Jase,

Check your corrugated 12" poly pipe again. Is it dual-wall 12" culvert? (It would look smooth on the inside, not corrugated.)

If it is dual-wall culvert, then that stuff is pretty tough. Further, even if it cracked, it should be mostly in packed clay. I think the preferred path of the water would still be down the pipe, rather than working around the outside of the pipe and washing out your dam.

Just my two cents. There are lots of actual dam building experts in the forum. Hopefully, one of them will come along and address your concerns.

Good luck!