The outfall of the storm drain system is a key component which impacts the physical and hydraulic characteristics of the system. The hydraulic designer should consider the requirements and characteristics of the area in which the outfall facility is located. Important considerations in the identification of an appropriate system outfall include the following:

Whether the outfall is enclosed in a conduit or is an open channel, the hydraulic designer should assess its ability to convey design flows. The hydraulic designer should consider that the outfall may need to be modified to minimize any significant impact to the receiving channel or the surrounding property. Detention upstream of the outfall may be an option to channel modification.

An outfall for a Department storm drain system must be operated for the life of the system. This implies that the Department must have access to all parts of the outfall for purposes of maintenance to ensure adequate operation of the drainage system. In some instances an outfall right-of-way (drainage easement) must be purchased to assure accessibility and that the discharge from the outfall will not be restricted.

Backflow preventers such as flap gates may be installed when necessary to prevent the outfall tailwater from backing into a storm drain system. However, backflow preventers are also maintenance intensive items which should be avoided if at all possible. The best solution is to design the storm drain system to prevent backflow from causing damage.