RAH, I think the flapper will be fine, its a pvc flapper and will all but float open and let gravity water thru, in his case he will have considerably more pull actually enough to pull the water up an 8' elevation by dropping more then that down the other side,
Like Esshup said, the vortex it creates could be a problem, I think if he is far enough underwater it should be fine, there is not a lot of velocity, just a steady flow, even then, it will take a little while to lower that size body of water several ft.
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