When the sediment concentration is high (approximately larger than 3,000 mg/l), the settling of particles is reduced due to return flow, particle collisions, increased mixture viscosity, increased buoyancy, and wake formation. This process is referred to as hindered settling. Of the sediment particle settling velocities presented above, only the Soulsby (1997) formula considers this process. When using other particle settling velocities hindered settling is considered using a modified form of Richardson and Zaki (1952)

1) \frac{\omega _{sd}}{\omega _{sd0}}=\left(1-C_{tV}\right)^{n}

where

ωsd = sediment particle settling velocity for turbid water [L/T]

ωsd0 = sediment particle settling velocity for clear water [L/T]

CtV = total sediment concentration by volume [-]

n = empirical exponent [-]

The empirical coefficient n ranges between 3.75 to 4.45 for medium to fine sands (approximately 4.0 for normal flow conditions and particles in the range of 0.05 to 0.5mm). The above formulation differs from Richardson and Zaki (1952) in the inclusion of the maximum suspended sediment concentration which may be set to the bed dry bulk density. This is physically correct since the particle velocity should become zero when the concentration is equal to the bed dry bulk density. The empirical exponent that varies from is a function of the particle Reynolds number but is set to user-defined constant here for simplicity.