Nationwide Subsidence in Bahrain Island: Drivers and Implications for Relative Sea-Level Rise

Abstract

The IPCC-AR6 report suggests that the sea-level in the Persian Gulf is expected to rise by 2.1-4.9 mm/year by 2100, which is lower than the global projections of 4-14 mm/year. However, a central nationwide ground subsidence in Bahrain's low-lying island can aggravate these figures. The island is only 14 km wide, yet we observe a 7 × 12-km ground subsidence with vertical rates up to 20 mm/year for 2016-2021. We forecast a subsidence of up to 1.5 m by 2100 by extrapolating the average rate. To characterize the subsidence, we use the following methods: i) SBAS-InSAR with 94 and 131 C-band Sentinel-1A orbital radar acquisitions in ascending and descending directions for 2016-2021, respectively, and ii) Stacking-InSAR with eight L-band ALOS-1-PALSAR acquisitions for 2007-2010 and six ALOS-2-PALSAR-ScanSAR acquisitions for 2016-2021. We suggest three causes associated with this subsidence: First, exploitation of the Awali oil field; second, local aquifer depletion; and third, subsurface dissolution of anhydrites and chalky limestones. To assess the potential impact of subsidence on the shoreline, we measure the coastline evolution at three undisturbed beaches on the West coast from 1985 to 2021, excluding areas that underwent land reclamation. Using subpixel shoreline detection analysis from 308 Sentinel-2 and Landsat (L5, L7, and L8) acquisitions, we observe that the selected shores remain stable. However, observations reveal a shoreline retreat of up to 5 m/year on the southwestern coast for 2003-2014, surpassing the estimated rate of 0.08 m/year derived from tide gauge data. This nationwide subsidence should be considered when forecasting coastal infrastructure planning in Bahrain. We recommend performing a similar analysis in other low-lying Gulf islands, where oil exploitation occurs.