Floods are natural disasters that occur in major cities throughout Indonesia. The causes of floods can be attributed to various factors, including topography, high rainfall intensity, blockage of water channels, and so on. Flooding resulting from river overflow occurs almost every year in the Welang Pasuruan river basin. Retention ponds serve as flood control structures in urban areas. The function of retention ponds is to temporarily contain floods or serve as temporary storage during peak river flood times. When the river flow subsides, the water in the retention pond is returned to the river. The planning of retention ponds is expected to reduce the risks associated with flooding. By processing rainfall data for the past 10 years using the Log Pearson III Method, and calculating the planned flood discharge using the Nakayasu Synthetic Unit Hydrograph Method, the analysis of flow profiles is conducted using the HEC-RAS 6.3.1 program under existing conditions with the calculated Q10th. The peak flood discharge is calculated to be 635,531 m3/s with a required time of 4.5 hours to reach the peak, resulting in a river runoff of 3,300,796.49 m3. The total planned area for the retention pond is 398,642 m2 with a depth of 4.5 meters. The maximum storage capacity of the pond is calculated to be 1,793,889 m3, reducing the flood discharge by 1,506,907.491 m3, with a percentage of 54.35%. Therefore, the planning of this retention pond is highly effective in addressing the floods in the Welang River
