Settling Tank Depth Design

I'm working on the design of a rectangular concrete sedimentation basin (settling pond). We're doing a preliminary calculation to see if the cost is feasible for the client to run a full design/construction. This is fairly new to me so I've been doing some research. Essentially, we have a potato wash station and they need to remove 65% of the solids from the water before it is released into another pond for treatment. I've attached a spreadsheet I'm working on to assist with the calculations (source links are on the "source" tab). I've figured out the design overflow rate and particle settling velocities, and eventually was able to iterate on a basin width and length until it worked. The only issue I'm having is that the depth doesn't seem to matter. The particles are falling at some constant velocity, so decreasing the depth reduces the horizontal velocity (thus increasing its relative downward velocity) but increases (by the same factor) the distance at which it has to fall. Therefore it seems the depth doesn't matter. The only thing that matters is the storage volume of solids, but even after 2 weeks between cleanings (at 0.09% concentration by mass), the total depth of solids is only 1/3 of an inch. So I could theoretically make the entire tank about 1 foot deep, have an 10 inch weir, and 1/3 inch of solids storage. That doesn't seem realistic to me; I feel that the tank needs to be deeper because they usually are. Is there something flawed with my reasoning or calcs?

Thanks,
Steven