Rule of Thirds (Fifty-Ninety Rule)

Definition

Assumes a sinusoidal rate profile over a six-hour half-cycle. In successive hours the stream runs at roughly one third, two thirds, three thirds, three thirds, two thirds and one third of maximum rate. The related fifty-ninety rule states that after one hour the stream has reached about 50 per cent of maximum and after two hours about 90 per cent. Both fail where the profile is distorted by shallow-water effects.