Sounding Cross Section Surveys at Alaska Bridge Crossings

Since 1993 the U.S. Geological Survey (USGS) has worked with the Alaska Department of Transportation and Public Facilities (ADOT&PF) to provide hydraulic assessments of scour for bridges throughout Alaska. As part of this effort, repeat channel cross section surveys, or channel soundings, have been collected at either the upstream or downstream side of bridges on an annual or as needed basis. Streambed and bank elevations are measured using USGS sounding weights and reels, weighted measuring tapes, acoustic Doppler current profilers, multibeam echo sounders and light detection and ranging and are referenced to the datum of as-built plan set to provide context for the streambed elevations in relation to bridge structures. Channel soundings are collected on an annual basis at most sites, however, as need soundings are collected due to flooding or periods of scour at select sites. Repeat channel soundings are used to access stream stability related to seasons, stage, and long-term aggradation or degradation as well as providing greater context for fixed mount streambed elevation monitoring.