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Sizing a circular culvert inverts Manning's equation for full-bore flow, giving diameter proportional to (Qn/sqrt S)^(3/8). A 0.85 m3/s design flow at 1% slope in concrete needs about 0.66 m, so a 750 mm pipe would be specified. Velocity should stay between about 0.9 m/s to avoid silting and 4 m/s to avoid scour.
Full-bore diameter
D = (3.208 Q n / sqrt(S))^(3/8)
Velocity check
v = Q / (pi D^2 / 4)
Because pipes are manufactured in fixed diameters, and undersizing risks surcharge and upstream flooding.
Design usually targets part-full flow at the design storm, leaving freeboard for blockage and larger events.