"NY" wrote in message
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"Tim+" wrote in message
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Why not syphons? I mean, pumps use power, quite a lot of it and are
relatively expensive. For a syphon, you just need a suitably sized bit of
pipe.

I suppose they would still need a pump to "prime" the syphon, and it would
have to remain "in circuit" afterwards,

No it wouldnt.

because would be very difficult to remove safely when there's a lot of
water flowing,

No it isnt. Just a couple of quick release valves like those
used with street fire hoses would work fine.; Have one
near the pump that is closed when priming, and another
between the pump and the bottom of the syphon.

Once the syphon is primed, open the valve that
lets the water out of the syphon near the pump,
close the one between the pump and the syphon,
remove the pump.

and an unpowered pump may significantly reduce the possible flow rate.

Thats why its removed, so it can be used to start another syphon.

It also depends on there being a significant head of water between the
inlet point and the point where it discharges into the spillway after
going over the "hump" between reservoir and spillway.

Yes, but by definition any dam has that.

I wonder whether the power needed to drive the pumps does decrease once
water is flowing and syphon conditions exist,

No it doesnt.

or whether it is substantially the same no matter if the outlet is at the
same level as the input or if the output is a lot lower than the input.

Thats correct.