Optimization Tailwater
Only a subset of available methods are supported in Optimization. These methods are: Linked or Input, Base Value Only, Base Value Plus Lookup Table, Stage Flow Lookup Table, and Coefficients Table. Selection of a method other than these results in an error. The method selected in the Optimization Tailwater category should correspond to the one selected in the simulation Tailwater category.
Tailwater Elevation may be part of the Optimization problem, and it is handled differently for each method. Below is a description of the behavior of Tailwater Elevation.
See Tailwater in Objects and Methods for details on the Simulation methods.
If Tailwater Elevation is not input, it is replaced as follows:
(5.13)    TailwaterBaseValue: Tailwater Elevation = Tailwater Base Value
Slots Specific to This Method
Tailwater Base Value
Type: Series
Units: Length
Description: See Linked or Input in Objects and Methods for details.
Opt Base Value Only
If Tailwater Elevation is not input, it is replaced as follows:
(5.14)    (Tailwater Base Value(t) + Tailwater Base Value(t-1) ) / 2
Slots Specific to This Method
Tailwater Base Value
Type: Series
Units: Length
Description: See Base Value Only in Objects and Methods for details.
Tailwater outflow relationship. Not drawn to scale.
Opt Base Value Plus Lookup Table
If Tailwater Elevation is not input, this slot is replaced by a mathematical expression based on Tailwater Base Value and a numerical approximation of Tailwater Elevation as a function of Outflow (Numerical 2-D Approximation), as defined by the user in the Tailwater Table.
Tailwater Elevation is constrained as follows:
(5.15)    (tailwaterBaseValue(t) + tailwaterBaseValue(t-1)) / 2 + tempTWLookupValue
where tempTWLookupValue is a Numerical 2-D Approximation of increase in Tailwater Elevation due to flow, as described in the Tailwater Table. The Tailwater Table Lookup LP Param table values for outflow are used as approximation points, indexing the Tailwater Table to determine tempTWLookupValue from the Tailwater Elevation column.
Slots Specific to This Method
Tailwater Base Value
Type: Series
Units: Length
Tailwater Table
Type: Series Slot
Units: Flow vs Length
Description: Reservoir outflow vs either the tailwater elevation or the tailwater elevation increment
Information: If the Tailwater Base Value is non-zero, the Tailwater Table gives values of incremental increase in Tailwater Elevation over the Base value. If the Tailwater Base Value is zero, the table simply gives the Tailwater Elevation values. The Tailwater Table should have increasing values of outflow. Tailwater Elevation is required to be a convex function of outflow. The preferred order of approximation is substitution, piece-wise, two-point line, tangent. See Object / Simulation documentation for details.
I/O: User input
Tailwater Table Lookup LP Param
Type: Table Slot
Units: Flow, Flow, Flow
Description: LP Parameter approximation points for Outflow.
Information: The Tailwater Table Lookup LP Param table values for outflow are used as approximation points indexing the Tailwater Table to determine tempTWLookupValue from the Tailwater Elevation column.
I/O: User input only
temp Tailwater Lookup Value
Type: Series
Units: Length
Description: Numerical 2-D Approximation of increase in Tailwater Elevation due to flow as described in the Tailwater Table.
Opt Stage Flow Lookup Table
Tailwater Elevation is numerically approximated as a function of Stage and Flow, as defined by the user in the Stage Flow Tailwater Table. The tables are queried depending on method selection in the Tailwater Linearization Automation category, as follows:
• If the selected method in the Tailwater Linearization Automation category is None, the user-input Tailwater LP Param table values are used.
• For the Range Input Automation method, the automatically developed points are used.
Tailwater Elevation is approximated numerically (3-D approximation) as a function of Outflow and Tailwater Base Value based on the Stage Flow Tailwater Table. The Tailwater Elevation LP Param table Tailwater Base Value and flow values are used as approximation points indexing the Downstream Stage and Outflow columns of the Stage Flow Tailwater Table, respectively.
Slots Specific to This Method
Tailwater Base Value
Type: Series
Units: Length
Stage Flow Tailwater Table
Type: Table Slot
Units: Flow vs Length vs Length
Description: Reservoir Outflow vs Downstream Elevation (Tailwater Base Value) vs Tailwater Elevation
Information: Data must be entered into the table in increasing blocks of the same Outflow value for the three-dimensional table interpolation to work correctly. For every block of same Outflows in column 1, Downstream Stages should be listed in increasing order in column 2, and the corresponding Tailwater Elevations in column 3. Tailwater elevation is required to be a concave function of Outflow. The preferred order of approximation is substitution, piece-wise, two-point line, tangent. Internally, the Stage Flow Tailwater Table is rearranged to use the Stage as the primary index (column) with outflow as the secondary index (column). This rearranged table may be labeled as “Convolved Stage Flow Tailwater Table” in output messaging. Also, because of this rearrangement, it is desirable to repeat stage values for each outflow.
Defined by: User input
Tailwater Elevation LP Param
Type: Table Slot
Units: Length, Flow, Flow, Flow
Description: Specifies the fixed tailwater base value point and the outflow points used to take the tangent, line and piecewise approximations for tailwater elevation linearization
Information: This table is used for linearization. The best Tailwater Base Value point to choose for tangent approximation would be an outflow equal to the expected value of outflow during the run; for the line approximation, the minimum and maximum values expected during the run; for piecewise approximation, the minimum and maximum values expected during the run plus additional intermediate values to more closely fit the tailwater curve.
Defined by: User input unless the selected method in the Tailwater Linearization Automation category is not None.
Tail Water Reference Elevation
Type: Table Slot
Units: Length
Description: Lowest Reservoir discharge Elevation when there are no backwater effects from a downstream pool (reservoir)
Information: Although this is part of the Stage Flow Lookup Table method (and its corresponding Opt method), this slot does not influence optimization. See Object / Simulation documentation for details.
I/O: User input
Opt Coefficients Table
This method sets up the physical Tailwater Elevation constraints using the same equation and coefficients used in the Coefficients Table tailwater method.

Slots Specific to This Method
Tailwater Base Value
Type: Series
Units: Length
Description: See Coefficients Table in Objects and Methods for details.
Tailwater Coefficients
Type: Table
Units: multi
Description: See Coefficients Table in Objects and Methods for details.
Information: If the Power Surface Approximation method is selected, then there are additional restrictions on the coefficients that can be specified. With the Power Surface Approximation, having terms for Outflow[t-1], Tailwater BaseVal[t] and Tailwater[t-1] are not allowed.
Revised: 08/02/2021