JohnR...that "low spot" in the corner of your dam is, in all likelihood, your emergency spillway in case a huge rain event occurs that the primary drain can't handle. You will need to lower your pond level and build a coffer dam to get in and repair the old corrugated steel drain. Concrete is not going to seal the pipe, plus the chemicals in the concrete will speed rusting. Chances are high that water will flow outside and alongside the dam's through pipe, and eventually, it can cause total dam failure. BTW, lowering the water level for a while is almost always a benefit.
I sent you a PM
