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19.05.2014 admin
As a general rule, the optimum slope-setting of a sluice is around one inch of drop per linear foot of box. There is no exact formula for setting the proper water velocity through a sluice box which will work optimally under all conditions for all the different types of riffles being used today.
In setting up a sluice, if feasible, it is desirable to have enough water flow to move the material through the box as fast as you can shovel (or dredge) it in at full production speed. How much water velocity is directed over the box directly affects how much material will stay behind the riffles. Too much force of water through a sluice box will put too much turbulence behind the riffles.
This means that the size of riffles affect how much water velocity is optimum through the box, how much classification of material is necessary and how fine in size your effective gold recovery will be. The correct amount of flow is usually found to be just enough to move the material over the box to keep up with your feed of material. Once you have your sluice box set up the way you think it ought to be, it is a good idea to run a sizable portion of gold-bearing material through the box and then pan some samples of the tailings. On the other hand, it is not good practice to run volume-amounts of water flow over a sluice box without some material being constantly or regularly fed through.
So if you will not feed more material into your sluice for a while, it is a good idea to cut your water flow back to reduce turbulence behind the riffles until you are ready to feed again. Even when a sluice box is set properly, occasional larger-sized stones or rocks can become lodged within the riffles.
On most suction dredges, the volume of water being moved through the sluice continues at the same steady flow during production speed. When placing a sluice box within a stream or creek for its water flow, the water velocity can be adjusted by either changing the slope of the box, by varying the volume of water being directed through the box, or by placing the sluice at different sites in the stream or creek where the water is moving at different depths and speeds. Usually, you will have little trouble arriving at the correct velocity through your sluice box when placing it in a fast stream of water.
In a situation where you must set your sluice into slower water, you will find it is generally more difficult to get the flow you need, because you have to create more water velocity than is presently there.

If the flow of the stream itself is not enough to move material through your box, you will sometimes find that changing the slope of the box within the stream has little or no effect on speeding up the flow through the sluice.
In this situation, there are several things that might be done to channel enough flow through your box so that you can run material through at production speed. If the water at the work site is moving too slowly, or for some reason a water director or elevator will not work in a particular location, it will be necessary to either set up your sluice in a different location where the water is moving faster, or use a motorized pump to feed water into your sluice.
Therefore, rather than give you a formula, I will attempt to give you an understanding of what affects the proper amount of water velocity will cause in a sluice box, and also what the affects are of too much or too little water velocity. When the correct amount of water force is being put through a sluice, its riffles will run about half full of material, and the material can be seen to be dancing and vibrating behind the riffles (concentrating) when the water is flowing.
Since gold is around 6 times heavier than the average material that will pass through a sluice, there is usually some margin for error if velocity is a little faster than necessary.
Too much material dumped at once into a sluice box has a tendency to overload the riffles and choke off the concentrating-action behind the riffles. This is because the scouring-action from the water flows will continue to further-concentrate materials trapped behind the riffles, causing heavier materials to be washed out of the box. These should be picked or flipped out of the riffles with minimum disturbance to the remaining portion of the sluice.
Getting the right flow of water to pass through a sluice box out in the field is not difficult. You can use river rocks to make a foundation within the stream so your box can sit level from side to side.
By doing so, and by placing your sluice where the moving water spills over the top, you might create more than enough water flow through the box to meet your needs. Or, in some situations, it is possible to siphon water into your box from a higher point upstream. Water velocity can be increased by either putting more water through the sluice box or by moving the same amount through faster.
When this happens, little or no visible vibrating action behind the riffles will be seen and material will not be moving through the box fast enough to allow you to feed the sluice at production speed without loading up the entire box.

When you have too much water velocity, as material is shoveled into the box, it passes through very quickly and has little time to make contact with the riffles. This will cause gold to wash right through the sluice box as if there were no riffles present at all.
A sluice box operated for extended periods with no new material being fed to it has an increased chance of losing some of its fine gold values. Then, once you are dredging, if you will stop feeding streambed material into the suction nozzle for any period of time, it is wise to block the nozzle with a larger-sized cobble to slow the water flow through your sluice box. By allowing different amounts of water volume through the box, and by changing its downward slope, you can work out a combination that does the job. A water deflector, or barrier, like this can sometimes be built by throwing river rocks out into the stream to make more water flow into and through the sluice.
It really does not take very much volume of water through a medium-sized sluice box to get the right amount of velocity, if the water is moved through the box at speed. Another test is to mix some pieces of lead in with some material, run it through the sluice, and see where the lead stops. For example, in a location where the water is moving slowly, you might be able to direct more water through the sluice and gain the amount of water velocity that you need.
It is good to have a length of nylon cord along with you for securing the sluice box to a rock or some other object upstream. In the case of a short elevator (dam), the water level might only need to be raised up slightly to increase the downward slope of the box enough to create the needed water velocity. You can shovel gravel into the box while trying the different combinations to see what effects the changes have on water velocity.

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