Sponginess of Sylhet City report

Rapid urbanisation in Bangladesh has intensified flood risk in cities like Sylhet, where unplanned development, wetland loss, and expanding impervious surfaces have disrupted natural hydrological processes.

 This study, conducted under SNV's Sustainable Urban Water Cycles (SUWC) project, assesses the baseline "sponginess" of Sylhet City Corporation—its capacity to absorb, store, and release stormwater—using the Sponge City framework for nature-based, water-sensitive urban planning. A GIS-based Multi-Criteria Evaluation approach was applied, integrating twelve spatial, environmental, and hydrological indicators, including vegetation cover (NDVI), built-up intensity (NDBI), soil moisture (NDMI), slope, drainage density, and land surface temperature (LST). These indicators were normalised, weighted, and combined via weighted overlay analysis to generate a composite Sponginess Index, classified into five categories ranging from very low to very high. 

Results show that 68.79% of the study area exhibits moderate sponginess, 19.45% high, 9.69% low, and only 2.05% very high, with pronounced spatial variation: the hilly, vegetated north retains greater sponginess, while the densely built urban core shows markedly reduced capacity due to impervious surfaces and the loss of natural water bodies. These findings underscore the city's uneven hydrological resilience and the urgency of targeted interventions. 

The study recommends ecological zoning to protect high-sponginess northern zones, restoration of degraded ponds and canals in the urban core, stricter controls on impervious development, integration of permeable pavements, rain gardens, bioswales, and green corridors into infrastructure planning, and mandatory rainwater harvesting provisions in building codes. Incorporating the sponginess map into the Sylhet Master Plan is proposed as a mechanism to institutionalise water-resilient planning. This research offers a replicable, evidence-based framework for evaluating urban water resilience and provides actionable guidance for policymakers and planners seeking to balance urban development with ecological sustainability in Sylhet and comparable South Asian cities.