Land subsidence dynamics and their interplay with spatial and temporal land-use transitions in the Douala coastland, Cameroon

Abstract

The Douala coastland (DCL), situated within the Douala sedimentary basin along the Gulf of Guinea, is characterised by its low elevation and alluvial geology, making it particularly susceptible to coastal erosion, land subsidence, and relative sea-level rise. The DCL is home to numerous rapidly growing cities, such as Douala, Tiko, and Limbe, which are currently experiencing alarming rates of coastal erosion, frequent flooding, and significant loss of land. Regional and continental investigations have provided evidence of coastal subsidence in this region; however, knowledge of its drivers and impact on the DCL remains limited. To address this knowledge gap, interferometric synthetic aperture radar (InSAR) datasets from the Sentinel-1 C-band satellite were used to quantify vertical land motion (VLM) between 2018 and 2023 with respect to the IGS14 global reference frame and assumed to represents absolute VLM. Digital Elevation Model datasets were used to analyse the elevation of the study area. The results revealed that the rate of VLM ranges from -17.6 to 3.8 mmyr-1 (standard deviation of 0.2 mmyr-1), with a mean and median land subsidence rate of 2.7 and 2.5 mmyr-1. The analysis of land cover datasets from 1992 to 2022 suggests that urbanisation increased fivefold from 1992 to 2022 and that all contemporary urban areas experienced land subsidence, with the highest rates observed in non-residential zones with building heights ranging between 3 and 6 m. Subsidence rates of the DCL are inversely proportional to the time at which a particular land use and land cover (LULC) class changed into an urban area, highlighting the impact of the timing of LULC changes and urban expansion on present-day subsidence. The land subsidence rates decreased with an increase in building height, suggesting shallow land subsidence, where tall building usually with deep foundations are less prone to subsidence caused by groundwater extraction from the phreatic aquifer.