I want to construct an artificial stream that will be fed from a pond then pump back to the pond. What is the least gradient that would give a reasonable flow or can I assume anything as long as it is not level? At the moment I haven't calculated the natural fall over the ground that I will construct the stream but it will be a stream and not a waterfall.
You don't actually want much gradient at all. And you should include areas where there is none, or "puddles", so that when the pump is switched off it does not immediately go dry. Think of a series of ponds connected by waterfalls, but sort of "squashed" into a stream. You say you don't want waterfalls, but presumably you want water flowing over / around rocks etc. If you want to simulate a deep stream, you'd want more like a pond stretched out - i.e. there may be no gradient at all. I was going to do one at the end of the garden, where the "ends" dissappear into bushes etc, such that it would look like a river flowing past my garden. I was going to have a duck that swam round and round. I would probably be rewarded for my realism with a rowingboat and cox suddenly zooming past ! Simon.
When I say I don't want waterfalls I mean that I want a water course that flows like a small stream rather than the sort of waterfall that you often see built into ornimental ponds. As you say I guess that it does not have to be very much of a fall. Using the main sewer as an example, that flows along at a fair old rate so something less than the fall on a sewer is required.
Kevin
You will have to experiment with pond liner / pump / rocks etc to get the effects you want before final design. Warning: don't skimp on the liner, get a butyl rubber one - they are stretchy and stronger than the other types. The stretchiness means they are more likely to stretch than rip when confronted with a stone. There are some good books around with lots of pictures. Go to a good garden centre. Simon.
Definitely only a degree or so of fall - I recall a botched attempt I made with a visible slope to the 'stream-bed' - water wooshed along it like guttering instead of dawdling restfully. Personally I'd use staged sections of reverse fall now, so that each section fills upstream and only reluctantly 'overflows' towards the pond end.
I'd make that no fall - or even a (very slight) negative fall - between waterfalls/rapids so that the water course is full even when it's not flowing.
That seems to be what the peanut smuggler on Groundforce does. Not so much a fall as a series of steps, so one section fills, overflows into the next, then that overflows into the next. These 'weirs' need only be an inch or two high for each section.
So I guess you decide on maximum weir height, divide the total fall by that, and you have the number of sections.
As long as the start point is higher than the finish point all should be OK.
John makes a good point about the water course remaining full.
Dave
Don't make a stream - when you cut the flow, it dries out almost instantly. Instead make a series of pools with cascades between them.
These pools give you far more "displayed water" for a particular flowrate. They also preserve their contents and your plants if you're temporarily low on water, or if you switch the pump off.
Height between pools is a question of style and how much noise you want (yes, noise is an issue to think about - hard to change later) An inch is about the minimum, two inches "the size of a single pebble" is good, anything over that starts to look like miniature waterfalls and a model village.
If you want a flowing water effect without running dry then you dont want a fall, the more the fall the faster the pump will have to work to keep up the return supply.
We made one in the garden as a series of three pools, the top too being quite shallow (3 to 9 inches) and the bottom one being a lot deeper for the pump. They are separated by two "waterfalls" of about 2 inch drop which gives an excuse/opportunity to split the liner. A standard pump works quite well with a 1" diameter pump to recycle the water.
The total length is about 5 yards.
If I was doing it again, I would try and make it straighter and try to use one liner (with pleats) throughout. I would also probably make it deeper (sharper sides). If the pump stops (power failure) or slows (weed) then there is sufficient water held in the ponds to allow plants to survive and the birds to drink.
You don't need any gradient to get a flow if you pump from one end - just look at canals where water is extracted - you get a gradual flow! The flow rate will be what your pump can handle - use the biggest pipe you dare - we didn't take the advice of the shop who said 1 1/2 inch and regret it, although we get a reasonable flow, since the pump could probably deliver more.
If you're not alarmed by a bit of maths and want to work it out, the relevant formulae are the Chezy and Manning formulae. It involves the gradient, cross sectional area and the wetted perimeter, IIRC. A Google search may turn up some information; you might even find a program to calculate it for you..
The problem I anticipate, IMHO, is that any such channel of a significant cross sectional area will shift a large volume of water, even if laid at a a shallow gradient (as with the sewage pipe) . You'd probably need a fairly large pump to feed it. This may be why most garden water features have low flow rates trickling down waterfalls, rather than sedately flowing streams.
You don't need any fall at all - the pump will raise the level in the pond, water will flow back to the pump.
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