for long term was observed, even though the degree of the retreat was not significant as compared to that of east coast area. It has been causing the narrowness of beach space and obstacle in the utilization of the beach during high tide.
(4) Necessity for Improvement at Kuta
Kuta is one of the Phase-1 Project beaches and the beach nourishment was undertaken from runway at the south end to Alam Kul-kul Hotel at the north end with 4.2 km alongshore distance. However, as presented in Chapter 6, the significant sand outflow after undertaking the beach nourishment has been observed at the south reef area of Kuta. The chronology of Kuta project is as follows:
a) Originally, the beach nourishment with three headlands and one groin system was proposed in the detailed design which was carried out in 1998 as shown in Figure 11.2.2 (1);
b) Before the commencement of implementation, a strong objection on the construction of a headland system was suddenly blown up by the communities and related NGO group. This movement against the project caused the disruption for undertaking the project;
c) To find the solution and proceed with the project, the socialization between communities and project side was conducted more than 100 times. This process took almost three years;
d) Finally, it was concluded that the communities agreed to carry out the project employing beach nourishment with three offshore breakwaters with their proposed crown height as shown in Figure11.2.2 (2). Even though the effectiveness to maintain the nourished sand became low, the project side should agree with their recommendation in order to proceed with the project;
e) Significant outflow of sand has been anticipated due to the change of design from headland system at that time. Thus, the consultant carried out further analysis to predict the outflow of sand, and proposed to prepare the sand stockpile; and
f) Based on the monitoring result which continued to carry out since completion of the nourishment, significant outflow of sand was observed at the south part of nourishment area which is located at Kuta reef area. According to the monitoring result, the total amount of sand loss during the last three years since 2008 to 2011 was estimated at
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roughly 80,000 m3 (20% of the total quantity of nourishment). This quantity of sand loss was significantly higher than that in Sanur and Kuta (10% for eight years) (see Figure 11.2.3).
(1) Planned (2) Actual Figure 11.2.2 Planned and Actual Layout at Kuta Project (Phase-1 Project) (Source: JICA Study Team)
Sand Remaining at Sanur & Nusa Dua Sand Remaining at Kuta (South & North Part) 110% 110% 103% 100% 100% 97.81% 100% 96% 100% 97% 97% 100% 93.43% 91% 100% 97.07% 91.05% 91% 90.71% 91% (%) 94% 90% 90% 94% 91.21% (%) 88% 88.18% 88% 87.98% 88%
80% 80% 83% 81% Remaining
Remaining 70% 70% Sanur Beach Nusa Dua Beach Sand 60% Sand 60% Kuta South Kuta North 50% 50% 0Y 1Y 2Y 3Y 4Y 5Y 6Y 7Y 8Y 0 Y1 Y2 Y3 Y Year s After construction (1) Sanur and Nusa Dua (2) Kuta
Figure 11.2.3 Remaining Sand after the Nourishment in Phase-1 Project (Source: JICA Study Team)
Local people and communities, who have strong objection in the Phase-1 Project, have already realized the insufficiency of the present structures to control the sand movement after they identified the difference in maintaining the nourished sand between Kuta and Sanur, Nusa Dua. Thus, it is expected that there will be no further social problems to undertake the improvement. Now, the improvement to reduce the sand outflow at Kuta is one of important issues of the Indonesian government.
Based on the abovementioned chronology and resulting current situation at the beach, it is recommended to conduct the improvement measures at Kuta to mitigate the further outflow of sand, even though Kuta belongs to the Phase-1 Project and it is out of the study area.
(5) Necessity of Japanese Technology and Experience on Beach Conservation Project
The leading western countries for coastal and beach conservation have already realized that the previous protection method using only artificial coastal facilities such as seawall, groin, etc., could not solve the problem of beach erosion. The only effective method for beach conservation is “beach nourishment”.
Japan is the only country that have many experiences on this process of beach conservation including not only successful but also unsuccessful experiences in Asian countries. Japan has been the leading country in Asia to have “lessons learned” on beach conservation in both
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management and technical points of view such as planning, design, and implementation after the project.
Indonesia’s recent economic growth is significant. On the other hand, several adverse impacts to the ecosystem have been exposed. The beach erosion problem and its control based on the required technology and management system is one of the urgent issues to be solved. Only Japan can support in this field of beach conservation among Asian developing countries for both technical and management field.
11.3 Objectives of the Project
(1) Candidasa
a) Facing Problems
Candidasa is one of the international beach resort areas in Bali Island, even though the tourism contribution is lower than that of south resort area due to its remote place from the south of Bali area. However, Bali Province has an active development strategy at the east coast area to decentralize the tourism area from the crowded south area. The extension of the highway road and newly opened international cruise port nearby Candidasa was following this strategy plan. Candidasa has great potential to contribute to the tourism at the east coast area.
On the other hand, due to the serious beach erosion and unsuccessful beach protection measures which has not been carried out until now, Candidasa is facing the following problems in approximately 5 km alongshore area.
Protection (Photos 11.3.1 and 11.3.2)
¾ Loss of land due to the beach erosion; and ¾ Damage to facilities and vegetation due to wave overtopping and decrease of life cycle of facilities (hotel facilities and houses) due to splash water. Beach Utilization (Photos 11.3.3 and 11.3.4)
¾ Disturbance of free walking across the beach due to existence of seawalls, groins, and brick walls. ¾ Disturbance of easy access to the beach due to the absence of suitable access road and the existence of brick walls. ¾ No space for beach activities for tourists such as sunbathing, swimming, etc. ¾ No space for religious events for residents; and ¾ Insufficient boat parking area. Environment and Landscaping (Photo 11.3.5)
¾ Deterioration of natural view as an international beach resort due to the existence of seawall and a lot of groins; and ¾ Disturbance of open view due to the existence of brick walls.
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Photo 11.3.1 Beach and Land Erosion Photo 11.3.2 Wave Overtopping
Photo 11.3.3 Disturbance of Walking Across Beaches
Photo 11.3.4 No Space at the Beach Photo 11.3.5 Deterioration of Natural View
Photo 11.3.6 Good Water Quality Photo 11.3.7 Corals on the Reef (Source: JICA Study Team)
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b) Requirement on Environment
The sea water at Candidasa maintains the good quality with high transparency as compared to that of the south resort areas such as Sanur, Nusa Dua, and Kuta as shown in Photo 11.3.6. This sea water condition is one of the advantages at Candidasa, therefore it is required to maintain this condition. Furthermore, Candidasa is one of the famous snorkeling and diving spots in Bali Island, and there are a lot of live corals and fish habitat at the coral reef area. There are also existing corals on the reef flat, especially near the peninsula located at the east side of the site as shown in Photo 11.3.7. Thus, these marine environmental conditions should be given attention.
c) Objectives of the Project
The objectives of the project at Candidasa are as follows:
¾ To prevent further beach and land erosion and to minimize the damage of facilities due to wave overtopping and splash water. ¾ To achieve easy access to the beach and free walking along the beach. ¾ To recover the beach space to be utilized for marine activities of tourists as international beach resort, and for religious events for residents. ¾ To recover the natural landscape as international beach resort. ¾ To maintain the present coastal environment such as good sea water quality and coral habitat. To achieve the abovementioned objectives, it is aimed to increase the tourism contribution at Candidasa as world famous beach resort by collaborating with other infrastructure developments near Candidasa, which are the extension of highway road at the east coast area and the new international cruise port.
The area to be rehabilitated is approximately 5 km in total from the east end of peninsula (Tanjung Nti) to Alilla Mangis Resort Hotel.
(2) North Kuta〜Legian〜Seminyak
a) Facing Problems
North Kuta〜Legian〜Seminyak at the southwest coast is the most attractive and high tourism contributing area in Bali Island. This beach area is maintaining the continuous natural sandy beach without artificial coastal structures. Due to this advantage, both foreign and local tourists come to this beach for many beach activities, such as sunbathing, surfing, body boarding, walking, jogging, and sunset viewing, etc. The tourism development is now further expanded to Canggu which is located at the north side of these area. Development of big hotels, apartments, and private villas are actively on-going in this area.
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Photo 11.3.8 Wave Runup to Property Photo 11.3.9 Narrowness of Beach Space (Seminyak) (Legian)
Photo 11.3.10 Narrowness of Beach Space Photo 11.3.11 Damaged Facility (Seminyak) (Canggu)
Photo 11.3.12 Illegal Development Photo 11.3.13 Breaking Waves (Canggu) (Legian) (Source: JICA Study Team)
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The problems on current beach condition at southwest coast are as follows.
¾ Urgent problems on beach erosion are not exposed and the foreshore area still remains. However, the wave run-up sometimes intrudes into the properties of hotels and restaurants during high tide, and it disturbs their operation (Photo 11.3.8.). ¾ During the high tide, beach space at this area becomes significantly narrow. According to the long term shoreline change based on aerial and satellite photos, the beach recession was estimated at 5 m to 15 m in Legian, Seminak area. The narrowness of the beach space disturbs the activities of tourists on the beach (Photos 11.3.9 and 11.3.10). ¾ Some public facilities were damaged by wave action at Canggu area (Photo 11.3.11). ¾ According to the regulation of Badung Regency, 100 m width from Highest High Water Level (HHWL) was defined as the beach riparian zone and no construction of facility is permitted at this area. However, illegal constructions and developments at foreshore area of the beach were observed at several points (Photo 11.3.12).
b) Requirement on Environment
This area is formed by common sandy beach and no corals exist. Therefore, basically, high consideration on marine environment might be unnecessary. However, some of the offshore area are utilized for fishing activities. This area is one of the world’s famous surfing spots due to the long crested wave breaking condition as shown in Photo 11.3.13, therefore, it is very important to maintain this wave breaking condition.
c) Objectives of the Project
The objectives of the project at the southwest coast are as follows:
¾ In order to maintain the beach activities at this area as the most attractive beach resort area in Bali Island, the sufficient beach space shall be secured. The target area for recovering the beach is from Alam Kul Kul Hotel at north Kuta (end of beach nourishment area in the Phase-1 Project) up to Kudeta at Seminyak with 2.9 km alongshore distance. The target of recovering beach width was assumed as the same level as that of 30 years ago, which is 15 m to 20 m increase of the width compared to the present condition; and ¾ As the principle of requirements on beach utilization and landscaping, 1) the natural viewing, which is the most advantageous point, shall be maintained, 2) present wave breaking condition shall be maintained as the world’s famous surfing spot.
(3) Improvement at Kuta (the Phase-1 Project Area)
a) Facing Problems
The sand outflow after the beach nourishment was continued at the south part of nourishment area. The quantity for the outflow of sand at this part was estimated roughly at 80,000 m3 for three years, which is 20% of the total quantity of nourishment sand. On the other hand, the sand outflow at Sanur and Nusa Dua is indicated as 10% for eight years as shown in Figure 10.2.2. The quantity of outflow at the south part of Kuta is significant compared to that of Sanur and Nusa Dua, and it is required to reduce the outflow of sand at this area.
b) Objective of the Project
The objective of the improvement at Kuta is to reduce the sand outflow than its present condition, especially at the area between the south end of sand stopper and 1st Offshore
11-10 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in the Republic of Indonesia Final Report (Simple Version)
Breakwater (BWN1), and between 2nd Breakwater (BWN2) and last Breakwater (BWN3). However, the difficulty was realized at surrounding BWN3 area due to the particular condition on geographical and littoral sand dynamics. Thus, the aim of the countermeasure surrounding BWN3 is not to recover the sandy beach, but to maintain certain thickness of sand layer at the toe part of revetment to avoid damage of the revetment.
11.4 Scope of the Project
(1) Candidasa
The scope of the project at Candidasa is as follows:
a) Modification of Existing Seawall to Permeable Slope-type Revetment
The existing vertical seawall was constructed by each property owner individually. Due to lack of unity for alignment, tourists cannot walk along the beach. Furthermore, the sandy beach is hard to maintain in front of the existing seawall due to acceleration of scouring. Thus, it is recommended to modify the existing revetment to permeable slope-type revetment as the same type as that of the Phase-1 Project. The alignment of the revetment is recommended to have set-back at a certain distance to enhance and maintain the sand.
b) Modification of Existing T-shape Groins
The crown height of the existing T-shape groin was set at +2.2 m (0.5 m below HWL) and the function to interrupt the westward littoral drift was absolutely insufficient. Furthermore, the crown part was very slippery due to seaweed clinging on the surface, and the tourists cannot utilize the crown part. Thus, it is recommended to modify the exiting T-groins to function effectively and interrupt the littoral drift sand and enhance the effective utilization of the crown part. Some of the existing groins will be removed or changed the position based on the result of future conducted detailed design.
c) Beach Nourishment
The possibility to increase the sand source on the coral reef is not expected based on the comparison of beach conditions by aerial and satellite photos. Therefore, it is definitely required to fill the sand in recovering the sandy beach. On the other hand, the reef width at Candidasa is not wider than that of Sanur and Nusa Dua. Thus, it may be difficult to secure the sandy beach as the same image as Sanur and Nusa Dua. To minimize the sand outflow after the nourishment, it is necessary to consider the appropriate width of the beach not to be wide.
d) Walkway
The crown part of the revetment is recommended to be used as walkway. The purpose of the walkway is not only to enhance the beach utilization to be able to walk continuously along the beach, but also to identify the boundary between private and public areas after the nourishment.
e) Public Facilities
Public facilities such as toilet, lamp, dust box, chair, gazebo, etc., are recommended to enhance the convenience of beach utilization for both tourists and residents.
11-11 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in the Republic of Indonesia Final Report (Simple Version)
(2) North Kuta〜Legian〜Seminyak
The scope of the project at the southwest coast is as follows:
a) Beach Nourishment
The purpose of beach conservation at the southwest coast is to maintain the beach utilization for a long period of time with the same condition as present. From this point of view, beach nourishment without artificial structures is recommended. The area for the beach nourishment will be determined taking into account the present use of the beach and the degree of beach retreat for long term.
b) Walkway (partially)
The purpose of the walkway is to support easy walking along the beach and to identify the boundary between private and public (beach) areas. From the former point of view, it seems it is not required taking into account the present beach use. Furthermore, the walkway or sidewalk exist in some areas at Legian, Seminyak. On the other hand, the illegal developments intruding into the beach riparian area are sometimes observed at Seminak and Canggu areas. To avoid this, it is required to identify the boundary between private and public properties. The construction of walkway becomes one identification. Thus, construction of the walkway is recommended partially.
c) Public Facilities
Public facilities such as toilet, lamp, dust box, chair, gazebo, etc., are recommended to enhance the convenience of beach utilization for both tourists and residents.
(3) Improvement at Kuta (the Phase-1 Project Area)
The scope of the improvement at Kuta is as follows:
a) Modification of Existing Offshore Breakwaters
Originally, the beach conservation measures at Kuta were planned to undertake the beach nourishment with the construction of three headlands and one groin. On the other hand, the final layout plan for the coastal structures which was accepted by all stakeholders during the Phase-1 Project was to construct three offshore breakwaters and the crown height was lower than that of the original plan.
Usually, the tombolo is formed behind the offshore breakwater due to diffraction effects of offshore breakwater. The littoral sand movement is interrupted by the formation of tombolo as long as the tombolo is surely stable. On the other hand, from the monitoring result after the nourishment, it was observed that the tombolo behind BWN3 has almost disappeared. Further, the area exposing the revetment surrounding BWN3 is still expanded slightly for both sides and it was concerned to affect the stable condition of the tombolo behind BWN2. One of the reasons why the tombolo was not maintained in stable condition this may be due to the insufficient interruption effect of the northward littoral drift. Taking into account the present condition, it is recommended to modify the exiting offshore breakwaters to add the groin part in order to enhance the interruption effect.
b) Construction of Additional L-shape Groin with Refilling of Sand at BWN3
As presented before, it is difficult to maintain the sandy beach with certain width nearby BWN3. The requirement to improve the present condition is to avoid the further retreat of the
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beach and to secure the revetment. The additional study to improve the present condition was carried out by the consultant after the completion of the Phase-1 Project. Referring to the result, it was recommended to construct the additional L-shape groin at the south side of BWN3 as well as conducting the sand filling at shadow area (north side of the L-shape groin).
11.5 Study for Suitable Beach Conservation Measures
(1) Comparison Study at Candidasa
The comparison study for beach conservation measures at Candidasa was carried out by assuming three alternatives as shown below.
Alternative-1: Modification only of the existing seawall
Alternative-2: Modification of the existing seawall + beach nourishment without new coastal structures (maintain the existing T-shape groins and offshore breakwaters)
Alternative-3: Modification of the existing seawall + beach nourishment with modification of existing T-shape groins and demolition of existing offshore breakwaters.
The result is shown in Figure 11.5.1. Alternative-3 is the most suitable beach conservation measures at Candidasa taking into consideration the points of technical aspect, beach utilization, landscaping, environmental impact, and public opinion.
(2) Comparison Study at North Kuta〜Legian〜Seminyak
The comparison study for beach conservation measures at the southwest coast was carried out by assuming three alternatives as shown in Figure 11.5.2.
Alternative-1: Protection by revetment
Alternative-2: Beach nourishment
Alternative-3: Beach nourishment with offshore breakwaters
The results showed that Alternative-2 is the most suitable beach conservation measures at the southwest coast taking into consideration the points of technical aspect, beach utilization, landscaping, economical aspect, and public opinion.
(3) Comparison Study at Kuta (Validity for L-shape Groin)
After understanding the significant sand outflow surrounding BWN3, the detailed study was carried out in the “Study on Countermeasure for Beach Recession at North Kuta Reef Area) in 2010 by the consultant (Nippon Koei Co., Ltd). As the selection of conceivable alternatives, the following four alternatives were considered as shown in Figure11.5.3.
Alternative-1: Sand refilling + increase of crown height of BWN3 + construction of groin at down coast
Alternative-2: Sand refilling + construction of offshore breakwater
Alternative-3: Sand refilling + construction of L-shape headland
Alternative-4: Sand refilling + construction of oblique groin to connect BWN3
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Based on the preliminary evaluation for each alternative, Alternative-3 was selected as the basic idea of countermeasure. Detailed case studies for four different layout plans as shown in Figure 11.5.4 were conducted by using the numerical analysis. Figure 11.5.5 shows the calculated sand loss with and without refilling of sand to make it easy to show only the effect of the structure. Even though the difference for each case is not significant, Case 3 showed the most minimal result for sand loss.
Predicted seabed elevation at toe position of the revetment is compared at two points (P1 and P2) as shown in Figure 11.5.6. As shown in Figure 11.5.7, the significant effect to secure the certain seabed elevation due to construction of an L-shape headland is observed at P1 (30 m far from the trunk position of L-shape headland). At P2, which is positioned near the open space between tip of L-shape headland and BWN3, even though the decrease of seabed elevation is significant in the initial stage, certain elevation can be secured by construction of an L-shape headland.
From the results, if the refilling of sand with too much volume will be conducted, most of the sand may flow out. Thus, it is recommended that the refilling of sand shall be conducted only for the purpose of maintaining the certain seabed elevation at serious retreated part.
11-14 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in the Republic of Indonesia Final Report (Simple Version)
Seawall
breakwater
Sand nourishment Existing
of
Offshore
Protected minimized.) Alternative-3 Recommended Possible to minimize to Possible Same asAlternative-2 Same Same as Alternative-2 as Same Same as Alternative-2 as Same Same as Alternative-2 as Same existing
Modification Demolishof tourism area. And sand re-filling after the project as maintenance can be can as maintenance project the after re-filling sand area. And tourism (This alternative can fulfill the request from stakeholders and requirement as requirement and stakeholders from request the fulfill can alternative (This Some degree of disturbance is expected is disturbance of degree Some but obstruction structures of num. the on reduce landscaping color, armor shape, of will be significantly improved due to change Modification of Existing Seawall Existing of existing of Modification + Demolish structures coastal new with Nourishment + Beach breakwaters offshore coastal of construction by interrupted be can drift littoral Westward same as maintained can beach sandy and groin), (headland, structures conservation Dua Nusa & as Sanur image Seawall
structures)
Existing
of
(without required.) Protected
Alternative-2 Not recommended Modification Sand nourishment continuous sand re-filling as maintenance of the beach after the project is project the after beach the of maintenance as re-filling sand continuous To recover the natural sandy beach can be fulfill the request from stakeholders request from the beach becan fulfill sandy natural recoverTo the (It is difficult to maintainthe filling sand due to existence of littoral sand drift.The Beach nourishment is main construction work Alternative-2. in during on the sand thebeach consider to filling necessary Thus, and is It sand at turbidityoffshore mining quality. seawater and area corals be might existing to impacted suitable construction method to minimize turbidity. Open landscaping at offshore side can be secured if all existing coastal structures coastal existing all if secured be can side offshore at landscaping Open demolished are 1) Beach activities (walking, sun-bathing, swimming on the beach) the on swimming sun-bathing, (walking, activities 1) Beach beach the along walkway on Walking 2) snorkeling & diving for offshore to go to point Access 3) Possible to minimize as far as sandy beach is secured is beach as sandy far as minimize to Possible Modification of Existing Seawall Existing of Modification structures coastal new without Nourishment + Beach 2) Existing groins are not effective on function of interrupt of drift sand due to due sand drift area of this on interrupt exists of drift littoral function 1) Westward on effective not are groins Existing 2) low crown height 3)Thus, itis difficult to maintain the sandy beachwithout coastalstructures -To secure the space for beach activity as resort area resort as activity beach for space the secure -To beach the to access continuous the enhance -To Seawall
Drift
Existing
×◎◎ ○△△ △◎○ △○◎ ◎◎◎ ×△◎ △○◎ of
Protected Alternative-1 Not Recommended environment is expected. is environment Westward Littoral Only Modification of Existing Seawall Existing of Modification Only Modification Still Occur (Same as present condition) as present (Same Occur Still -To enhance the continuous access to the beach the to access continuous the enhance -To construction of beach protection facilities is not fulfill their request. their fulfill not is facilities protection beach of construction side can be protected. The requirement as tourism area cannot fulfill.) area cannot tourism as requirement be The protected. can side Stakeholders strongly requested to recover the natural sandy beach.Only sandy requestednatural to recover the Stakeholders strongly (Sandy beach cannot be recovered by Alternative-1 even though the land the though even Alternative-1 by recovered be cannot beach (Sandy Alternative-1 is without sand filling, therefore no significant impact to marine to impact significant no therefore filling, sand without is Alternative-1 Disturbance due to a lot of existing T-groins & headlands & T-groins existing of a lot to due Disturbance condition) present as (same 1) Walking on walkway along the beach the along walkway on Walking 1) snorkeling & diving for offshore to go to point Access 2) 1) Existing groins and offshore breakwaters are not effective function due function effective not are breakwaters offshore and groins Existing 1) to mainly insufficient crown condition) as present (same height exists beach 2) No sandy and length Land Wave Beach Securing of Sandy overtopping Protection of Protection Image Survey) Concept Evaluation Alternative and Corals) and Landscaping Environment Public Opinion (From Interview Beach Utilization (Natural viewing) (Natural Impact to Marine (Seawater Quality of View of al Point al Technic Figure 11.5.1 Comparison of Alternatives for Beach Conservation Measures at Candidasa (Source: JICA Study Team)
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(Toohigh) Protected ificant impact to marine activities) × Alternative-3 49.4 million U$ Not Recommended Not Same as Alternative-1 Same Beach Nourishment with offshore breakwaters Strong objection (Sign 1) No significant littoral driftanalysis. was So, observed it is not necessary in this to construct areainterrupt the based coastal Alternative-2 the structure on than littoral secured several be more to drift will beach Sandy 2) 1) Beach activities (walking, sun-bathing, swimming) can be almost be can swimming) sun-bathing, most are (walking, which board) body and activities Beach 1) (surfing activities marine but maintained, important as present are maintained activities residents for culture events, in this area 2) Religious will significantly disturbed. A lot of breakwaters disturb natural landscaping and image as world as image and landscaping natural disturb breakwaters of lot A famous resort beaches, significantly Sun-set viewing (this is one of strong advantage of thisdisturbed area) is also Marine activities such as surfing, swimming and body board are the most attractive this in beach. Construction of offshore breakwaterimpact significantly to current wavebeach. condition Further, cost which is too high. is one of the attractive points of this
Protected 9.4 million U$ Alternative-2 Beach Nourishment Same as as Alternative-1 Same If necessary (beach is under retreat condition), agreed Recommended Width of the beach can be increased than present condition and this canenhance the beach activity with no impact to marine activities and landscaping. Thus, this only conservation method (nourishment) can the fulfill purpose of beach conservation at this beach. 1) No significant littoral drift was observed in this area based on several area based on this in observed was drift littoral No1) significant but analysis. 0.5m/yr, to 0.2m with So, coastal so significant not is area this at retreat structures Beach 2) to interrupt thethe littoral beach drift will be is gradually not necessary. retreat by natural wave action. 1) Beach activities condition. as present same as (walking, sun-bathing, be maintained board) can body and swimming) and marine(surfing present as activities maintained are residents for culture events, Religious 2) Same natural viewing as present can be maintained at both beach-side and beach-side both at be maintained can as present viewing natural Same sea-side . ◎◎◎ ○○◎ △◎△ △◎× ○○○ ○○ Protected 9.2 million U$ Alternative-1 Not Recommended Not
Protection by Revetment Objection (Artificial structure was not desirable) The purpose of beach conservation at this beach is not to protect the land, the protect to not is beach this at conservation beach of purpose The but to maintain present necessary beach not is it construct to the revetment. utilization as highest tourism area in Bali. Thus, 1) No significant littoral but 0.5m/yr, to not necessary. drift 0.2m is So, drift analysis. coastal with structures littoral was the to interrupt observed significant so not is area in this this at area retreat Beach 2) based on several the beach be will gradually retreat by natural wave action. 1) Beach activities such as sunbathing, bodyboad, etc. are restricted due to due restricted are etc. bodyboad, sunbathing, as such activities Beach 1) narrowness to due restricted are residents for etc) (disappearing (ceremony, events 2) Religious in the future) of beach spaceinsufficient space of beach side than present due to construction of artificial structures. Natural artificial landscaping of construction to due present at than sea-side side is maintained, but deteriorated at beach- Project beach is volcanic sandy beach and there was no corals. Even corals. no was there and beach sandy volcanic is beach Project though the turbidity will occurredbeach, the during influence to marine environment the is not so sand significant comparing mining and beaches coral the fillingto on the y) Land Beach Securing of Sandy of Protection of Protection Item Image Method Evaluation Landscaping Environment Public Opinion Beach Utilization (Natural viewing) (Natural Impact to Marine Construction Cost View from InterviewSurve Point of Point ( Technical Figure 11.5.2 Comparison of Alternatives for Beach Conservation Measures at North Kuta~Legian~Seminak (Source: JICA Study Team)
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Alternative Image Comment Evaluation - Theoretically northward longshore transport can be trapped by north groin. But offshore transport Alt-1 due to increase of depth was also suggested from this study, and long groin may be required Sand refilling (unrealistic of construction cost as countermeasure, × negative impact for beach utilization、effect of north Increase of crown height of A BWN3 area ) New groin at north - Additional structure at south side from BWN3 may be required to keep the beach shape - Sand outflow from south to north through new Alt-2 offshore breakwater still exists if sufficient periodical refilling of sand for maintenance is not carried out (same problem as present will be Sand refilling × happen at south side of new offshore breakwater New offshore breakwater A at south
- it is expected to keep the certain sand at shadow area of L-shape headland
Alt-3 - northward sand movement can be stopped at south side of L-shape headland. At least, it can be ○ Sand refilling more secure at south side. A New L-shape headland - Northward transport still exists at north from L- shape headland. So if sufficient periodical re-filling is not conducted, finally almost sand will disappear.
- For the functional point of view, similar effect can be expected. Alt-4 - Deterioration of water exchange and negative impact for landscaping and utilization (strong △ Sand refilling enclosed image) Oblique groin A - Cost become higher than Alt-3
Figure 11.5.3 Preliminary Evaluation for Each Alternative at North of Kuta Reef (Source: Nippon Koei Co., Ltd.)
Figure 11.5.4 Case Study for Numerical Analysis (Source: Nippon Koei Co., Ltd.)
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12000 40,000
35,000 10000 30,000 ） ） 8000 25,000 m3 m3 （ （ 6000 20,000
15,000 Sand loss 4000 Sand loss 10,000 2000 5,000
0 0 Case1 Case2 Case3 Case4 Case1 Case2 Case3 Case4
(1) Without Refilling Sand (2) With Refilling Sand Figure 11.5.5 Comparison of Calculated Sand Loss for Each Case (Source: Nippon Koei Co., Ltd.)
P1 P2
Figure 11.5.6 Checking Points of Seabed Elevation (Source: Nippon Koei Co., Ltd.)
5 5 case1 case2 4 4 case3 case4 no refill 3 3 ) m ( . L . D.L. (m) D.L. 2 D 2 case5 case2 1 case3 1 case4 no refill 0 0 0123401234 year year
(1) P1 (2) P2 Figure 11.5.7 Calculated Seabed Elevation at Toe Part of Revetment (P1 and P2) (Source: Nippon Koei Co., Ltd.)
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11.6 Basic Layout and Design at Candidasa
(1) Design Principle
a) Modification of Existing Seawall
¾ The existing seawall shall be modified from vertical impermeable type to sloping permeable type revetment. To achieve this, armor-type sloping revetment is planned; ¾ The slope gradient is set at 1:2.5 considering easy approach to the beach for tourists and insufficient beach space (1:3 was adopted at Sanur, Nusa Dua, and Kuta in the Phase-1 Project); ¾ The crown elevation shall be kept as the same elevation as present elevation of the property. Furthermore, the elevation shall be above the backshore elevation, and is set at +5.0 m; ¾ The alignment of modified revetment will be set back as much as possible to enhance the securing of nourished sand. However, the final alignment will be determined as the result of consensus building with each owner of property and it is difficult to finalize in this stage. Thus, the alignment of revetment in this study is tentatively assumed based on the existing condition for the land use of each property; and ¾ Taking into account the harmony on landscaping with light colored white-yellow sand that exist on the coral reef, the limestone with light yellow color is recommended as armor material.
Figure 11.6.1 Image for Modification of Existing Seawall (Source: JICA Study Team)
b) Modification and Demolition of Existing T-shape Groin
¾ The existence of many number of groins disturb the beach utilization and natural view as beach resort area. Therefore, it is desirable to minimize the number of groins as much as possible. On the other hand, the function of groin is to interrupt the littoral sand drift and to secure the stable condition. To achieve this, it is required to set sufficient number of groins. These two requirements are under the relation of “trade-off”. The important consideration on layout design is to make a balance between these two requirements;
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Figure 11.6.2 Image for Modification of Existing Groins (Source: JICA Study Team)
¾ The crown elevation for existing T-shape groin was at +2.2 m in average (0.5 m below HWL) and the function to interrupt the littoral sand was insufficient. Thus, the crown height for modified groin shall be higher than the expected elevation of sand. Considering this point of view, the elevation at trunk part of the groin shall be kept at the same elevation as that for the revetment (+5.0 m); ¾ The crown elevation of the modified groin can decrease following the presumed beach slope. Considering this and to minimize the construction cost, the crown elevation of the modified groin is planned to gradually decrease toward the head part. However, in order to take account the use of tourists on the crown part, the elevation of trunk part with certain length shall be set to keep it above the HWL; ¾ In order to keep the permeability and to harmonize with revetment on the point of landscaping, the armor type groin with the same material (limestone) shall be applied as structure type of modified groin; ¾ In order to enhance the utilization condition on the crown part for tourists and to achieve easy approach from the groin to the beach, side slope gradient is set to keep the same value as that for revetment, 1:2.5; ¾ The toe position of modified groin shall be offshore side to the presumed toe position of nourishment to interrupt the drift sand. Based on the experience for Phase-1 design, toe position is set 10 m to 20 m offshore side from the toe position of the filling sand. As mentioned below, the beach width is presumed to be 40 m to 50 m. Thus, the length of the groin is assumed at 60 m to 70 m; and ¾ In order to mitigate the existing corals which attach to several existing groins and offshore breakwaters, it is possible to utilize the head part of modified groin as the media for coral transplantation.
c) Beach Nourishment
¾ Considering the narrow width of the original beach at Candidasa before beach erosion occurred, and the narrowness of the coral reef width and the deepening of the depth on the coral reef flat due to coral mining, difficulty is expected to recover the sandy beach as the same level (20 m to 30 m increase of backshore width) as Sanur and Nusa Dua. As the
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minimum requirement, it is planned to recover only foreshore part to minimize the sand outflow after the nourishment; ¾ The material to be utilized for beach nourishment shall keep the same characteristics of the existing sand on the beach as much as possible. The high priority for the requirement is grain size and color; ¾ The beach slope is greatly influenced by the grain size. The beach slope becomes milder in accordance with the increase of fine contents of sand, and becomes easy to shift to offshore. Thus, it is desirable to use coarse sand to enhance the stability of the beach as far as adverse impact on beach utilization is unexpected; ¾ Based on the results of grain size and beach slope which was obtained on site, the beach slope is assumed at 1:10 in case that the grain size (D50) is assumed to be 0.3 mm-0.4 mm; ¾ The top elevation of the nourishment is set at +4.5 m to assume the same elevation of existing berm top. As the elevation of coral reef flat is -0.5 m to +0.5 m, the beach width from toe part to the top is assumed from 40 m to 50 m in average; and ¾ The color of sand is also one of the important requirements. White or yellow colored sand is required to secure the image as resort beach on coral reef.
d) Walkway
¾ The walkway shall be constructed for the whole project area. The walkway will be set at crest part of modified revetment. The width of the walkway is set at 1.5 m taking into account the available space behind the revetment, and the required width that the pedestrians can pass each other; and ¾ To minimize the wave runup into the properties, flower boxes with 60 cm height and 30 cm width is planned to be installed at the land side of the walkway as prevention facility against wave runup.
(2) Layout Plan
The position and required length of the modified groins and the resulting presumed shape of beach shall be examined by applying the further detail numerical analysis taking into account the actual beach evolution obtained on site and collected data. On the other hand, this study is still at the feasibility study stage and the detailed numerical analysis is not planned to be conducted. Therefore, the layout of groins was tentatively determined based on the result of the existing beach shape obtained during the site investigation, and the presumed incident wave direction obtained in the numerical computation for waves. The further detailed analysis will be carried out in the detailed design stage.
The basic layout image for beach conservation measures at Candidasa is shown in Figures 11.6.3 and 11.6.4. As explained before, it is desirable to minimize the number of groins (headlands) as far as securing of stable beach condition. Based on this opinion, seven modification groins and six new groins are arranged in 5 km alongshore distance.
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Figure 11.6.3 Layout Plan at Candidasa (Area 1) (Source: JICA Study Team)
Figure 11.6.4 Layout Plan at Candidasa (Area 2) (Source: JICA Study Team)
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As presented in Chapter 12, there are two possibilities for project execution taking into account the total project cost, its economical contribution, and social condition for each area. One idea is to implement at the whole area with 5 km alongshore, in which area is faced with the problem of beach erosion (Areas 1 and 2 in Figure 10.8). Another idea is to implement at Area 1, initially. And the implementation at Area 2 will be decided after obtaining the project contribution at Area 1. The distance between each groin (headland) at Area 2 seems longer than that of Area 1. The layout plan at Area 2 is recommended to be finalized after obtaining the detailed analysis at Area 1 as required.
There is one suggestion from the Indonesian government to make a few sections without beach nourishment. The purpose of this suggestion is to make clear the necessity of beach nourishment for each section enclosed by both groin (headland) and to make clear the possibility to recover the sand without beach nourishment. Basically, in this report, it is shown that beach nourishment for all sections should be done. However, taking into consideration this suggestion, the candidate section for without beach nourishment is shown, as required.
(3) Structure Design
a) Design Criteria
i) Design Tide Condition
The design tide condition, which was employed in the Phase-1 Project, is as follows:
H.W.L. : +2.6 m M.W.L. : +1.3 m L.W.L. : 0.0 m According to the tide observation carried out in this study (Section 10.2 in Chapter 10), the obtained tide change at Candidasa was not so different from that of south area. Therefore, the same tide condition can be applied as design tide condition at Candidasa.
ii) Offshore Design Waves
Offshore design waves are presented referring to the Project Completion Report (PCR) in the Phase-1 Project.
There are two seasons in Bali, the wet season that is governed by tropical monsoon from December to March when the west winds prevail, and the dry season from May to October where the east to south east winds prevail. The U.S. Navy Marine Climatic Atlas of World Volume 3 Indian Ocean (1976) presents the overall offshore wave climate to the south of Bali Island in the Indian Ocean, including the statistical data of wave heights, wave periods, and wave directions for the last 120 years. On the basis of this statistical wave data, the offshore wave characteristic in the project site can be visualized. The predominant wave directions near the site are south-east during the wet season and south-west during the dry season. The predominant offshore wave periods are 5-10 seconds. These offshore waves, when approaching Bali Island, develop into swells at 10-15 seconds.
In the existing design wave height with return periods of 1 to 120 years were developed from the following historical wave data.
¾ U.S. Navy Statistic Data (Marine Climatic Atlas of World); ¾ Wave forecast data offshore of Indonesia; and ¾ Wave observation data at Kuta Beach (1991–1992)
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On the basis of the above data source, the wave height with several return periods were hindcasted as outlined in Table 11.6.1. For the designing of shore protection facilities in Bali Beach, the wave heights with a return period of 50 years have been adopted. The design wave height, averaging the three results, has been determined as (Ho)50 = 5.5 m.
The actual wave observation at Kuta indicates that the predominant wave periods offshore of the reef are 10 to 15 seconds, and occasionally reach 16 seconds when high waves attack the beach. Considering the combined probability of wave heights and wave periods, the offshore wave period (To) has been set at 16 seconds. The wave directions vary depending on the prevailing wind directions during wet and dry seasons. The prevailing wave directions of the southern coast of Bali Island are distributed between south-east and south-west.
Table 11.6.1 Statistical Offshore Wave Height with Each Return Period Actual (Kuta) U.S. Navy Statistic Return Period Forecasting Wave Observation Data (Year) (m) (m) (m) 1 3.03 4.09 > 1.0 5 4.14 5.01 2.70 10 1.53 5.37 3.40 30 5.16 5.75 4.50 50 5.44 5.91 4.95 100 5.80 6.12 5.40 120 5.89 6.17 5.70 (Source: PCR in Phase-1 Project)
Table 11.6.2 Offshore Design Waves (50-year Return Period)
H0 (m) T0 (s) θ0
5.5 16 SW to SE
(Source: PCR in Phase-1 Project)
On the basis of the above consideration, the design offshore waves have been determined as shown in Table 11.6.2.
iii) Design Waves for Coastal Structures
Prior to setting up the design waves for coastal structures, the equivalent deep water wave height (Ho’) shall be estimated by using the following formula.
' 0 = rd HKKH 0 , 1 = TT 0 3 Where,
' H 0 : Equivalent deepwater wave height (significant waves)
H 0 : Offshore wave height (significant waves)
T0 : Offshore significant wave periods
, KK rd : Diffraction and refraction coefficients
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The refraction and diffraction coefficients were obtained by the numerical computation for offshore wave field as shown in Chapter 7. The obtained equivalent offshore waves
for each wave direction are shown in Table 11.6.3. As the result, H0’=4.1 m (θ: SSE) was adopted at Candidasa area as shown in Table 11.6.4.
Table 11.6.3 Equivalent Offshore Wave Height for Each Wave Direction
H0 (m) 5.5
T0 (s) 16
θ0 SSW S SE Kr 0.25 0.38 0.74
H0'(m) 1.4 2.1 4.1
T1/3 (s) 16 θ S SSE SSE (Source: JICA Study Team)
Table 11.6.4 Design Wave Height for Coastal Structures
H0'(m) 4.1
T1/3 (s) 16 G.L. (m) (at reef edge) 0.0 H.W.L. (m) +2.6 depth (m) 2.6 Bottom slope at offshore 1:20
H0'/L0 0.01
ηmax/H0'0.17 Reef Width (m) 150 H1/3 (m) 1.4 (Source: JICA Study Team)
The design waves for coastal structures such as revetment, groin, and headland are calculated by using the Takayama formula (1977) which is applied to the estimation of wave height on the coral reef as shown below.
H 1 3 ⎡ x ⎤ h2 ' exp⎢−= AB ' ⎥ + α ' H 0 ⎣ H 0 ⎦ H 0 Where, x : Shoreward distance from reef end : Water depths added by water lend rise at a given distance h 2 shoreward from reef end A, α : Constant (A = 0.05, α = 0.33)
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And B and h2 can be developed as follows:
⎛ ⎞ ⎜ H 1 ⎟ ⎝ 3 ⎠ x=0 h2 B = ' −α ' H 0 H 0
1 ⎧ 2 ⎫ 2 2 ⎡⎛ ⎞ ⎤ ⎪ ⎜ H 1 ⎟ ⎪ h2 ⎪⎡()h +η00 ⎤ ⎢⎝ 3 ⎠x=0 ⎥ ⎪ = 989.0 ⎨⎢ ' ⎥ + 21.0 ' ⎬ H ' ⎢ ⎥ 0 ⎪⎣ H0 ⎦ ⎢ H0 ⎥ ⎪ ⎩⎪ ⎣ ⎦ ⎭⎪ Where, ho : Water depth above reef surface : Significant wave height at (x = 0) developed from f breaking (H ) 1/3 x = 0 limit wave height formula by Dr Goda)
η0 : Wave set up at reef edge
Applying the geographical condition of coral reef at Candidasa, the design wave height for costal structures are calculated H1/3=1.4 m.
b) Structure Design for Groins and Headlands
The required weight of armor material for groins and headlands was calculated by using the Hudson formula as shown below.
3 γ r H W = 3 SK rD − cot)1( θ Where,
W : Minimum weight requirement of armor unit
γ r : Unit weight of armor material
Sr : Specific gravity of armor material against seawater θ : Angle of mound slope with horizontal H : Waves height at mound installation point
Kd : Stability coefficient
Calculated result is shown in Table 11.6.5. According to the common design standard for coastal facility design, it is recommended to increase the armor size at head part. Thus, the size of armor material for each part of groins and headlands are set as shown in Table 11.6.6.
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Table 11.6.5 Required Weight for Armor Material
Kd 2.0 (rough, breaking waves)
H1/3 (m) 1.4 Slope 1:2.5
W (ton) 0.47 (Source: JICA Study Team)
Table 11.6.6 Armor Size for Each Part of Groin, Headland
Trunk Part 0.5 ton Head Part 0.7 ton
(Source: JICA Study Team)
c) Structure Design for Revetment
The required weight of armor material for revetment was basically the same as that for groins and headlands. However, there is no need to consider the increase of armor size. Therefore, the size of the armor for revetment was set at 0.5 tons.
d) Idea for Cost Reduction to Armor Layer for Groin and Revetment
Two layer system was applied for the armor layer taking into account of the stability of armor layers. And same kind of armor material was commonly employed. In the Phase-1 Project, light colored limestone was employed in order to consider the landscaping as world tourism area. The limestone was transported from Sumbawa Island to fulfill the required specifications, and the same quarry site was recommended in the Phase-2 Project. However, the price of limestone becomes higher than that of andesite which was employed as under-layer and core layer about 2 times more or less, due to increase of transportation cost. As the experience of the Phase-1 Project, it was lessoned and learned that the armor stone which was placed at the 1st layer was never exposed to the surface, and didn’t disturb to the landscaping even if the other kinds of armor stone was placed to the 1st layer. Considering this experience, it was recommended to employ the andesite for the 1st layer of armor layer for groins, headlands and revetment as cheaper stone in order to reduce the construction cost.
e) Design for Beach Nourishment
i) Selection of Beach Nourishment Materials
i-1) Potential of Sand Borrow Site
The potential area of sand borrow site was investigated as presented in the Section 10.5 of Chapter 10. It is desirable to find the sand borrow area near the project area in order to minimize the construction cost and to avoid social problems. The candidate area was evaluated taking into account the grain size, color, and potential quantity for the nourishment. The fundamental requirements as sand borrow material are as follows:
¾ Grain size shall be coarse and with high content of fine sand (approximately less than 0.2 mm of grain size) is not allowed; ¾ Color of sand shall be maintained light gray or light brown color similar to the existing foreshore sand as much as possible; and ¾ The quantity of sand is expected to fulfill the requirement for the nourishment.
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Even though some difficulty was experienced in looking for the sand borrow site to fulfill the abovementioned requirements, some locations have potential, which is at the eastern part of Candidasa and near Patangbai. According to the bottom sampling on the surface, the grain size (D50) was 0.5 mm or more. Further detailed investigation shall be required to ensure the potential for sand borrow. But it was assumed that the specified sand will be utilized.
i-2) Relation between Grain Size and Foreshore Slope
It is very important on the design of beach nourishment to know the relation between the grain size and foreshore slope on the beach. By taking the sand samples and checking the foreshore slope at the points where the sandy beach still remain, the relation between the grain size and foreshore slope is shown in Figure 11.6.5. The grain size is distributed from D50=0.2 mm to 0.6 mm and foreshore slope is varied from S=1/12 to 1/5. Representative grain size at foreshore slope is 0.35 mm and S=1/10.
Figure 11.6.5 Relation Between Grain Size(D50) and Foreshore Slope (Source: JICA Study Team)
ii) Cross Section Dimension of Beach Nourishment
The following dimensions are mainly required to be determined for the design of beach nourishment (Figure 11.6.6).
¾ Beach width ; ¾ Backshore elevation; and ¾ Foreshore slope.
Beach width
As mentioned before, the difficulty to recover the beach width with sufficient backshore area was expected. Thus, the beach width was considered only at foreshore area. Based on the elevation on the coral reef and foreshore slope, the beach width was set at approximately 40 m to 50 m
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Backshore elevation
The back shore elevation shall be set taking into account the existing condition. Based on the survey result, the elevation at backshore was approximately +4.5 m. Based on this result, the same elevation with +4.5 m was set as the elevation at backshore area.
Foreshore slope
The foreshore slope was determined taking into account the existing condition of the beach and the grain size of borrow material. Referring to Figure 11.6.6, the foreshore slope was set at S=1/10.
Beach Width
Backshore Foreshore Outer Beach BackshoreWidth
Backshore Elevation
Forshore Slope
Outershore Slope
Remark Offshore : Offhore Area from Breaking Point (B.P.) Outershore : Area from B.P. to Shoreline at Low Tide Foreshore : Area from Shoreline at Low Tide to Wave Run-up Point Backshore : Land-side Area from Shoreshore Figure 11.6.6 Determination of Cross Section Dimension (Source: Coastal Engineering Manual (CEM), 2006)
(4) Typical Cross Section Drawing
The typical cross section drawing for beach nourishment, revetment, and groin are shown in Figures 11.6.7 to 11.6.9.
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Figure 11.6.7 Typical Cross Section for Beach Nourishment (Source: JICA Study Team)
Figure 11.6.8 Typical Cross Section for Revetment (Source: JICA Study Team)
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Figure 11.6.9 Typical Cross Section for Groin (Source: JICA Study Team)
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11.7 Basic Layout and Design at North Kuta〜Legian〜Seminyak
(1) Design Principle
a) Beach Nourishment
Considering the technical point of view on expected direction of littoral drift and the advantage of beach in this area, the beach nourishment with no coastal structure is planned.
b) Walkway
The wall, road, and walkway exist at most part of the target area. Thus, the walkway is only set at some parts of the northern area.
(2) Layout Plan
The target area for beach nourishment is from the north boundary at the end of beach nourishment at the Phase-1 Project until Kudeta at Seminyak with 2.9 km alongshore taking into account the condition of waves and littoral sand drift. The basic layout image for beach conservation measures at the southwest coast is shown in Figure 11.7.1.
Figure 11.7.1 Layout Plan for Beach Nourishment at Southwest Coast (North Kuta - Legian - Seminyak)) (Source: JICA Study Team)
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(3) Design for Beach Nourishment
a) Selection of Beach Nourishment Materials
i) Potential of Sand Borrow Site
The potential area of sand borrow site was investigated near the project area as presented in Section 10.5 of Chapter 10. The candidate area was evaluated mainly taking into account the grain size and the following results were obtained;
¾ The contents of fine sand (D50 < 0.2 mm) were significant at the north offshore area; ¾ On the other hand, the suitable fine sand and grain size can be found at the outer reef area of Kuta; ¾ Further detailed investigation will be required to ensure its potential for sand borrows. However, it was assumed to borrow the sand at the outer reef area of Kuta.
ii) Relation between Grain Size and Foreshore Slope
The relation between the grain size and foreshore slope at this area is shown in Figure 11.7.2. The grain size is distributed from D50 =0.2 mm to 0.6 mm and the foreshore slope is varied from S=1/15 to 1/7. The representative grain size at foreshore slope is 0.35 mm and S=1/10.
Figure 11.7.2 Relation between Grain Size (D50) and Foreshore Slope (Source: JICA Study Team)
b) Cross Section Dimension of Beach Nourishment
Beach width
The target width for beach nourishment is set at 20 m increase at the elevation of HWL taking into account the beach retreat for the past 30 years.
Backshore elevation
The backshore elevation shall be set taking into account the existing condition. Based on the survey result, the elevation at backshore was approximately +5.5 m. Based on this result, the same elevation with +5.5 m was set as the elevation at backshore area.
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Foreshore slope
The foreshore slope was determined taking into account the existing condition of the beach and the grain size of borrow material. Referring to Figure 11.7.2, the foreshore slope was set at S=1/10. This slope is the same as that for beach nourishment at north Kuta carried out in the Phase-1 Project.
(4) Typical Cross Section Drawing
The typical cross section drawing for beach nourishment is shown in Figure 11.7.3.
Figure 11.7.3 Typical Cross Section for Beach Nourishment (Source: JICA Study Team)
11.8 Basic Layout and Design for Improvement at Kuta
(1) Design Principle
The target for the improvement at Kuta area (the Phase-1 Project area) is as follows:
¾ To reduce sand outflow and to maintain the sandy beach from the south end of sand stopper to BWN2; and ¾ To avoid further decrease of elevation at the toe part of revetment surrounding BWN3 area.
One of the main reasons that causes continuous beach retreat between BWN1 and BWN2 was due to insufficient function to interrupt the northward littoral drift. To improve this, it is recommended to consider the original design concept, which is to replace the existing offshore breakwater BWN1 and BWN2 to headland type structure. For this, it is basically recommended to add the groin part for BWN1 and BWN2.
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To improve the present condition surrounding the BWN3 area, the construction of a new L-shape groin at the south side of BWN3 was recommended as shown in Figure 11.8.1. In addition, the sand filling was also recommended at shadow area for this L-shape groin to recover the present urgent condition. However, it is recommended not to overfill sand at the shadow area of L-shape groin in order to avoid sand loss.
(2) Layout and Typical Cross Section Drawing
The layout and typical cross section drawing for the recommended improvement at Kuta are shown in Figures 11.8.1 to 11.8.3.
Figure 11.8.1 Layout Drawing for the Improvement at Kuta (Source: JICA Study Team)
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B - B Figure 11.8.2 Typical Cross Section for the Improvement of Existing Offshore Breakwater (BWN2) (Source: JICA Study Team)
B - B Figure 11.8.3 Typical Cross Section for New L-shape Groin (Source: JICA Study Team)
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11.9 Principle for Planning of Beach Control at Other East Coast Area
(1) Beach Erosion
Among the east coast beaches, Candidasa was selected as candidate site of Phase-2 Project. This selection was taking into account both natural and economical point of view. However, beach erosion has seriously occurred in this area from Unda River mouth to Ayung River mouth with 25 km alongshore as shown in Figure 11.9.1.
As explained in the section 9.3 of Chapters 9, the serious beach erosion with more than 200 m retreat of the beach at a maximum at this area was caused by significant decrease of sand inflow from the river, which was mainly caused by the improvement of water control for the river and large scale sand mining at the river mouth.
Even though this area is out of the selected area for Phase-2 Project, the principle for the planning of beach conservation measures are suggested based on the examined beach condition.
Figure 11.9.1 Shoreline Change at East Coast between 1981 and 2011 (Source: JICA Study Team)
(2) Principle for Planning of Beach Conservation
If beach erosion persists and the shoreline continues to retreat, the efficiency of artificial coastal protection facilities such as seawall, revetment, etc., are limited. Even if protection facilities were constructed, it is impossible to reduce beach erosion without solving first the fundamental cause of erosion, which is to prevent the decrease of sand inflow from the river.
On the other hand, if beach erosion continues almost at an even condition and the retreat of the beach is observed not significant, the artificial protection facilities will be effective to
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ensure protection of the land. However, in case the protected beach is utilized as local tourism area or as religious ceremony area, it was not recommended to protect the beach using artificial hard structures. Beach nourishment is the suitable measure taking into account the protection and utilization of the beach.
At Rubih Beach which belongs to this area, the rubble-type revetment was constructed to protect the land. It seems temporarily effective to protect the land side as far as constructed revetment can be secured against beach erosion. However, the retreat of the beach still continues and if it exceeds the structural limit of the revetment, the revetment will be damaged and finally collapsed. There are a lot of cases in Indonesia where the coastal protection facilities failed due to this reason.
The change of shoreline for 30 years between 1982 and 2011 was compared in this study, and significant beach retreat was observed. On the other hand, large-scale sand mining from the river has been prohibited since 2007. In the interview survey at this area, some of the residents informed that the rate of beach retreat slowed down in the latest several years. This change of rate might be due to the prohibition of sand mining. Although the analysis of shoreline change in this study is only for two periods (between 1982 and 2011), it is necessary to clarify the recent change of shoreline in order to ensure the condition of beach retreat.
The principle for planning the beach conservation under progress of beach erosion is as follows:
1) Basically, there was no alternative to reduce beach erosion and beach retreat without solving the potential causes, which is to prevent the decrease of sand inflow from the river. To achieve this, it is strongly required to control the transfer of sand between river and coast, without causing further decrease on the inflow of sand;
2) If it is still difficult to conduct sand control at the river, the following two possibilities should be considered; (i) one is to keep the buffer zone for further beach retreat based on the prediction on future beach retreat and (ii) no construction of land facilities should be planned in the future development plan of this area;
3) The other is to reduce the external forces (waves) which directly induced the drift of sand. For this, it is necessary to construct some hard structures at offshore side such as offshore breakwaters. However, the construction of offshore facility might cause further problems and change the movement of littoral sand. Also, the cost to construct the coastal facility at offshore area is high. Thus, the costs and benefits shall be compared carefully.
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