What you need is a big tub in which you can stand a small two-stage sluice box. You then need several jerrycans of water. You fill the tub, collect your paydirt feed it, in small amounts, into the sluice. You can then ladle water from the tub into the sluice. A two or three litre milk carton cut down but with the handle still attached makes an excellent bailer.
Alternately you can put a small marine bilge pump in the tub and power it with a small battery that you keep charged with a solar panel. A constant stream of water can then be recycled down the sluice box. This may not always be legal, but a friendly mining inspector contacted the Chief Mining Warden in Perth and obtained permission for us to run this small-scale contraption.
The major problem is getting rid of the tailings. It is necessary to make very frequent stops to remove the worked gravel from the tub.
You will obviously lose water during this process and will need to have plenty more jerry cans on hand.
With clean gravel I have been able to work all day with 5 or 6 jerry cans of water. With dirtier gravel I have had to use up to 12 jerry cans. Obviously you need
a water source somewhere within range and substantial transport capacity. While often. all we get is a bit of colour, we sometimes collect a gram or two. The reward is in the journey not the destination.