soo, if i may add in here.
I've been Gravity dredging for about 6 years now and i'll say the setups, math and physics i've seen could use some help.
firstly, don't bother with the corex rolls or the pvc the. best and cheapest rout i'v found that works vary well. is to use the triple wall drain pipe 10ft $7-10.
Advanced Drainage Systems 4 in. x 10 ft. Triplewall Pipe Solid-4550010 - The Home Depot
than couple these together with 10ft , lengths of corex. no more than 30 ft please with less than 60ft of triple wall or about 4+ft drop. this is your flexible mining hose you'll be holding on to, "THIS IS NOT USED AS A SLUICE"
4 in. x 10 ft. Corrugated HDPE Drain Pipe Solid with Bell-End-4540010 - The Home Depot
you'll need this to put the corex together with the triple wall.
Eastman 4 in. x 4 in. Flexible Rubber Coupling with Clamps-86007 - The Home Depot
attach the rubber coupler to the corex don't over tighten.
when you find a good spot with some rapids or fall start laying your triple wall out from the top down "BELL ENDS FACING UP" or it will suck air and try to fall apart.
hold everything down with rocks, don't worry about sealing the joints just try to keep them under water if you can so if they do leak a little itll suck in water. usually they pull themselves sealed.
the more you use the pipes they will get harder to get apart (3" anyway they can give ya a fight 4s not so much).
once you have it layed out you'll want to build up a bit of a rock dam at the head of the pipe. have the pipe as low in the water as you can, and stack the rocks on top of it, leave the whole bell sticking out.
at this point the pipes should start running on their own. hold all the corex you want to use under the water, get the air out.
attach it to the triple wall bell. should be sucking like a sob.
this setup is self starting, self clearing, fully modular, cheap, easy, and fairly durable.
be nice to your piping and itll last a while, the corex will start to get little tiny holes from wear. but as long as you keep it under water it cant suck in air.
the triple wall is somewhat flexible. will kink with too much bend. set the end of the pipe chain in a normal 12" sluice, have some water running into the sluice from the stream/river too not just the pipe. this will help eliminate build ups in your sluice, and help even out the blasting force from the pipe. I hold the pipe down with a rock in the sluice.
I've been able to get a 30ft setup running with ~2ft of fall, and still have plenty of power to work with. double that and oh mama. look out power dredge.
the pipes will wear after a while I usually get new pipes every year. not too bad. probably a couple months worth of continuous use. you'll notice the difference between new and old pipes. old pipes loose syphon not as much git. you can also see how marked up the inside of the pipe gets.
they work well for a while though.
edit: also shake and tap on with fist to bounce and send down any caught material out of the corex ridges. be careful with detaching the couple too. gold may get trapped there.