Also, what does this excerpt from Wikipedia mean
However, since with open channel flow, the water surface is open to the atmosphere, the pressure term between two points has the same value and is therefore ignored.
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Because head pressure is the weight of the water standing on top of the region being measured. Even though you can visualize an open channel as a slope you will not see a difference in pressure if you measure one foot deep at any point on the slope; this is because you are only measuring the weight of the water above the measuring point.
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$\begingroup$ Okay. And what are the two points that are considered in the sentence in the question? $\endgroup$ May 8, 2017 at 20:33
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$\begingroup$ two points along the length of the channel... $\endgroup$ May 8, 2017 at 21:33
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$\begingroup$ Any 2 points along the open channel. If you look at a 2ft deep sluice that drops 5 ft over a 50 ft run and observe the pressure 1ft down on the bottom at the top it will be the same as 1 ft down at the top. This is because it is only measuring 1 ft of water pushing down on the pressure gauge. If it were a solid filled pipe you would read 1 ft depth at the top and 5 ft at the bottom because the solid run of pipe has the same amount of water pushing down on the gauge as a 5 ft deep tank. $\endgroup$ May 9, 2017 at 15:31