Just a long shot, a very long shot

Working on a little project; weekend one with a few friends. Well the need has arisen to make a dam

As we're all over 30, the details matter and the longer it takes, arguably the better. But not too long or we'll forget what we are doing.

So are there any dam engineers around?

Don't worry it's not a big challenge...quite small in fact, small but has to be well formed.

wow.. i was pro at backing up the local stream as a kid! But that was mainly done with shopping trolleys, bricks and twigs :P

Sandbags.. that's what you need!

What sort of size river did you need to block/divert?

I'd suggest going for this approach

Civil/Structural Engineer here, not sure what use I will be to you though :P

What sort of dam are we going for?

Rock fill, Earthwork, Gravity, full-on Concrete...?

marine ply in a gripe 300' underground

Without saying too much it's a 2' (widest) clean (is by hand) cut channel that has a highly variable water range, between dry and 18" deep; more in exceptional circumstances, we're trying to steady that flow. Partly done now with a siphon to keep the channel watered in dry times; yes this is a dry time.

So it's not a big project, but still has a channel about 200' long 18" deep 2' across to hold up

Questions I have are along the lines of a weir or spout and pipe to handle overflow etc. The biggest dam I've built is on a beach and it took most of the day with rocks, then the sea came in. I learnt then that engineering is flawed without planning

I think you need a weir, not a dam, then. It might be acceptable to just build a wall part-depth in your channel, as long as you're not risking over-topping the channel sides, or could put in cut waters to rechannel the water that over-topped.

Edited 10 December, 200817 yr by KenONeill
Officious swear filter!

hmmm... I am a civil engineer but dams are not my speciality. I do have access to 14,000 others in the company though so someone might know if you want me to ask... Seriously I could ask our water and drainage guys upstairs.

Strangely Mark, my response was going to be along the same lines

Again, there are some water boys I could call upon...

Steve

A weir, yes. We're hoping to use the water to create power, so we first thought of a dam...then use the flow / height drop to convert to energy. Plenty of resources out there for us to do that from a stream side etc...

But we need to be as high as possible, thinking overshot; which puts us in risk of overspilling the pathways; this is not acceptable.

Now in the best conditions this channel will be fed by a 3" spihon so max flow in is 3" diameter pipe.

I'm simply thinking to cope with flood we need a

|   
| Y
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| /
_|/
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_|___
|   \
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That should be a Y spout below max height to allow water to flow out.... or if it's a weir then it would spill over; either works for me, the later I think is far easier.

If it's a wier we need to channel the overflow to our energy device... which given water levels will vary will not work all the time if we are taking from the top...do'h!

The problem for us is we need to raise the water level in order to gain height/drop then keep that water to power during dry. Or we go for undershot / turbine.

As such the only way I can see this working 24/7/4/12 is to take the water out of the base in a small diameter pipe to generate a higher pressure of flow, but not so much as to drain the dam as in the lower _ shown above.

Ken, by cut channels would you effectively mean a bit of 3" gutter pipe laid in on one side, lower than max height to catch overflow?

Edited 11 December, 200817 yr by ColinD

Well, what you're building is a weir, because what you're trying to do is maintain a constant head above the point you're building at in conditions of varying flow. If you know the maximum depth of the flow, and the depth of the channel, then you can build a weir slightly less than that difference high, and the water shouldn't overflow the channel except in a hundred year storm. Whether this would give you enough mass flow rate and head to generate worthwhile power is another set of questions entirely

A cut water is a low divertor wall designed to push flowing water off a flat plate. If you look at Iowa class battleship - Wikipedia, the free encyclopedia , the dark grey V-shape between the anchors and A-turret in the picture is a separating cut water, to protect the turrets from waves breaking over the bow. What you would need, just downstream of the weir at the upstream end is a merging cut water, to push the overflow back off the sides (footways?) into the channel.

Edited 12 December, 200817 yr by KenONeill

Generate power as well?

Getting far to technical now.

Employ a family of diligent beavers, they've been doing it for centuries. And you have the added bonus of making beaver jokes at the slightest provocation!

Employ a family of diligent beavers, they've been doing it for centuries. And you have the added bonus of making beaver jokes at the slightest provocation!

....and also brings us onto that internet classic - the Beaver Dam letters.....

snopes.com: Beaver Dam Letter

marine ply in a gripe 300' underground

Without saying too much it's a 2' (widest) clean (is by hand) cut channel that has a highly variable water range, between dry and 18" deep; more in exceptional circumstances, we're trying to steady that flow. Partly done now with a siphon to keep the channel watered in dry times; yes this is a dry time.

So it's not a big project, but still has a channel about 200' long 18" deep 2' across to hold up

Questions I have are along the lines of a weir or spout and pipe to handle overflow etc. The biggest dam I've built is on a beach and it took most of the day with rocks, then the sea came in. I learnt then that engineering is flawed without planning

I'm an Asset Manager / Maintenance Planner and help look after about 15 dams ranging from 300 MegaLitre puddles up to 2,000,000 Megalitres plus ~150km of pipeline & 60km of canals. I've only been doing it a small while (aircraft maintenance planner previously) but from what I've seen I think you might need a V-notch weir. During low water it lets out a trickle but during high water it allows the excess to escape without too much backup.

If you have the capacity to allow the water to build up then you need a weir with an outlet pipe (preferably adjustable).

It's only tiny so lay a pipe & valve surround it with a bit of steel cage embedded in the walls & some concrete.

Or you can put some vertical channels in and fit stopboards (dropboards in some places) made out of timber - the leakage will probably be enough to keep the channel wet.

If I'm reading this right this is happening underground.

If so do you know that you can raise the water level without potentially diverting water to another location. I was involved in a minewater treatment project where raising the level of the discharge approximately 300mm would have diverted the water to another discharge point about 3 miles away.

I'm not completley convinced on what you are doing but my thoughts.

You want power but do not seem to have much head to generate this even with a dam you say 18" high.

I would therefore have thought a high flow speed would be desirable in which case I would be thinking along the lines of a penstock set up on top of the water. This can then be lowered into the water creating a head behind the penstock and forcing a shallower flow through the bottom of the channel at higher speeds.

By adjusting how low the penstcok is you can adjust how much flow goes through. In addition for flood protection you can have a hole in the penstock (stirrup shaped) which sits below top water level and so if the flow underneath is too low and it is backing up it gets to a level where it flows through the cut out.

How much power do you think you will generating?

Our environmental flows range from 1ML to 12ML per day and have about a minimum 10m head / 6,000GL behind them. We're just doing a project at the moment to fit small hydro power generators on to them and the output is quite low (can't remember the figures, only got the project last week).