The lower catchment of the Palar River Basin is drastically experiencing rainfall-runoff during monsoon, which induces soil erosion, surface runoff, and flooding due to rapid changes in landuse/land cover, and other human-induced impacts. The catchment covers an area of 2945 km2, endowed with 564 tanks and pools for runoff storage and irrigation purposes. The annual average rainfall is 1260 mm, and the maximum rainfall occurs in the northeast monsoon. Estimation of rainfall-runoff is vital for understanding impacts on water storage under landuse/land cover (LULC) change conditions. The Soil Conservation Service-Curve Number (SCS-CN) model in Hydrologic Engineering Center-Hydrologic Model Systems (HEC-HMS) is used to estimate the runoff rate under the LULC change in 2014 and 2024. The Landsat ETM and OLI images are used to detect the LULC changes. Flood hydrographs data for a 100-year flood have been incorporated with LULC conditions in 2014 and 2024. The LULC change revealed a significant increase of 59.66% in built-up areas in the coastal region between 2014 and 2024. The study provides quantitative runoff rates at the site-specific level that are directly influenced by LULC changes, which in turn affect peak runoff discharges and volumes in the lower outlets across the coastal floodplains of the study area. The findings provide primary data sources for the sustainable management of catchment resources in the long-term scenario.