Chapter 10 Field Survey and Sediment Analysis for the Candidate Site
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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) 10-3 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version) (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). 10-4 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version) 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) 10-5 The Project for the Preparatory Survey on Bali Beach Conservation Project-Phase II in The Republic Of Indonesia Final Report (Simple Version) 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.