Until now we have discussed open channel hydraulics assuming that the flowrate in the channel
never changes - in other words, Q is “Steady” - and we have also assumed that the channel
geometry does not change – meaning it is “Uniform”.
While Steady, Uniform flow is possible to achieve in an open channel it is also safe
to say that steady uniform flow will not always be the case.
Let’s consider the open channel profile shown here.
The simple fact that the slopes are not equal means that we have a non-uniform condition
here.
Now I draw in the critical depth as a red dotted line and the normal depth as a solid
blue line.
As you can see, this channel starts at a mild slope then transitions to a steep slope.
If we simply connect the normal depth lines in this channel we are assuming that the normal
depth will continue at a given velocity until the channel grade break, then immediately
speed up to the velocity resultant in the steep channel.
Not physically possible.
What is easier to see is that the water will begin to accelerate from the mild channel
into the steep channel creating a tapering effect in the mild channel.
This effect continues into the steep channel until reaching normal depth.
Each of these sections is referred to as a gradually varied flow section.
Now let us examine what could be the opposite scenario: transitioning from steep to mild
slopes.
Drawing in the critical and normal depth lines we can see that we are transitioning from
supercritical flow in the steep channel to sub critical flow in the mild channel.
There are two possible situations that can result from this type of transition.
First, it is possible that the water enters the mild channel moving so quickly that it
pushed the water in the mild channel downstream.
This interaction slows the flow rate down in the mild section
to the point where a hydraulic jump forms . This is known as the rapidly varied flow
section.
The second scenario can occur when the flow depth in the mild section is so deep that
the flow is backed up into the steep section.
If this occurs there will be a hydraulic jump followed by a GVF profile which completes
the transition to normal depth in the mild channel.
In order to help organize our calculations and help qualitatively assess non-uniform
flow we classify the GVF sections.
The GVF classifications have two digits.
The first digit states the type of channel slope where the transition is ocurring: M
for mild and S for steep.
So we write the first letter for each section close to where the GVF is labeled.
The next digit in the classification system describes the flow depth relative to the normal
and critical depth.
For example, if the depth of flow is decreasing downwards relative to the channel bed, and
is transitioning between the normal depth and the critical depth, we call this a Zone
2 profile.
This is what is happening in the mild to steep transition section.
When the flow depth is increasing to a depth that is greater than the normal depth and
the critical depth then we call this a Zone 1 profile
On our examples shown here this is occurring when there is a hydraulic jump in the steep
channel and the GVF that follows is Zone 1 because the depth is greater than both the
normal and critical depth.
Finally you can have an increasing flow depth but where the flow is less than both the critical
depth and the normal depth.
This is a Zone 3 profile.
This can occur when the steep flow enters the mild channel and suddenly find itself
below the critical depth and the normal depth just before it transitions into a hydraulic
jump.
Once all of the GVF profiles are classified it makes it easier to approach the quantitative
analysis.