Semarang land subsidence rate, among world’s fastest

by: Hartatik

Semarang – The tidal flood was still visible in the Tanjung Mas Port area on the third day after the sea water barrier burst on Monday (23/5). The condition of the tidal flood as high over one meter deep today shows how negligent management of future disasters will be.

“Many factors contributed to the occurrence of tidal flooding, but focusing on broken embankments as the cause of flooding can make us careless in handling future disasters,” said Nila Ardhianie, Director of the Amrta Institute for water literacy, in a written release, Thursday (26/5).

She assessed that there are five factors related to the magnitude of the impact of the tidal flood that surrounded the north coast of Central Java. First, the early warning system is not optimal. Second, the poor quality of the construction and maintenance of the embankment. Third, land subsidence. Fourth, matters related to sea water, both sea level, wave speed and others. The fifth factor is the competence and authority of the relevant agencies.

Ardhianie, a student in the Environmental Science Doctoral Program at Diponegoro University, regrets the ineffectiveness of the early warning system routinely issued by the Meteorology, Climatology and Geophysics Agency (BMKG). “Has the socialization carried out by BMKG been effective? Have all potentially affected parties received information,” she said.

Furthermore, Ardhianie questioned whether parties that received the information ignored the warning or if something else happened. Videos circulating shortly after the levee burst depicting workers running from their workplaces and many motorbikes completely submerged in seawater showed the warning was not working optimally.

“In some countries, a properly adhered to early warning system brings great benefits. When potentially affected residents are warned, they work from home to avoid bigger losses,” she said.

 

Furthermore, the embankment factors are related to the quality of the materials and methods used during construction. In other words, when the embankment was built, it was designed to be able to withstand sea waves of what height, speed and other technical matters. This information is very important to be included in every decision making. This is also related to the maintenance of the embankment which must be in accordance with the initial design and construction and the environmental changes that occur.

“Is the appropriate periodic maintenance carried out or not? Usually major damage must have an indication in the form of minor damage first,” Ardhianie questioned.

The factor of land subsidence is a phenomenon that occurs in various coastal cities in the world. Ardhianie also cites one of the results of a recent study (2022) from Pei-Chin Wu, Meng (Matt) Wei, and Steven D’Hondt with the title “Subsidence in coastal cities throughout the world observed by InSAR” which concluded that Semarang is a city with the second fastest land subsidence among the 99 coastal cities studied.

The study recorded highest subsidence rates in Tianjin in China, Semarang and Jakarta with a maximum displacement rate. The data used in this research are PS Interferometric Synthetic Aperture Radar method and Sentinel-1.

“Soil subsidence is an important factor when floods occur, both tidal floods and due to rain water,” she added.

As for the land that has fallen, it increases the capacity of water on land, thus making the puddles deeper and difficult to drain into the sea. At high tide conditions become more severe, because the water will stay longer on land and difficult to flow into the sea.

Furthermore, Ardhianie added, land subsidence is when the land surface sinks as a response to geological events or causes related to human activities. For Semarang, land subsidence is related to excessive groundwater extraction, loading of buildings and structures, and compaction/consolidation of young alluvial sediments, especially in the lower Semarang area. Tectonic events under Semarang can also cause land subsidence.

Banner photo: Floods surrounded Semarang, Central Java, (Saturday, February 6, 2021). The flood was caused by heavy rains that flushed the capital of Central Java for 12 hours since Friday night. (BanGhol/shutterstock.com)

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