For experimental purposes.
You can seal your dam bottom by laying a tarp or fertilizer bags along the bottom over the lumps
and up your dam wall.The water pressure will mould the material and seal around the obstructions.
you can then shovel sand , dirt etc on top to help hold the front edge down in higher flows.
Remmember to tie everything back to something solid so it doesnt end up in the neihbors place due
to flooding.........don't ask.
If you can get to two feet of head then a breast shot may be possible.
The usual question,,, how much head , flow do you have ?
cheers
Keith