When connecting a subsoil drain system by gravity to a storm sewer, what must be used?

When connecting a subsoil drain system to a storm sewer by gravity, a backwater valve is essential to prevent any potential flooding or backup of stormwater into the subsoil drain system. This situation can occur if the storm sewer system becomes overwhelmed during heavy rainfall, causing water to flow back into the drains.

The backwater valve functions as a one-way device, allowing water to flow out of the subsoil drainage system while preventing any reverse flow. This ensures that excess water does not enter the drain, which could lead to flooding in the surrounding areas and could compromise the functionality of the drainage system.

Considering the other options: a check valve, while also preventing backflow, is designed for use in situations involving pressurized systems and is not typically suited for gravity drainage applications. A holding tank is not applicable in this context as it is used to store liquid rather than facilitate drainage. A reduction fitting pertains more to altering the pipe sizes in a plumbing system rather than addressing backflow issues in drainage systems. Hence, the backwater valve is the correct choice for safeguarding the subsoil drain system in this scenario.