Influence of the choice of the altimeter surface on the precision of urban hydraulic models in a comparison between the Earth Gravitational Model 2008 (EGM08) and the geodetic network of Southern Bénin

Abstract

In east-southern Benin's coastal urban areas, the reliability of elevation data remains a critical factor in the accuracy of hydraulic modelling for drainage and stormwater management. The objective of the study was to evaluate how the selection of vertical reference surfaces namely EGM08 and the Beninese National Geodetic Network (RNGB) affects key hydrological computations in the cities of Cotonou, Porto-Novo and Abomey-Calavi. Differential GNSS measurements were processed using two correction methods: geoid undulation from the EGM08 model and benchmarks from the national geodetic network. Corrected elevation data were used to compute hydraulic parameters including slope, time of concentration, and peak discharge using standard empirical formulas. Comparative analysis revealed notable discrepancies in elevation and simulated runoff volumes, particularly in large catchments where volume differences reached up to 12.4%, as opposed to less than 1.5% in smaller basins. EGM08-derived slopes tended to be underestimated compared to RNGB-based computations, which may lead to suboptimal infrastructure design. While the spatial consistency of the geoid undulation was generally confirmed, anomalies were observed near wetlands and low-lying zones. These findings highlight the limitations of global geoid models like EGM08 for localized, high-precision applications. The RNGB emerges as the most reliable vertical reference for hydraulic modelling in the region. However, its limited spatial coverage underscores the need to develop a refined local geoid and integrate advanced elevation technologies such as LiDAR or photogrammetry to enhance digital terrain models. Such improvements are essential for optimizing urban stormwater management and infrastructure planning in West African coastal cities.