There is a widely popular idea that putting gravel in the bottom of container gardens and planters is good for drainage. This isn't true and in fact, putting gravel into the bottom of pots reduces the capacity of potting soil to drain.
The problem is that water (and keep in mind here that physics isn't my strong suit) flows through similar textures and when it hits a different texture, gravel for example, the flow stops and the water backs up into the soil.
According to Dr. Linda Chalker-Scott, "gravitational water will not move from a finely soil texture into a coarser material until the finer soil is saturated. Since the stated goal for using coarse material in the bottoms of containers is to “keep soil from getting water logged,” it is ironic that adding this material will induce the very state it is intended to prevent.
For more info on how to keep your plants from drowning.