Juba, the capital city of South Sudan and home to an estimated 525,000 residents in 2024, has experienced a 200% increase in flood-related road closures between 2012 and 2024, driven by rapid unplanned urbanisation, expanding impervious surfaces, and a chronically under-maintained drainage network inherited from the pre-independence era. This study presents the first systematic engineering analysis of urban road drainage failure in Juba City, integrating drainage network condition assessment, hydrological modelling, failure mode analysis, and rehabilitation planning optimisation. A structured field survey of 26.4 km of drainage infrastructure across ten urban districts catalogued 1,166 failure events, of which Pareto analysis identifies inadequate hydraulic capacity, sediment and debris blockage, and structural collapse as the three dominant failure modes, together accounting for 64.1% of all events. Intensity-Duration-Frequency (IDF) curves were derived from 42 years of pluviograph records at Juba International Airport using the Sherman equation, and design rainfall intensities for return periods of 2 to 100 years were calibrated against three independent gauge stations. The Rational Method was applied to compute peak runoff discharges for 28 sub-catchments, revealing that 15 sub-catchments exhibit drainage capacity ratios (Q_peak / Q_capacity) exceeding 1.5 — indicating systemic undersizing. OLS regression confirms a statistically significant relationship between impervious cover and runoff coefficient (r = 0.94, p < 0.001), underscoring the compound effect of urbanisation on drainage loading. Four rehabilitation scenarios — ranging from cleaning-only to a combined green infrastructure and full reconstruction approach — were evaluated using net present value (NPV) and