Copper River Channel Migration and Its Effects on the Copper River Highway

Abstract

The Copper River in southcentral Alaska drains an area of more than 62,700 square kilometers. Near its mouth, it flows into a large alluvial plain known as the Copper River Delta. The banks and streambeds of the Copper River Delta are readily erodible and thus channels constantly scour and migrate. The Copper River Highway traverses this dynamic and complex network of braided channels via 11 bridges. Over the last decade, several of these bridges and the highway have sustained serious damage from high flows in excess of 12,000 cubic meters per second and channel instability. Scour monitoring, LIDAR data, bathymetry, hydrologic data, and hydrodynamic modeling are being used to investigate the cause and timing of the channel migration and future impacts to the highway and its bridges. The distribution of flow through these bridges was relatively stable until the mid 1990s. In 1991, up to 68 percent of the Copper River flowed through 3 bridges on the western side of the Delta. In 2004, these same bridges conveyed only 8 percent of the flow while 90 percent of the overall discharge flowed through 3 bridges on the eastern side of the delta. Migration of the river across the delta and redistribution of discharge has resulted in streambed scour at the bridges, overtopping of the road during high flows, prolonged highway closures, and formation of new channels through forests. Scour monitoring equipment and channel soundings at the eastern bridges have recorded up to 13.4 meters of fill at one bridge and 7.3 meters of scour at another bridge. The scour and fill are the product of the overall redistribution of flow and local channel migration at the bridges. A combination of LIDAR and aerial photography taken at various times from 1950 to the present indicate that the channel migration from west to east begins immediately downstream of where the river splits from a single channel to numerous braids. Longterm delta evolution would result in periodic channel migration back and forth across the delta, and in south-central Alaska, tectonics and isostatic adjustments may affect geomorphic processes. The immediate cause of the channel migration could be the result of natural adjustments following large floods in 1981, 1995, and 2006, or the result of channel formation during ice jams. Current observations indicate the eastern channels are becoming more established and will continue to threaten sections of the highway and several bridge crossings.