A cutter-suction dredge cuts sand from the seabed and a centrifugal pump lifts the water–sand mixture ("slurry") through a pipeline. Whether the sand actually reaches the discharge point — instead of settling out and plugging the line — depends on the mean slurry velocity v = Q / A versus the Durand–Condolios limit-deposit velocity:
A = π(D/2)² pipe cross-section
v = Q / A mean slurry velocity
F_L ≈ 1.3·(1 − e^(−d50/0.3mm)) Durand deposit factor (rises with grain size)
V_L = F_L·√(2·g·D·(s−1)) critical (limit-deposit) velocity, s = ρ_solid/ρ_water ≈ 2.65
ρ_m = ρ_w + Cv·(ρ_s − ρ_w) mixture (slurry) density
Q_solids = Cv·Q·ρ_s solids mass flow → production rate
If v ≥ V_L the sand stays suspended and transports efficiently. As v drops toward V_L, coarser grains start to saltate along the pipe invert; below V_L they settle out, raising the risk of a plugged pipeline — exactly why real dredge operators watch this margin. Coarser sand (larger d₅₀) needs a higher critical velocity to stay suspended; a wider pipe at the same flow lowers velocity and needs more concentration or flow to compensate.
- Slider interactions — raising pump flow or shrinking pipe diameter raises v; raising particle size or pipe diameter raises V_L.
- Stockpile — grows on the right at the delivered production rate; pause and watch it stop.
- F_L above is an educational approximation of the empirical Durand curve, not the full multi-branch correlation used in dredge engineering design.