Conveyance and Cross-section Property Display

It is possible to display the calculated channel conveyance against stage for a RIVER section unit, either on a cross-section plot or on its own. A common cause of instabilities in a model is a decrease in conveyance with increasing stage, therefore the conveyance plotting tool gives the modeller a quick reference to check that conveyance increases monotonically and smoothly with stage. A decrease in conveyance as the water level goes out-of-bank may be rectified by adding a panel marker at bank level.

The conveyance may also be shown in tabular form alongside other cross-section properties for a range of water levels, namely depth, area, top width and wetted perimeter. Flow (discharge), velocity and Froude number at normal depth are also displayed, having been calculated from Manning’s equation,    , using the slope value   , which must be entered by the user in the RIVER section data form.

Displaying Conveyance on a Cross-Section Plot

When a cross-section plot is displayed, the conveyance is automatically calculated. It can be shown on the plot by selecting “Edit Series” from the context menu (Ctrl-D) and selecting the Conveyance checkbox. Note: if the node immediately upstream and/or downstream of this node is also a RIVER section unit, conveyance can be displayed simultaneously for these too by selecting the Upstream and/or Downstream Conveyance checkbox as appropriate.

Figure 1: Conveyance shown on a cross section plot - note the decrease in conveyance at bank level, indicating that this section would benefit from insertion of panel markers

The stage for the conveyance plot is displayed on the same vertical axis as the section data, providing the modeller an easy reference for the relationship between conveyance and section data. Conveyance is displayed on the top axis, with cross chainage on the bottom axis.

Tabulating Cross-Section Properties

Cross-section properties can be displayed in tabular form by highlighting a single or a number of RIVER section units and selecting Model Review Tools > 1D Models >Tabulate cross section properties from the Flood Modeller Toolbox. The following items are displayed for stage values over the range of the section data: flow, depth, velocity, Froude number, conveyance, width and wetted perimeter. Flow, velocity and Froude number are calculated from normal depth, using Manning’s equation and the slope, which must be entered by the user in the RIVER section data form. The node label and slope are also displayed for reference purposes.

The tabulated data can be copied to the clipboard, to paste in another application, or saved to file in csv (comma separated variable) format, for further analysis by selecting the relevant buttons at the bottom of the window.

Figure 2: Tabulated cross-section property data

 The data can also be plotted by selecting the “Conveyance Plotting” tab. The default plot is for Conveyance against Stage, but any two variables in the table may be plotted against each other. Selecting the “Plot” button displays the data on screen, and the “Print” button will send the current plot to the printer. The plot format can also be customised using the “Edit chart settings” button.

Figure 3: Cross-section property plotting