ITB Meteorology Lecturer Reveals the Facts Behind the 2025 Bekasi Flood: Dikes and Dams Are No Longer Enough
By Mely Anggrini - Mahasiswa Meteorologi, 2022
Editor M. Naufal Hafizh, S.S.
BANDUNG, itb.ac.id – In early March 2025, Bekasi area was hit by a major flood following an extreme rainfall event that lasted up to 36 hours. In response to this incident, Dr. Edi Riawan, S.Si., M.T., a lecturer from the Atmospheric Sciences Research Group at the Faculty of Earth Sciences and Technology, ITB, along with his team, conducted a rapid assessment through field surveys and numerical simulations to understand the causes and dynamics of the flood.
The initial survey revealed that the flooding affected not only Bekasi but also extended to Jakarta and Karawang. In Bekasi, 171 flood points were recorded, with water levels reaching up to 3.32 meters and covering an area of approximately 380 hectares. Interestingly, nearly all of the affected areas were already protected by dikes ranging in height from 0.95 to 3 meters. However, unlike the major flood in 2020, no cases of dike breaches were found, only minor leaks were observed in several locations.
Rainfall analysis shows that on March 3, 2025, heavy rain began to fall in the upstream area of the Bekasi watershed (Bahasa: Daerah aliran sungai, DAS) around 5:00 PM local time. Over time, the rain moved downstream and reached its peak uniformly between midnight and 3:00 AM on March 4. This rain propagation pattern worsened water discharge accumulation in the downstream area, as confirmed by Windy radar data.
The 2025 and 2020 floods had different characteristics. In 2020, the major flood was triggered by rainfall in the downstream area, resulting in a combination of urban and riverine flooding. In contrast, the 2025 flood was primarily caused by heavy upstream rainfall that moved downstream.
Through numerical simulations using hydrological and hydraulic models, the team found that the flood peak lasted for 15 hours, from 3:00 AM to 6:00 PM, with an approximate time difference of 8 hours between rainfall in the upstream and downstream areas.
Three scenarios were tested in the simulation: flood without weirs and dikes, with weirs only, and with both weirs and dikes. The results showed that the presence of both weirs and dikes—reflecting actual field conditions—resulted in the largest flood inundation area, approximately 720 hectares. Although the dikes and weirs helped protect against the initial overflow, these infrastructures also made it more difficult for floodwater to recede, prolonging inundation since the water could not quickly flow back into the main river.
In a more detailed explanation, under the scenario without weirs and dikes, flood inundation was concentrated in the downstream area, with a maximum water depth reaching 4 meters in the river, while the flooding on land was lower. The simulated Water Level Height (TMA) for this scenario was recorded at 2.27 meters, compared to 3.32 meters based on field survey data.
In the second scenario, which included the Bekasi Weir with no dikes, the simulated TMA was 2.58 meters. Meanwhile, in the actual scenario with both the weir and dikes, the simulated TMA reached 3.51 meters, closely matching the field measurement. These data indicate that the combination of weir and dikes plays a significant role in shaping the flood inundation characteristics in the Bekasi area.
Over the past five years, Bekasi has experienced two major floods—in 2020 and 2025. Although both were triggered by extreme rainfall, the differences in rainfall patterns and resulting impacts suggest a clear change in the characteristics of the Bekasi watershed.
One of the main factors driving this change is land conversion. The Cikeas Sub-Watershed (Sub-DAS) has experienced a greater degree of land conversion compared to the Cileungsi Sub-DAS. This conversion has contributed to an increase in flood discharge and sedimentation, both of which exacerbate the risk of flooding. The land cover change has occurred gradually, with a rate of less than 1% per year, making its impacts felt gradually as well.
Data shows that residential areas in the Bekasi watershed increased from 30% in 2000 to nearly 50% in 2024. Additionally, the loss of swamps and historical lakes along the Bekasi River, as noted in Junghuhn’s 1901 study, has become a significant phenomenon that warrants further investigation. A special study is needed to understand the extent to which the loss of these swamps and lakes has contributed to the increasing frequency and intensity of floods in Bekasi and its surroundings. These changes are suspected to have made the Bekasi watershed more vulnerable to major flood events.
To address this issue, a planned and long-term management strategy is required. These efforts should include the revitalization of water catchment areas, reorganization of riverbank areas, conservation of swamps and lakes, adaptive management of dikes and weirs to accommodate flow dynamics, as well as the implementation of early warning systems based on upstream observations. Furthermore, strict control over land conversion is crucial, especially in areas functioning as water catchment regions.
"This is not a fully mature conclusion, given the short study period, but at least we can take lessons from this incident in Bekasi. Hopefully, it will be useful for future mitigation steps to minimize flood risks in Bekasi," said Dr. Edi.
This research, although still an early study with limited data, provides important insights into the new challenges faced by the Bekasi watershed. The integration of physical infrastructure development, environmental preservation, and data-driven mitigation systems is key to building the region's resilience to future floods. Strong collaboration between researchers, practitioners, the government, and the wider community is essential to achieve more effective and sustainable flood management in Bekasi and its surrounding areas.
Reporter: Mely Anggrini (Meteorologi, 2022)
Translator: Indira Akmalia Hendri
