Rod Hutchinson in The Carp Strikes Back reckoned the carp would use the top of the gravel bar during the day, hanging around and moving along, then would feed at the base of the bar.
I've had fish from the top of gravel bars, Brackens Pool especially, where the bars could be 15ft above the lakebed, but on other lakes as well.
I've also done well on either side at the base of the bars.
Pretty much the only way is to fish both and see which the fish prefer.