Spatial variability of combined sewer overflow responses to rainfall events in Brussels, Belgium

Brussels' complex sewer network, with around hundred regularly-spilling combined sewer overflows (CSOs), contributes significantly to its surface water courses pollution, threatening compliance with the European Water Framework and the new Urban Wastewater Treatment Directives. Identifying which specific rainfall events trigger different CSO occurrences is essential for implementing solutions like real-time control. However, many studies on CSOs rely on sparse or low-resolution rainfall data. This study investigates CSO occurrences for rainfall events in 2023 and 2024 using high-resolution rainfall data in the Brussels Capital Region, specifically radar-based precipitation with 1 km spatial and 5-minute temporal resolution. Nine overflow locations were selected, which discharge into two different surface water bodies at varying frequencies. We quantify overflow frequency and duration, and link these to rainfall event metrics. Future work will include more overflow locations, and will aim at identifying specific rainfall patterns that disproportionately contribute to CSO at the different locations.