Open Channel Velocity Profiling 
part 4 of 13

Choosing the Method 

All profiling methods can be used in a site that produces a typical profile and has sufficient level to measure three point velocities.  If you cannot avoid sites with non-typical profiles or low flows, the following guidelines will help in choosing a method that will give the best results.  Keep in mind that choosing the method will become easier as you gain experience in profiling. 

bulletLow Flows - The 0.9 x Vmax method is recommended in flows of less than two inches.
bulletRapidly Changing Flows - A flow that is changing more than 10% in three minutes or less can be classified as rapidly changing.  The 0.9 x Vmax method or 0.4 methods take the least amount of time.  However, these methods usually require a typical profile shape for accurate results.
*Comment: Check the level several times during the profiling procedure. If the level has changed, but the change is less than 10%, average the level measurements and use the average in the flow calculation.* 
bulletAsymmetrical Flow - The 2-D method is recommended for asymmetrical flows.  An asymmetrical flow will have a difference of 30% or more between the right and left side velocities.
bulletVertical Drop (outfalls) - The 2-D method is recommended for outfalls.  Remember to measure the level on the same plane as the velocity profile.  Outfalls should be avoided wherever possible. 
bulletNon-typical Profile Shape - If you suspect a profile shape may not be typical, use the 2-D method.

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