Flow - BC

Owned by Kester Scandrett
Last updated: 14 Jun, 2023
4 min read

For the flow boundary condition, first, use the Add button to add the cells for this boundary condition. Then use the Flow Time Series drop-down list, as shown in Figure 1 to link flow tables with the concentration table or time series. The Edit button allows the user to edit the associated time series. More details of each frame are provided below. 

In the Flow Boundary Conditions tab (as shown in Figure 1), the Number of Flow groups and the Number of Flow Series are listed at the top.

Figure 1. Flow Boundary Conditions form.

Boundary Group Information

  • Current Group: the current group is displayed
  • Group Name: enter the group name in the box
  • Flow Multiplier Switch: the drop-down box provides a number of options, from normal inflow to various lateral inflow/outflow options

Current Boundary Cell

The number of cells in the boundary group is listed with detailed locations (L, I, and J ). initial WSEL, Depth, and Bottom values.  The Add, Remove and Remove All buttons are used to edit the boundary cells.

Flow Assignment

  • Flow Time Series: this is the drop-down list (as shown in Figure 1) link flow tables with the concentration table or time series.
  • Edit: the button is used to edit the associated time series of the current boundary cell
  • All: the button allows the user to quickly assign all the boundary cells in the current group to the Flow Time Series.  For example, pressing the All shown in Figure 1 would set the flow series number for all cells in the group to the current “QSER_2” table settings.
  • Time Series Flow Split Factor: If the user has an input flow series for a river that requires distributing the river flow across several cells, the single time series of flow can be specified in the Flow Time Series and the Distribute Factor can be set to split the flow appropriately among the cells.  The Distribute Factors sets the factor based on cell volumes
  • Constant Flow: A constant inflow can also be distributed in a similar manner to the Time Series Flow Split Factor by using the Distribute Flows button

Momentum Flux

All calculations internal to the model grid allow for the momentum from cell to cell. However, when volume enters a cell from an inflow boundary, for example, a tributary at the edge of a river, the volume goes into a cell with no momentum. Adding the momentum to one of the cell edges relative to the rest of the grid allows the simulation something more like the real conditions. The default option is that the volume goes into the cell, but the momentum from the inflow of the flow boundaries is neglected. The momentum flux option allows the modeler to decide if they want the flow coming into the river from the edge to be parallel to the flow of the river or perpendicular to it.  If the modeler thinks the inflow should be parallel to the river then the momentum flux should be added to one of the faces to allow that. The control parameter NQSMF in C24 indicates if the volumetric sources and sinks have an associated momentum flux.

If the Momentum Options is selected, the user can select Flow Into Cell  or Flow Out of Cell and enter Momentum Width. The moment option will add the momentum fluxes generated by current inflow/outflow at the boundary. The momentum flux QMF (m4/s2) at a boundary cell is calculated as: 

QMF = QF*QF/( HPK*QWIDTH )

Where QF is the flow (m3/s), HPK (m) is the layer thickness, QWIDTH (m) is the width of the discharge for momentum. This width should be equivalent to that of the pipe or the outfall being used. 

The Momentum Options frame for the flow boundaries provides more control over how EFDC handles momentum flux on the cell faces. The options now include:

  • Inflow Momentum Flux Ignored (default option)
  • Momentum Flux on West U Face
  • Momentum Flux on South V Face
  • Momentum Flux on East U Face
  • Momentum Flux on North V Face

Depending on the cell position and the characteristic of the flow boundary (inflow or outflow), the "Flow into cell" or "Flow out of cell" should be selected. For example, a boundary cell of an inflow is located on the west edge of the model domain, as shown in Figure 2. In this case, to add the momentum flux, you must choose the "Momentum Flux on West U Face" combined with "Flow Into Cell."  Alternatively, selecting the "Momentum Flux on East U Face" combined with "Flow Out Of Cell" will give an almost identical result.  

Figure 2. Momentum flux options.

Boundary Group Concentration Assignments

  • Time Varying Concentration: the option is used with time series flows only. The user can select the time series for concentration by using the drop-down list in the Data Series list. 
  • Constant Concentration: the option is used with constant flow only. For example, if the user defines a constant salinity concentration of 5 ppt in the Constant Concentration frame but with zero constant flow, even if the boundary has flows assigned via a time series, the salinity input to the model will be zero.

Note: the concentration settings, for both time series and constant, are fixed for all cells for the same group.