Philippe,
Having worked in clay soils most of my life I tend to think the same way. In fact, even in sandy soils, most bunkers get tile these days.
When designing a green plan, I have probably already roughed out the drainage scheme – either pipe, ditch, pond, whatever, although draining into nearby creeks is getting rarer these days due to environmental regulations.
After plotting the basic green and bunker locations and sizes (which will be modified as the plan goes on) I start figuring the drain pipe outlet elevation and its distance from the lowest bunker, green tile outlets (usually the front of the green) and even any bunkers or grass bunkers on the far side of the green from the drain outlet.
To keep pipes “self cleansing” and avoid clogging, I have minimum grades I use for the typical 4” (1%), 6” (0.67%) and 8” (0.5%). If using 8” outlet pipe and with the bunker 200 feet from the outlet, the pipe flow line must be 1 foot above the outlet. Add in another 8” for perforated tile and gravel base in the bunker, and 6” for sand and a fudge factor and I would set the minimum bunker base elevation at about 2 ft. above the drain outlet.
I also consider the “natural” floor elevation, which is what it would be if I cut it into the soil up to a foot and the “maximum” it might be, although it usually looks unnatural to fill a bunker base more than a little bit. Sometimes, we must, though.
Then, I consider the minimum and natural green front elevations. A lot of things go into the green elevation like visibility from the fairway, fitting to trees and natural grades, mowable slopes, cut and fill balance or ability to import fill, ADA ramps and cart path access, etc.
On rolling ground, most factors favor setting a base green and bunker elevation pretty near their center point elevations. The result is a bunker about as deep as the differences between those two naturally are. But, I look at the hole design, too. On a long par with a “natural bunker depth” of ten feet, is that too much penalty considering the rest of the hole? Or is it in a place in the round where it’s a good change of pace and gut check, and does it contrast nicely with the other long par 4’s which might feature harder tee shots, or easier green hazards, etc.
However, on flat ground, I may be faced with a situation where the bunker is 3 feet above the outlet, and the natural green front is only 4’ above it. Then, I need to decide if I want a one foot deep bunker, another hazard like chocolate drop mounds, or to fill the green, still within the parameters above. If there is a fill source nearby I will probably fill the green to get at least 3-4 feet of bunker depth, all other factors being equal, but costs usually keep me from filling the green 8-9’, even if it’s a short approach that might benefit from a stout bunker. I go through the same process of comparing it to other short holes, the difficulty of tee shot, whether I can make green contours or other things the key feature of the approach, etc.
It’s a circular process to say the least. And the land always has some input to the process, but at some point, I have to say, “Hmmmm, what exactly do I want this shot to play like?” And that’s when some rudimentary math comes in to play simultaneously with both art and intuition.
I agree its weird to obsess over some theoretical bunker depth to feet and inches. Given the typical upslope and natural warp of the greens and bunkers, some parts of the bunker will be relatively deeper than others, so all I try to do is get a general idea of shallow, moderate, or deep bunker, considering topo and play. Also, in general, the more bunkers there are around the green, the shallower I generally make them. If there is one bunker, I feel it can be deeper, so there are more places to avoid them. Even with multiple bunkers, I may make one, or one side the "master bunker", i.e. the deep one the astute player knows he needs to avoid.
Mike also makes an interesting point about slope in the bunkers. Behind and above greens my tendency is to slope bunkers up for vision, and up more quickly to keep the bunkers relatively small and close to the green. Some players complain about the downhill lie to a downhill green - especially if aiming at a pond on the other side, which sometimes is the case. That - and the length of tile required to drain bunkers on the high side - often becomes a practical reason to follow Ross and limit backing bunkers. (If he did. And, if he did, I guess its for the same reason)
Other complaints on lateral bunkers include flash slopes near the green, where a shot missing by five feet might bury, and one missing by ten finds the comfortable flat lie in the bottom of the bunker. Personally, I love the look of flash bunkers, and hate to see just "slivers" of sand. I find sloping my bunker bases up 5-8% makes even a "flat" bunker visible.
The only actual writings I recall about any of the ODG on bunker depth, slope, etc. was Ross writing that the inside of a fw bunker ought to be shallower than the outside, to create proportional penalty. I believe he also liked a continuous slope away from the bunker edges to assist in collecting balls near the middle rather than in an unplayable lie. Other than that, I don't think they gave it as much thought as some would think.
Frankly, I am just starting to at this point in my career. For a long time, bunkers just came out where they came out. Now, my bunker depths still often "just happen" but I do like to think about them before they wander off on their own!