Fluid Density

Fluid density and viscosity are the two basic physical properties of fluids that matter most in sediment transport. ^[1] Density - commonly referred to by the symbol ρ (rho) - is defined as mass per unit fluid volume. ^[1] The fluids of primary interest in sediment transport are air, water, and water containing various amounts of suspended sediment. ^[1]

Effects on Sediment Transport

Density affects the magnitude of forces that act within a fluid and on the bed, as well as the rate at which particles fall or settle through a fluid - particles settle more slowly through denser fluids. ^[1] Density particularly influences the movement of fluids downslope under the influence of gravity. ^[1] This downslope influence is most visible in density currents - turbidity currents, for example, flow along the seafloor precisely because their density, elevated by suspended sediment, is greater than that of the surrounding clear water.

Water vs. Air: A Critical Contrast

Density varies with different fluids and increases with decreasing temperature of a fluid. ^[1] The density of water - 0.998 g/mL at 20°C - is more than 700 times greater than that of air. ^[1] This enormous density contrast is the main reason water can transport particles of much larger size than those transported by wind - a denser fluid exerts far greater drag and lift forces on grains resting on the bed. ^[1] The practical consequence is that rivers can carry cobbles and boulders while the strongest desert winds move little more than sand and dust.

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