Investigating the impact of low impact development implementations on water quantity and quality

Within the scope of this study, the effects of Low Impact Development (LID) practices on urban surface runoff and water quality were examined. The study area selected for numerical modeling was the Istanbul University-Cerrahpaşa Büyükçekmece Campus. The EPA Storm Water Management Model (EPA-SWMM) was used for the hydrological-hydraulic and diffuse pollutant modeling. LID practices are nature-based sustainable infrastructure solutions developed to reduce flood risks and pollutant loads caused by impervious surfaces. In this study, five different LID applications were identified for implementation on the campus: bioretention systems, rain gardens, permeable pavements (permeable asphalt, permeable sidewalks, etc.), green roofs, and rainwater harvesting tanks. Each was defined in the EPA SWMM program with its technical specifications. Before the modeling process, a comprehensive field study was conducted because stormwater and sewer line projects for the campus could not be obtained from either the university or İSKİ (Istanbul Water and Sewerage Administration). Through on-site measurements, manhole locations, pipe diameters, and elevation data were obtained in collaboration with İSKİ's technical team. These data were converted into a digital model infrastructure using AutoCAD, Global Mapper Pro, and Google Earth software. Subsequently, 50 sub-catchments were defined across the campus, and hydrological parameters such as area, slope, imperviousness ratio, and surface type were entered into the model for each sub-catchment. For the diffuse pollutant model, Total Suspended Solids (TSS), Chemical Oxygen Demand (COD), Total Nitrogen (TN), and Total Phosphorus (TP) parameters were defined. Simulations were run separately for both pre-LID and post-LID scenarios using rainfall data from the Florya Meteorological Station for 5-, 10-, 25-, 50-, 100-, and 500-year return periods, followed by comparative analyses. The results show that LID practices reduced surface runoff volume by 30% to 56% and peak flow by 11% to 25%. Additionally, in terms of water quality, reductions of 7% to 63% were observed in TSS, COD, TP, and TN concentrations. Particularly, bioretention and rain garden applications demonstrated the highest performance in terms of infiltration and pollutant removal. This thesis study is one of the first examples in Turkey to evaluate LID applications at the campus scale through integrated modeling, providing scientific and technical contributions to sustainable infrastructure planning for local governments.