Physical Vulnerability of Reef Islands in Jakarta Bay, Indonesia

Abstract

The purpose of this study is to assess the vulnerability of reef islands in Jakarta Bay through investigation of the morphological and sedimentological characteristics of reef islands in Jakarta Bay. Reef islands are considered highly vulnerable to erosion and inundation as they are low lying and constructed of unconsolidated sediment. Reef islands are also dynamic systems frequently exhibiting morphological changes and are sensitive to changes in environmental conditions. Coral reef ecosystems in Jakarta Bay are influenced by anthropogenic impacts, such as pollution, over-fishing, and sand and coral mining, which can subsequently affect island stability. The morphology and composition of eight reef islands was assessed through detailed topographic surveys and surficial sediment sampling. Geospatial analysis is conducted using aerial photographs collected between 1927 and 2013 and GPS surveys undertaken during January-February 2014. Peak elevation of each island ranges from 2.17 m and 3.43 m above MSL. Textural analysis indicates that the sediments are sand-size grains with average mean grain size from 0.68 phi to 1.26 phi (medium sand to coarse sand). Corals are the dominant sediment constituent over all islands studied, accounting for 70% to 80% of sediment particles. Over an 87 year record (available for Bidadari, Ayer Besar, and Kelor) for shoreline analysis, Kelor shows the most severe erosion. Over a 51 year record (available for Bokor, Rambut and Untung Jawa), Bokor is shown to be relatively stable. Geospatial analysis over the 20 year record indicates severe erosion for Nyamuk Kecil. Shoreline change is considered within in the context of each island’s physical characteristics in order to explore potential drivers of coastal change. The results of sediment analysis (grain size, sorting and component analysis) do not show a clear relationship to island shoreline change rates, so sediment characteristics cannot be ascertained as a driver of shoreline change in this study. As it has been previously suggested that anthropogenic factors (such as pollution and hard structures) can affect reef island stability, anthropogenic modification remains a likely driver of shoreline change in Jakarta Bay. The vulnerability of the islands studied to sea level rise driven inundation is examined in the context of IPCC sea level projections for late 21st century according to RCP2.6, RCP4.5 and RCP8.5 emission scenarios. It is shown that Kelor, Untung Jawa, Bidadari, Ayer Besar, Bokor, Nyamuk Kecil, and Damar Kecil are more vulnerable (to varying degrees) than Rambut. Of these, Kelor is the most physically vulnerable to inundation. Key Words: Reef islands, Sediments, Shoreline change, Jakarta Bay, Kepulauan Seribu