I'd visit the local NRCS office and get their recommendation on overflow pipe size which is calculated by watershed, type of soil, vegetation and black magic. They do the calcs and size it accordingly for a flood.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).