On 04/08/2019 20:20, NY wrote:
"Tim+" wrote in message
...
Why not syphons?Â* 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, because would be very
difficult to remove safely when there's a lot of water flowing, and an
unpowered pump may significantly reduce the possible flow rate.

You can start a syphon by placing the whole pipe in the water, letting
it fill, closing a valve on the end, lifting the end over and down and
opening the valve. A job for a mobile crane.

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.

If the outlet were placed at the bottom of the dam, there would be a
decent head until the reservoir was virtually empty.

Even if the flow were not as great as with the pumps, more syphons could
be used or they could be just used to supplement the pumps and drain the
reservoir faster.

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

I would presume that the syphon effect would assist the pumps, at least
be reducing the back pressure and counterbalancing frictional effects.

SteveW