Boundary Condition Alternatives
The boundary condition alternative allows you to define boundary condition settings for outfall elements. The following columns are available:
| Column | Description |
|---|---|
| ID | Displays the unique identifier for each element in the alternative. |
| Label | Displays the label for each element in the alternative. |
| Elevation (User Defined Tailwater) | The user defined tailwater value. This column is only available for elements that use the User Defined Tailwater Boundary Condition Type. |
| Boundary Condition Type | Lets you specify the type of boundary condition to be used at the associated outfall element. The following choices are available: Free Outfall —For a free outfall control, it is assumed that the downstream discharge conditions do not directly affect the hydraulic response of the structure. This is equivalent to assuming that the downstream TW elevation never rises above the controlling structure outfall invert of the structure. Time-Elevation Curve —For this type of boundary condition, a time-elevation curve is specified to establish at the outfall a tailwater condition that varies over time. Elevation (User Defined Tailwater) —This type of boundary condition allows you to directly enter a tailwater value. Elevation Flow Curve —For this type of boundary condition, an elevation-flow table is specified to simulate at the outfall a channel or outlet structure where flow rate varies over time. Boundary Pond —This type of boundary condition allows you to specify a pond element as the outfall. Normal —When this type of boundary condition is chosen, the boundary condition is the normal depth of the upstream link. If there is more than one upstream link, the one with the highest normal depth is used as the boundary condition. Tidal —For this type of boundary condition, a cyclic time-elevation curve is specified to establish at the outfall a tailwater condition that varies over time in a cyclical fashion. |
| Tidal Gate | Lets you specify whether or not there is a tidal gate at each outfall in the alternative. |
| Cyclic Time-Elevation Curve | Clicking the Ellipsis (…) button in this field opens the Cyclic Time-Elevation Curve editor, allowing you to define the cyclic time vs. elevation curve for each outfall in the alternative that uses the Tidal Boundary Condition Type. |
| Boundary Element | Contains list boxes that display all of the elements that are available to be used as boundary ponds. This column is available only when the Boundary Pond Boundary Condition is specified. |
| Time-Elevation Curve | Clicking the Ellipsis (…) button in this field opens the Time-Elevation Curve editor, allowing you to define the time vs. elevation curve for each outfall in the alternative that uses the Time-Elevation Curve Condition Type. |
Note: (Applies only when current solver is Explicit (SWMM Solver)): EPA-SWMM supports an outfall drain to a catchment. With this connection option, a modeler may send excess runoff from one sub-network as overland runoff entering another stormwater collection sub-network. To model this in Bentley software: Select an outfall, set its Boundary Condition Type = "User-Defined Tailwater". In addition to entering the water surface elevation value of the outflow, select a catchment in the model with the attribute "Route to Catchment". The "Route to Catchment" attribute option is only visible when current solver is Explicit (SWMM), and otherwise ignored by other solvers.