It becomes apparent that water flow velocity is far more important than water volume with respect to proper riffle function and recovery of any heavy materials, including very fine gold. If the flow and slope are set up properly, these riffles work like gangbusters, but if anything is even slightly ‘off’ they only function about as well as conventional riffles. The problem with this particular style of riffle is that it is extremely sensitive with respect to flow velocity and material composition so it is not necessarily a very good design for a ‘general purpose’ sluice box. The horizontal lip by itself only contributes a very small amount to the creation of that vortex. It is the combination of this high-pressure area behind the riffle and the horizontal lip at the top of the riffle bar that contributes to the creation of the concentration vortex. In fact it may be the ‘secret’ to effective vortex creation. This very low laying, backwards moving, high pressure wave is what contributes significantly to the creation of the rather large concentration vortex on the downstream side of these riffles. No other types of riffles exhibit this trait. There is a very significant backwash or high-pressure area created very low on the upstream side of the riffles. Pebbles are actually pushed ‘backwards’, essentially moving upstream behind the riffles. This design behaves very differently than all other riffle styles, in that there are two different components that come into play, while other riffles typically have only a single component to their functioning. If there is a fault it is that this riffle is too effective and captures virtually everything, so it is very sensitive to how and what is being fed down the chute.Īt low flows it appears that the space between the riffles gets filled with the larger gravel particles, but if you watch what is happening this doesn’t seem to effect the operation of the riffles and lighter materials will still keep getting ejected back up into the laminar flow and the heavier particles are driven towards the bottom of the sluice by the centrifugal force of the vortex. This riffle style needs a lot of fast moving water and a steep slope to perform well. The big drawback is that this riffle will not function properly with larger gravels at very low flow velocity and packs up very fast with anything much larger than number 8-mesh materials if the water volume is on the low side.
Rather than a fairly aggressive vortex creation, it has only a minor improvement over the vertical angle type riffle.Įven at very low flow velocities this style riffle builds up a very large concentration vortex that is not only long but also very high and extremely aggressive in action. In reality the increased efficiency is very slight. Compound (Hungarian) Rifflesįrom a performance standpoint, the compound Hungarian Riffles supposedly combine the best features of all riffle types. Basically the lip simply increases the length of the vortex in comparison to a sloped flat bar riffle but not the height or intensity of the concentration vortex.
The horizontal lip helps to a point, but not nearly as much as ecpected. The vertical types of Angle Iron (or aluminum angle sections) do not seem to perform much better than the sloped flat bar riffles, even though the upper horizontal lip supposedly aids in vortex creation. The big advantage of these riffles is that they don’t get loaded up even at high feed rates.
Even though they are thought to create more turbulence, they actually create less and they also create more of a back pressure wave which we is vitally important. In other words bars sloped 30-degrees from the vertical performed better than bars sloped at 45-degrees. The performance improvement on closer spaced bars is significant but still not enough to actually warrant anybody using these riffles in a modern sluice unless you’re running a wham-bam operation wanting to run the maximum amount of material in the least amount of time.įlat bars at more upright slopes perform better with respect to vortex creation. However, this type of riffle can be very closely spaced to improve performance and the 3.5-inch spacing found on most production riffle sets can be reduced to 2-inches or even less. As a result capture of heavy materials was primarily based upon gravity so retention of fine gold is very poor. In general, flat bar riffles at virtually any slope failed to generate efficient concentration vortexes and seemed to operate basically as dams to interrupt and slow the water flow just enough so that the normal gravity settling characteristics of the various particles came into play. Riffle Shapes And Design Sloped Flat Bar Riffles
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