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<br />, ~---~ <br /> <br />~ <br /> <br /> <br />Pressure Head <br /> <br />~ <br />'" <br />- <br />" <br />~ <br /> <br />I Velocity Head <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />______ _J-'"' <br />I~Crit i ca I Dept h <br />I <br />I <br /> <br />- <br />Q. <br />'" <br />C <br /> <br />Hb Ha <br />Total Energy Head <br /> <br />Fig. 3.01. Relationship depicting critical depth <br /> <br />Water flowing initially ata depth A, marked on the ordinate <br />axis in fig. 3.01. has a total energy head marked Ha on the abscissa <br />axis. If discharge is held constant and the water depth 4110wed to <br />decrease, as in the case of water approaching a free overfall, velocity <br />head will increase, pressure head will decrease, and total energy <br />will decrease toward a minimum value where the rate of decrease in the <br />pressure head is just counterbalanced by the rate of increase in <br />velocity head. This point is critical depth, and velocity head will <br />not continue to increase beyond the value at critical depth (i.e.. <br />critical velocity) unless additional energy is added to the flow as <br /> <br />3.04 <br />