Shields parameter, threshold of motion কী?

Example 20.11 (What the river moves)

In the river $2\,\mathrm{m}$ deep on a slope of $0.001$, $\tau_b = 19.6\,\mathrm{Pa}$. For grains of $20\,\mathrm{mm}$, $\theta = 19.6/(1651.8 \times 9.81 \times 0.020) = 0.060$, just above their threshold of $0.055$: they move. For grains of $40\,\mathrm{mm}$, $\theta = 0.030$, half the threshold: they stay. The river moves its bed up to coarse gravel; for sand of $1\,\mathrm{mm}$, $\theta = 1.21$, nearly forty times the threshold of $0.031$.