A terminal outfall is a point of a stormwater drainage system where collected stormwater or wastewater is released into a larger body of water, such as a river, lake, or ocean. In hydrology, outfalls are terminal nodes of the drainage system used to define final downstream boundaries under dynamic wave flow routing. For other types of flow routing, the terminal outfall behaves as a junction. Only a single link can be connected to a terminal outfall node.
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The boundary conditions at the terminal outfall can be described by any one of the following stage relationships:
- Critical or normal flow depth in the connecting conduit
- Fixed stage elevation
- Tidal stage described in a table of tidal elevation versus hour of the day
- User-defined time series of water surface elevation versus time
The primary input parameters for the terminal outfall include:
- Invert elevation
- Boundary condition type and stage description
- The presence of a flap gate to prevent backflow through the outfall
- Downstream link slope for connecting elements, such as pipes, culverts, etc.
Terminal Outfall Design Considerations
The following considerations should be followed while designing terminal outfalls in the drainage system:
- Tailwater elevation
The tailwater depth or elevation in the storm drain outfall must be considered carefully. For most design applications, the tailwater will either be above the crown of the outlet or located between the crown and the critical depth of the outlet. The tailwater may also occur between the critical depth and the invert of the outlet.
- Orientation of outlet structure
The orientation of the terminal outfall is another important design consideration. Where feasible, the outlet of the storm drain should be positioned in the outfall channel so that it is pointed in a downstream direction. This will reduce turbulence and the potential for excessive erosion.
- Hydrological and hydraulic design
Designing a terminal outfall requires careful consideration of the following hydrological and hydraulic factors:
- Flow Capacity: The system must accommodate peak flow rates expected during heavy rainfall or storm events.
- Backflow Prevention: To prevent water from backing up into the drainage system during high tide or flood conditions, terminal outfalls may include flap gates or other backflow prevention devices.
Defining Terminal Outfall in Stormwater Projects
The GeoSTORM software allows the user to incorporate and define terminal outfalls into a stormwater project. The user can draw or assign terminal outfalls by either drawing or assigning nodes on the Map View using the following commands:
- Draw Terminal Outfalls
- Assign Terminal Outfalls
Refer to this article in our knowledge base to learn how to incorporate terminal outfalls into the stormwater project.
After adding terminal outfalls to the stormwater project, the Terminal Outfall Data command can be used to define and manage additional data and view the corresponding output results. During stormwater computations, the following parameters can be derived for terminal outfall:
- The peak flow rate that occurred at the terminal outfall
- Date and time when the peak flow rate occurred at the terminal outfall
- The maximum water surface elevation (WSEL) that occurred at the terminal outfall
- The maximum water depth that occurred at the terminal outfall
- The total accumulated discharge volume that has occurred at the terminal outfall
Refer to this article in our knowledge base to learn how to define terminal outfall data using the Terminal Outfall Data command.