Land subsidence in coastal city of Semarang (Indonesia): characteristics, impacts and causes

Semarang is the capital of Central Java province, located in the northern coast of Java island, Indonesia. Land subsidence in Semarang has been widely reported and its impacts can be seen already in daily life. Based on the estimation from Levelling, Interferometric Synthetic Aperture Radar (InSAR), Microgravity and Global Positioning System (GPS) survey methods, land subsidence with rates of up to about 19 cm/year were observed during the period of 1999 up to 2011. Results derived from GPS since 2008 up to 2011 show that land subsidence in Semarang has spatial and temporal variations, with spatial average rates of about 6 to 7 cm/year and maximum rates that can go up to 14-19 cm/year at certain locations. The northern region of Semarang along the coast exhibits higher rates of subsidence compared to its southern region, and this subsidence is believed to be caused by the combination of natural consolidation of young alluvium soil, groundwater extraction and load of buildings and constructions. The impact of land subsidence in Semarang can be seen in several forms, mainly the wider expansion of (coastal) flooding areas, cracking and damage of buildings and infrastructure, and increased inland sea water intrusion.