This report provides the results of a detailed Level II analysis of scour potential at structure
BARTUS00050166 on U. S. Route 5 crossing the Barton River, Barton, Vermont (figures
1–8). A Level II study is a basic engineering analysis of the site, including a quantitative
analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of
a Level I scour investigation also are included in Appendix E of this report. A Level I
investigation provides a qualitative geomorphic characterization of the study site.
Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT)
files, was compiled prior to conducting Level I and Level II analyses and is found in
The site is in the New England Upland section of the New England physiographic province
of north-central Vermont in the town of Barton. The 65.2-mi2
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the banks have a
combination of dense woody vegetation coverage, brush, and field grasses.
In the study area, the Barton River has an incised, sinuous-to-meandering channel with a
slope of approximately 0.0065 ft/ft, an average channel top width of 58 ft and an average
channel depth of 4 ft. The predominant channel bed material is gravel (D50 is 75.6 mm or
0.25 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 19, 1994, indicated that the reach was stable.
The U. S. Route 5 crossing of the Barton River is a 126-ft-long, two-lane bridge consisting
of one 60-foot steel beam span with two steel-beam approach spans (Vermont Agency of
Transportation, written communication, August 4, 1994). The bridge is supported by two
concrete piers. The left bank has a concrete retaining wall that is attached to the US face of
the left pier; consequently this pier functions as an abutment for the analysis because no
flow occurs to the left of the pier. For the purposes of computing scour, this pier was
considered an abutment. The channel is skewed approximately 40 degrees to the opening
while the opening-skew-to-roadway is 25 degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the right pier
during the Level I assessment. Scour protection measures at the site consist of type-1 stone
fill (less than 12 inches diameter) along the entire base length of both piers. Additional
details describing conditions at the site are included in the Level II Summary and
Appendices D and E.
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.
Contraction scour for all modelled flows ranged from 1.1 to 2.4 ft. Abutment-type scour
was computed for the left pier; scour ranged from 9.1 to 11.3 ft. Abutment scour at the right
abutment ranged from 6.1 to 11.3 ft. Pier scour, computed for the right pier, ranged from
31.3 to 33.3 ft. The severity of the pier scour was directly related to the attack angle of 25
degrees. The worst-case scour in all computations occurred at the 500-year discharge.
Additional information on scour depths and depths to armoring are included in the section
titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths,
are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is
presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive
material and a homogeneous particle-size distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 37 (BARTUS00050166) on U.S. Route 5, crossing Barton River, Barton, Vermont