This study was carried out to assess land subsidence due to excessive groundwater abstraction in the northern region of Semarang City by integrating the application of both numerical models and geodetic measurements, particularly those based on the synthetic aperture radar interferometry (InSAR) technique. Since 1695, alluvial deposits caused by sedimentations have accumulated in the northern part of Semarang City, in turn resulting in changes in the coastline and land use up to the present. Commencing in 1900, excessive groundwater withdrawal from deep wells in the northern section of Semarang City has exacerbated natural compaction and aggravated the problem of land subsidence. In the current study, a groundwater model equivalent to the hydrogeological system in this area was developed using MODFLOW to simulate the hydromechanical coupling of groundwater flow and land subsidence. The numerical computation was performed starting with the steady-state flow model from the period of 1970 to 1990, followed by the model of transient flow and land subsidence from the period of 1990 to 2010. Our models were calibrated with deformation data from field measurements collected from various sources (e.g., leveling, GPS, and InSAR) for simulation of land subsidence, as well as with the hydraulic heads from observation wells for simulation of groundwater flow. Comparison of the results of our numerical calculations with recorded observations led to low RMSEs, yet high R² values, mathematically indicating that the simulation outcomes are in good agreement with monitoring data. The findings in the present study also revealed that land subsidence arising from groundwater pumping poses a serious threat to the northern part of Semarang City. Two groundwater management measures are proposed and the future development of land subsidence is accordingly projected until 2050. Our study shows quantitatively that the greatest land subsidence occurs in Genuk District, with a magnitude of 36.8 mm/year. However, if the suggested groundwater management can be implemented, the rate and affected area of land subsidence can be reduced by up to 59% and 76%, respectively.

In many coastal and marine environments, the blue economy is being developed to enhance community welfare. The possibility of a tsunami disaster, which could destroy the infrastructure supporting the blue economy, is one of the development challenges. The Sindangkerta beach and the Pamayang harbor in Tasikmalaya, West Java, Indonesia, are the two locations where this study discusses the possibility of tsunami debris occurring. Infrastructure that supports life may be threatened by tsunami debris. To ascertain the origin of the debris, a field survey was used in the study. When a tsunami is approaching, beachfront houses and fishing boats are easily swept towards land. The number and mass of houses and boats are counted. In order to calculate the magnitude of the tsunami impact force, the material stiffness of the two types of objects was also examined. The potential tsunami debris's runup and rundown directions are also estimated. The study's findings show that houses that serve as restaurants and shops for tourists predominate at the Sindangkerta tourist destination while fishing boats serve as the primary source of debris at the Pamayang fishing port. In addition, the debris dispersal area due to run-up and run-down is also depicted.