Hydraulic grade line is a hypothetical line used to indicate the relative elevation of the total energy head (of the water) at any point in the pipeline.The concepts of the hydraulic are illustrated in the figure below. Under no flow conditions, the pressure head available anywhere in the system equals the difference in elevation between the ground elevation and the hydraulic gradeline elevation. In the first part of the illustration, the elevation of the hydrulic gradeline is level because there is no flow in the pipe and therefore no frictional losses. The pressure available at houses A and B is equal.
Under flow conditions, the hydraulic grade line slopes in the direction of the flow. For the low flow condition, the hydraulic grade line slopes moderately because the velocity in the pipe is low and the frictional losses are low. When the demand is high, the hydraulic grade line drops more rapidly because of the higher flow velocities in the pipeline. Available pressure is consequently lower at high flow because the elevation of the hydrulic grade line is lowered.
