Urbanization-Driven Waterlogging in Coastal Bangladesh: A Case Study of Narail Sadar Municipality Using Remote Sensing and Hydrological Modeling

Abstract

An integrated approach combining satellite imagery, hydrological analysis, and field-based data was employed to assess the causes, spatial distribution, and severity of waterlogging in Narail Sadar Municipality. Land Use Land Cover (LULC) analysis from 2014 to 2024 reveals a significant expansion of built-up areas, increasing from 318 ha to 659 ha, accompanied by a loss of over 500 ha of vegetated land. The waterlogged area significantly increased from 476 ha to 789 ha during the post-monsoon period (October), as identified using the Modified Normalized Difference Water Index (MNDWI). The average soil infiltration rate in the study area was calculated as 2.31 mm/hr, based on a comparative assessment of hydraulic conductivity, soil texture, LULC, and underlying lithological characteristics. Topography and slope maps reveal that most of study area lies between 7 meters near the riverbank areas (with slopes below 1.73°) and less than 3 meters above msl in the flood basin zones, indicating limited gravitational drainage potential. The average water level rises from 2.21 m in the dry season (October–June) to 3.40 m during the monsoon (July–September), leading to reduced percolation and increased surface runoff under heavy rainfall. The municipality’s drainage system, comprising six outlets (two natural streams and four engineered drains), has a maximum discharge capacity of 10,152 m³/hr. However, the estimated infiltration volume is 26,425 m³/hr, with an additional 974 m³/hr of wastewater generated by existing population (156,000), collectively exceeding the drainage capacity and contributing to frequent waterlogging. A numerical model developed using the Rational Method indicates that the existing drainage system can accommodate a maximum rainfall of 38.5 mm/day or an intense precipitation rate of 3.2 mm/hr without triggering waterlogging. However, Rainfall nearing or exceeding 41.5 mm/day or 4.9 mm/hr surpasses the 2-hour drainage threshold, posing a critical risk for increased frequency and severity of urban waterlogging.