Reconsidering Inverted Crown Crossfall in Local Urban Roads: A Reassessment of Drainage Design for Residential Access Streets

Abstract

Efficient surface drainage is fundamental to urban road geometry, yet conventional crossfall practice in Gulf cities often underperforms under the combined effects of low longitudinal grades, flat urban topography, and dense driveway access. Current manuals, the Oman Highway Design Standards (OHDS, 2017) and the Saudi Highway Code 301 (2024): Geometric Design of Roads, recognize only normal-crown and one-sided crossfalls, both of which are inherited from traditional international standards developed for different topographic and urban contexts. Field evidence from Muscat and Riyadh shows that these outward-sloping systems frequently cause frontage ponding, edge erosion, and high maintenance dependence. This paper re-examines the inverted crown, where both halves of the carriageway slope inward toward the centerline (with or without a shallow flow line), using HEC-22 formulations with Manning’s equation to compare one-sided, normal-crown, and inverted geometries under identical rainfall and grade conditions derived from local IDF curves. Results from a representative local access street in Riyadh indicate that the inverted crown achieves equal or better hydraulic efficiency (lower water-film depth and spread) while eliminating driveway and curb conflicts. Despite proven field performance in recent Riyadh applications, the study concludes that the inverted crown performs best under the modeled scenarios and observed segments in Gulf cities, and recommends its inclusion in forthcoming revisions of Code 301 and OHDS.