Flooding increasingly endangers the arid Wadi Ibrahim catchment that drains central Makkah. This study quantifies the evolving flood hazard under climate change by coupling CORDEX-downscaled rainfall (RCP 4.5 and RCP 8.5) with high-resolution two-dimensional HEC-RAS Rain-on-Grid modelling for 50-, 100- and 200-year return periods. Simulations adopt a dry-bed initial state and 48-h horizon to capture the ephemeral flash-flood cycle typical of the region. Results show a pronounced intensification of flood metrics. Present-day flood volumes rise from 18.9 × 10 6 m 3 (50-year) to 24.8 × 10 6 m 3 (200-year); these volumes almost double under RCP 4.5 and triple under RCP 8.5, reaching 44.5 × 10 6 m 3 and 86.1 × 10 6 m 3 , respectively. Average inundation depth increases from 0.2 m today to 0.6 m (RCP 4.5) and 0.8 m (RCP 8.5). Spatially, high-hazard zones (> 2 m depth) expand from 7.2–9.8 km 2 under current conditions to 5.3–12.0 km 2 under RCP 4.5 and 9.5–16.6 km 2 under RCP 8.5. Peak discharges for the 100- and 200-year events exceed the established Saudi Arabian envelope curve, evidencing non-stationary extremes. These findings highlight the need to update design standards and prioritize adaptive strategies, such as enlarged culverts, detention basins, and real-time early-warning systems; to safeguard lives, the Grand Mosque environs, and critical infrastructure against escalating flood hazards in a warming climate.
Hidayatulloh et al. (Sun,) studied this question.