There are a few general principles that I try to follow, from which the bunker depths evolve. Greenside bunkers tend to be deeper than fairway bunkers for one because the recovery shot is with a more lofted club. The depth of the bunker is a function of the type of recovery shot that can be played, the size of the bunker, the existing topography of the hole or hole features as well as the severity of the hazard you are trying to create. This should be related to other spatial issues like the amount of run-up room, the length of the hole or shot, the location of the bunker relative to the green position, etc. When all the the factors relating to the strategy of the hole are put together, the bunker depth starts to develop relative to the severity of the hazard desired. The existing topography will also dictate bunker depth to a degree. Deeper bunkers might feel more natural on an undulating piece of ground, whereas on a floodplain site bunkers might tend towards the shallower end of the scale.