If the first pond is a combination forage/silt pond, I would think that you might want out be able to clean all the muck out easily, accessible from all edges. You would have to be careful not to flush all the sediment down with the forage fish when sending the forage fish to the big pond. Others might give better advice, but you may want to consider two smaller settling pond rather than a big 1/2 acre one. I seem to remember Bob Lusk having a short video somewhere on a forage pond and the +/-'s of having one. IIRC it wasn't that deep, maybe 5 '?
Here is the video I was thinking of. It is for a hatchery pond, but might be of some help.
http://www.youtube.com/watch?v=bwQ3Wm4qGCc 
