TMK, The depth of the pond below the discharge pipe has no relevance, because the pressures will be lateral. The only pressure that will be applied to the flashboards are relatively low and come from the blocked water only. These pressures will rise as the level of blocked-off water rises. It is fairly simple to create a flashboard riser system but to seal it is much more difficult, but can be done. On a discharge pipe (vertical Riser) that is surrounded by ice, failure will probably occur, however, if the discharge pipe is lateral the ice expansion pressure is far less.