EVALUATION OF SUPERCRITICAL SUBCRITICAL FLOWS IN HIGH-GRADIENT CHANNEL

The Quail Creek Dike failure flood, near Hurricane, Utah, which occurred December 31, 1988, and January 1, 1989, affords the opportunity to evaluate the hydraulics of a high-gradient channel. For the first mile (1.6 km) downstream from the dike, this unnamed channel has an average slope of 0.032 until it enters the Virgin River at a point where the average channel slope is 0.003. Model-computed depths and discharges are compared with field-measured depths and indirect discharge measurements. Initially, flows are modeled as supercritical in the 1-mi (1.6-km) reach having a 0.032 channel slope. However, the field-measured depths can be reconstructed only by modeling the complete reach as subcritical. This paper questions whether supercritical flows occur in natural channels except for very short distances.