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

Scientific Investigations Report 2017-5149
Prepared in cooperation with the Alaska Department of Transportation and Public Facilities
By: , , and 
https://doi.org/10.3133/sir20175149

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Abstract

Streambed scour potential was evaluated at 52 river- and stream-spanning bridges in Alaska that lack a quantitative scour analysis or have unknown foundation details. All sites were evaluated for stream stability and long-term scour potential. Contraction scour and abutment scour were calculated for 52 bridges, and pier scour was calculated for 11 bridges that had piers. Vertical contraction (pressure flow) scour was calculated for sites 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 flood-frequency estimation techniques did not apply. Scour also was calculated for large recorded floods at 13 sites.

Channel instability at 11 sites was related to human activities (in-channel mining, dredging, and channel relocation). Eight of the dredged sites are located on active unstable alluvial fans and were graded to protect infrastructure. The trend toward aggradation during major floods at these sites reduces confidence in scour estimates.

Vertical contraction and pressure flow occurred during the 0.2-percent or smaller annual exceedance probability floods at eight sites. Contraction scour exceeded 5 feet (ft) at four sites, and total scour at piers (pier scour plus contraction scour) exceeded 5 ft at four sites. Debris accumulation increased calculated pier scour at six sites by an average of 2.4 ft. Total scour at abutments exceeded 5 ft at 10 sites. Scour estimates seemed excessive at two piers where equations did not account for channel armoring, and at four abutments where failure of the embankment and attendant channel widening would reduce scour.

Suggested Citation

Beebee, R.A., Dworsky, K.L., and Knopp, S.J., 2017, Streambed scour evaluations and conditions at selected bridge sites in Alaska, 2013–15: U.S. Geological Survey Scientific Investigations Report 2017-5149, 67 p., https://doi.org/10.3133/sir20175149.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Methods
  • Results of Flood Frequency and Scour Assessments
  • Scour Calculations
  • Summary and Conclusions
  • Acknowledgments
  • References Cited
  • Appendix 1. Stream Stability Cross Sections
Publication type Report
Publication Subtype USGS Numbered Series
Title Streambed scour evaluations and conditions at selected bridge sites in Alaska, 2013–15
Series title Scientific Investigations Report
Series number 2017-5149
DOI 10.3133/sir20175149
Year Published 2017
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Alaska Science Center Water
Description Report: vi, 67 p.; Appendix
Country United States
State Alaska
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details