Great question Jason!
And your answer was spot on. Without the discharge ports the material in the trap would fill up to the top of the grizzly and incoming gold would scour out before it had a chance to settle down into the trap. Besides, the less material above the fluidizing tube the more fluidized the trap will remain. Also the diverter baffles located between the intake grizzly and the overflow ports insure that any floating gold will get submerged to prevent it from flowing out the overflows.
Red arrow points to a diverter baffle......
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I am working on a new design this winter for my 4" dredge sluice to do away with the riffles and mats altogether and just run a fluid bed trap at the end of the sluice as it's only source of gold recovery. Of course the trap will be a slightly different design than the one I'm using now. (more tubes and longer plus the discharge port will be at the end like the Bazooka gold trap instead of the sides)
It will make clean outs simple and fast and should eliminate any worry's of gold blow out due to a sudden burst of flow after clearing a hose clog etc.
* The trade off for eliminating mats and riffles will be that the new system will only be capable of recovering gold small enough to fit through the 3/8" holes in the grizzly. However since here in Indiana 99.99% of the gold is less than 1/8" it shouldn't be a problem.
Here is the one currently in use on the dredge.
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GG~