Title
Hydraulic Conditions and Scour at I-90 Bridges on Blackfoot River after Removal of Milltown Dam, Spring 2009
Abstract
The 2008 removal of Milltown Dam on the Clark Fork, near Bonner was predicted to affect pier and abutment foundations of nearby bridges. The substantial contraction of streamflow at the bridge exerted hydraulic forces on these structures not experienced when they were in the backwater of Milltown Reservoir. Countermeasures used to mitigate predicted scour and erosion included substantial modification of piers and abutments of the I-90 bridges that cross the Blackfoot River near the confluence with the Clark Fork. With Milltown Dam removed and scour countermeasures completed, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, began monitoring hydraulic and scour conditions at the I-90 bridges beginning with the spring 2009 runoff season. The purposes of the multi-year monitoring effort are to measure the effects of high-flow conditions on the I-90 bridge piers and abutments and nearby stream channel morphology, to compare measured hydraulic and scour data to scour and erosion predictions from earlier models, to characterize the extent to which scour and erosion effects are due to local hydraulic factors versus more broad-based geomorphic factors, and to provide stream-velocity data for this reach to aid in evaluating the possibility of bull trout passage. Real-time hydroacoustic instrumentation was installed at bridge piers, bathymetry of the streambed was surveyed before and after runoff, and topography of the riprap blanket at bridge abutments and cross sections near bridges also were surveyed. Finally, hydraulic data including water stage and slope and flow velocity were collected.
Start Date
5-3-2010 3:30 PM
End Date
5-3-2010 4:00 PM
Document Type
Presentation
Hydraulic Conditions and Scour at I-90 Bridges on Blackfoot River after Removal of Milltown Dam, Spring 2009
The 2008 removal of Milltown Dam on the Clark Fork, near Bonner was predicted to affect pier and abutment foundations of nearby bridges. The substantial contraction of streamflow at the bridge exerted hydraulic forces on these structures not experienced when they were in the backwater of Milltown Reservoir. Countermeasures used to mitigate predicted scour and erosion included substantial modification of piers and abutments of the I-90 bridges that cross the Blackfoot River near the confluence with the Clark Fork. With Milltown Dam removed and scour countermeasures completed, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, began monitoring hydraulic and scour conditions at the I-90 bridges beginning with the spring 2009 runoff season. The purposes of the multi-year monitoring effort are to measure the effects of high-flow conditions on the I-90 bridge piers and abutments and nearby stream channel morphology, to compare measured hydraulic and scour data to scour and erosion predictions from earlier models, to characterize the extent to which scour and erosion effects are due to local hydraulic factors versus more broad-based geomorphic factors, and to provide stream-velocity data for this reach to aid in evaluating the possibility of bull trout passage. Real-time hydroacoustic instrumentation was installed at bridge piers, bathymetry of the streambed was surveyed before and after runoff, and topography of the riprap blanket at bridge abutments and cross sections near bridges also were surveyed. Finally, hydraulic data including water stage and slope and flow velocity were collected.