In GeoSTORM, an outfall (or terminal outfall) is a terminal node in the stormwater drainage system that defines the downstream boundary condition of the modeled network.
While an outfall acts as a physical structure that releases flow to a large receiving body, such as a river, lake, or ocean, its primary role is to establish the boundary condition of the system’s downstream end. Refer to this article in our knowledge base to learn more about terminal outfalls.
Since an outfall defines the downstream boundary of the network, correctly specifying its boundary condition is essential for obtaining reliable simulation results. An improperly defined boundary condition may lead to inaccurate hydraulic grade line (HGL) calculations, backwater effects, or flow routing errors.
Types of Outfall Boundary Conditions
In GeoSTORM, the boundary conditions at the terminal outfall can be described by any one of the following conditions:
Critical Depth
When Critical Depth is selected as the outfall boundary condition, the software will compute the boundary stage as the minimum of critical depth and normal depth. The user can select this boundary condition when the outfall discharges freely into the air, such as a culvert discharging above ground or into a steep drop.
Normal Depth
When Normal Depth is selected as the outfall boundary condition, the software will compute the boundary stage at the outfall node based on the normal depth of flow in the connecting conduit. The user can select this boundary condition when the downstream behaves like a long, uniform channel.
Fixed Water Surface Elevation (WSEL)
When Fixed WSEL is selected as the outfall boundary condition, the specified elevation remains constant throughout the simulation. The user can select this boundary condition when a constant downstream water level is known (e.g., discharging into a pond, lake, or controlled pool).
Tidal Boundary
When Tidal Boundary is selected as the outfall boundary condition, the software will require a data table of tidal elevation versus hour of the day. The user can select this boundary condition when the outfall is subject to tidal fluctuations.
Time Series
When Time Series is selected as the outfall boundary condition, the software will require a user-defined stage (water surface elevation) time series covering the full simulation duration. The user can select this boundary condition when downstream water levels vary with time, such as when discharging into a river or reservoir.
In the Terminal Outfall Data dialog box, the user can select and apply these outfall boundary conditions using the Outfall type dropdown combo box entry.
Refer to this article in our knowledge base to learn more about the Terminal Outfall Data command.
Common Errors While Setting the Outfall Boundary Condition
When defining outfall boundaries, the user needs to avoid these common mistakes:
- Entering depths instead of elevations
For Tidal Boundary or Time Series outfall types, GeoSTORM requires absolute water surface elevations for computation, not depths relative to the invert.
Therefore, entering depth values instead of elevation values in the data grids for the aforementioned outfall types can lead to incorrect results.
- Not covering the full simulation duration with tailwater data
Data gaps can result in incorrect or constant water level assumptions.
- Using Critical Depth for backwater conditions
When backwater conditions exist downstream, using the critical depth outfall type may underestimate tailwater impacts and give unrealistic results.
- Entering negative depths at terminal outfalls
Negative depth values are invalid at terminal outfalls and using them for computation can result in errors.