Why should the freeboard on a pond be between 1 and 2 feet?

Free board for any pond or a lake or a reservoir is directlly proportion to its water spread area and the wind velocity in that area. This is to calculate the Fetch and height of wave the wind can form on a reservoir/pond. This is more useful in earthen pond/reservoir etc since the wave if it over flows the bank, the flow on the down stream slope of the earthen embankment/bund may get eroded/scoured and end in a breach of the earthen bund itself.