Assessing sponginess for urban resilience planning in metropolitan cities of Bangladesh: A GIS-based multi-criteria decision-making approach

Urban sponginess refers to a city's capacity to absorb, store, and gradually release stormwater through blue-green infrastructure such as wetlands, ponds, and permeable surfaces. This capacity offers a promising framework for building flood resilience in rapidly urbanizing cities. Operationalizing this concept, however, requires a reliable baseline measure of sponginess to guide planning and policy decisions in flood-prone metropolitan areas. This article has developed a sponge city index for two major Bangladeshi metropolitan cities, Khulna and Rajshahi, using a GIS-based multi-criteria decision analysis integrated with the Analytical Hierarchy Process (AHP). Nine indicators representing blue-green-grey ratio, runoff potential, moisture retention, and thermal environment were derived from Landsat imagery for 2015, 2020, and 2025. These indicators were combined through a weighted overlay to classify urban sponginess into five categories, ranging from very low to very high. Results show that moderate sponginess dominates both cities. In Khulna, very low sponginess declined from 18% to 15%, while very high sponginess rose from 10% to 12%. This indicates modest improvement despite ongoing urban expansion and wetland loss. In Rajshahi, very high sponginess increased from 6% to 7%, coinciding with documented plantation initiatives. However, rapid population growth is associated with pressure on blue-green assets in dense residential wards. This pattern signals a widening gap between ecological gains and the governance capacity needed to protect them. Large portions of both cities still remain in the low to moderate range. The index is grounded in macro-scale remote sensing proxies. It does not capture sub-parcel drainage performance or ground-truthed hydrological data. Translating index outputs into zoning or infrastructure investment decisions will also require closer integration with local planning institutions. Despite these limitations, the findings point to a clear need for integrated ecological restoration, wetland conservation, and drainage improvement. The index offers a practical, transferable baseline tool for urban resilience planning in Bangladesh and in other flood-prone, rapidly urbanizing regions.