This study presents a computational tool for designing and evaluating stormwater management devices in small urban catchments. The tool is highly versatile, supporting the testing of diverse hydrological models tailored to local conditions. It is particularly useful for designing retention tanks, sizing infiltration trench, and modernizing stormwater drainage systems. Using rainfall data from 32 cities across Poland, the study highlights the influence of regional rainfall patterns on the capacity of infiltration trenches. The highest soil capacity was observed during 15-minute convective rainfall events, with a maximum of 86 m³ in Nowy Sącz and a minimum of 37 m³ in Zakopane, influenced by the unique topographical conditions of mountainous areas. These findings underscore the need for a localized approach to stormwater system design, considering terrain and rainfall intensity. The tool facilitates sustainable stormwater management strategies, improving flood prevention and urban resilience amid dynamic climate change.