JW's GRAVITY DREDGE

[Steve Herschbach]

OK, here is the deal.

What we have a lot of in Alaska are "perched valleys". These are created when a large glacier gouges a large, deep valley. Smaller tributary glaciers create side valleys joining the main valley, but they have bases that are typically much higher. The perched valley has a gentle grade that ends abruptly with a steep drop into the larger deeper main valley.

This drop originally was a waterfall of 2000 feet or more. But erosion has been at work for awhile, and now we have gentle high elevation valleys that have a canyon in the last mile that stair steps rapidly down to the main valley. Lots of waterfalls and plunge pools, and some of those waterfalls are still pretty big.

The upper perched valleys contain little gold since the glaciers scoured them to bedrock, but there is gold scattered through the glacial gravel. The gravels slowly erode into the short canyon at the end of the valley which are natures sluice boxes. And like any poor sluice box they lose gold, and a lot ends up in what is often the richest area, in the main valley just below the canyon.

This scenario repeats over and over again in Alaska. Lean upper valley, with short canyon at end that has what is referred to as a "gulch placer" that dumps into a large relatively flat lower valley. The best gold is usually in the main channel below the canyon.

These were often mined with giants or hydraulic cannons/nozzles as a lot of drop is available to work with and power was even more valuable in the old days then it is now. I know locations right now that if I was going to work them for several summers in a row I'd have to consider putting in a pipe system from the upper valley to the lower. I'm not talking weekend warrior stuff, I'm talking dredging for a living and obtaining a free power source that lasts for years.

I'd lay pipe from the upper valley to a fixed point below, just like the old timers did. That will take time and money but will only need to be done once. From that point I'd run a 2" pressure hose to my dredge jet or nozzle.

The pipe would need to run down near to where I am dredging, but I'd want hose for the last 100 feet so I can move at will upstream. By using multiple dams it is easy to create a situation where you are getting a small siphon effect already with the dredge, with the jet or nozzle acting as a booster. As you dredge up the valley, pipe sections would be removed to move up the valley.

I've dredged for a living, and I have had to helicopter fuel near to a site at great expense. I've then had to pack that fuel six gallons at a time down to a dredge in a deep canyon down one of the few places one can even manage to climb down into the valley, and then had to hike up or down the valley to get to the dredge. And do it every day, twice a day. All summer long.

Having done that, in retrospect I can see immense value in spending a great deal of time, money, and effort to put in a hydraulic system to power the dredge. The work is up front, but the longer the time frame the mining occurs over, the more it would pay off.

I have a similar situation on my hands now and so this is real world at the moment for me. I'm WAY back in and fuel logistics are a killer. If my time is worth $20 an hour ( it's worth more than that) then I'm talking $50 plus a gallon for fuel.

[overtheedge]

Tis just my nature to attone. Most of my working life I spent part of my time instructing and hated it when I provided false or misleading info. My nature demands I make it right.

Both AceHand and JW have a good point. Hose will conform to the supporting surface. Each minor bend causes the flow to go from laminar to turbulent. The flow inside a straight pipe is laminar with the friction loss being confined to the inner surface of the pipe. Larger pipe, less surface area per volume and ergo less friction loss.

Each bend in the hose increases the friction loss and induces turbulence. This can cause the material to start clogging the hose at the bends. Think poor riffle section at each bend.

From what I can tell, the biggest reasons for gradually necking down large hydraulic installations was cost and reduced friction losses. Gradually necking from big to small got away from one reduction and the tremendous forces confined to just one spot. Then there was the cost of the pipe (smaller is cheaper) and the fact that the pipe could be nested inside (2" in 3" in 4" ... you get the picture) for a reduction in shipping costs. A hundred years ago, shipping costs to the gold fields was outrageous. (see chapter on costs, "Frozen Gold", John A Gould. and USGS Bulletin 263, Methods and Costs of Gravel and Placer Mining in Alaska)

From what I can grok out, JW's design is the best bang for the buck. It works with even a low head. I always discounted smaller pipe (JW's 2") because I thought it would have too much friction loss. Obviously, I thought wrong.

Already made up the upper hose section. Used 10' of flex hose and made the couplers and nozzle from a piece of 2" exhaust pipe with a 45 degree bend. Had lots of straight section on each side of the 45, so cut off 6" or so from each end for couplers. Already have 150' of 2" PVC in 5' sections with a bell end. Gonna build a flared header for the sluice out of a highway cone (already on hand) and plywood.

Because the gold in the Copper River Basin mostly runs <1mm, I'm going to put 1/4-1/8" screen over the nozzle with a hose clamp. I'll use a spading fork for digging out the big stuff and just vacuum up the finer material.

Thanks to one and all for the forward assist on the brain housing group.

[Steve Herschbach]

I think I'll build a siphon dredge also and give it a go this summer. For areas with a lot of drop the idea has a lot of merit. It makes far more sense than the system I described in my last post for areas where the drop is behind you, and is far more suited for short time frames. Thanks JW.

[AceHand]

You're both right. Depends on how you look at it. I was thinking difficulty of set-up and possibility of use in any location. Ease of use goes to the jet style when you think clogs and losing prime. I bet you figured out how to avoid those pretty quick, JW. That's gotta be a real pain when it happens.
I shouldn't even be posting. I have zero experience with dredging of any sort, having only witnessed a failed attempt at a gravity highbanker, but I know it'll work in our no mechanized equipment area. The information in this thread is invaluable towards my first attempt. I love a good problem and a great discussion like this too.
Does anyone have a ballpark figure of the pressure needed to make a jet work on a 2" or 3"? Then I could figure out minimum head required. A second opinion on the required head would be ok too, lol.
Tom

[Steve Herschbach]

Hi Tom,

I do not think it is near as much as you might think. Here is something I have done a lot. I work small very fast moving creeks. So standard operating procedure is to get all the rocks out of the work area that will not go through the nozzle. The way to do that is to build a small dam with the rocks below the work site, which builds up a pool of slower moving water that is easier to work in.

Now typically I'll float the dredge in the pool with the end of the sluice positioned over the dam so tailing flush away with no worries. But I've also had occasion to set the dredge below the dam with the hose looping over the dam into the pool. And then you find out that even with the motor shut down you get enough siphon effect to suck up sand - but not rocks. It is not going to take a huge boost for a jet or nozzle to get the suction you need if set up like this. I do think a larger orifice would be called for.

You guys tell me if this makes sense. A 5.5HP Honda with a Keene P180 pump has a 2.5" intake and 2" output. You can put a 2" hose on it and get about 120 foot of vertical head. What that means is that at 120 foot (more or less) you only get a dribble of water. The weight of that column of water exactly offsets the power delivered by the pump. So would it be that the same 120 foot of 2" pipe with a 120 foot head of water would be delivering the same power as the pump? And if hooked to the jet would replace the pump?

[overtheedge]

Steve, head is head. It is all about balancing vertical drop (head available) and length of pipe (friction loss) needed to get the drop. There would be a bit of friction loss from the length of pipe, but not that much if you use larger pipe than 2" to get 120' of head. At 50psi, you lose 4.6psi per 100' of 2" plastic. My book doesn't have tables of head loss for plastic larger than 2.5", but for steel pipe @ 50psi, 2" is 8.5 psi/100 feet, 3" is 2.3psi/100 and 4" is 0.3psi per 100'.

The gold in the Copper River Basin is in the same sort of terrain, perched valleys with narrow water carved gulches. Large alluvial deposits at the base level with the gold scattered throughout the deposit from top to bottom. With its small flakey nature, it is only profitable in isolated spots and with low expenses to operate.

I've avoided the gullies due to the work hauling in/out pumps and fuel. The siphon dredge concept makes operating in the gullies practical and holds costs to a minimum. More important to me is the low-head requirement. If I can find some paystreaks, I would happily haul in several 20' lengths of 4" ABS.

I stated pumps in the previous paragraph due to distances involved to pack it in and if you need a pump, you need a back-up or brand new one each season. I think it has something to do with Murphy. The greater the distance you pack a pump and the greater your need is for it to operate without a hitch, the greater the probability is that it will fail.

[AceHand]

Steve, I like your way of attacking the problem with logic. I'm guessing you're right on with 120' of head giving the max output for a 2" gas dredge. Also that a gravity jet dredge is more feasible than I thought.
Pretty tough to figure what the minimum would be. It's up the what is considered satisfactory by the individual. I suppose if you only had 20 or 30' you would need to do all you could to combat friction loss with pipe size, length, and orifice size. I bet 50' would work ok.
Thanks,
Tom

[Steve Herschbach]

Hi Tom,

Actually, we are talking enough power to run a 4" dredge, which is what a Honda 5.5HP w/Keene P180 pump will easily power.

A siphon dredge will do for where I'm headed this summer. Fly 60 miles back in. ATV for several miles to a point down the canyon to where you can't drive any more. Then pack at least another mile down the canyon to the work site. Endlessly hauling fuel in there is not only very expensive but a real time killer.

[kiwijw]

Hi Steve, I must say I am a bit gob smacked at your reply to me. I wasnt stating, fighting or defending that my method or any of the methods were better than the other. I dont know where you got that from. I cant see it in any of my posts
In my last post I was just describing what I did in this little creek that had bugger all water in it. There wasnt even enough water to run a siphon through a 2" pipe continuesly let alone the pressure & water needed to run a nozzle or power jet. I simply described how I damed up the little creek to get enough water depth to run my 2" suction nozzle dredge to work that flat bit.
I am not arguing at all that what I do is better just commenting that it suits me for the creeks I have been doing it in & if what I am saying & showing in my pics helps otheres & gives them a thought or idea to maybe try the same thing then that is all good.. As I said in a past post in this thread I am in the hobby dredgers world & do this sort of thing in my spare time which is mostly weekends & outside my day job which is a builder. I dont have the time to take miles of pipe up to the head waters of a creek & lay it all out & leave it there hoping it will still be there when I return the following weekend. Or a flood washers it all to buggery.
If enough head of pressure can be got to run a nozzle or power jet that is great & I arent stating for a second that it cant be done or hasnt been done.
What you call a perched valley we call them a hanging valley. Same thing though.

I think I will lay low for a while & lick my wounds. I may have over stayed my welcome.

Good luck for the coming season

Regards

JW

[Steve Herschbach]

Hi JW,

Try HERE for starters. "My gravity dredge is not using water pressure to drive a suction nozzle or a power jet. That wouldnt be practicle." overtheedge spends a great deal of time coming up with useful information that would make my brain explode and your response is "why bother". And you say that using water pressure to drive a suction nozzle ot powerjet is not practical. So yes, you are stating it is not a practical solution. A more pertinent example is at http://www.akmining.com/forums/showt...king-Questions Do not assume you know what frustrates me because you have no idea. I can speak for myself.

You do not need to give your opinion of other people's posts here. That is my job. You post, you let them post, I call the shots as to what is useful or not. The thing I want you to know is A. we all value your contributions and B. you have not over stayed your welcome. I owe you for giving me good ideas - not an insignificant thing, and I've applauded you for that. But please refrain from being critical of other persons posts. I do not think that is too much to ask.

Yeah, hanging valley. That is the proper term. Thanks JW.