The dredge system linked depending on pump size can be used at any depth, with a larger pump the more stuff that can be moved by it(the main limiting factor is your suction size) a 4" rock will not go up a 2" hose. If someone was so inclined to visit ebay and search gold dredge suction they could by chance see basic flow requirements. That model of water lift is what they use for gold dredging as long as the pipe is enclosed and you have enough pressure, you can move the material as far as you want. If you have multiple trash pumps you can also make a booster mid-way on you material handling hose.

As for making a separator/catch basin I also am an avid DIY kind of person I would have to recommend a silt fence. have never personally tried it but it is what that product is designed for retain silt and put water into drainage areas.

DISCLAIMER: I do not gold dredge, I do however use airlifts and waterlifts on a regular basis for excavating around pipelines(part of my job). This can be dangerous work do not put your head underwater unless you know what you are doing and preferably have a helmet on or at a minimum someone to call the local dive team to get your body.