When the water reaches the bend at the offset, it’s turned at right angles to its original flow and for a few pipe diameters. Downstream it will continue to flow at relatively highest velocity along the lower part of the horizontal pipe. Hence the slope of the horizontal piping is not adequate to maintain the velocity of flow that existed when the water reached the offset, the velocity of flow in these horizontal drain slowly decreases with a corresponding increase in the depth of flow until a critical point is reached where the depth of flow suddenly and sharply increases. This increases in depth is often the great enough to completely fill the cross sectional area of the pipe. It sudden rise in depth is called the “hydraulic jump.” The critical distance at which the hydraulic jump may occur varies. It’s dependents upon the entrance velocity, depth of water which may already exist in the horizontal drain when the new flow is introduced, roughness of the pipe, diameter of the pipe and the slope.
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