Abstract
<title>Abstract</title> <p>Road culvert failures are a major cause of infrastructure damage in developing regions. This study presents an integrated hydrologic–hydraulic framework for culvert siting and sizing at road–waterway intersections, using New Zakho City, Kurdistan Region, Iraq, as a case study. The methodology combines the Watershed Modeling System (WMS) for watershed delineation and rainfall–runoff simulation with two-dimensional HEC-RAS 2D for flood inundation modeling. Design storms were derived from maximum 24-hour rainfall records using frequency analysis, producing depths of 104 mm and 114 mm for the 50- and 100-year return periods, respectively. Four road–stream crossings were identified, draining watershed areas of 0.99–15.71 km² with design discharges of 13.1–111 m³/s. Mesh sensitivity analysis identified a 10 m resolution as optimal, balancing computational efficiency and simulation accuracy. Flood inundation results showed significant variations in depth and velocity between return periods, providing the basis for culvert sizing and scour protection. Box culverts ranging from 2.13 × 1.22 m to 3.66 × 3.05 m satisfied the allowable headwater-to-rise ratio of 1.5 for the 100-year event, while outlet velocities exceeding 3.0 m/s required riprap apron protection. The proposed open-data framework provides a transferable, cost-effective approach for culvert design in ungauged, data-scarce watersheds.</p>