This report provides the results of a detailed Level II analysis of scour potential at structure
TROYTH00290016 on Town Highway 29 crossing Beetle Brook, Troy, 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 Green Mountain section of the New England physiographic province in
north-central Vermont. The 8.70-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest.
In the study area, Beetle Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 42 ft and an average bank height
of 3 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D50) of 61.9 mm (0.203 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 8, 1995, indicated that the reach was stable.
The Town Highway 29 crossing of Beetle Brook is a 26-ft-long, one-lane bridge consisting
of one 23-foot log-beam span (Vermont Agency of Transportation, written communication,
March 7, 1995). The opening length of the structure parallel to the bridge face is 20.0 ft.
The bridge is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately 5 degrees to the opening while the measured opening-skew-toroadway is 10 degrees.
A scour hole 1.2 ft deeper than the mean thalweg depth was observed along the failed
downstream right wingwall during the Level I assessment. The scour counter-measures at
the site included type-2 stone fill (less than 36 inches diameter) at the upstream end of the
upstream right wingwall and at the downstream left wingwall. 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).
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 1.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.2 to
13.4 ft. The worst-case 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”. 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 16 (TROYTH00290016) on Town Highway 29, crossing Beetle Brook, Troy, Vermont