Flooding and sediment transport in a small alpine drainage basin in Colorado

Abstract

A short-duration, high-intensity rainfall on a 0.43-km 2 (105-acre) alpine drainage basin in the central Colorado mountains produced a peak discharge, including water and sediment, of 108 to 134 m 3 /s (3,810 to 4,730 cfs). This exceeds by an order of magnitude the discharge of the 100-yr flood as predicted by the procedures recommended by the U.S. Soil Conservation Service and exceeds by a factor of two the largest discharge ever reported in a basin of similar size. The Manning formula and superelevation calculations were two methods used to compute velocities and discharges. These two methods depend upon independent sets of assumptions, but they produced similar results. Calculations of the fluid-dynamic stresses required to transport boulders in the upper channel also suggest flood depths and velocities within the range of values computed through use of the Manning formula and superelevation calculations. In the lower channel, bed load moved as inertial grain flows. One flow studied in detail transported sediment onto a part of the alluvial fan not reached by deep flood water. The larger clasts were transported to the top and front of the grain flow. This type of sorting supports a theory of grain-flow deformation first suggested by Bagnold.