skip to main content
Objects and Methods : Storage Reservoir : User Methods : Tailwater
Tailwater
The Tailwater category is used to model the effects of the tailwater elevation on reservoir releases. The tailwater elevation represents the water surface elevation immediately downstream of the storage reservoir’s release structure.
* None
This is the default method. It performs no calculations and there are no slots associated with it.
* Input Tailwater Elevation
This method is used if the tailwater elevation is linked to the downstream reservoir pool elevation or directly input by the user. There are no tailwater elevation calculations associated with this method. In other words, the outflow does not affect the tailwater elevation other than by changing the downstream pool elevation. When the tailwater elevation is above the tailwater reference elevation, the effective head is used to compute the max release.
Slots Specific to This Method
 Tailwater Elevation
Type: Series Slot
Units: LENGTH
Description: The elevation associated with the downstream end of the head drop across a controlled release structure.
Information: The Tailwater Elevation is usually considered to be the pool elevation of the downstream reservoir.
I/O: Optional; may be linked or input
 Tailwater Reference Elevation
Type: Scalar Slot
Units: LENGTH
Description: The lowest reservoir discharge elevation where there are no effects from a downstream reservoir pool elevation
Information: This slot is the elevation of the downstream side of the controlled release structure. The Effective Head is computed using either this elevation or the Tailwater Elevation (whichever is greater).
I/O: Required input
 Effective Head
Type: Series Slot
Units: LENGTH
Description: The head difference across a controlled release structure
Information: Computed as difference between the reservoir pool elevation the greater value of either the Tailwater Elevation or the Tailwater Reference Elevation. If the Tailwater Reference Elevation is greater than the Tailwater Elevation, the Effective Head slot is not used.
I/O: Output only
 Head Vs Max Release
Type: Table Slot
Units: LENGTH vs FLOW
Description: The relationship between effective head and maximum reservoir release
Information: When the Tailwater Elevation is greater than the Tailwater Reference Elevation, the Effective Head is used with this table to determine the max release. Otherwise, the Pool Elevation and Max Release table are used to compute the Max Release (that is, the Effective Head is not considered).
I/O: Required input
Method Details  
In this method, Tailwater Elevation is either linked to the downstream reservoir pool elevation, or input by the user. Therefore, there are no calculations to determine the Tailwater Elevation. However, the Tailwater Elevation is used to compute the Effective Head, which is used to compute the maximum reservoir release. If the Tailwater Elevation is greater than the Tailwater Reference Elevation, the Effective Head is computed as follows:
    
If the Tailwater Elevation is less than the Tailwater Reference Elevation, Effective Head is computed as follows:
    
Note:  Important Max Release Calculation Details
If a tailwater method is not selected, the maximum reservoir release is computed using the average Pool Elevation and the Max Release table. The selection of a tailwater method allows the modification of the maximum reservoir release calculation. When the Input Tailwater Elevation method is selected, the average Effective Head (using the beginning of timestep and end of timestep values) is used with the Head Vs Max Release table to determine the maximum reservoir release. However, this is done ONLY if BOTH the beginning of timestep and end of timestep Tailwater Elevations are greater than the Tailwater Reference Elevation. Otherwise, the average Pool Elevation is used with the Max Release table to determine the maximum reservoir release.
Revised: 01/05/2024