Modeling of Flows and Flood Forecasting in an Urbanized Watershed: Case of the Gourou Basin in Abidjan, Côte d’Ivoire

Abstract

The Gourou watershed is characterized by its high urbanization and poor rainwater drainage due to malfunctions in evacuation infrastructure. This situation leads to recurring floods at the basin's outlet and surrounding neighborhoods, often causing traffic interruptions as well as sedimentation and unsanitary conditions in the Ebrié lagoon, the recipient of these waters. This study aims to model water flows and forecast flood risks in the Gourou watershed using the SWMM conceptual model. Model validation was possible through flow rates observed in 2018 at the Martyrs' Monument station using a RAVEN-YE RS485 flowmeter with a 5-minute time step. Horton's formula was used to evaluate soil infiltration capacity. The dynamic wave formula, which involves complete resolution of the Barré de St. Venant equations, allowed for runoff modeling. SRTM-3 images version 2 from the USGS site with a 1 arc-second resolution and covering the entire watershed led to the extraction of the Digital Terrain Model. The primary input data for the SWMM model is the Desbordes double triangle rainfall, providing hyetographs with a 2-hour duration and a 30-minute intense period. The estimation of physical parameters of the Gourou watershed reveals a maximum altitude of 149 m with an average slope of 7.67%. Infiltration is maximal upstream in the sub-basin S7 (20.60 mm/h) and gradually decreases downstream, with a minimum value of 6.23 mm/h in sub-basin S18, reflecting significant urbanization downstream. The Nash criterion of 92.52% and Pearson coefficient of 0.56 confirm that the SWMM simulation for the Gourou watershed provides excellent results. There is also a strong correlation between simulated and observed flow rates. No overflow is recorded at the watershed outlet for recurrence periods of 1, 2, 3, 5, 10, 20, 30, 50, and 100 years. Only the flood-limiting dams of Dokoui Est and Agban overflow during these identified return periods. Flow velocities vary from 0.01 m/s to 10 m/s in the sections of the main Gourou canal. Furthermore, all flood control dams receive rain runoff at more than 100% of their required capacities. Then the floods observed in recent years at the Indenié Crossroads are generally caused by drainage infrastructure malfunctions (obstruction, sedimentation, ruptures, wastewater connections). These findings are important and should serve as decision-making tools for policies implemented to combat floods in urban environments.