CressHelp - Scour - Scour in free flowing water

Cress Help - Scour - Scour in free flowing water - B28.1

Determination of critical flow velocity

General equations

For non-cohesive/loose grained the critical depth averaged velocity Uc can be determined from the following equation:

equation 1

in which:

d₉₀: grain diameter that is exceeded by 10% [m]

g: acceleration of gravity = 9.81 [m/s^2]

k_s: (= 3d₉₀) effective bed/bottom roughness [m]

y_c: critical mobility parameter [-]

y_c as a function of D* (=d₅₀(Dg/v²)^1/3)

y_c = 0.24 D*^-1 for D* =< 4

y_c= 0.14 D*^-64 for 4 < D* =< 10

y_c= 0.04 D*^-10 for 10 < D* =< 20

y_c= 0.013 D*^-29 for 20 < D* =< 150

y_c= 0.055 D* for D* > 150

Empirical relations for y_c (Van Rijn, 1984)

The equilibrium depth y_{m, e} can be calculated with:

equation 2

In which:

w: turbulence constant (= 1 + 3 r0) [-]