Select which tailwater conditions to use for each outfall in the system.
Outfalls
On the Outfalls tab in the Analysis Criteria, each outfall in the selected drainage system will be listed showing a banner with its name, current tailwater condition and level if applicable. Select the Outfall banner to expand the view to see all the outfall properties.
Tailwater Condition
Zoom to Object. Zooms to the outfall structure in the drawing.
Open Properties. Opens the structure properties dialog box for the outfall.
Type. Specifies the method in which the depth of tailwater at the outfall part is determined. This provides an initial level value for the hydraulic analysis of the pipe network when performing the analysis.
- Free: Specifies that the tailwater is based on free discharge. During analysis the lesser of the calculated critical depth and normal depth will be used as the tailwater level. Since the free discharge level gets calculated per rainfall event, the Rainfall event overrides are not displayed.
- Fixed Level: Specifies a user-defined level for the tailwater depth. Enter the default level in the field to the right of the drop-down list. You can override the tailwater level under Rainfall event overrides for each rainfall event currently selected on the Rainfalls tab.
- Level Curve: Specifies that the tailwater is based on a level curve that varies with time. Select the default level curve in the drop-down to the right of the Type. If no level curves have been defined yet, use ‘Open level curves’ to create a level curve. You can override the level curve under Rainfall event overrides for each rainfall event currently selected on the Rainfalls tab.
- Crown: Specifies that the tailwater depth is assumed to be equal to the crown of the pipe that is upstream from the outfall part. The crown level will be displayed (read only) in the field to the right of the drop-down list. Since the crown does not change per rainfall event, the Rainfall event overrides are not displayed.
- Normal Depth: Specifies that the tailwater is assumed to be at normal depth (as determined by Manning's equation). Since the normal depth gets calculated per rainfall event, the Rainfall event overrides are not displayed.
- Critical Depth: Specifies that the tailwater is assumed to be at critical depth. However, this tailwater condition is not supported by the Analyse Drainage System command, so the Free tailwater condition will be used instead when analysing the drainage system. Since the free discharge level gets calculated per rainfall event, the Rainfall event overrides are not displayed.
- Average (dc + D)/2: Specifies that the tailwater depth is the average of the critical depth and diameter of the pipe that is upstream from the outfall part. However, this tailwater condition is not supported by the Analyse Drainage System command, so the Free tailwater condition will be used instead when analysing the drainage system. Since the free discharge level gets calculated per rainfall event, the Rainfall event overrides are not displayed.
Rainfall Event Overrides
Configure tailwater conditions for individual rainfall events. This section is only displayed when the tailwater condition type is set to Fixed Level or Level Curve. For each rainfall event, the user can override the default level or level curve.
When a level or level curve is overridden, a reset icon will be displayed next to the override, a Reset All option will be displayed above the table and a blue dot will appear on the Outfall banner. Select the reset icon to set the value back to the default for the selected outfall.
Reset all. Will be displayed when a level or level curve is overridden. Select Reset All to reset all the overrides back to the default for the selected outfall.
Open Level Curves. This will open a separate dialog box to manage level curves. Learn more