Application of the USGS Multi-Dimensional Surface Water Modeling System (MD_SWMS) at Bridge 339, Copper River - Tim Brabets , USGS (co-authors: none)

ABSTRACT

The Copper River Basin, the sixth largest watershed in Alaska, drains an area of 24,200 square miles. This large glacier-fed river braids across a wide alluvial fan before it enters the Gulf of Alaska. Bridges along the Copper River Highway, which traverses the alluvial fan, have been impacted by channel migration. Due to a major channel change in 2001, Bridge 339 at Mile 36 of the highway now receives more of the river’s flow than it was designed for, resulting in excessive scour and damage to its abutments and approaches. During the open-water season, which typically extends from mid-May to September, the design discharge for the bridge often is exceeded. The approach channel to Bridge 339 shifts continuously, and from 2005 to 2008 shifted back and forth from the left bank to a course currently along the right bank nearly parallel to the road.

Maintenance at Bridge 339 has been costly and will continue to be so if no action is taken. Possible solutions to the scour and erosion problem include (1) constructing a guide bank to redirect flow, (2) dredging approximately 1,000 feet of channel above the bridge to align flow perpendicular to the bridge, and (3) extending the bridge. The USGS Multi-Dimensional Surface Water Modeling System (MD_SWMS) was used to assess these possible solutions. The major limitation of modeling these scenarios was the inability to predict ongoing channel migration. A hybrid dataset of surveyed and interpolated bathymetry was used in the approach channel, which provided the best approximation of this dynamic system. Under existing conditions and at the highest measured discharge and stage of 32,500 ft3/s and 51.08 feet, respectively, the velocities and shear stresses simulated by MD_SWMS indicate scour and erosion will continue. The model predicts that a 250-foot-long guide bank would not improve conditions because it is not long enough. The model also indicates that dredging a channel upstream of Bridge 339 would help align the flow perpendicular to Bridge 339, but because of the mobility of the channel bed, the dredged channel would likely fill in during high flows. Results from MD_SWMS suggest that the option of extending the bridge is most likely to be successful.

Discharges at Bridge 339 are expected to exceed 32,500 ft3/s as channel migration continues to capture additional flow from adjacent channels. New LIDAR (Light Detection and Ranging) data obtained in November 2009 provides better resolution and current location of channels and is currently being used with MD_SWMS to examine other possible solutions. These include temporary diversion channels to other bridges and determining the required conveyance for Bridge 339 for a given discharge in order to properly design an extension for the bridge.

Topic: Alaska Hydrology