The default method for ineffective flows is that the area defined as ineffective will contain water but have no conveyance (the velocity is assumed to be zero). This remains true until the water surface reaches a trigger elevation (an elevation set by the user, as to when the ineffective flow area should become effective again). Once the water surface is higher than the trigger elevation, the entire ineffective flow area becomes effective. Water is assumed to be able to move freely in that area based on the roughness, wetted perimeter and area of each subsection.

Occasionally you may have a need to have these ineffective flow areas remain ineffective permanently. The ineffective flow areas can be set to the permanent mode individually from the cross section editor, or through a table from the geometric data editor. To bring up the table, select Set Ineffective Areas to Permanent Mode from the Tools menu of the geometric data editor. When this option is selected a window will appear as shown in Figure 5-90.

Figure 5 90. Editor to Set Ineffective Flow Areas to Permanent

The editor for this option allows the user to select the river, reach, and river stations, of the cross sections in which you want to set the ineffective flow areas to the permanent mode. Add those locations to the box on the right side of the editor, by pressing the arrow button in the middle of the editor. Continue to do this until you have all of the cross section that you want. Finally, press the OK button and the data will be reversed.