<![CDATA[Manggala District in eastern Makassar City experiences recurrent urban flooding driven by rapid land-use conversion, undulating topography, and drainage channels whose hydraulic capacity has been degraded by sedimentation and waste. Previous studies in Makassar have largely evaluated open eco-drainage measures, infiltration wells, and retention ponds, but few have provided a reproducible, discharge-based design of a fully closed circular conduit network sized against a defined return period. This study addresses that gap by coupling frequency-based hydrological analysis with hydraulic design of a closed circular drainage system for the Zone 1 catchment of Manggala. Annual maximum daily rainfall from the Makassar station (2014–2024) was fitted to candidate distributions and evaluated with goodness-of-fit tests; the 10-year design storm was propagated to peak discharge using the rational method with land-use-weighted runoff coefficients, and circular reinforced-concrete conduits were sized with the Manning equation. The catchment was delineated into two zones by topography and runoff concentration. Zone I (857.1 ha) requires a conveyance capacity of 0.63–1.57 m³/s and 1.86 km of closed channel, whereas Zone II (272.4 ha) requires 0.35–0.80 m³/s and 14.39 km. Circular conduits of 600–1200 mm diameter provide full-flow capacities of 0.43–2.76 m³/s at velocities of 1.5–2.4 m/s, satisfying the self-cleansing range while remaining below the scouring limit. Field observation confirmed that inundation develops when rainfall persists beyond one hour in low-lying, obstructed reaches. The main novelty of the study is a zone-specific, return-period-referenced closed circular drainage model in which conduits are placed beneath the road corridor, releasing surface land for urban functions while maintaining hydraulic efficiency and reducing sediment retention relative to the trapezoidal open channels currently in service.]]>
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