Application of a sediment-transport model to estimate bridge scour at selected sites in Colorado, 1991-93

A bridge-scour study by the U.S. Geological Survey, in cooperation with the Colorado Department of Transportation, was begun in 1991 to evaluate bridges in the State for potential scour during floods. A part of that study was to apply a computer model for sediment-transport routing to simulate channel aggradation or degradation and pier scour during floods at three bridge sites in Colorado. Stream-channel reaches upstream and downstream from the bridges were simulated using the Bridge Stream Tube model for Alluvial River Simulation (BRI-STARS). Synthetic flood hydrographs for the 500-year floods were developed for Surveyor Creek near Platner and for the Rio Grande at Wagon Wheel Gap. A part of the recorded mean daily hydrograph for the peak flow of record was used for the Yampa River near Maybell. The recorded hydrograph for the peak flow of record exceeded the computed 500-year-flood magnitude for this stream by about 22 percent. Bed-material particle-size distributions were determined from samples collected at Surveyor Creek and the Rio Grande. Existing data were used for the Yampa River. The model was used to compute a sediment-inflow hydrograph using particle-size data collected and a specified sediment-transport equation at each site. Particle sizes ranged from less than 0.5 to 16 millimeters for Surveyor Creek, less than 4 to 128 millimeters for the Yampa River, and 22.5 to 150 millimeters for the Rio Grande. Computed scour at the peak steamflows ranged from -2.32 feet at Surveyor Creek near Platner to +0.63 foot at the Rio Grande at Wagon Wheel Gap. Pier- scour depths computed at the peak streamflows ranged from 4.46 feet at the Rio Grande at Wagon Wheel Gap to 5.94 feet at the Yampa River near Maybell. The number of streamtubes used in the model varied at each site.