If you have never seen how a sand bunker is built, this picture is for you.

This is what a finished bunker looks like right before sand installation. Drainage trenches are dug out 12" deep and a 4" pipe with holes in it are placed then backfilled with rice gravel. Outside the bunker solid pipe is run to a location for the drain water to exit. The white material you see on the banks is a special fabric that is stapled down to the soil. This will prevent washouts and soil from mixing with the sand. Bunkers can last from 15 to 50 years before they need to be rebuilt again depending on the style and location.