From my experience at the VS Fisheries carp farm and over a fair few years of specifically targeting carp, I have found that they will vary their feeding depth according to water temperature and quality and the location of food.
I have caught carp in large gravel pits at depths of between 25ft and 30ft, particularly in autumn as the water cools.
That said, on a typical gravel pit I would probably start by targeting the shallower areas of 10ft and also regularly try at least one rod a bit further down the slope.
If the water is cold, then try fishing in the deeper areas. Additionally, don’t be afraid to try fishing with zig rigs using an adjustable zig float, which makes it possible to fish a few feet beneath the surface even if the water is really deep.
Zig rigs may be a bit daunting to start with, particularly in really deep water, but take my word for it, they can be hugely effective!
The adjustable zig rig is perfect for fishing really deep water