What is Land Subsidence?
As the capital city of Indonesia urban development of Jakarta has grown very rapidly in the sectors of industry, transportation, real estate, and many others, which has caused several negative environmental impacts. In turns Jakarta become prone toward natural hazards of flooding and also land subsidence. Based on geodetic measurement methods of GPS Surveys, InSAR, every year land subsidence in Jakarta about rate of 3 to 10 cm, and the land subsidence observed in coastal, west and northeastern of Jakarta caused by deep groundwater extraction which also raising the risk of flood in the areas. These coastal flooding usually occurs in the areas with relatively large subsidence rates. In this situation land subsidence will have direct and indirect affects with flooding in Jakarta, both in coastal and inland areas.
Why Land Subsidence can occur?
Land subsidence has several reasons such as groundwater extraction, building/natural compaction and tectonic, the Groundwater extraction the middle and lower aquifers of Jakarta basin by individuals and by industries to be main factor the causes land subsidence. Excessive groundwater extraction will lead to the deepening of groundwater level, which in turn will cause land subsidence and seawater intrusion and hydrostatic pressure under land surface will reduce as much as ground water extracted then the aquifer layer compression occurs.
Building loads on land surface also one of the reasons of layer compression which experiencing soil article deformation, particle relocation and water or air expelling out from the pore that create volume changes of alluvial soil, through all these schemes processes trig land subsidence. Other variable that cause land subsidence is Tectonic movements which results in damage to the soil structure, instability of the land parcels due to certain processes.
In Jakarta the population density has grown due to urban development and demands of a growing population, its surrounding areas has expanded widely such as extensive conversion of prime agricultural areas into residential, industry, high rise buildings, massive constructions which lead to the high demand of water supply. On other hand water supply from PAM Jaya is not sufficient to whole area of Jakarta. Hence people do not have any option others than groundwater extraction.
Explanation of animation
Massive and excessive groundwater abstraction will lead to void of water in sand layer. In process to fill the void in sand layer, water will flow from clay located on above and below. Through that process the result is clay layer will be consolidated. Once the deep clay layer is recharged by water, clay cannot be possible return to the initial condition.
Where are the land subsidence areas?
Here are some locations in Jakarta where areas with both industrial and residential activities, as well as in few areas with high-rise building, and the combination of ground water extraction and sediments compaction due to building loads could explain the main rates of subsidence, in this below figure shows the subsidence rate about 20mm/10 years widely spread throughout the DKI and surrounding areas,
Land subsidence in 12 years (2007.01.31 – 2018.12.13)
How To Deal With The Land Subsidence?
Countermeasure of land subsidence conducted by 4 following activities :
Monitoring
Land subsidence necessarily being monitored to know the measurement of the changes that happen to the land and groundwater level, these are the activities of monitoring land subsidence for instance construct monitoring well and measure by GNSS survey.
Mitigation
The way to prevent and stop land subsidence is to fulfill water supply and stop deep groundwater extraction. Hence need activities such as enlarge coverage of water supply, construct rain water harvesting and establish regulation on Zero deep well at area where water supply fully fulfilled the area.
Adaptation
Once land subsidence happened, land level can not be possible return to the initial level. Hence impact of land subsidence such as flood/ inundation necessarily to be reduced. In order to reduce the impact it is required to conduct activities such as create inundation map and construct coastal dike.
Social awareness
Awareness of people on land subsidence phenomena is necessarily risen up . People need to be aware to reduce the utilization of groundwater and shift to surface water supply. People need to know better information about land subsidence by socialization and the exact land level comparing to sea level by watch sea-level sign board.