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Sedimentation basins are sized on surface overflow rate, since settling depends on surface area rather than volume — a particle settles if its settling velocity exceeds the overflow rate. Detention time then fixes the volume and hence the depth. Typical values are 20-40 m3/m2/day and 2-4 hours for water treatment.
Surface area
Area = flow / surface overflow rate
Volume and depth
Volume = flow x detention time / 24; depth = volume / area
Ideal settling theory shows that removal depends only on the ratio of settling velocity to overflow rate. Depth affects detention time and scour resistance but not, in theory, the critical particle size removed.
Flow taking a direct path from inlet to outlet, so the effective detention time is far below theoretical. Inlet baffles, proper outlet weirs and tracer testing are used to control and verify it.