I completely agree with RAH. He says it in about the simplest way possible.
Since that overflow pipe seems undersized, gives two choices. Tear out that one and put a bigger one in, or put in a E-Spillway that can handle a lot of water on a regular basis (kind of not the point of the E-spillway). Which ever way, better build it right. Sure wouldn't take long for the water to be pouring over that dam , like you had, before you lose the whole pond and cost you a lot like Deerman said.
