Tailwater Elevation is used in the computation of head — the difference between reservoir pool elevation (above the dam) and tailwater elevation (below the dam). Specification of Tailwater Elevation is required when you model power plants and/or pumps. You can add Tailwater Elevation to Dams, Diverted Outlets, Outlet Groups, or individual outlets.
When Tailwater Elevation is specified at the Dam, it applies to all outlets in the dam unless overridden by another Tailwater specification at a level closer to a specific outlet. For example, consider the two Tailwater nodes illustrated in "Figure: Reservoir Tree with Tailwater Nodes" — one Tailwater node was added to the Dam and another was added to the Pump. The Pump will use the Tailwater definition that was attached it, but the Power Plant will use the Tailwater Elevation specified at the Dam.
To add Tailwater Elevation:
In the Reservoir Tree, right-click on the dam, group, or outlet to which you want to add a Tailwater Elevation specification.
Select Add Tailwater Elevation from the component's context menu ("Figure: Dam Context menu - Add Tailwater Elevation").
- A Tailwater node will appear in the reservoir tree beneath the component you selected and the Tailwater pane will be displayed in the edit panel.
The Tailwater pane ("Figure: Tailwater Node and Tailwater Edit Pane") provides three options for specifying the tailwater elevation:
- Constant Elevation,
- Downstream Control (elevation of a downstream element), or
- Rating Curve.
You can choose one, two or all three of the options and ResSim will evaluate all those selected and use the highest value among them.
If you select Rating Curve, you can specify the rating as a Simple Rating for which you must enter the relationship between Flow and Stage in the table, or as a Rating Function for which you must define tailwater Stage (or elevation) as a function of a Model Variable, External Variable, State Variable, or Two Variables.
A rating curve usually relates STAGE at a gage to FLOW. And the measured quantity is usually STAGE, which would make STAGE the independent variable. But to ResSim FLOW is the known variable and it wants to lookup STAGE, so STAGE is seen as the dependent variable. So, be careful when entering the rating table to be sure FLOW is the first column of data and STAGE is the second.
In addition, each rating curve has an option for a Stage Datum to be provided. The Stage Datum (or gage zero) is the elevation of a STAGE of 0.0 at the gage. Each time ResSim looks up stage from a rating curve, it also computes elevation using the Stage Datum. If you do not provide a value for the Stage Datum, ResSim will use a default value of 0.0.