Tuflow and estry manual Version 3

3.13.7Effective Area versus Total Area

For XZ Cross-Sections, the flow area is calculated as an effective area (E flag) or a total area (T flag). The flag will override the global setting set by Flow Area.

If there is no variation in relative resistance across the cross-section there is no difference between effective and total areas. This is dependent on the relative resistance being 1.0 across the whole section. (An ERROR is produced if the relative resistance is not 1.0 somewhere along the cross-section when using effective area.)

The total conveyance of a cross-section is not affected by whether effective or total area is used.

The primary differences between using effective and total area are:

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  The channel velocity calculated is the depth and width average of the primary (normally mainstream) parallel sub-channel if using effective area, and the depth and width averaged of the whole cross-section if using total area.
  
• 
  Where the effective and total areas are significantly different, the channel velocities used in the 1D momentum equation will be significantly different. If the channel velocity is sufficiently high and different depending on whether effective or total area is used, the inertia terms in the 1D momentum equation may affect the results. Note the frictional (bed resistance) term in the momentum equation is NOT affected as the hydraulic properties for the cross-section are adjusted so that the total conveyance is the same irrespective of whether effective or total area is used.
  

The “purists” among us tend to favour effective area as it gives a more reliable calculation of the mainstream velocity, and therefore, a more accurate estimate of approach and exit velocities of structures, and more appropriate velocities for advection-dispersion and sediment transport calculations. Where velocities are not high or significantly changed when using effective or total area, the water level and flow results are usually identical or very similar.