Streambed scour evaluations and conditions at selected bridge sites in Alaska, 2012

Abstract

Streambed scour potential was evaluated at 18 riverand stream-spanning bridges in Alaska that have unknown foundation details or a lack of existing scour analysis. All sites were evaluated for stream stability and long-term scour potential. Contraction scour and abutment scour were calculated for 17 bridges, and pier scour was calculated for 7 bridges that had piers. Vertical contraction (pressure flow) scour was calculated for sites with overtopping floods (where the modeled water surface was higher than the superstructure of the bridge). In most cases, hydraulic models of the 1-and 0.2-percent annual exceedance probability floods (also known as the 100-and 500-year floods, respectively) were used to derive hydraulic variables for the scour calculations. Alternate flood values were used in scour calculations for sites where smaller floods overtopped a bridge or where standard floodfrequency estimation techniques did not apply. Scour was also calculated for large recorded floods at several sites. Equations for scour in cohesive soils were used for sites where streambed sediment was silt-sized or smaller. Channel instability at four sites was related to human activities (in-channel mining, dredging, and channel relocation). Three of the dredged sites are located on active unstable alluvial fans and were graded to inhibit aggradation. The trend toward aggradation during major floods at these sites greatly reduces confidence in scour estimates. Vertical contraction and pressure flow occurred during 1 percent or smaller annual exceedance probability floods at five sites, including three aggradation sites. Contraction scour exceeded 5 feet at two sites, and total scour at piers (pier scour plus contraction scour) exceeded 5 feet at two sites. Debris accumulation increased calculated pier scour at six sites by an average of 1.2 feet. Total scour at abutments including contraction scour exceeded 5 feet at seven sites. Scour estimates seemed excessive at aggradation sites where upstream sediment supply controls scour and deposition processes, at cohesive soil sites where conservative assumptions were made for soil strength and flood duration, and for abutment scour at sites where failure of the embankment and attendant channel widening would reduce scour.

Measurement of long-term channel change through repeated cross-section surveys at bridge crossings in Alaska

“…More detail is included for sites that have been part of the real-time streambed scour monitoring network. The scope of this report is limited to a discussion of methods and presentation of data; interpretation of these data is presented in other USGS publications (Heinrichs and others, 2001;Conaway, 2004;Conaway and Schauer, 2012;Beebee and Schauer, 2015;Beebee and others, 2017).…”

“…Since 1994, USGS and the Alaska Department of Transportation and Public Facilities (ADOT&PF) have been collaborating to assess, model, and monitor streambed scour at bridges in Alaska. This collaboration has generated long-term data sets of hydraulic and geomorphic variable, hydraulic models, and publications of streambed scour assessments (Heinrichs and others, 2001;Conaway, 2004;Conaway and Schauer, 2012;Beebee and Schauer, 2015;Beebee and others, 2017). Bridges with abutment or pier foundations determined to be unstable owing to (1) observed scour at the site or (2) the potential for severe streambed scour as determined from a scour evaluation are designated as scour critical (Arneson and others, 2012).…”

Streambed scour evaluations and conditions at selected bridge sites in Alaska, 2016–17

“…The threshold for "substantial scour" was considered 5 ft in some previous Alaskan studies (Beebee and Schauer, 2015;Beebee and others, 2017), while 4 ft was the threshold in Conaway and Schauer (2012). These numbers are somewhat arbitrary, and the influence of scour on the bridge structure ultimately depends on the depth of the bridge foundations.…”

“…The U.S. Geological Survey (USGS) has been studying scour at Alaskan bridges since 1964 (Norman, 1975), in cooperation with the ADOT&PF. In 1994, the USGS began a phased process to provide hydraulic assessments of scour for more than 400 bridges throughout Alaska (Heinrichs and others, 2001;Conaway, 2004;Conaway and Schauer, 2012;Beebee and Schauer, 2015;Beebee and others, 2017).…”

Streambed scour evaluations and conditions at selected bridge sites in Alaska, 2013–15

“…In cooperation with the ADOT&PF, the USGS began a phased process in 1994 to provide hydraulic assessments of scour for bridges throughout Alaska (Heinrichs and others, 2001;Conaway, 2004;Conaway and Schauer, 2012;Beebee and Schauer, 2015). This study follows the approach of Beebee and Schauer (2015), using one-dimensional models and site-specific information, but includes updated methods for addressing unsteady flow, channel stability, flood frequency, abutment scour, and scour at piers in coarse-bedded streams.…”