Adapting normative methods for highway drainage design to climatic nonstationarity in Uzbekistan: a risk assessment based on ERA5 reanalysis

Central Asia is classified as a region experiencing warming rates that exceed the global mean. Nevertheless, the drainage design standards currently applied in Uzbekistan (ShNK 2.04.03-19) still rely on the climate stationarity assumption and on legacy rainfall intensity-duration-frequency (IDF) curves derived from pre-1990 observations. This study evaluates the hydraulic adequacy of these standards under contemporary climatic conditions. High-resolution ERA5 atmospheric reanalysis data (1990-2024), validated against CHIRPS satellite observations and in situ station records, are used to reconstruct probabilistic precipitation models based on the Gumbel distribution. Comparative analysis reveals a pronounced safety gap between normative and observed intensities. For frequent events (return period P = 2 years), the discrepancy is moderate (8.1 %), whereas for rare extremes ( P ≥ 50 years) the current standards underestimate precipitation intensity by 27.9-29.3 %. These findings indicate a high risk of hydraulic overload for critical infrastructure designed under the existing codes. The study substantiates the need to introduce a dynamic “climate factor” ( k c c ≈ 1.3) into national construction standards to reduce flood-related losses, currently estimated at USD 395 million annually.