respectively. The shear stress is computed by the formula:

1

2

τc = ρ f **U**

(6)

2

where *f *is a dimensionless friction factor calculated assuming a logarithmic velocity profile (see

for example Sleath, 1984):

-2

8⎡

⎛ *d * ⎞⎤

(7)

25 ⎣

⎝ 15*H *⎠⎦

where *d*50 is the median grain size. The critical shear stress is computed from the critical Shields

parameter which is either estimated as a constant or calculated using the formula proposed by

Soulsby and Whitehouse (1997).

1.5

⎛

τ ⎞

⎛1

⎞

(8)

τc ⎠

⎝2

⎠

⎝

Note that *A *is a monotonically increasing function of *τ*c and therefore **U**.

7