snrub,
I did plan a sediment settling pond much like yours.
My only design modification was to use a gravel-filled gabion basket for the water interchange between the settling pond and the main pond. I was going to put geo-fabric on the bottom and the "downstream" sidewall, and then fill it with the recommended size of gravel.
This system should allow a slow exchange of water from the settling pond to the main pond, but no exchange of fish or fry if you did want to try to manage the forage pond.
It has the added advantage of doubling as an emergency spillway for that 100-year rain event. If you had 2' of freeboard, and installed the top edge of a 20' long gabion basket 1' above normal pool, then you would have 20 square feet of outflow (equal to a 42" pipe).
If you had it perfectly level, it would go from a gushing waterfall to a trickle fairly quickly as your run-off water crested. Hopefully, there would only be a very short window for adventurous LMB to swim upstream.
As FireIsHot found out with his forage (now grow out) pond, it takes only a short window for LMB to go "upstream" and get where they are not supposed to be.
I took some updated pictures today that shows the elevation (or actually lack thereof) challenge I was facing. It has rained recently and all 4 ponds were right at full pool with maybe a quarter inch of water depth flowing in all the overflows. Ponds are as picture perfect as they are ever going to be as far as all being full at the same time.
I'll post the pictures in order so that a person can best get the lay of the land. In the first picture, the shed on the right that is closest to the center, the tiny pre-sediment pond is just to the right of that shed and behind that pile of dirt (that eventually will be removed). As a person follows through the picture sequence, keep the right hand end of that shed in mind as a reference point (right hand of that shed is to the south, left end is to the north).
I think this series of pictures will show how little elevation difference I had to work with to make the sediment pond. The reference shed is right on the peak of the hill, as is the culverts pictured in the pre-sediment pond picture. Main 3 acre pond is in the foreground, forage pond to the left, sediment pond in front of the reference shed, and pre-sediment pond to the right (south) of the shed.
The very last picture is looking back the opposite direction from the pre-sediment pond (looking down hill towards the main pond).
These pictures show the relationship of the ponds a lot better than some of the earlier ones at varying water levels.
