A Foundation in Flux
The Nakdong River Delta is a geologically young feature, formed over the last 7,000 years as the mighty Nakdong River deposited vast quantities of sediment into the sea. This process created a low-lying plain of soft sand, silt, and marine clay that extends up to 46 meters deep in some areas. While natural compaction of these Holocene-era sediments occurs slowly, human activity has dramatically accelerated the process. The primary driver of the delta's sinking, or subsidence, is the extensive extraction of groundwater for agricultural, domestic, and industrial use.
When water is pumped from the aquifers between sediment grains, it reduces the underground pore pressure. The loss of this pressure allows the layers of fine clay to compress under the weight of the land above, causing the ground level to fall. Monitoring with GPS and satellite-based InSAR (Interferometric Synthetic Aperture Radar) reveals that large parts of the delta are sinking by an average of 2 centimeters annually. In some heavily developed areas, such as the reclaimed land of the Busan New Port, localized subsidence has reached as high as 12.5 centimeters over a four-year period. This rapid compaction places immense strain on the urban infrastructure built upon this unstable foundation.
The Weight of a Megacity
Busan's western districts, where a significant portion of the city's 3.3 million residents, are built directly on the sinking delta plain. Critical infrastructure, including Gimhae International Airport and major industrial parks, face increasing risks. The Busan New Port, a massive facility built on 72 million cubic meters of dredged and reclaimed land, is particularly vulnerable to differential settlement, where the ground sinks at uneven rates. This can damage buildings, port equipment, and underground utilities.
The sinking land magnifies the threat of coastal flooding. As the ground elevation drops, the delta becomes more susceptible to storm surges and high tides, a problem compounded by global sea-level rise. Subway construction and other large-scale excavations can further disrupt groundwater flow, leading to ground collapse and sinkholes, which have become a growing concern for the city. Saltwater intrusion also occurs. As freshwater is pumped out and the land subsides, saline water from the sea can penetrate inland aquifers, contaminating the remaining groundwater and threatening agricultural activity on the fertile delta plain.