Root depth Crab and Dogwood

I planted a long line of White Dogwoods and Robinson Crabapples only a foot and one-half away from a ditch. The ditch has a plastic drain pipe covered by trap rock. The pipe carries runoff ground water to a catch basin. WILL THE TREE ROOTS COMPROMISE THE DRAINAGE SYSTEM by invading the pipe? HOW DEEP do the roots of these two trees go down? The pipe is no less than 20" deep. Our condo board needs assurance that all is well, or I'll have to move all of the trees. Thanks
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