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SITE SELECTION
A site that produces the typical profile shape will give the most accurate results. In a majority of the
cases, problem sites can be identified by a visual inspection. Site inspection guidelines are as fol-
lows:
• The channel should have as much straight run as possible. Where the length of straight run is
limited, the length upstream from the profile should be twice the downstream length.
• The channel should be free of flow disturbances. Look for protruding pipe joints, sudden changes
in diameter, contributing sidestreams, outgoing sidestreams, or obstructions. Clean any rocks,
sediment, or other debris that might be on the bottom of the pipe.
• The flow should be free of swirls, eddies, vortices, backward flow, or dead zones. Avoid areas
that have visible swirls on the surface.
• Avoid areas immediately downstream from sharp bends or obstructions.
• Avoid converging or diverging flow (approach to a flume) and vertical drops.
• Avoid areas immediately downstream from a sluice gate or where the channel empties into a
body of stationary water.
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 nontypical 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.
Low flows - The 0.9 x Umax method is recommended in flows of less than two
inches.
Rapidly 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 Umax 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.
Asymmetrical flow - The 2-D method is recommended for asymmetrical flows. An asym-
metrical flow will have a difference of 30% or more between the right
and left side velocities.
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