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Inundation occurred in high density urban areas is a major issue especially in low land because it potentially damage the affected infrastructure. In this research, a case was taken in Kelayan Regional Inundation Handling Unit in Banjarmasin City, South Borneo. The main problem of inundation that occurs is the lack of the existing drainage channel capacity to drain discharge due to rain. The hydrological analysis was calculated by using the rational method to get discharge, using data from Sungai Tabuk Rain Station that 24 years long. From the discharge obtained, then the required channel dimensions could be calculated and compared with the existing channel dimensions. The hydraulic analysis was performed by HEC RAS model with the discharge as boundary condition, and topographic data to determine elevation of the channel. As the results of running models, the existing drainage channels can only cover 37.19% of the entire area, and 85% of these existing channels cannot drain the discharge that occurs, therefore it becomes an inundation. For example, channel number 4.B.16b the existing channel has 0.8m width and 0.5m depth, it caused inundation as high as 10cm upstream. After normalized the channel by increasing the dimensions to 0.9m width and 0.7m depth, the discharge does not overflow so there is no inundation.