Effects of River Channel Structural Modifications on High-Flow Characteristics Using 2D Rain-on-Grid HEC-RAS Modelling: A Case of Chongwe River Catchment in Zambia

Le résumé fourni par la source

With accelerating climate change and urbanization, river catchments continue to experience structural modifications through dam construction and concrete-lining of natural channels as adaptation measures. These interventions can alter the natural hydrology. This necessitates assessment of their influence on hydrology at a catchment scale. However, such evaluations are particularly challenging in data-scarce regions such as the Chongwe River Catchment, where hydrometric records capturing conditions before and after structural modifications are limited. Therefore, we applied a 2D rain-on-grid approach in HEC-RAS to evaluate changes in high-flow characteristics in the Chongwe River Catchment in Zambia, where structural interventions have been implemented. The terrain was modified in HEC-RAS to represent 21 km of concrete drains and ten dams. Sensitivity analysis was conducted on five model parameters and showed that Manning’s roughness coefficient had by far the largest impact on peak flows. Model calibration and validation showed strong performance with R² = 0.99, NSE = 0.75 and PBIAS = – 0.68 % during calibration and R² = 0.95, NSE = 0.75, PBIAS = – 2.49 % during validation. Four scenarios were simulated to determine the hydrological effects of channel concrete-lining and dams. The results showed that concrete-lining of natural channels in the urban area increased high flows at the main outlet by approximately 4.6%, generated very high channel velocities of up to 20 m/s, increased flood depths by up to 11%, and expanded flood extents by up to 15%. The existing dams reduced peak flows by about 28%, increased lag times, reduced flood depths by about 11%, and reduced flood extents by up to 8% across the catchment. The findings demonstrate that enhancing stormwater conveyance through concrete-lining must be complemented by storage to manage high flows, while future work should explore nature-based solutions to reduce channel velocities and improve sustainable flood mitigation.

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