Alternative study of coastal protection buildings for mitigating rob floods at Sirenja Beach

Due to the earthquake that occurred on September 28, 2018, in Donggala Regency, especially in Sirenja District, Tompe Village, the phenomenon of land subsidence may be the occurrence of land subsidence to cause damage to existing coastal protection infrastructure and the entry of sea tides into land (rob floods), on the other hand, the possible causes of rob floods on Sirenja Beach are influenced by abrasion, namely erosion of coastal areas that caused by waves and currents to damage the existing infrastructure of coastal protection. This study aims to determine the causes of rob flooding and the handling of Sirenja Beach as a result of the rob flooding that occurs. This research was conducted by analyzing topographic and bathymetric changes with the overlay cut and fill method before and after the disaster, then a SWOT analysis was carried out to select the right building to deal with rob flood that occurred. The results of topographic and bathymetric change analysis showed before the earthquake and tsunami disaster, the sea depth value reached ± -97, 943 m, and the land height was at ± 13 m after the earthquake and tsunami sea depth conditions reached ± - 59.0555 m with land height at ± + 8.48 masl then there were avalanches and piles with a total avalanche area of 0.100 km2 with a total volume of 931,531 m3 and a total stockpile area amounting to 0.308 km2 with a total volume of 3,017,743 m3. Then from the results of the SWOT analysis of the selection of the right building to deal with rob flooding on Sirenja Beach from each alternative building to overcome problems on Sirenja Beach in conditions with the treatment of Seawall buildings as polders getting a value of 47, treatment with Revetmen as polders getting a value of 31, with breakwater treatment obtained a value of -32 and a condition score value with treatment with Groin of -34. With the changes in topography and bathymetry between before and after the disaster, it showed significant results that on Sirenja Beach, there was an average land subsidence of ± 5 m, and based on cut and fill analysis, there was a change in the seabed where the volume of stockpiles was more dominant. So it can be concluded that the occurrence of rob flooding on Sirenja Beach is caused by land subsidence, and the right building in protecting Sirenja Beach from rob flooding is a Seawall with a polder system.