The problem with flat ground when drinking (too much or enough) isn't how much you drink, it's the flat ground itself! The flat ground just doesn't know how to be. It should be a steep hill or in this case a high pile of gravel so that it's easy to walk on it when under the influence of alcohol. Same thing when being drunk in a big ocean vessel that rocks on waves. There the flat floor knows how to behave. Swinging floor is easier than a floor that stays still when drunk.