Judging from the picture that was posted, a hydrostatic relief valve or drain tiles may not be needed. The pond is obviously on a slope, so there should not be any great water accumulation. Any risk that may occur in a very heavy rainfall will be dictated by the present soil substrate permeability and/or the present depth of the impermeable layer or bedrock.
If this location is part of the watershed of a nearby public waterway, permitting may or may not be required for the installation of an overflow.
"Everybody is ignorant, only on different subjects." - Will Rogers