Analysis of Block-Based RDII Using Water Level Observation Data in a Separate Sewer System

In separate sewer systems, Rainfall Derived Inflow and Infiltration (RDII) creates operational issues such as manhole overflow and overloading of treatment plant capacity during rainfall events. Previous studies proposed a block-based water level monitoring system and a method for determining middle-block monitoring priority. However, studies on procedures for ascertaining whether and why RDII occurs based on the collected water level data remain scarce. This study classifies block outlets into independent and linked operation types, depending on whether they are impacted by inflow from upstream blocks, and proposes an analysis method for each type. For independent blocks, a dry weather-wet weather level separating analysis is performed to calculate the direct sewer level, and RDII influence patterns are classified as W/O RDII, Delayed RDII, or Instantaneous RDII based on the time lag between rainfall onset and level rise. For linked blocks, this study confirms that RDII cannot be reliably analyzed by simply combining upstream and downstream observations, and instead proposes a procedure in which the RDII status of upstream independent points is assessed and addressed first, allowing downstream blocks to be treated as independent in sequence. Applying the methodology to water level data from City A (May-July 2024, 12 rainfall events) showed that 18 of 41 small blocks (43.9%) exhibited no RDII influence and could be relocated to the next-priority point, while eight blocks (19.5%) displayed patterns suggestive of cross-connection inflow requiring detailed inspection. These results show that, even under limited monitoring resources, relocation and inspection priorities can be determined based on objective criteria.