Sluice Set-Up and Tuning
Proper set-up and tuning of your sluice box is very important. When it is humming, it is working at peak efficiency. No setup is perfect. All of them will lose gold. But with the proper set up you can minimize your losses in a big way.
Many websites and videos explain how to properly set up and run a sluice box. I am just amazed at all of the bad advice out there. Some will tell you the sluice must be set at an angle of 4 - 7 degrees. Others say it should be 5 - 8 degrees. There is even a tool you can buy called Sluice Setter that will help you set the correct angle. Don't waste your money.
None of that advice accounts for how YOU use your sluice in YOUR location and under YOUR conditions. One size does not fit all.
Previously, I described what a sluice is and how it works. Here, I will build upon that to show you how to get the very best performance from your sluice, no matter what type you have, or what the stream conditions might be.
We know that the most important condition is that the water must be flowing fast enough to wash material over the riffles and out of the sluice. If the water is not flowing fast enough, we will need to speed it up. Remember that we want the riffles to be actively working without turbulence or excess scour.
If the water is too fast, try moving the sluice to another part of the stream. If that is not possible, place a rock or some obstacle upstream to reduce the flow into the sluice box.
If the water is too slow, try finding a part of the stream with more current. If that is not an option, try building a wing dam to divert more water into the flare of the sluice. You can also increase water speed by making the angle of the sluice steeper or making it more level to slow it down.
You have probably heard that the correct angle is 1 inch of drop for every foot of length. So a five-foot-long sluice would need to drop from head to foot by five inches. This is a good place to start but is not the best set-up. What if the creek is only 4 inches deep? Then what?
What is important is the volume and speed of the water flowing through the sluice. The angle is secondary. The angle is only used to adjust the speed of the water. During Spring runoff when the stream is running high and fast, I run my sluice almost flat level. I watch my riffles and if they are working as designed, all is good. I get a good clean-out when I'm done.
Once you have your sluice in the water, make sure that the water going through is only an inch or two deep over the top of the riffles. If the water is "rooster-tailing" over the riffles, it is too turbulent. Slow it down. You may need to allow more water in so it is deeper and it can smooth out the upper-level flow. Remember that turbulence is bad, you want a nice laminar flow that rides over the riffles and causes them to work efficiently. The upper level carries the lights out of the sluice and the heavies get pushed along the bottom.
You can see if the riffles are working when you start to feed material into the sluice. The light materials will get washed out of the sluice within a couple of seconds. The heavy stuff will be caught behind the riffles and appear to be dancing. Not slow-dancing - really jumping around, but not leaving the riffle. There should be a "clean" spot between the trapped material and the next riffle. That is your exchange zone. If it fills with sand, increase your velocity. Be careful that you are not boiling everything out of the riffle. It should be riffle, then heavy materials, clear space, then riffle again (see below).
Make sure your sluice is level from side to side. You should have an equal depth of water on both sides of the sluice. If not level, then one side will have a higher velocity and scour than the other. The material will drop into the riffle, then slide sideways and get washed out.
Check the foot of the sluice. If the foot of the sluice is in the water, as the water flows out of the box and into the stream, there will be back pressure at the foot. Make sure the flow is strong enough to push material out of the box. The best situation is to have the foot at or above the water level. This is not always possible. My favorite spot to set up a sluice is on rapids. They provide enough drop that I can angle the sluice for the right water speed and still keep the foot out of the water. You can do something similar by building a small dam. Nothing big, but enough to raise the water level where you will place your sluice. That allows you to raise the head-end higher than the foot. In the image below, the operator should have moved the sluice over into the faster water and then leveled out the sluice.
When everything looks like it is working properly, there is one last step. Place your fingertip lightly on the bottom of the sluice. You should feel it vibrating. Adjust your sluice until that vibration is at its maximum. As the water flows over the riffles creating their vortex, the vibration helps with the exchange of heavies and lights in the riffle vortices. When I do this, my sluice is humming, literally. When the vibrations are at maximum and I have my head near the sluice box, I can hear it humming. If you can't hear it, don't worry. Use your finger and feel the vibrations. Now, using what you know about sluices, what can you tell about this one?
It has no flare to allow stream water to enter.
It has no slick plate to allow the materials to begin separating.
It uses large riffles on top of expanded mesh over matting.
The water flow will be almost flowing over the sides to get over the top of the riffles.
Most likely this is a homemade dredge riffle setup. Somebody needs to study sluice design.
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