Evaluation of the effectiveness of low impact development (LID) practices on stormwater management using SWMM

One of the most important environmental threats faced by modern cities is that extreme rainfall, increasing under the influence of climate change, significantly raises peak runoff because impervious surfaces formed by uncontrolled and dense concretization prevent infiltration. This situation exceeds the capacity of aging grey infrastructure systems, causing surface runoff to reach unmanageable levels and making it necessary to develop sustainable alternative solutions. In this thesis, the hydrologic and hydraulic effectiveness of Low Impact Development (LID) practices was evaluated using the EPA SWMM 5.2 modelling tool to reduce urban flood risks and increase infrastructure resilience, taking Konya Province's Selçuklu District as a case study. In the modelling studies, both a 1-hour 2-year return-period design storm (12.6 mm) and 4-hour observed rainfall data from a rain gauge station (35.7 mm) were used. Under the design storm, the existing drainage system produced a flood volume of 4.35 million m³, and under the station rainfall it produced 16.87 million m³; these findings show that the risk of flash flooding is high even for short return-period storms and, consistent with field observations, reveal the area's vulnerability. To prevent such easily triggered floods, three different LID scenarios, whose effectiveness has been proven in the literature, and which are based on controlling stormwater at its source, were implemented: rainwater harvesting on all roof areas, converting 60% of impervious surfaces outside roof areas into permeable surfaces, and integrated implementation of these two measures. Under design-storm conditions, rainwater harvesting reduced the total flood volume by 35%, while permeable-surface applications achieved an 87.5% reduction; in the integrated LID scenario, flood formation was prevented as source storage, infiltration, and routing mechanisms operated together. However, in analyses conducted under the station rainfall reflecting climate-change impacts, it was observed that the system reached saturation and that the effectiveness of routing was limited due to the long rainfall duration; under these challenging conditions, the integrated LID system reduced flood volume by 63.6% but could not prevent flooding entirely, demonstrating the need for support from grey infrastructure. Considering Konya's arid climate conditions and flat topography, rainwater harvesting and permeable-surface applications appear to have significant potential not only for flood control but also for water-supply security; this study demonstrates the role of parcel-scale stormwater management in reducing urban-scale flooding.