TEGAL BESAR BEACH REHABILITATION WITH SCALLOPED CONCRETE BLOCK REVETMENT

Klungkung has a coastline of 113 km out of a total of 5780.06 km 2 the coastline in the province of Bali, but about 25 km is eroded. One of the critical areas occurs in Tegal Besar beach which caused erosion of 0.08875 to 3.0915 m/yr. Revetment with scalloped concrete blocks is expected to protect land from the wave attack. Concrete blocks have uniform size and shape, showing the aesthetic aspect so as not damage the beauty of the beach. Analysis of wind, tidal, bathymetry and soil data are carried out to obtain the structure and stability of the revetment. The results of this design can be used to rehabilitate the condition of the Tegal Besar beach. From the analysis, the design wave height with a return period of 25 years (H25) = 0.891 m, while the height with the breaking wave (Hb) = 1.003 m and the breaking wave depth (db) = 1.068 m and the design water level elevation (DWL) = 2.061 m calculated from MSL. The revetment structure has a height of 4.00 m, a peak width of 1.50 m, the weight of the first layer of protection is 300 kg and the second is 30 kg with a thickness of 1.00 m. Toe protection are 1.25 m high, 3.00 m wide and weigh 150 kg. The results showed that the revetment was stable against overturning, shearing, and the bearing capacity of the soil was declared safe with the results of overturning stability 22,075 > 2, slidding stability 2.249 > 1.5 and the bearing capacity of the soil 57,993 > 3 so that the revetment can be eligible to be applied at the site.


INTRODUCTION
Bali is one of the provinces in Indonesia which is famous for its scenery and the coastal area with a coastline of 633.35 km. However, currently, more than 215.82 km of coastline in Bali has been identified as experiencing erosion. Coastal erosion is a problem that threatens the existence of the coast, caused by a combination of factors such as sea level changes, storms, and human intervention (Kementrian Pekerjaan Umum Dan Perumahan rakyat, 2020). Causes of coastal damage due to human intervention, such as the reduced supply of sediment to the beach due to the construction of dams, mining of coral and sea sand and the construction of coastal protective structures (Zamdial et al., 2017). Beaches in Bali that experience moderate to critical erosion levels are in the southern part of Bali, including in Pengambengan (Jembrana Regency), Candidasa (Karangasem Regency), Padang Galak (Denpasar City), Lebih (Gianyar), Kuta (Badung) , and Tegal Besar (Klungkung Regency). Coastal erosion in Klungkung and Gianyar regencies began to occur significantly from 2007-2017 (Hariyanto et al., 2018), (Aryastana et al., 2016), this also occurred in the coastal area of West Bali, marked by a decrease in coastline and damage to coastal buildings .
Currently, the impact of erosion on the Tegal Besar beach is clearly visible from the changes in the coastline that occur. According to Nugraha et al., (2017), the rate of change of the coastline of Tegal Besar due to erosion ranges from 0.8875 to 3.09155 m/yr. To overcome the damage to the coastline due to coastal erosion, the government and the community have build a revetment. Revetment that has been built, is currently broken and is no longer functional as shown in Figure 1. Therefore, it is necessary to carry out alternative treatments to overcome coastal erosion that occurs in Tegal Besar Beach  Figure 2 shows location of the study. An alternative treatment for coastal erosion is urgently needed at Tegal Besar Beach. Therefore, it is necessary to re-plan the revetment using different materials. Materials used to redesign the revetment at Tegal Besar Beach is a scalloped concrete block. Scalloped concrete block has a lighter armor weight than precast concrete in general but has higher stability due to scalloped concrete blocks has a scalloped structure as a lock between the units so that doesn't happen shift in position between the concrete block units that are arranged. the results of planning with almost similar concrete blocks produce a fairly high stability of the structure (Sanjaya, 2020), (Putra et al., 2017). Attractive finish as the installed concrete blocks have uniform size and shape, showing the aesthetic aspect so as not to damage the beauty of the beach. Revetment with scalloped concrete blocks has been applied in several areas such as Happy Beach in Buleleng Bali and Muko-Muko Beach in Bengkulu, with result looks good provides an attractive aesthetic atmosphere so it is suitable for the beach which is a tourist attraction (Sulaiman, 2018). In this research, revetment planned with scalloped concrete blocks as an alternative in overcoming coastal problems what happened at Tegal Besar Beach so that it was a setback shoreline is not happening anymore.

METHODS
The planning stage begins with reviewing and observing the location, identifying problems, collecting data and analyzing data. In planning the revetment, it is necessary to collect data which includes 1) topographic maps and bathymetry to determine the depth of the seabed at that location, 2) wind, 3) tides, and 5) soil data.
The stages of data analysis used in this study are, 1) Wind Data Analysis to obtain wind speed correction (UA), 2) Fetch Length Analysis, and 3) Wave Analysis Calculation is carried out by calculating return wave calculations using the Weibull and Gumbel method, Calculation of Breaking Waves, Calculation of Design Waves, Calculation of Design Water Level, and Calculation of Revetment Dimensions.

Wind Data Analysis and Fetch Calculation
Based on wind data from the Meteorology, Climatology and Geophysics Agency in Bali, wind data is used for 10 years from 2010 to 2019. Wind rose is made to get the most dominant wind direction and wind speed as shown in Figure 3. After knowing the dominant wind direction then proceed with the calculation of the corrected wind speed. The wind data used is the dominant wind data each year which then follows converted to units of m/s, where 1 knot is 0.514 m/s, Wind data were analyzed to obtain wind speed correction (U A ) using Eq. (1) and then Wind stress correction (U A ) was used in wave forecasting.
The fetch is limited in the form of land surrounding the sea as measured by the potential distance of wave formation between the observation site and the surrounding islands, Tegal Besar Beach has a fetch to the east. Calculation of the effective fetch length (F eff ) using Eq (2). The calculation of the effective fetch length (F eff ) is then used in calculation of wave forecasting and calculation of water level elevation plan due to wind (wind set-up). (1) Feff = ∑ . / ∑ (2)

Wave Calculation
Determining factors in the safety of coastal buildings from waves based on wave history, power proneness waves, and the possibility of tidal waves in a certain period (Mangare, 2016), therefore, need to be forecasting waves in the deep sea to find out the wave height significant (HS) and significant wave period (TS) can be calculated based on corrected wind speed (UA) and effective fetch length using Eq. (3) and (4) as follows: The propagation of waves from the deep water to the coast changes shape due to changes in sea depth, then the calculation of breaking waves can use the following equation.

Run-Up
The wave run-up value will be used to determine the peak elevation of the revetment, using the following equation: Ir = (7)

Sea Level Rise
The increase in water level due to global warming can be calculated using Figure 4 below, there are estimates of sea level rise from 1990 to 2100.

Protected Layer Stones
The thickness of the main protective stone and the second protective stone as well as the core layer is given by the following formula: While the amount of the main protected stone layer is given by the following equation:

Toe Protection
The weight of the toe protection on the revetment is given by the following equation:

RESULT AND DISCUSSION Return Period Analyze
Based on the calculation results, the maximum wave at Tegal Besar Beach with a return period of 25 years, the wave height (H 25 ) = 0.996 m with the wave period (T 25 ) = 4.624 s is obtained. Therefore, the results of this calculation will be used in the calculation of the design wave.

Calculation of Design Wave Height
Based on the topographic map and bathymetry of Tegal Besar Beach, at a depth (d) = 2 m, the design wave height is calculated using the following equation (1)

Wave Breaking Calculation
The waves propagate from the deep sea to the coast, changing shape due to the influence of changes in sea depth. Calculation of breaking waves can be seen in the calculations below: The equivalent sea wave height at a depth of 2 m is calculated using the equation

Revetment Structure Design
The main structure of the revetment using scalloped concrete blocks in this plan is designed to have a slope of 1:2 so that the run-up height is obtained as follows: The run-up revetment value of the scalloped concrete armor block material with KD = 4 has the same value as the crushed stone material, then runup wave value is obtained as follows:   The results of the calculation of the stability of the bearing capacity of the soil at the building site are declared safe with a stability value of = 57.993, which is greater than the number of safety factors, which is 3 (Hardiyatmo, 2018;Indriasari, 2018), because it is more influenced by soil characteristics, generally the value of soil bearing capacity produced far exceeds that of safety factor (Indriasari, 2018).

CONCLUSIONS
Based on the results of the analysis, the revetment with scalloped concrete blocks can be applied to rehabilitate the Tegal Besar beach which was previously protected by revetment but has been damaged. The revetment structure has a height of 4.00 m, a peak width of 1.50 m, the weight of the first layer of protection is 300 kg and the second is 30 kg with a thickness of 1.00 m. Toe protection are 1.25 m high, 3.00 m wide and weigh 150 kg. The results showed that the revetment was stable against overturning, shearing, and the bearing capacity of the soil was declared safe with the results of overturning stability 22,075 > 2, shear stability 2.249 > 1.5 and the bearing capacity of the soil 57,993 > 3 so that the revetment can be eligible to be applied at the site.