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 it's name, current tailwater condition, and elevation 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 elevation 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 elevation. Since the free discharge elevation gets calculated per rainfall event, the Rainfall event overrides are not displayed.
- Fixed Elevation: Specifies a user-defined elevation for the tailwater depth. Enter the default elevation in the field to the right of the drop-down list. You can override the tailwater elevation under Rainfall event overrides for each rainfall event currently selected on the Rainfalls tab.
- Elevation Curve: Specifies that the tailwater is based on an elevation curve that varies with time. Select the default elevation curve in the drop-down to the right of the Type. If no elevation curves have been defined yet, use Open elevation curves to create an elevation curve. You can override the elevation 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 elevation will display (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 Analyze Drainage System command, so the Free tailwater condition will be used instead when analyzing the drainage system. Since the free discharge elevation 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 Analyze Drainage System command, so the Free tailwater condition will be used instead when analyzing the drainage system. Since the free discharge elevation 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 Elevation or Elevation Curve. For each rainfall event the user can override the default elevation or elevation curve.
When an elevation or elevation 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 display when an elevation or elevation curve is overridden. Select Reset All to reset all of the overrides back to the default for the selected outfall.
Open Elevation Curves.
This will open a separate dialog box to manage elevation curves.
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