The
basic principle under which saddle siphon works/functions is summarized briefly
as follows.
1. When the water starts to rise in the reservoir above the Normal
water level (NWL), then water starts to spill from the siphon spillway.
2. When the water rise just above the depriming hood, the duct of
the siphon is locked y the water from both the end.
3. In order to increase the rate of priming of the siphon
spillway, bay priming arrangement is made.
4. The flowing water from the crest of the spillway will suck the
entrapped air inside the duct.
5. This will cause the gradual increase of vacuum pressure
(sucking pressure) inside the duct.
6. The negative pressure will suck more water from the reservoir
and finally the duct will be running full of the water.
7. The siphon is in the full operation now.

NOTE: The time period between the instant the water just starts to spill from
the crest to the instance that siphon runs in full is called priming.

When the water level of the reservoir decreases and reaches just to the inlet of
the depriming hood, air will be passed to the throat section and negative
pressure developed will be released and siphon will stop running. If the
deprimer is not provided then water level will decrease just to the upper level
of the inlet pipe which will cause the loss of water.