Investigation of the effects of wetland location and storage with overflow and controlled release on watershed-scale flood mitigation

Arid and semi-arid regions are characterized by water scarcity and prolonged droughts, leading to the misconception that flooding are negligible due to low annual rainfall. However, recent flood events demonstrate that these areas are highly vulnerable to floods. Current strategies for wetland at enhancing hydrologic resilience often overlook the spatial arrangement of wetlands within a watershed. This gap largely stems from limited understanding of how wetland location influences hydrologic functioning, especially in semi-arid watersheds. This study evaluates the effectiveness of open-water wetlands flood control in the Aqhsu River watershed in northeastern Iran. Three scenarios were examined: (1) wetlands placed upstream and downstream with equal storage volumes; (2) wetlands located only upstream with varying storage capacities; and (3) controlling released flow by using dynamic operation strategy with outlet valves which were applied for the second scenario. In the first scenario, eight wetlands upstream and six downstream were designed with equal storage, and results showed that the allocation of upstream wetlands reduced the floodplain area by 47% more than downstream wetlands relative to the Do-Nothing scenario. The results were greatly influenced by increase in the basin lag time (∆t). This timing factor has significant effect on flood control in the semi-arid watersheds with limited lag time (t_lag) and high curve number (CN). In the second scenario, wetlands covering 7% of the upstream area produced the best performance, reducing flooded area by 90% and flood depth by 86% in urban area. Due to limited space for wetland construction in watershed, the third scenario incorporated regulated outflow. Results show that in this case study, dynamic storage management, converting 5% of the upstream area can decreases 93% flooded area rather than do-nothing scenario. This research reveals that combining nature-based solutions with traditional engineering methods can substantially enhance flood resilience and can be adapted to semi-arid watersheds with high effectiveness.