Okay, I'm not wanting to be disagreeable with anyone here
but, here are the facts about gardening in a raised be, also called Square Foot Gardening. Here
http://www.squarefootgardening.com/index.php/The-Project/what-is-square-foot-gardening.html is the leading authority on this method of gardening and as you can see, by utilizing a raised bed there are a ton of benefits. Now, I'm sure that the gravel in my method wasn't totally necessary, but I felt that due to my existing soil being of poor quality, the gravel just 'aids' in the drainage because of the extra 3 inches. The water then just gets absorbed in the ground below. However, had I placed my beds on lets say concrete, then you are right, I would have had to make drainage provisions, like piping or something.
As for the depth, I have always tended to go over board and wanted my soil deeper than was the bare minimal. Also, by being in a raised bed instead of a row garden, you
can grow more plants per square foot as I have successfully done last year without having overcrowding issues.
Enough of my ramblings....Check out the link, I think it will clear up any confusion.
Taken from the above website:
Spacing - Seeds and Plants
To figure plant and seed spacing and to eliminate the need to thin seedlings, read the back of your individual seed packets. If it says thin to 12 inches, plant 1 per square foot; thin to 6 inches, plant 4 per square foot; thin to 4 inches, plant 9 per square foot; thin to 3 inches, plant 16 per square foot. This applies to everything you plant - vegetables, herbs and flowers.
Basically, an extra-large plant is 1 per square, large is 4 per square, medium is 9 per square and small is 16 per square.
Since most people are not comfortable planting just one seed, we suggest they plant a pinch of seeds in each hole. How many in a pinch? It usually depends on the size of the seed. For tiny seeds, plant 2 to 4; for large seeds only 2 are needed. If only one sprouts, you are all set. If two or more sprout take a pair of scissors and cut off the weakest looking sprout or sprouts. Don't fall to the temptation of transplanting it because while you are digging it up, you will disturb the plant you are trying to save in that space. So just be bold and snip off the extra sprout or sprouts, leaving just the one you want to remain.