I see two options to keep from draining and do the repair.

Cofferdam the pond, dig out old and rebuild the dam. Cost of cofferdam rental & installation, continuous pumping of water while work is taking place. Can take a lot of time to get cofferdam to seal and a lot of labor installing.

Pump grout in and around the old pipe. May not have local. Hard to estimate how much it will take but probably more than you think. Cement grout isn't cheap.

The cheapest and probably best option will be draining, removing the old pipe and rebuiding the dam. Take the opportunity while water is low to make other improvements.



"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge." Stephen W. Hawking