Originally Posted by tadpole
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.
That is my thinking. If you think that a drainage system needs to be in place drail tile running the lengh out the far end with the pipe day lighted out would be my suggestion. Looking at the slope I agree that I don't think any drainage is needed.