Impacts of retention basins on downstream flood peak attenuation in the Odaw river basin, Ghana

Abstract

Study region: Odaw River Basin (ORB) in Ghana Study focus: We simulated and examined the impacts of retention basin as a hydraulic structure in attenuating the flood peaks downstream of the ORB with a coupled HEC-HMS and HEC-RAS models for different flood scenarios. Calibrated and validated HEC-HMS models were used to forecast floods in terms of peak flows and results forced into the 2D Unsteady flow HEC-RAS model to simulate flood inundation areas.New hydrological insight for the region: The calibrated and validated HEC-HMS model indicated good performance (NSE > 0.65) in simulating the runoff characteristics of the basin. Forecasted peak flows with the calibrated model for return periods of 30-to-1-year were 131.1 m3/s, 121.6 m3/s, 106.4 m3/s, 88.8 m3/s and 59.3 m3/s respectively for the “without” a retention basin scenario within the ORB. In the “with” retention basin scenario, attenuated peak flows were observed downstream the ORB by 8.1 % (120.5 m3/s), 8.4 % (111.4 m3/s), 9.1 % (96.7 m3/s), 13.6 % (76.7 m3/s) and 13.2 % (51.5 m3/s) respectively for the different return periods. Flood volumes reaching downstream of the basin reduced by 2.3 %, 2.5 %, 2.7 %, 3.0% and 3.8 % respectively due to the presence of the retention basin. The retention basin increased the basin lag time by an average of 4 h across all different flood scenarios. The introduction of retention basins will serve a pivotal role in flood management and planning in the ORB.

Keywords

• Flood arrival time
• Flood attenuation
• HEC-RAS
• Odaw river basin (ORB)
• Retention basin
• Return period