Bruce, I think for your application the conical tank is the best option.
1) concentrate the sediment to the bottom.
2) quick flush of the sediment.
The only thing that will be proven by testing with the actual tank is whether the sediment will have enough weight to it so it falls all the way to the bottom of the cone, rather than get hung up on the sloped sides. That's why a circular current is so desirable in aquaculture tanks with a center drain. No siphoning of solids - the current does the work.
Maybe with the sloped sides less current would be required to get the sediment to the center of the cone?
I don't think you'll have an issue with stirring up the sediment when the valve is opened. I'd want the cone to dump directly into the valve and not have a short standpipe in the tank - more crevices for the solids to hang up.
