The uplift pressure on the roof of a siphon aqueduct is maximum when

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UPPSC AE Civil 2013 Official Paper II
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  1. canal is running at full supply level and drain is dry.
  2. drain is running at high flood level and canal is dry.
  3. canal is running dry
  4. drain is running dry. 

Answer (Detailed Solution Below)

Option 2 : drain is running at high flood level and canal is dry.
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Detailed Solution

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Syphon Aqueduct:

 In a syphon aqueduct, canal water is carrier above the drainage but the high flood level (HFL) of drainage is above the canal trough. The drainage water flows under syphonic action and there is no presence of atmospheric pressure in the natural drain.

F1 N.M M.P 31.07.19 D 5

As the syphon aqueduct passes from below the super structure, if the drain is running at high flood level then it will generate maximum pressure at the roof. Hence, to counter this pressure, maximum uplift pressure is generated in the form of reactions.

Important points:

HFL of drain is much higher above canal bed in case of syphon aqueduct.

Water surface level of flood is depressed when it passes under the canal trough.

Bed of the drain is also lowered.

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