Rethinking Drainage Design: Insights from Rainfall Trends and Urban Growth Using the Rational Method

Debate on the stationary versus non-stationary approaches in drainage system design is still ongoing. Under the stationary concept, the rainfall statistics are considered unchanged over time, even though recent observations demonstrate clear signs of change. This study aims to provide insight into how long-term changes in rainfall patterns influence surface runoff. Average rainfall intensities in Indonesia ranging from 1950 to 2024 were used as input for the analysis. In addition, the effect of the land-use change on the surface runoff was investigated through a range of imperviousness scenarios. Peak runoff was estimated using the Rational Method, which offers a conceptually straightforward yet operationally robust approach. The results showed that changes in land cover, intensified the peak runoff. Increase in rainfall intensities also led to higher calculated peak runoff, with values rising by up to 44%, particularly when comparing rainfall intensities from 1950 and 2024. Level of peak runoff escalated as the effect of combined land cover changes and rainfall intensification. These findings indicate that stationary assumptions may underestimate the runoff conditions. In other words, the findings support the adoption of non-stationary design approaches for modern drainage networks.