This report provides the results of a detailed Level II analysis of scour potential at structure
TUNBTH00450033 on Town Highway 45 crossing the First Branch White River,
Tunbridge, 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 Appendix D.
The site is in the New England Upland section of the New England physiographic province
in central Vermont. The 86.4-mi 2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream
of the bridge, while woody vegetation sparsely covers the immediate banks.
In the study area, the First Branch White River has an incised, sinuous channel with a slope
of approximately 0.003 ft/ft, an average channel top width of 68 ft and an average bank
height of 7 ft. The channel bed material ranges from sand to gravel with a median grain size
(D50) of 27.1 mm (0.089 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on October 18, 1995, indicated that the reach was laterally unstable due to
a cut-bank present on the upstream right bank and a wide channel bar in the upstream reach.
The Town Highway 45 crossing of the First Branch White River is a 67-ft-long, one-lane
bridge consisting of one 54-foot timber thru-truss span (Vermont Agency of Transportation,
written communication, March 23, 1995). The opening length of the structure parallel to the
bridge face is 53.5 ft. The bridge is supported on the right by a vertical, concrete abutment
with an upstream wingwall, and on the left by a vertical, stone abutment. The channel is
skewed approximately 20 degrees to the opening while the computed opening-skew-toroadway is 10 degrees.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. Scour countermeasures at the site include type-1
stone fill (less than 12 inches diameter) along the upstream right wingwall, type-2 stone fill
(less than 36 inches diameter) along the right abutment, and type-3 stone fill (less than 48
inches diameter) along the upstream right bank. Additional details describing conditions at
the site are included in the Level II Summary and Appendices D and E.
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge was determined and analyzed as another potential worst-case scour scenario.
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
Contraction scour for all modelled flows ranged from 0.0 to 3.0 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 12.8
to 31.0 ft. Right abutment scour ranged from 9.8 to 19.0 ft. The worst-case left and right
abutment scour occurred at the 500-year discharge. Additional information on scour depths
and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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 33 (TUNBTH00450033) on Town Highway 45, crossing the First Branch White River, Tunbridge, Vermont