Abstract
<title>Abstract</title> <p>Stormwater flowing into sanitary sewers (RDII) creates serious urban management challenges, especially in coastal areas where water levels change frequently. This study assessed RDII patterns in the Lagoa da Conceição system (Florianópolis, Brazil) using the RTK unit hydrograph approach. Eight rain events at two pump stations (Ponte PS and Rendeiras PS) were reviewed using flow, rainfall, and tide/lake-level data. Key findings revealed that Ponte PS responded to events with a quicker and larger RDII flow, indicated by higher R coefficients and a greater vulnerability to stormwater entry. The timing and shape (T and K parameters, respectively) of the hydrograph at Ponte PS were significantly affected by lake level, confirming the presence of a backwater effect. In contrast, Rendeiras PS showed a slower, less pronounced response, with its RDII largely shaped by the sewer network design. Therefore, setting RTK values for coastal systems should include downstream effects rather than relying on generic figures. These findings highlight the need for targeted repairs and upgrades to enhance resilience against severe weather in coastal sanitation systems, clarifying where interventions are most needed.</p>