For 15 high velocity flows in the Mount St. Helens area, velocity profiles were used to compute ratios of mean velocity to the velocity measured nearest the surface, as shown in table 4 (data from Dinehart, 1987). Ratios ranged from 0.68 to 0.87, with a mean value of 0.78. For natural channels, Rantz and others (1982) recommend using a ratio (surface-velocity coefficient) of 0.86 to estimate mean velocity. The excessive velocity observed in the outer region of the flow (at points above 0.2 times depth) has been observed in other steep channels (Marchand and others, 1984).
