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Diversion Data – Selecting a Diversion Method

A diversion is an element with two outflows, main and diverted, and one or more inflows. Inflow comes from other elements in the basin model. If there are multiple inflows, all inflows are added together before computing the outflow. A diversion can be used to represent weirs or pump stations that divert flow into canals or off-stream storage. The diversion element includes various methods for limiting the diverted flow that will be taken out of the channel. All flow that is not diverted becomes the main outflow. Diverted flow can be connected to an element that is computationally downstream or can simply be lost out of the system.

In GeoHECHMS, the diversion methods can be selected from the General Specifications section of the Diversion Data dialog box. Each diversion element may use a different method, or several diversions may use the same method.

Follow the steps below to select a diversion method:

  1. From the Input ribbon menu, click the Diversions dropdown menu, and then select the Diversion Data command.
    Diversion Data input ribbon menu command
  2. The Diversion Data dialog box will be displayed.
    Diversion Data dialog box
  3. From the Diversion ID dropdown combo box, select the diversion to assign a diversion method.
  4. From the Diversion method dropdown combo box, select the diversion method.
    Diversion method dropdown combo box
  5. To enter the data for the selected diversion method, select the Diversion Data option from the Diversion Specifications dropdown combo box.
    Diversion Specifications dropdown combo box - Diversion Data option
  6. The Diversion Data panel will be displayed. Note that the Diversion Data panel content changes based upon the diversion method selected in the General Specifications section.
    Diversion Data panel

The following sections describe the different diversion methods and how to enter the parameters for each method in the Diversion Data panel.

Diversion Method: None

When None is selected for the diversion method, the Diversion Data dropdown combo box entry is disabled (i.e., grayed out).
Diversion Method: None data panel

If None is selected for the diversion method, the diversion will pass all flow down the main connection and no flow will be diverted.

Diversion Method: Constant Discharge

This method uses a constant diversion flowrate for the entire simulation. It is best suited to event simulation when the diversion flow is more likely to be constant. If the diversion flowrate changes during the year according to a reliable pattern, the optional percentage pattern can be used to adjust the constant rate throughout the year.

When Constant Discharge is selected for the diversion method, the following data panel will be displayed:
Diversion Method: Constant Discharge data panel

The following input parameters are provided in the data panel:

  • Constant discharge
    This entry field allows the user to define the discharge being diverted. If the total inflow into the diversion element is less than this specified discharge, the diversion flow rate will be limited to the total inflow.
  • Constant discharge annual pattern
    This dropdown combo box allows the user to select an already defined annual pattern. The annual pattern provides a percentage that varies throughout the year. If no annual pattern is selected, then the software will use the same diversion flowrate for each time interval in the simulation. Clicking on the pencil icon allows the user to rename the percent pattern ID. Clicking on the […] browse button will display a Diversion Constant Discharge Annual Pattern Data dialog box, which allows the user to define a new user-defined annual pattern as a percentage.
    Diversion Constant Discharge Annual Pattern Data dialog boxNote that the user can define a percentage value ranging from 0 to 1000%.

Diversion Method: Inflow Diversion Table

This method uses a functional relationship between inflow and diverted flow to determine the amount of flow that should be diverted for each time step. The defined range of inflows should cover the complete range of total inflow from the upstream elements. Usually, the first defined inflow value is defined as zero. The last defined inflow value should be greater than the maximum anticipated inflow into the diversion element. Diversion flow is a dependent variable and must be specified for each corresponding inflow value.

When Inflow Diversion Table is selected for the diversion method, the following data panel will be displayed:
Diversion Method: Inflow Diversion Table data panel

The following input parameters are provided in the data panel:

  • Inflow
    This data column allows the user to define the discharge entering the diversion structure from upstream sources.
  • Diverted Flow
    This data column allows the user to define the corresponding discharge being diverted from the downstream routing reach.

Diversion Method: Lateral Weir

The only method currently available for computing flow over the lateral weir is the broad-crested spillway method. Flow depth in the channel is computed using a rating curve. It is assumed to be level with a uniform head along the length of the weir computed using the rating curve. Tailwater is similarly computed using a rating curve that represents the characteristics of the area where the weir discharges the diverted flow.

When Lateral Weir is selected for the diversion method, the following data panel will be displayed:
Diversion Method: Lateral Weir data panel

The following input parameters are provided in the data panel:

  • Computational method
    This dropdown combo box entry allows the user to select the weir computational method to be used. Currently, only the Broad Crested Weir option is available.
  • Weir discharge coefficient
    This entry field allows the user to define the weir discharge coefficient to be used in the weir computations. Note that different coefficients may be selected, depending on the project data units: Metric (SI) or US Units. By default, the software uses a value of 2.6 for US Units. Clicking on the […] lookup button will display a Weir Discharge Coefficient dialog box.
    Weir Discharge Coefficient dialog box
  • Weir crest elevation
    This entry field allows the user to define the elevation of the weir crest. This value needs to be in the same vertical datum as the rating curves are defined in.
  • Weir crest length
    This entry field allows the user to define the length of the weir crest. It should be the total width through which water passes over. Clicking on the […] browse button causes the dialog box to temporarily disappear and allows the user to measure the weir crest length from the Map View.

Rating Curve Definition

This section allows the user to define the rating curve data. Any defined rating curve should provide the stage for the entire range of inflows that will occur during a simulation. The defined curve must be monotonically increasing.

The following tabbed panels are provided:

Channel

This panel allows the user to define a channel rating curve. It represents the elevation versus discharge at the upstream side of the flow diversion.

The following entries are used for defining the rating curve:

  • Elevation
    This data column allows the user to specify the corresponding elevation for the defined rating curve.
  • Discharge
    This data column allows the user to specify the corresponding discharge for the defined rating curve.

Tailwater (Optional)

This optional panel allows the user to define a tailwater rating curve. The rating curve should define the tailwater stage in the area where the diverted flow is discharged. It is used to automatically account for the submergence of the weir. If there is no tailwater rating curve defined, then the software computes the flow over the weir assuming no tailwater influence.
Rating Curve Definition - Tailwater (Optional) panel

Note that the entries of the Tailwater (Optional) panel are similar to that of the Channel panel.

Diversion Method: Pump Station

A pump station diversion method is designed to represent one or more pump units extracting water from the main channel and discharging it into a diversion. The head-discharge curve describes the pumping capability of the pump as a function of the total head. The total head is the sum of the equipment loss and the dynamic head. The dynamic head is first estimated as the difference between the water surface elevation in the channel and the line elevation. If the water surface elevation is above the line elevation, then the estimated value will be zero. Secondarily, the estimate is adjusted for tailwater submergence. This second stage is only necessary if an optional tailwater rating curve is specified. When specified, the tailwater water surface elevation is compared to the line elevation. If the tailwater exceeds the line elevation, then the depth of submergence over the line elevation is added to the initial estimate of the dynamic head. The head-discharge curve is used to calculate the diverted flow given the calculated total head.

When Pump Station is selected for the diversion method, the following data panel will be displayed:
Diversion Method: Pump Station data panel

The following input parameters are provided in the data panel:

  • Head-discharge pump curve
    The head-discharge curve defines the pumping capacity of the pump as a function of the total head. The dropdown combo box allows the user to select an already defined pump curve. Clicking on the pencil icon allows the user to rename the elevation discharge ID. Clicking on the […] browse button will display a Head Discharge Pump Curve Data dialog box, which allows the user to define a new user-defined pump curve.
    Head Discharge Pump Curve Data dialog box
  • Number of pumps
    This entry field allows the user to define the number of identical pumps in operation at the diversion. It allows pump data to be defined only once when there are multiple pump units of the same type being used throughout the model.
  • Pump intake elevation
    This entry field allows the user to define the elevation for the pump intake.
  • Pump highest line elevation
    This entry field allows the user to define the highest elevation in the pump pressure line from the pump to the discharge point.
  • Pump ON elevation
    This entry field allows the user to define the trigger elevation for the pump to turn on. Once the pump turns on, it will remain on until the stage in the intake drops below this trigger elevation.
  • Pump OFF elevation
    This entry field allows the user to define the trigger elevation for the pump to turn off. Once the pump turns off, it will remain off until the stage in the intake exceeds the pump on elevation. This elevation must be below the pump on elevation.
  • Minimum pump ON time
    This optional entry field allows the user to define the minimum run time. If it is used, once a pump turns on, it must remain on for the specified minimum run time—even if the intake water surface elevation drops below the trigger elevation to turn the pump off. The only exception is if the water surface elevation drops below the intake elevation and the pump starves for water. In this situation, the pump will shut off.
  • Minimum pump OFF time
    This optional entry field allows the user to define the minimum rest time. If it is used, once a pump turns off, it must remain off for the specified minimum rest time—even if the intake water surface elevation exceeds the trigger elevation to turn the pump on.
  • Pump head loss
    This entry field allows the user to define the energy head loss of the pump and associated equipment. It includes all energy losses between the intake and discharge points. These losses are sometimes called static losses because they do not change very much even as the water surface elevation at the intake fluctuates. Components of this loss include entrance losses at the intake, losses in the pump, pipe friction losses, pipe bend losses, and exit losses at the discharge. The defined loss is added to the dynamic head to determine the total head against which the pump must operate.

Note that the Rating Curve Definition section of the Pump Station diversion method is similar to that of the Rating Curve Definition section of the Lateral Weir diversion method.

Diversion Method: Time Series

The time series flow diversion method is designed for situations where the flow diverted from the channel is measured with a flow gage. This method allows the user to enter the time series of discharges that are diverted from the channel. If the specified diversion discharge exceeds the total inflow to the diversion, then the diversion will be limited to the inflow volume.

When Time Series is selected for the diversion method, the following data panel will be displayed:
Diversion Method: Time Series data panel

The following input parameter is provided in the data panel:

  • Time series diverted flow
    This dropdown combo box allows the user to select an already defined time series. Clicking on the pencil icon allows the user to rename the diverted flow ID. Clicking on the […] browse button will display a Diverted Flow Time Series Data dialog box, which allows the user to define a new user-defined time series.
    Diverted Flow Time Series Data dialog box

Pros and Cons of Diversion Methods

MethodProsCons
Constant Discharge
  • It is best suited for event simulation when the diversion flow is more likely to be constant.
  • It requires a percentage pattern to adjust the constant rate throughout the year.
  • Inflow Diversion Table
  • It is best suited for determining the amount of flow that should be diverted from the channel.
  • Diversion flow is the dependent variable and must be specified for each corresponding inflow value.
  • Lateral Weir
  • This method is best suited for large rivers.
  • Only broad-crested spillway method is available for computing flow over the lateral weir.
  • Pump Station
  • This method is best suited when there is limited space for a diversion.
  • The pump station data must be accurate. Otherwise, analysis results can be affected.
  • Time Series
  • Most appropriate for situations where the flow diverted from the channel is measured with a flow gage.
  • This method cannot be used without specifying the time series discharge gage.
  • About the Author Chris Maeder

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