Hydraulic riffles were first developed in New Zealand as far as I know. I first encountered them on a floating wash plant when I was doing a job there in 1989. Hydraulic riffles are a system of riffles that have a built in manifold that injects water into the gold recovery mats, and fluidizes the gravel that is retained in a specially designed deep riffle system for a fluidized bed that greatly enhances fine gold recovery.
Sizing the material prior to introducing it to this system is very critical for these to work properly. I have found that 1/8 inch minus feed generally works the most efficiently. They handle a pretty substantial feed rate with surprisingly good recoveries. The larger size gravels will tend to clog up the riffles and impact your fine gold recoveries. These riffle systems are sensitive to feed rate and feed size but when you balance them out, they enhance many operations that are plagued by black-sand and other heavy alluvial minerals. The system above is one I did on a test of a property in western Montana.
We did not increase our recovery with these,on this particular test. Angle iron and expanded metal was satisfactory in this case. The downside of hydraulic riffles is the tremendous amount of cleanup concentrate. If the values are sufficient then your cleanup circuit will be a bit more complicated with a cleanup jig and or a concentrating table.