Since you want the volume in yards (really cubic yards) I think it is easiest to change all the dimensions to yards immediately. There are 3 feet in a yard so 21 feet is 21/3 = 7 yards. There are 36 inches in a yard so 4 inches is 4/36 = 1/9 yards.
The volume of a circular cylinder is
where r is the radius and h is the height. You want half a cylinder so the volume is
1/2 72 1/9 = 8.6 cubic yards.