Bedrock-controlled Fluvial Geomorphology and the Hydraulics of Rapids on the Colorado River

Abstract

The fluvial geomorphology of the Colorado River cutting across the Colorado Plateau in the western United States is bedrock controlled and largely governed by rapids. Rapids on the Colorado River control the water-surface profile and influence the bathymetry, the storage of sand, and the aquatic ecology. Despite their importance, little data on the hydraulics, sediment transport, and long-term stability of rapids have been collected. By comparing water-surface profiles, the average rate of aggradation at the head of 91 rapids in Grand Canyon between 1923 and 2000 was calculated to be 0.26 ± 0.15 m. In addition, while in 1923, 50% of the cumulative drop through the river corridor occurred in just 9% of the distance, by 2000, the cumulative drop over the same distance increased to 66%. A new hydraulic model, incorporating one-dimensional step-backwater theory, was constructed for the Colorado River in Grand Canyon. The model includes 2,690 cross sections and simulates discharge up to 5,600 m³/s, offering the opportunity to simulate large floods, rare under the current regulated flow regime. Flow velocities were measured directly in rapids using three separate flow measurement instruments. An acoustic Doppler velocimeter (ADV) was used to measure velocity in five Grand Canyon rapids. While the instrument was able to measure velocity in three dimensions up to 3.0 m/s, limitations rendered data unusable for flow above 3.0 m/s. An acoustic Doppler current profiler (ADCP) was used to measure the flow field in rapids throughout the water column in Cataract Canyon. The peak average velocity measured by the ADCP was roughly 4.0 m/s. Similarly, average flow velocity of 5.2 m/s was measured in a Cataract Canyon rapid using a pitot-static tube. The pitot-static tube measured instantaneous flow velocities up to 6.5 m/s, one of the fastest velocity measurements made in a river. Using the combination of the ADCP and pitot-static tube, the flow structure and nature of turbulence within rapids were analyzed. Finally, techniques were developed to enable the measurement and construction of detailed water surface, shoreline, and bathymetric maps directly in rapids on the Colorado River.