MIke,
You are so right about the size of bunkers versus depth. A six foot deep pot bunker is a lot more intimidating than a large bunker of the same depth. So, steepness should factor in as well.
It's always possible for bad luck to cause your ball to be unplayable. In my experience, you can't - even if you wanted to - eliminate every unadvanceable directly to the pin shot on a course, especially in bunkers! But, it surpizes me how many golfers will go out during construction and say "What if I end up right under the lip?" wanting me to do something to eliminate the "unfair" situation.
Donald Ross said he contoured bunkers to make sure the ball rolled at least a little bit away from the lip. On most courses today, the edges are flatter, because the cost of maintaing such steep sand is deemed prohibitive.
I know this group abhors "formulas" but without some consideration of the physics of the golf ball and its flight, it is very possible to design an unpopular course, because it is not playable. (don't ask me how I know this
)
Generally, though, I agree with Pat. Each bunker should be carefully considered so that it accomplishes as much as possible - looks good, provides framing and definition (yes!) and good strategic value. It's hard to believe that a designer wouldn't approach the design of individual bunkers without at least some idea of what it's strategic value should be.
As the old chinese proverb says, "If you don't know where you are going, you can board any train at the station"
Every so often, its nice to have a bunker that screams "stay out of me" because it is so deep (let's give 'em something to talk about) or is easy enough to recover from to really tempt someone into making the carry required. I naturally have some ideas about where each type should be used. But to make a choice continuously interesting, there must be a balance between risk (too tough and no one will take it) and reward (hazard to easy, no one won't take the risk). By and large, most fairway bunkers should be deep enough to install about a 50% chance of full recovery, and should allow at least some advance forward, unless you are really unlucky -see above.
Of course, this is not a firm and fast rule, just a starting point. In fact, my only real firm and fast rule about bunkers is to never again build one that will get me accosted at the hockey game!
Accomplishing this means varying the depth and, as you say, the steepness of the total slope on the line facing the green, in relationship to the longest reasonably anticipated club (for a 150 yard bunker recovery shot, some golfers may hit a nine iron, others a 7, still others a 5, presuming they DO get into the same bunker by virtue of using multiple tees).
If we expect mostly 7 iron recovery shots, with a loft of about 40 degrees, I think the bunker slope is ideally just under 20% (each 1% is about 2 degrees). Shorter holes should generally have steeper banks, and longer ones should not only be shallower, but have flatter slopes. I think a two iron is about 23 degree loft, no? so it should have about 10 or 11% of upslope facing the green.
Conceptually, I think you could have the same depth for all fairway bunkers, but less slope but the bunker would probably become far too large in most cases.
Before any one goes putting a slope measuring device on any of my bunkers, I will say that few probably fit this "formula", in part because I just made it up while I was typing! Anyway, the job of a professional golf course architect is to think about things in terms like that. Making a concept work is a matter of engineering it in. So, you and Patrick have given me something to think about.....
I do agree with Jeremy that there are usually lots of different things going on, and in the end the tendency is to make it fit the topo best. There is lots of wiggle room, and no firm rules, but if we don't think of bunkers in terms of playability as Patrick Mucci suggests, we often find them the objects of critisism (sometimes not so) later.