Land subsidence occur in some large cities in Java Island, Indonesia. Among those cities, Pekalongan suffer relatively large and consistent rate of land subsidence compared to other cities. Impact of this long-term land subsidence manifested as worsen of flood, infrastructure damage, and decline of ground water quality. To see the continuation of land subsidence in Pekalongan, we use Sentinel-1 data from 2016 to 2023 from both ascending and descending direction to spatio-temporaly mapped both magnitude and rate of the land subsidence. Persistent Scatterer InSAR (PS-InSAR) method is used to process the SAR data, also use DEMNAS and Generic Atmospheric Correction Online Service (GACOS) to reduce topographic and atmospheric error consecutively. We use CPKL CORS station, which managed by Badan Informasi Geospasial (BIG) for control point for the final subsidence rate calculation. Our result show that the area of subsidence does not change significantly for the last 7 years, with the maximum rate reach 16 cm/year of subsidence occur in the western side of Pekalongan City. However, the land subsidence also occurs in the coastal part with lower rate of 5 cm/year. The subsidence rate in most of the places are constant, with exception during 2020-2021 period.

Pekalongan City is located on the northern coast of Java Island, where climate crises, floods, and tidal waves pose significant threats exacerbated by rising sea levels and land subsidence. This study aims to mitigate of flood future losses and emphasizes its potential as input for Indonesian regulators to enhance current flood hazard classifications. This study conducts a comprehensive flood risk analysis, emphasizing a detailed review of flood hazard classification quantitatively with flood hazard mapping. The results showed flood hazard mapping, comparing NDMA, MPW, and proposed classifications, indicates a tendency for MPW to overestimate hazards, particularly categorizing depths of 0.5 m and above as “High”. The flood depth according to MPW with land subsidence produces a flood inundation area of 14,145 Ha. This shows that land subsidence recorded in risk mapping can increase the overall hazard level. This study proposed flood depth and velocity classification, balancing NDMA’s underestimation and MPW’s overestimation, offering a more comprehensive approach.

Cisokan River is one of the main inflow rivers of Cirata Reservoir aside from Citarum River. Cisokan River is the second largest contributor to Cirata inflow after Citarum River. In May of 2023 Cisokan River experienced instances of flash flood disasters that submerged houses in Cianjur Regency, one of the affected villages is Ciranjang Village. Several news outlets reported that this flood also left debris and mud in the affected areas, which indicated that the flood was debris flow. This study conducts the risk analysis resulting from debris and non-debris flow with the changes in river cross-section due to sedimentation. In this study, modeling was carried out employing a Digital Elevation Model (DEM) map and utilizing the Hec-RAS software. One focus of the modeling was on simulating the Newtonian (non-debris) and non-Newtonian (debris), comparing the results and analyzing the risk from both. The result showed that the non-Newtonian flow’s flood inundated area is larger than the Newtonian flow’s, which means the high flood risk area of debris flow is 12.5% larger than the non-debris flow’s high flood risk area.

Due to land subsidence at Pekalongan, some areas at Pekalongan coast submerged at a rate up to 10 cm/year. This study was conducted to examine the effect of land subsidence on the hydrodynamic conditions in Pekalongan through a numerical model simulation using Delft3D. There are 2 scenarios for the numerical model, scenario 1 in 2016 and scenario 2 in 2021 after the occurrence of land subsidence at a rate of 10 cm/year. Model calibration was carried out on water level elevation and current velocity by comparing the simulation data and field data based on the variation of the Chezy number. The calibrated Chezy number parameter was 60 m^0.5/s. The sensitivity test is carried out by changing the input parameters such as water level elevation, river discharge, and wave height. The sensitivity test results shows that the most sensitive parameter is wave height. The comparison of the simulation results of the two scenarios shows that the current velocity decreased by about 0.05 m/s to 0.1 m/s and the wave height increased by about 0.05 meters to 0.15 meters after land subsidence occurred. Land subsidence causes the waters in the study area to become deeper, and the results of the study show that current velocity tends to decrease and wave height tends to increase because of the land subsidence.

This study examines the condition of sediment transport on the Pekalongan coast by a numerical model computation using the Delft3D Flow module. The research was started by collecting and analysing field data both primary and secondary. Thus, the hydrodynamic and sediment transport model are then calculated in two scenarios which are representing the conditions before (2016) and after (2021) land subsidence by assuming a land subsidence of 10 cm/year. The numerical model is then calibrated and verified on the data of tidal water level elevation and current velocity, the best results are obtained at a chezy number of 60 m^0.5/s, and sensitivity tests are carried out on changes in sediment grain size, wave height and wave direction in order to determine the characteristics of the numerical model. The sensitivity analysis of sediment grain size is the most sensitive effect to sediment transport characteristic. The results of the numerical model are then compared based on the scenarios before (2016) and after the occurrence of land subsidence (2021). The results indicate that after land subsidence, the sedimentation rate is increasing by 56.34%, whereas the erosion rate is decreasing by 3.98% after one neap-spring tidal cycle (around 15 days) in the study area.