If there are further undulations, then the back pressure effects add together. If H1 is greater than H2, then some water can flow, but the full pipe hydraulic head H3 will not be reached and so flow is much less than expected. If H2 is greater than H1, then you have a full air lock, and the water level in the up leg Lo2 to Hi2 stops at H1 and no further water can flow. This exerts a pressure on the trapped air of either H2 m of water (WG = water gauge) or H1, whichever is less. As more water flows down, the upward leg Lo2 to Hi2 fills up. If the water flow velocity is below the rising velocity of air bubbles, then water trickles down to the low point Lo2 and traps the remaining air between Hi1 and Lo2. and low points between them such as Lo1, 2 etc., then if the pipe is filled from the top, and was empty, the pipe fills OK as far as Hi1. If the ground in which the pipe is laid has high points – such as Hi1, 2 etc. Fig 1 shows a reservoir which feeds a gravity distribution system – for drinking water or irrigation.
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