Urban expansion driven by coexisting diffusion and coalescence processes in a rapidly growing Sub-Saharan city

Understanding urban growth dynamics is essential for sustainable land management and planning. Contrary to the widely accepted global model that assumes a sequential transition from diffusion to coalescence, this study provides empirical evidence that these processes can occur simultaneously in rapidly transforming urban systems. However, such non-linear growth patterns remain insufficiently documented, especially in Sub-Saharan secondary cities. This study investigates the rapid expansion of Abomey-Calavi, a secondary city in Benin shaped by demographic pressures resulting from Cotonou’s urban overflow and rural migration. Landsat imagery from 1988 to 2023 was classified using a supervised maximum likelihood algorithm, and landscape metrics including the Urban Expansion Intensity Index $$\:\left(UEII\right)$$ and patch analysis were applied to quantify spatial and morphological transformations along the urban-rural gradient. Results reveal a tenfold increase in built-up areas, from 2,213 ha in 1988 to 23,601 ha in 2023, largely replacing vegetation/fields, which declined by 45%. The mean annual urban growth rate reached 6.11%, with $$\:UEII$$ values indicating a shift from medium speed on 1988–2001 to high-speed expansion on 2001–2023. Spatial processes were dominated by new patch creation and vegetation dissection. Growth modes varied by distance, with coalescence within 3 km of the core generated dense urban clusters, while diffusion beyond 6 km characterized peri-urban zones. This simultaneous operation of diffusion and coalescence challenges conventional sequential models. The findings highlight Abomey-Calavi’s role as a major receptor of metropolitan spillover and provide critical insights for managing land conversion and infrastructure in fast-growing African secondary cities.