Sag Operation

This category appears at the selection of any regulation discharge method in the Regulation Discharge category, so is related to the operating level balancing approach to flood control. Once a flood hydrograph recedes to the normal regulating discharge, a sag operation can be used to reduce the regulating discharge for a few timesteps to allow time for local field drainage. A sag operation will continue for a prescribed number of timesteps during which the regulation discharge will be reduced to the sag discharge. After the sag operation has continued for the specified number of timesteps, operations resume at the normal regulating discharge.

None

This default method means no sag operations are performed.

Sag Operation

This method implements sag operations. A sag operation is explicitly turned on and continues until it is explicitly turned off, although the regulation discharge is only reset to the reduced sag value for a first number of timesteps equal to the sag period. Sag operations are tracked in the Sag Operation series slot where an incremented number appears in each timestep that is part of a sag operation. Timesteps not in sag operation are assigned zero.

Slots Specific to This Method

Sag Period

Type: TableSlot

Units: NONE, FLOW

Description: Number of timesteps that the hydrograph should sag below regulation discharge to allow for local field drainage, and the temporary discharge to be used for the sag period.

Information:

I/O: Required input

Links: Not linkable

Sag Operation

Type: SeriesSlot

Units: NONE

Description: Contains an incremented number for each timestep that is part of a sag operation, allowing tracking of sag operations across simulation timesteps. Timesteps not in sag operation contain a zero.

Information: Other than an initial value to indicate the status of sag operations at the beginning of the run, this slot is output only.

I/O: Output with initial input

Links: Not linkable

Sag Tolerance

Type: ScalarSlot

Units: DECIMAL

Description: When a Sag Operation is ongoing, this is the proportional amount by which the total inflow for the current timestep must be greater than the calculated regulation discharge for current forecast timestep in order for the Sag Operations to be turned off.

Information: The default value of 0.03 is from the 3% value coded into the COE’s Super program code.

I/O: Required input

Links: Not linkable

Method Details

The Sag Operation method does the following:

1. Check for an ongoing sag operation. There is an ongoing sag operation if the previous simulation timestep had a nonzero number written to the Sag Operation slot.

2. If there is an ongoing sag operation, test to see if it needs to be turned off. If the total inflow for the current simulation timestep is 1 plus the value in the Sag Tolerance Slot (default is 0.03) times greater than the calculated regulation discharge for the current simulation timestep, turn off sag operations and assign zeros to the Sag Operation slot for all timesteps in the forecast period. Regulation discharges for the forecast period will remain unchanged due to sag operations.

3. If sag operations are ongoing and are not turned off, continue incrementing the numbers in the Sag Operations slot for all timesteps in the forecast period. Any timesteps in the forecast period that have an incremented number less than or equal to the sag period defined in the Sag Period slot should have their regulation discharges set to the temporary sag discharge value also entered in the Sag Period slot.

4. If there is no ongoing sag operation, check to see if one needs to be turned on. A sag period is turned on if the forecast inflow for every timestep in the forecast period is less than that timestep’s regulation discharge, and if the previous simulation timestep’s total flow is greater than the current simulation timestep’s regulation discharge.

5. If a sag operation is turned on, assign the number 1 to the Sag Operation slot for the current simulation timestep and assign incremented numbers to all other timesteps in the forecast period. Any timesteps in the forecast period that have an incremented number less than or equal to the sag period defined in the Sag Period slot should have their regulation discharges set to the temporary sag discharge value also entered in the Sag Period slot.

Revised: 01/11/2023