What is the most likely cause of an immediate significant increase in flow rates in combined sanitary sewers due to rainfall?

The most likely cause of an immediate significant increase in flow rates in combined sanitary sewers during rainfall is direct inflow. Direct inflow refers to the rapid entry of stormwater into the sewer system through means such as roof leaders, catch basins, and improper connections where stormwater drains are inadvertently connected to the combined sewer. This influx occurs quickly as the rain begins, leading to a sudden spike in flow rates.

In contrast, groundwater infiltration typically occurs more slowly over time as groundwater seeps into the sewer system, making it less impactful in the immediate aftermath of a rainfall event. Stormwater runoff, although it contributes to overall flow, does so through various drainage systems and is less direct compared to inflow. Exfiltration would involve wastewater escaping from the sewer system, which would not contribute to an increase in flow rates but rather reduce the volume within the sewer system. Thus, direct inflow is the primary factor in causing the significant and immediate rise in flow rates in combined sewer systems as rain falls.